Title of Invention	STRENGTH ENHANCING INSERT ASSEMBLIES
Abstract	Insert assemblies of high specific strength to reduce stress concentration at location where multidirectional stresses act on sandwich structure have been designed based on mapping stress distribution and failure initiation. The insert assembly comprises of insert, potting material, core, lower face-plate, upper face-plate and attachment. The insert materials are slelected from 2D woven composites,3D thermoelastic isotropic woven composites, 3D woven composites, 3D woven composites with multiple inserts and 3D functionally gradient woven composites. Specific strengths of inserts of present invention are higher than the inserts of prior art.

Title of Invention

STRENGTH ENHANCING INSERT ASSEMBLIES

Abstract

Insert assemblies of high specific strength to reduce stress concentration at location where multidirectional stresses act on sandwich structure have been designed based on mapping stress distribution and failure initiation. The insert assembly comprises of insert, potting material, core, lower face-plate, upper face-plate and attachment. The insert materials are slelected from 2D woven composites,3D thermoelastic isotropic woven composites, 3D woven composites, 3D woven composites with multiple inserts and 3D functionally gradient woven composites. Specific strengths of inserts of present invention are higher than the inserts of prior art.

Full Text	FORM - 2 THE PATENTS ACT, 1970 (39 OF 1970) COMPLETE SPECIFICATION (See section 10; rule 13) TITLE OF INVENTION "STRENGTH ENHANCING INSERT ASSEMBLIES" (a) INDIAN INSTITUTE OF TECHNOLOGY Bombay (b) having administrative office at Powai, Mumbai 400076, State of Maharashtra, India and (c) an autonomous educational Institute, and established in India under the Institutes of Technology Act 1961. The following specification particularly describes the nature of the invention and the manner in which it is to be performed. 19-10-2005 Field of the Invention This invention relates to insert assemblies of high specific strength to reduce stress concentrations at locations where multidirectional stresses act on sandwich structures designed based on mapping stress distribution and failure initiation. Background of the Invention Light weight sandwich structures are used in structural applications such as vehicles, aerospace industry, framework etc. because of their superior strength and stiffness properties along through-the-thickness direction under bending loads. The use of inserts is essential to strengthen the sandwich structures to withstand localized loads. Further, when the external members or sub-structures are attached to sandwich structures, inserts become a necessity. Sandwich structures are made of a three layer type of construction consisting of two thin sheets of higher stiffness and strength material between which a thick layer of lower strength and density is sandwiched. The two thin sheets on either side are called the face-plates, and the intermediate layer the core. The face¬plates resist the in-plane and bending loads. These are generally made of aluminum or polymer matrix composites. The core of a sandwich structure is generally made of foam or honeycomb. Sandwich structures are characterized by very high flexural stiffness-to-weight ratio. For a given set of mechanical loads, sandwich structures often result in a lower structural weight than do other configurations [Plantema, F. J. 1966. Sandwich Construction, John Wiley & Sons, Inc., New York;.Vinson, J. R. 1999. The Behavior of Sandwich Structures of Isotropic and Composite Materials, Technomic Publishing Co., Inc., Lancaster]." 2 The specific strength of an insert assembly is a ratio of load at failure initiation to weight of the insert assembly, which should be as high as possible to achieve effective utilization of sandwich structures with inserts. In practice, these inserts are made of aluminum alloys, other metals/alloys etc. High density of metals/alloys increases the weight of insert assembly resulting in undesirable reduction in the specific strength. Further, the difference in material properties at the interface between the insert and the potting material such as resin leads to high stress concentration at the interfaces between insert and potting material and between potting material and core. For structural members having a constant cross-section or a section with gradual change of contours, stress distribution is generally obtained using the elementary formulae. For such cases, stress distribution follows a regular trend. But, actual structural members are complex in nature, sometimes consist of two or more materials. For such cases, localized high stresses occur generally at the location of material discontinuities or geometrical discontinuities. This localization of high stresses is known as stress concentration [Peterson, R. E. 1974. Stress Concentration Factors, John Wiley & Sons, New York; Naik, N. K. and Rajaiah, K. 1984. Minimum stress concentration factor for two neighbouring holes in disks by photoelasticity, ASCE Journal of Engineering Mechanics, Vol. 110, pp. 654-659]. US Patent 5240543 discloses a basic procedure to seat a fastener insert in a honeycomb panel. A fastener insert made of alloy attached to a mounting fixture is inserted into a hole drilled in a honeycomb panel. The mounting fixture has a rod with a disc at an end descending from a base and a chimney ascending from the base. Two bores are located in the base and are enclosed by the chimney. The mounting fixture base overlaps the head of the fastener insert which has two bores in the head. The bores in the fastener insert head are aligned with bores in the base of the mounting fixture. Epoxy is inserted into one of the bores in the base of the mounting fixture continuously until it returns out of the second bore. 3 The epoxy is allowed to cure and thereafter the mounting fixture is removed by a turning motion. Though this patent describes the method of installation of inserts into honeycomb sandwich structures, it does not address the issue of minimization of the insert assembly weight and a means to increase specific strength. US Patent 5082405 and US 4941785 disclose the geometry of the inserts and the method to attach the insert to the attachment. In this insert assembly, epoxy resin is used as potting compound. The insert is a metallic member, made of stainless steel, aluminum alloy, and carbon steel. However, metal insert increases weight of the insert assembly resulting in reduction of specific strength. US Patent 5053285 discloses the method and apparatus for making corrugated aluminum inserts. These inserts are made from aluminum foil strips by passing the strips through the corrugating device. Though the weight of such inserts is less, stresses at the interfaces of different materials increase because the elastic properties of corrugated aluminum inserts are less resulting in higher stress concentrations. US Patent 6055790 discloses construction of an insert wherein the conduction and radiation are improved through the inserts. To improve the heat transfer rate, one of the face-plates is made of metal sheet. The insert material is aluminum alloy. In this arrangement, a higher thermally conducting path is provided from one side of the insert assembly to the other side. However, these inserts suffer from the deficiency such that the surrounding potting material made of resin which is not a good conductor of heat that leads to thermal gradient along the radial direction. This causes higher thermal stresses in the sandwich structure. The use of metal for face-plate increases weight of insert assembly thereby decreasing the specific strength. 4 US Patent 3271498 discloses an improved method of installation of inserts. However it does not address the issue of weight of an insert assembly. A method of fabricating a honeycomb core structure with embedded fastener is disclosed in US Patent 4716067. The bonding material is epoxy resin. The method comprises laying down a first nonmetallic synthetic layer, which may be cured or uncured. Bonded to the first nonmetallic synthetic layer is a honeycomb core layer into which one or more flush head fasteners are inserted into holes in the honeycomb core that have an indentation at one end configured to mate with the underside of the head of a flush head fastener. Bonding is accomplished by priming the walls of the honeycomb core layer adjacent the surfaces of the layer, and the fastener, with a suitable primer, preferably before the fastener is installed, and, coating one of the surfaces of the first nonmetallic synthetic layer with a suitable adhesive before joining the first nonmetallic synthetic layer to the surface of the honeycomb core layer containing the head of the flush head fastener. Thereafter, the cells of the honeycomb core that surround the fastener are filled with a potting material. Then, a second nonmetallic synthetic layer is bonded to the surface of the honeycomb core remote from the surface to which the first nonmetallic synthetic layer is bonded. There are several drawbacks of this method and insert assembly. The fasteners are made of an alloy. The density of the alloys used is more compared to the composites. Significantly high interfacial stresses develop between the alloy insert and the potting resin material. Further, this patent describes the method of installation of inserts into honeycomb sandwich structures. It does not address the issue of enhancing the specific strength of insert assemblies. The shortcomings in prior art vis-a vis use of inserts in sandwich structure are • Use of metal for inserts which leads to decrease in specific strength of the insert assembly. • Though geometry of the inserts and different methods of installation of these inserts into sandwich structures is addressed, the issue of 5 minimization of the insert assembly weight and a means to increase specific strength of insert assembly has not been addressed. • " There are no methods of reliably mapping stress distribution and prediction of failure initiation in sandwich structures with diverse ! geometrical configurations. Summary of the Invention The main object of the invention is to provide insert assemblies of high specific strength to reduce stress concentrations at locations where multidirectional stresses act on sandwich structures based on mapping stress distribution and failure initiation in sandwich structures. It is another objective to provide inserts of composite materials. It is yet another object of the invention to reliably map stress distribution in sandwich structures with insert assemblies. It is yet another object of the invention to reliably to select diverse geometrical configurations and materials by mapping stress distribution and obtaining failure initiation. It is yet another object of the invention to provide insert assemblies with through-the-thickness, fully potted and partially potted geometrical configurations. It is yet another object of this invention to explore the use of • 2D composites • 3D thermo elastic isotropic woven composites • 3D woven composites • 3D woven composites with multiple inserts, • 3D functionally gradient woven composites as inserts 6 and their combination for insert assemblies Thus in accordance, the invention of the insert assembly comprises of. • insert • potting material • core • lower face-plate • upper face-plate • attachment wherein attachment is linked to insert and insert is bonded to core, lower face¬plate and upper face-plate by means of potting material wherein insert materials are selected from • 2D composites • 3D thermoelastic isotropic woven composites • 3D woven composites • 3D woven composites with multiple inserts • 3D functionally gradient woven composites as inserts and their combination for the insert assemblies wherein high of specific strength is achieved using appropriate geometrical configurations of the insert assembly Detailed Description of the invention Features and advantages of this invention will become apparent in the following detailed description and preferred embodiments with reference to the accompanying drawings. Figure 1 Cross section and plan view of through-the-thickness insert assembly Figure 2 Cross section and plan view of fully potted inserts Figure 3 Cross section and plan view of partially potted inserts 7 Geometrical configurations The geometrical configurations are a) Through-the-thickness insert configuration b) Fully potted insert configuration c) Partially potted insert configuration Fully potted inserts are having potting material thickness equal to the thickness of the core, and the length of the insert equal to or less than the thickness of the core; Partially potted inserts are having potting material thickness less than the thickness of the core and equal to or more than the length of the insert inserted into the core. Through-the-thickness inserts is a special case of fully potted inserts in which the length of the insert is equal to or more than the total thickness of the core and face-plates [European Space Agency. June 1987. Insert Design Handbook, ESA PSS-03-1202, Issue 1, Paris]. a) Through-the-thickness insert configuration Figure 1 shows schematic of the through-the-thickness insert assembly configuration. Insert 10 is cylindrical in shape with flanges 11, 12 that are integral to provide shear resistance. The inserts are strongly attached with attachment 13 with bonding / threading. The localized external loads are applied to sandwich structures through the attachment 13. The insert-attachment assembly is held in sandwich structure by using potting materials 14 as shown in Figure 1. The potting materials are different types of resins. Reinforcement is added to the resins to increase the stiffness and strength. Upper face-plate 15, lower face-plate 16 and core 17 are the other components of the insert assembly. In one of the embodiments of through-the-thickness insert assembly configuration, material of insert is 2D composite. 8 In another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D thermoelastic isotropic woven composite. In yet another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D woven composite. In another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D woven composite with multiple inserts. In yet another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D functionally gradient woven composite. In another embodiment of through-the-thickness insert assembly configuration, material of insert is a combination of the above mentioned. b) Fully potted insert configuration Figure 2 shows schematic of the fully potted insert assembly configuration. Insert 20 is cylindrical in shape with flanges 21, 22 that are integral to provide shear resistance. The inserts are strongly attached with attachment 23 with bonding / threading. The localized external loads are applied to sandwich structures through the attachment 23. The insert-attachment assembly is held in sandwich structure by using potting materials 24 as shown in Figure 2. The potting materials are different types of resins. Reinforcement is added to the resins to increase the stiffness and strength. Upper face-plate 25, lower face¬plate 26 and core 27 are the other components of the insert assembly. In one of the embodiments of fully potted insert assembly configuration, material of insert is 2D composite. 9 In another embodiment of fully potted insert assembly configuration, material of insert is 3D thermoelastic isotropic woven composite. In yet another embodiment of fully potted insert assembly configuration, material of insert is 3D woven composite. In another embodiment of fully potted insert assembly configuration, material of insert is 3D woven composite with multiple inserts. In yet another embodiment of fully potted insert assembly configuration, material of insert is 3D functionally gradient woven composite. In another embodiment of fully potted insert assembly configuration, material of insert is a combination of the above mentioned. c) Partially potted insert configuration Figure 3 shows schematic of the partially potted insert assembly configuration. Insert 30 is cylindrical in shape with flanges 31, 32 that are integral to provide shear resistance. The inserts are strongly attached with attachment 33 with bonding / threading. The localized external loads are applied to sandwich structures through the attachment 33. The insert-attachment assembly is held in sandwich structure by using potting materials 34 as shown in Figure 3. The potting materials are different types of resins. Reinforcement is added to the resins to increase the stiffness and strength. Upper face-plate 35, lower face¬plate 36 and core 37 are the other components of the insert assembly. In one of the embodiments of partially potted insert assembly configuration, material of insert is 2D composite. 10 In another embodiment of partially potted insert assembly configuration, material of insert is 3D thermoelastic isotropic woven composite. In yet another embodiment of partially potted insert assembly configuration, material of insert is 3D woven composite. In another embodiment of partially potted insert assembly configuration, material of insert is 3D woven composite with multiple inserts. In yet another embodiment of partially potted insert assembly configuration, material of insert is 3D functionally gradient woven composite. In another embodiment of partially potted insert assembly configuration, material of insert is a combination of the above mentioned. Insert materials The insert materials are selected from I) 2D woven composites II) 3D woven composites III) 3D thermoelastic isotropic woven composites IV) 3D functionally gradient woven composites V) 3D woven composite with multiple inserts I) 2D composites 2D composites are those in which only in-plane reinforcements are provided. In laminated composites made of unidirectional layers, different layers are oriented accordingly to achieve required elastic and strength properties. In woven fabric composites, reinforcements are provided along mutually perpendicular directions in the same layer by the process of weaving. Weaving is the process in which the woven fabric is formed by interlacing warp and fills (weft) yarns in regular 11 sequence of under and over. Based on the sequence of placing the yarns under and over, the woven fabrics are classified into plain, twill and satin. Specifically, one under and one over sequence is the plain weave. II) 3D woven composites 3D woven performs are fully integrated continuous fiber assembly having multiaxial in-plane and out of plane fiber orientations. In such preforms, reinforcement is also provided in through-the-thickness direction in addition to planar directions. These preforms are made using the process of 3D weaving. Based on the weave pattern, the preforms can be orthogonal interlock woven or angle interlock woven. Further, it can be classified into through-the-thickness woven and layer to layer woven. Using the 3D preforms and resin transfer molding, 3D composites are made. 3D composites are 3D orthotropic and macrospecically homogeneous materials. Such materials are characterized by 9 elastic properties and 9 strength properties. III) 3D thermoelastic isotropic woven composites These are a class of 3D composites with special characteristics. For such materials, elastic and thermal properties are the same along all the directions. In other words, such composites are thermoelastically isotropic. IV) 3D functionally gradient woven composites 3D functionally gradient woven composites are the ideal materials for making inserts. In a typical insert assembly with single insert material, there is a significant difference in material elastic properties between insert and potting material and potting material and core. Through-the-thickness elastic properties of insert, potting material and core are of the order of 40 GPa, 2.5 GPa and 0.31 GPa respectively. This leads to higher stress concentrations at the interfaces between attachment and insert, insert and potting material, potting material and core. An ideal way of reducing the stress concentrations and increasing the specific strength is to use a material system for inserts with gradually varying elastic and strength properties along the radial direction. Such a material is called 12 functionally gradient. A composite insert made of functionally gradient material has through-the-thickness elastic properties nearly matching with that of the attachment along the inner circumference and through-the-thickness elastic properties matching with that of potting material along the outer circumference. Consequently, elastic properties of the insert with such a configuration would be higher along the inner circumference and lower along the outer circumference and varying radially in a functionally gradient way. For such insert materials, the stress concentrations at the interfaces would be minimum leading to higher specific strength. V) 3D woven composites with multiple inserts Inserts made of 3D woven composite with multiple inserts is a class of inserts made of 3D functionally gradient woven composite. In this insert, the elastic and strength properties are not varied gradually along the radius from the inner circumference to the outer circumference. Instead, the elastic and strength properties are varied in a stepped manner. This amounts to using multiple inserts instead of a single insert. If the material properties are varied in three steps, there would be three different 3D woven composites. These three different 3D woven composites are referred as material 1, material 2 and material 3. The materials and geometry of the insert assembly is decided by mapping stress distribution for obtaining failure conditions by the following process: A method is described to map stress distribution in sandwich structures with inserts under localized through-the-thickness tensile / compressive loading. The core is relatively thick and compressible whereas the face-plates are relatively thin. The core is analyzed using higher-order sandwich plate theory whereas the face-plates are analyzed using classical plate theory. The behavior of the sandwich structure is represented using a set of 24 governing differential equations. For the geometry of the insert assembly and the loading conditions 13 In the above method the stress distribution is mapped and failure initiation is obtained in steps comprising ♦ Establishing frame of reference for sandwich structures with inserts ♦ Applying equilibrium equations, continuity conditions and constitutive relations of the core and the face-plates to obtain 24th order partial differential equation consisting of fundamental variables such as Mid-plane radial displacement of upper and lower face-plates in radial direction, Mid-plane circumferential displacement of upper and lower face¬plates in circumferential direction, Transverse displacement of upper and lower face-plates, Derivative of transverse displacement with respect to radius of insert assembly of upper and lower face-plates, Derivative of transverse displacement with respect to circumferential direction of upper and lower face-plates and divided by radius of insert assembly, Bending moment resultant of upper and lower face-plates in radial direction, Twisting moment resultant in the plane of radial and circumferential coordinates of upper and lower face-plates, 16 All the Cjj are stiffness constants and are calculated using elastic properties of the material. The normal and shear stress distribution within the entire insert assembly can be obtained using this novel method. The induced stress state can lead to initiation of failure within insert assembly. Initiation of failure is characterized using the following through-the-thickness quadratic interaction failure criterion. (5) Here, 15 considered the boundary conditions are specified. In this case, there are 24 boundary conditions. Using the differential equations and the set of 24 boundary conditions, the deformation behavior and stress state within the insert assembly are calculated. Specifically, the following quantities are determined: transverse displacement of top and bottom face-plates (w1 and w2), transverse shear stress {Trz), mid-plane radial displacement of top face-plate (ul0l), mid-plane circumferential displacement of top face-plate (vl0l). The novel method to reliably map stress distribution is described in the form of flow diagrams in Figures 4 and 5. The input parameters are: geometry of the insert assembly, elastic properties of different materials used for making the insert assembly and the loading condition. This novel method enables to map the following parameters reliably: • transverse and radial displacements of the insert assembly • normal and shear stress components throughout the insert assembly Equations used for mapping stress distribution for obtaining failure conditions are: The transverse displacement of the core material, (2) Radial displacement in the core material, 14 In-plane normal stress resultant in radial direction of upper and lower face-plates, In-plane normal stress resultant in the plane of radial and circumferential coordinates of upper and lower face-plates, In-plane shear stress resultant in the plane of radial and circumferential coordinates of upper and lower face-plates, Transverse shear stress component of core in the plane of radial and circumferential coordinates, Shear stress in circumferential direction on a plane perpendicular to through-the-thickness direction in the core, Derivative of shear stress in circumferential direction on a plane perpendicular to through-the-thickness direction in the core with respect to radius of the insert assembly, Derivative of transverse shear stress component of core in the plane of radial and circumferential coordinates with respect to radius of the insert assembly, and further to obtain 24 first order coupled exact differential equations ♦ Applying boundary conditions to the 24 first order coupled exact differential equations and solving two point boundary value problem to obtain stresses (equation 2), displacements (equations 1,3,4) and failure initiation (equation 5) wherein, frame of reference is established based on I. The attachment is infinitely rigid II. Insert and potting material are an integral part of the core for mathematical formulation 17 III. In-plane shear stress and in-plane normal stresses are neglected in the core material IV. Core material is flexible in nature V. Effective shear modulus of the honeycomb core is considered in modeling VI. Insert assembly is circular in shape VII. Interaction between two adjacent inserts is negligible VIII. Interaction between the insert and the honeycomb core along the circumference of the insert assembly is negligible IX. Face-plates are homogeneous and isotropic / quasi-isotropic X. Classical plate theory is applicable for the analysis of the face-plates and wherein, the steps to obtain 24 first order coupled exact differential equations involve a. Representing the behavior of the sandwich structure with an insert assembly using a set of plurality of equations based on equilibrium equations, constitutive relations and continuity conditions b. Combining the core and face-plate equations to obtain 24th order governing partial differential equation with 24 unknown fundamental variables c. Rearranging the governing partial differential equation to 24 first order coupled partial differential equations in terms of 24 fundamental variables, their derivatives with respect to circumferential angle and radius using plurality of equations d. Eliminating the dependency of derivatives of circumferential angle in the 24 first order coupled partial differential equations using Fourier expansions to convert them into 24 first order coupled exact differential equations and wherein, stresses, displacements and failure initiation are obtained by 18 I. Specifying 24 boundary conditions with respect to 24 first order coupled exact differential equations with 12 boundary conditions at the interface of attachment and insert and remaining at simply supported outer edge of the insert assembly. II. Constituting a two point boundary value problem comprising 24 first order coupled exact differential equations and boundary conditions III. Converting two point boundary value problem into a series of initial value problems by dividing the sandwich structure into a number of segments along radial direction IV. Solving the series of initial value problems numerically using multi-segment method of direct integration for 24 fundamental variables at each segment using continuity conditions between two adjacent segments to determine the stresses and displacements throughout the insert assembly for given loading conditions V. Obtaining the failure initiation within the insert assembly using quadratic failure criterion and the corresponding failure initiation load VI. Obtaining transverse, radial and circumferential displacements (equations 1,3,4), through-the-thickness normal (equation 2) and shear stress components in the core, induced normal stress resultants, induced shear stress resultants and induced bending moment resultants in the face-plates and specific strength of the insert assembly based on geometrical, mechanical and physical properties of the insert assembly and loading conditions. The method described above was used to obtain geometrical configuration of the inserts and the displacement and failure initiation were predicted and experimentally verified. The present work establishes the superiority of the inserts of the present invention over inserts of prior art. 19 Example 1 Experimental studies ♦ Fabrication of Through-the-Thickness Insert Assembly The insert assembly comprises of six constituents. They are: insert, potting material, foam core, lower face-plate, upper face-plate and the attachment. Lower face-plate and the upper face-plate are made of woven fabric E glass and epoxy resin using matched-die molding technique. The core is made of polyurethane foam. The attachment is made of mild steel. The material used for composite inserts is glass. The potting material is epoxy resin. Three insert assemblies were constructed using the above with aluminum, 2D woven composite and 3D woven composite as insert materials. • Measurement of load, displacements and failure initiation • The insert assembly was placed on a support ring and then located on Hounsfield Test Equipment-450 KS, 50 KN UTM. • Compressive load was applied through the attachment on to the insert assembly. • The displacement of the attachment, lower face-plate, upper face-plate and the corresponding load were measured at loading rate of 0.25 mm/min. • Failure initiation of the insert assembly is obtained from sudden change in the load-displacement plot. 20 Experimental Results Transverse displacement as a function of compressive load for through-the-thickness inserts is presented in Figures 6-8. For the same geometrical configurations and material properties (Tables 1 and 2), analytically obtained transverse displacement plots, compressive load at failure initiation and specific strength of inserts are presented in Figures 6-8 and Tables 3 and 4. The compressive loading was applied until the failure initiation took place. Failure functions (equation 5) are plotted as a function of compressive load in Figure 9. It is observed from Tables 3 and 4 that the compressive load at failure is higher for the case of 3D woven composite compared to the aluminum as insert material. The specific strength of insert is significantly higher for the case of 3D woven composite compared to the aluminum insert case. For the case of 2D woven composite insert, it is in between aluminum and 3D woven composite. Table 1. Geometrical configuration of the insert assembly for the experimental studies. Configuration Diameter, mm Thickness, mm Attachment, Da Insert, D, Potting material, DD Support, Dh Upper plate, ti Lower plate, t2 Core, c 1 5 10 40 120 2 2 10 21 Table 2. Material properties of foam core sandwich structure with inserts: used for experimental studies. Material Young"s modulus, Ez (GPa) Shear modulus, Grz (GPa) aluminum 70 27 2D woven composite 6 2.5 3D woven composite 10 4.5 Epoxy resin 2.5 0.93 Foam core 0.025 0.009 Face-plate 1 10* 4.2* Face-plate 2 10 4.2* Table 3. Specific strength of through-the-thickness inserts with different materials:experimental studies. Volume of insert, V=1.22x 10-6 m3 Density of aluminum = 2800 Kg /m3 Density of 2D woven composite = 1700 Kg/m3 Density of 3D woven composite = 1700 Kg/m3 S.No. Insert At failure initiation Mass of insert, m(Kg) Specific strength of insert = (Max. compressive load/ weight of insert) % increase in specific strength of insert % decrease in mass of insert Compre ssive load, Qc(N) Displace ment, w (mm) 1 Aluminum 5610 5.9 3.416xl0"3 0.1642xl06 Reference Reference 2 2D woven composite 5420 5.2 2.074 xlO-3 0.2613xl06 59.10 39.29 3 3D woven composite 5695 7.7 2.074 xlO-3 0.2746 xlO6 67.23 39.29 22 Table 4. Specific strength of through-the-thickness inserts with different materials: analytical predictions for experimental configurations. Volume of insert, V= 1.22x10"6 m3 3D woven composite insert: Zt = 45 MPa, Srz = 36 MPa 2D woven composite insert: Zt = 27 MPa, Srz = 36 MPa Aluminum insert: Zt = 150 MPa, Srz = 30 MPa S.No. Insert At failure initiation Specific strength of insert = (Max. compressive load / weight of insert) % increase in specific strength of insert % decrease in mass of insert Compressive load, Qc (N) Displacement, w (mm) 1 Aluminum 6355 6.1 0.186x106 Reference Reference 2 2D woven composite 5950 5.4 0.287x106 54.30 39.29 3 3D woven composite 7040 7.3 0.339* 106 82.25 39.29 Example 2 Comparison of through-the-thickness inserts of present invention with inserts of prior art Using the method of the present invention, compressive load at failure initiation, failure function and specific strength of inserts are mapped for the prior art disclosed in US Patent 50532285 (corrugated aluminum insert) and for the insert (3D woven composite insert) of the present invention with the same geometry as used for the prior art (R = 30 mm). The results are given in Figure 10 and Table 5. Further, the geometry of the insert of the present invention was modified (R = 10 mm) to reduce the insert assembly weight. With such a modified configuration, compressive load at failure initiation, failure function and specific strength of inserts are mapped. From Figure 10 and Table 5, it is established that the inserts of the present invention are having higher specific strength compared to the insert of prior art. 23 Table 5. Specific strength of through-the-thickness inserts with corrugated aluminum and 3D woven composite: analytical studies. Density of Corrugated aluminum = 459 Kg / m3 Density of 3D woven composites = 1700 Kg / m3 Insert Compressive load at failure initiation, Qc (KN) Mass of insert, m (Kg) Specific strength of insert = (Max. compressive load / weight of insert) % increase in specific strength of insert % decrease in mass of insert Corrugated aluminum 8.27 0.038 21.62x103 Reference Reference 3D woven composite with radius of 30 mm 15.85 0.065 24.23 x103 12.1 -71.0 3D woven composite with radius of 10 mm 14.08 0.0068 206.77x103 856.6 82.2 24 Example 3 Analytical studies with different insert materials Using the experimentally validated method for mapping of stresses, displacements and failure initiation, the results for aluminum, 2D woven composite, 3D thermoelastic isotropic woven composite, 3D woven composite, 3D woven composite with multiple inserts and 3D functionally gradient woven composite are obtained for the geometrical configuration of the insert assembly as given in Table 6. Material properties of the insert assembly are presented in Tables 7 and 8. Maximum displacement, maximum normal stress and maximum shear stress corresponding to maximum compressive load at failure initiation are presented in Table 9. Specific strength of insert for different materials is also presented in Table 9. Failure function as a function of compressive load for different insert materials is presented in Figure 11. Percentage increase in specific strength of insert and percentage decrease in mass of insert for the composite inserts compared to the aluminum insert is presented in Table 9. It is observed that there is significant increase in specific strength and decrease in mass of insert for the composite inserts. The maximum gain is for the case of 3D functionally gradient woven composite inserts. Table 6. Geometrical configuration of the insert assembly for the analytical studies. Configuration Diameter, mm Thickness, mm Attachment, Da Insert, Di Potting material ,DD Support ,Dh Upper plate, Lower plate, t2 Core, c 1 10 20 50 120 2 2 10 25 Table 7. Material properties of the insert assembly. Volume of insert, V= 4.006 x10"6 m3 3D woven composite insert: Zt = 60 MPa, Srz = 36 MPa, 2D woven composite insert: Zt = 27 MPa, Srz = 36 MPa Aluminum insert: Zt = 150 MPa, Srz = 30 MPa, 3D thermoelastic isotropic woven composite insert: Zt = 55 MPa, Srz= 36 MPa Epoxy : Zt = 38 MPa, Zc = 85 MPa, S - 42 MPa 3D woven composite with multiple inserts: Zt = 60 -> 38 MPa, Srz = 36 MPa 3D functionally gradient woven composite insert: Zt = 55 -> 38 MPa, Srz = 36 MPa Material Young"s modulus, Ez (GPa) Shear modulus, Gz (GPa) Aluminum 70 27 2D woven composite 9 4 3D thermoelastic isotropic woven composite 37.7 3.71 3D woven composite 43 3.55 Insert material 1 37.7 3.71 Insert material 2 22.0 2.5 Insert material 3 8 1.5 3D functionally gradient woven composite 37.1 -- 2.5 3.71 - 0.93 Honeycomb 0.310 0.138 Epoxy resin 2.5 0.93 Face-plate 1 15* 4.5 Face-plate 2 15* 4.5* * in-plane properties 26 Tables 2, 7 and 8 are based on the following references. 3D functionally gradient woven composite inserts are analyzed for the range of properties given in Table 7. References: 1. Naik, N. K. and E. Sridevi. 2002. An analytical method for thermoelastic analysis of 3D orthogonal interlock woven composites, Journal of Reinforced Plastics and Composites, Vol. 21, pp. 1149-1191. 2. Naik N. K. et al. 2001. Stress and failure analysis of 3D orthogonal interlock woven composites, Journal of Reinforced Plastics and Composites, Vol. 20, pp. 1485-1523. 3. Naik, N. K. and V. K. Ganesh. 1996. Failure behavior of plain weave fabric laminates under on-axis uniaxial tensile loading: II - analytical predictions, Journal of Composite Materials, Vol. 30, pp. 1779-1822. 4. Shembekar, P. S. and N. K. Naik. 1992. Elastic behavior of woven fabric composites: II - laminate analysis, Journal of Composite Materials, Vol. 26, pp. 2226-2246. 5. Engineered Materials Handbook, Vol. 1, Composites, 1989, ASM International, Materials Park, OH. Table 8. Elastic properties of orthotropic composite inserts (at 0=0). Young"s modulus Shear modulus Poisson"s ratio Material Er (GPa) Ee (GPa) Ez (GPa) Gr6 GPa) Grz (GPa) G9z (GPa) »rB »rz v6z 3D thermoelastic isotropic woven composite 37.1 37.5 37.7 3.71 3.71 3.71 0.111 0.106 0.106 3D woven composite 46.7 16.4 43.1 3.57 3.55 3.55 0.088 0.222 0.225 27 Table 9. Specific strength of through-the-thickness inserts with different materials: analytical studies. Volume of insert, V= 4.006x 10"6 m3 Density of Aluminum = 2800 Kg / m3 Density of 2D and 3D woven composites = 1700 Kg / m3 Density of 3D functionally gradient woven composite = 1700 -> 1100 Kg / m S.No. Insert At failure initiation Mass of insert, m (Kg) 10~3 Specific strength of insert = (Max. compressive load / weight of insert) % increase in specific strength of insert % decrease in mass of insert Compre ssive load, Qc (KN) Displace ment, w (mm) 1 Aluminum 13.15 1.40 11.217 0.117x106 Reference Reference 2 2D woven composite 14.65 1.48 6.810 0.215x106 83.93 39.29 3 3D thermoelastic isotropic woven composite 14.45 1.42 6.810 0.212x106 81.19 39.29 4 3D woven composite 14.90 1.49 6.810 0.219x106 87.18 39.29 5 3D woven composite with multiple inserts 15.90 1.32 6.009 0.265x106 126.50 46.43 6 3D functionally gradient woven composite 16.35 1.62 5.176 0.316x106 170.09 53.86 28 We Claim: 1. An insert assembly of high specific strength for sandwich structures comprising a core with an opening, sandwiched between an upper face¬plate and a lower face-plate; an insert-attachment assembly consisting of a flanged tubular insert and an attachment means positioned inside the said flanged tubular insert; the said insert-attachment assembly positioned inside the said opening of the core, extending from the upper face-plate to the lower face-plate; and potting material bonded to the core and the said insert-attachment assembly; wherein materials for making the tubular insert are selected from • 2D composites • 3D thermoelastic isotropic woven composites • 3D woven composites • 3D functionally gradient woven composites and their combination thereof, and the geometrical configurations of the insert assembly selected from: • fully potted having potting material thickness equal to the thickness of the core and the length of the tubular insert equal to or less than the thickness of the core; or • partially potted having insert-attachment assembly extending from the upper face-plate and terminating above the lower face-plate; potting material thickness being less than the thickness of the core; or • through-the-thickness having insert-attachment assembly extending from the upper face-plate to the lower face-plate; length of the tubular being equal to the total thickness of the core and the two face-plates; and wherein the said materials and geometry of the said insert assembly are decided by mapping stress distribution for obtaining failure conditions, as described herein. 30 2. An insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein potting material is selected from resins such as epoxy, polyester, polyimide and their like. 3. An insert assembly of high specific strength for sandwich structures as claimed in claims 1-2 wherein upper face-plate is selected from material with density of 1700 - 7800 Kg / m3, Young"s modulus of 5 - 200 GPa, Shear modulus of 2 - 77 GPa and Poisson"s ratio of 0.108 - 0.35. 4. An insert assembly of high specific strength for sandwich structures as claimed in claims 1-2 wherein lower face-plate is selected from material with density of 1700 - 7800 Kg / m3, Young"s modulus of 5 - 200 GPa, Shear modulus of 2 - 77 GPa and Poisson"s ratio of 0.108 - 0.35. 5. An insert assembly of high specific strength for sandwich structures as claimed in claims 1 & 3 wherein the profile of the upper face-plate is flat or curved or a combination thereof. 6. An insert assembly of high specific strength for sandwich structures as claimed in claims 1 & 4 wherein the profile of lower face-plate is flat or curved or a combination thereof. 7. An insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein specific strength of the insert is enhanced by about 200 % with respect to aluminum and about 500 % with respect to other metals. 8. An insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein upper face-plate is selected from materials such as aluminum, aluminum alloys and fiber reinforced plastic composites. 31 9. Insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein lower face-plate is selected from materials such as aluminum, aluminum alloys and fiber reinforced plastic composites. Dated 2nd day of December 2004 Dr. Prabuddha Ganguli Agent on behalf of Applicant 32

Full Text

FORM - 2
THE PATENTS ACT, 1970
(39 OF 1970)
COMPLETE SPECIFICATION
(See section 10; rule 13)
TITLE OF INVENTION
"STRENGTH ENHANCING INSERT ASSEMBLIES"
(a) INDIAN INSTITUTE OF TECHNOLOGY Bombay (b) having administrative office at Powai, Mumbai 400076, State of Maharashtra, India and (c) an autonomous educational Institute, and established in India under the Institutes of Technology Act 1961.

The following specification particularly describes the nature of the invention and the manner in which it is to be performed.
19-10-2005

Field of the Invention
This invention relates to insert assemblies of high specific strength to reduce stress concentrations at locations where multidirectional stresses act on sandwich structures designed based on mapping stress distribution and failure initiation.
Background of the Invention
Light weight sandwich structures are used in structural applications such as vehicles, aerospace industry, framework etc. because of their superior strength and stiffness properties along through-the-thickness direction under bending loads. The use of inserts is essential to strengthen the sandwich structures to withstand localized loads. Further, when the external members or sub-structures are attached to sandwich structures, inserts become a necessity.
Sandwich structures are made of a three layer type of construction consisting of two thin sheets of higher stiffness and strength material between which a thick layer of lower strength and density is sandwiched. The two thin sheets on either side are called the face-plates, and the intermediate layer the core. The face¬plates resist the in-plane and bending loads. These are generally made of aluminum or polymer matrix composites. The core of a sandwich structure is generally made of foam or honeycomb. Sandwich structures are characterized by very high flexural stiffness-to-weight ratio. For a given set of mechanical loads, sandwich structures often result in a lower structural weight than do other configurations [Plantema, F. J. 1966. Sandwich Construction, John Wiley & Sons, Inc., New York;.Vinson, J. R. 1999. The Behavior of Sandwich Structures of Isotropic and Composite Materials, Technomic Publishing Co., Inc., Lancaster]."
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The specific strength of an insert assembly is a ratio of load at failure initiation to weight of the insert assembly, which should be as high as possible to achieve effective utilization of sandwich structures with inserts. In practice, these inserts are made of aluminum alloys, other metals/alloys etc. High density of metals/alloys increases the weight of insert assembly resulting in undesirable reduction in the specific strength. Further, the difference in material properties at the interface between the insert and the potting material such as resin leads to high stress concentration at the interfaces between insert and potting material and between potting material and core.
For structural members having a constant cross-section or a section with gradual change of contours, stress distribution is generally obtained using the elementary formulae. For such cases, stress distribution follows a regular trend. But, actual structural members are complex in nature, sometimes consist of two or more materials. For such cases, localized high stresses occur generally at the location of material discontinuities or geometrical discontinuities. This localization of high stresses is known as stress concentration [Peterson, R. E. 1974. Stress Concentration Factors, John Wiley & Sons, New York; Naik, N. K. and Rajaiah, K. 1984. Minimum stress concentration factor for two neighbouring holes in disks by photoelasticity, ASCE Journal of Engineering Mechanics, Vol. 110, pp. 654-659].
US Patent 5240543 discloses a basic procedure to seat a fastener insert in a honeycomb panel. A fastener insert made of alloy attached to a mounting fixture is inserted into a hole drilled in a honeycomb panel. The mounting fixture has a rod with a disc at an end descending from a base and a chimney ascending from the base. Two bores are located in the base and are enclosed by the chimney. The mounting fixture base overlaps the head of the fastener insert which has two bores in the head. The bores in the fastener insert head are aligned with bores in the base of the mounting fixture. Epoxy is inserted into one of the bores in the base of the mounting fixture continuously until it returns out of the second bore.
3

The epoxy is allowed to cure and thereafter the mounting fixture is removed by a turning motion. Though this patent describes the method of installation of inserts into honeycomb sandwich structures, it does not address the issue of minimization of the insert assembly weight and a means to increase specific strength.
US Patent 5082405 and US 4941785 disclose the geometry of the inserts and the method to attach the insert to the attachment. In this insert assembly, epoxy resin is used as potting compound. The insert is a metallic member, made of stainless steel, aluminum alloy, and carbon steel. However, metal insert increases weight of the insert assembly resulting in reduction of specific strength.
US Patent 5053285 discloses the method and apparatus for making corrugated aluminum inserts. These inserts are made from aluminum foil strips by passing the strips through the corrugating device. Though the weight of such inserts is less, stresses at the interfaces of different materials increase because the elastic properties of corrugated aluminum inserts are less resulting in higher stress concentrations.
US Patent 6055790 discloses construction of an insert wherein the conduction and radiation are improved through the inserts. To improve the heat transfer rate, one of the face-plates is made of metal sheet. The insert material is aluminum alloy. In this arrangement, a higher thermally conducting path is provided from one side of the insert assembly to the other side. However, these inserts suffer from the deficiency such that the surrounding potting material made of resin which is not a good conductor of heat that leads to thermal gradient along the radial direction. This causes higher thermal stresses in the sandwich structure. The use of metal for face-plate increases weight of insert assembly thereby decreasing the specific strength.
4

US Patent 3271498 discloses an improved method of installation of inserts. However it does not address the issue of weight of an insert assembly.
A method of fabricating a honeycomb core structure with embedded fastener is disclosed in US Patent 4716067. The bonding material is epoxy resin. The method comprises laying down a first nonmetallic synthetic layer, which may be cured or uncured. Bonded to the first nonmetallic synthetic layer is a honeycomb core layer into which one or more flush head fasteners are inserted into holes in the honeycomb core that have an indentation at one end configured to mate with the underside of the head of a flush head fastener. Bonding is accomplished by priming the walls of the honeycomb core layer adjacent the surfaces of the layer, and the fastener, with a suitable primer, preferably before the fastener is installed, and, coating one of the surfaces of the first nonmetallic synthetic layer with a suitable adhesive before joining the first nonmetallic synthetic layer to the surface of the honeycomb core layer containing the head of the flush head fastener. Thereafter, the cells of the honeycomb core that surround the fastener are filled with a potting material. Then, a second nonmetallic synthetic layer is bonded to the surface of the honeycomb core remote from the surface to which the first nonmetallic synthetic layer is bonded. There are several drawbacks of this method and insert assembly. The fasteners are made of an alloy. The density of the alloys used is more compared to the composites. Significantly high interfacial stresses develop between the alloy insert and the potting resin material. Further, this patent describes the method of installation of inserts into honeycomb sandwich structures. It does not address the issue of enhancing the specific strength of insert assemblies.
The shortcomings in prior art vis-a vis use of inserts in sandwich structure are
• Use of metal for inserts which leads to decrease in specific strength of the insert assembly.
• Though geometry of the inserts and different methods of installation of these inserts into sandwich structures is addressed, the issue of
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minimization of the insert assembly weight and a means to increase specific strength of insert assembly has not been addressed.
• " There are no methods of reliably mapping stress distribution and
prediction of failure initiation in sandwich structures with diverse ! geometrical configurations.
Summary of the Invention
The main object of the invention is to provide insert assemblies of high specific strength to reduce stress concentrations at locations where multidirectional stresses act on sandwich structures based on mapping stress distribution and failure initiation in sandwich structures.
It is another objective to provide inserts of composite materials.
It is yet another object of the invention to reliably map stress distribution in sandwich structures with insert assemblies.
It is yet another object of the invention to reliably to select diverse geometrical configurations and materials by mapping stress distribution and obtaining failure initiation.
It is yet another object of the invention to provide insert assemblies with through-the-thickness, fully potted and partially potted geometrical configurations.
It is yet another object of this invention to explore the use of
• 2D composites
• 3D thermo elastic isotropic woven composites
• 3D woven composites
• 3D woven composites with multiple inserts,
• 3D functionally gradient woven composites as inserts
6

and their combination for insert assemblies
Thus in accordance, the invention of the insert assembly comprises of.
• insert
• potting material
• core
• lower face-plate
• upper face-plate
• attachment
wherein attachment is linked to insert and insert is bonded to core, lower face¬plate and upper face-plate by means of potting material wherein insert materials are selected from
• 2D composites
• 3D thermoelastic isotropic woven composites
• 3D woven composites
• 3D woven composites with multiple inserts
• 3D functionally gradient woven composites as inserts
and their combination for the insert assemblies wherein high of specific strength is achieved using appropriate geometrical configurations of the insert assembly
Detailed Description of the invention
Features and advantages of this invention will become apparent in the following detailed description and preferred embodiments with reference to the accompanying drawings. Figure 1 Cross section and plan view of through-the-thickness insert
assembly Figure 2 Cross section and plan view of fully potted inserts Figure 3 Cross section and plan view of partially potted inserts
7

Geometrical configurations
The geometrical configurations are
a) Through-the-thickness insert configuration
b) Fully potted insert configuration
c) Partially potted insert configuration
Fully potted inserts are having potting material thickness equal to the thickness of the core, and the length of the insert equal to or less than the thickness of the core; Partially potted inserts are having potting material thickness less than the thickness of the core and equal to or more than the length of the insert inserted into the core. Through-the-thickness inserts is a special case of fully potted inserts in which the length of the insert is equal to or more than the total thickness of the core and face-plates [European Space Agency. June 1987. Insert Design Handbook, ESA PSS-03-1202, Issue 1, Paris].
a) Through-the-thickness insert configuration
Figure 1 shows schematic of the through-the-thickness insert assembly configuration. Insert 10 is cylindrical in shape with flanges 11, 12 that are integral to provide shear resistance. The inserts are strongly attached with attachment 13 with bonding / threading. The localized external loads are applied to sandwich structures through the attachment 13. The insert-attachment assembly is held in sandwich structure by using potting materials 14 as shown in Figure 1. The potting materials are different types of resins. Reinforcement is added to the resins to increase the stiffness and strength. Upper face-plate 15, lower face-plate 16 and core 17 are the other components of the insert assembly. In one of the embodiments of through-the-thickness insert assembly configuration, material of insert is 2D composite.
8

In another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D thermoelastic isotropic woven composite.
In yet another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D woven composite.
In another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D woven composite with multiple inserts.
In yet another embodiment of through-the-thickness insert assembly configuration, material of insert is 3D functionally gradient woven composite.
In another embodiment of through-the-thickness insert assembly configuration, material of insert is a combination of the above mentioned.
b) Fully potted insert configuration
Figure 2 shows schematic of the fully potted insert assembly configuration. Insert 20 is cylindrical in shape with flanges 21, 22 that are integral to provide shear resistance. The inserts are strongly attached with attachment 23 with bonding / threading. The localized external loads are applied to sandwich structures through the attachment 23. The insert-attachment assembly is held in sandwich structure by using potting materials 24 as shown in Figure 2. The potting materials are different types of resins. Reinforcement is added to the resins to increase the stiffness and strength. Upper face-plate 25, lower face¬plate 26 and core 27 are the other components of the insert assembly.
In one of the embodiments of fully potted insert assembly configuration, material of insert is 2D composite.
9

In another embodiment of fully potted insert assembly configuration, material of insert is 3D thermoelastic isotropic woven composite.
In yet another embodiment of fully potted insert assembly configuration, material of insert is 3D woven composite.
In another embodiment of fully potted insert assembly configuration, material of insert is 3D woven composite with multiple inserts.
In yet another embodiment of fully potted insert assembly configuration, material of insert is 3D functionally gradient woven composite.
In another embodiment of fully potted insert assembly configuration, material of insert is a combination of the above mentioned.
c) Partially potted insert configuration
Figure 3 shows schematic of the partially potted insert assembly configuration. Insert 30 is cylindrical in shape with flanges 31, 32 that are integral to provide shear resistance. The inserts are strongly attached with attachment 33 with bonding / threading. The localized external loads are applied to sandwich structures through the attachment 33. The insert-attachment assembly is held in sandwich structure by using potting materials 34 as shown in Figure 3. The potting materials are different types of resins. Reinforcement is added to the resins to increase the stiffness and strength. Upper face-plate 35, lower face¬plate 36 and core 37 are the other components of the insert assembly.
In one of the embodiments of partially potted insert assembly configuration, material of insert is 2D composite.
10

In another embodiment of partially potted insert assembly configuration, material of insert is 3D thermoelastic isotropic woven composite.
In yet another embodiment of partially potted insert assembly configuration, material of insert is 3D woven composite.
In another embodiment of partially potted insert assembly configuration, material of insert is 3D woven composite with multiple inserts.
In yet another embodiment of partially potted insert assembly configuration, material of insert is 3D functionally gradient woven composite.
In another embodiment of partially potted insert assembly configuration, material of insert is a combination of the above mentioned.
Insert materials
The insert materials are selected from
I) 2D woven composites
II) 3D woven composites
III) 3D thermoelastic isotropic woven composites
IV) 3D functionally gradient woven composites
V) 3D woven composite with multiple inserts
I) 2D composites
2D composites are those in which only in-plane reinforcements are provided. In laminated composites made of unidirectional layers, different layers are oriented accordingly to achieve required elastic and strength properties. In woven fabric composites, reinforcements are provided along mutually perpendicular directions in the same layer by the process of weaving. Weaving is the process in which the woven fabric is formed by interlacing warp and fills (weft) yarns in regular
11

sequence of under and over. Based on the sequence of placing the yarns under and over, the woven fabrics are classified into plain, twill and satin. Specifically, one under and one over sequence is the plain weave.
II) 3D woven composites
3D woven performs are fully integrated continuous fiber assembly having multiaxial in-plane and out of plane fiber orientations. In such preforms, reinforcement is also provided in through-the-thickness direction in addition to planar directions. These preforms are made using the process of 3D weaving. Based on the weave pattern, the preforms can be orthogonal interlock woven or angle interlock woven. Further, it can be classified into through-the-thickness woven and layer to layer woven. Using the 3D preforms and resin transfer molding, 3D composites are made. 3D composites are 3D orthotropic and macrospecically homogeneous materials. Such materials are characterized by 9 elastic properties and 9 strength properties.
III) 3D thermoelastic isotropic woven composites
These are a class of 3D composites with special characteristics. For such materials, elastic and thermal properties are the same along all the directions. In other words, such composites are thermoelastically isotropic.
IV) 3D functionally gradient woven composites
3D functionally gradient woven composites are the ideal materials for making inserts. In a typical insert assembly with single insert material, there is a significant difference in material elastic properties between insert and potting material and potting material and core. Through-the-thickness elastic properties of insert, potting material and core are of the order of 40 GPa, 2.5 GPa and 0.31 GPa respectively. This leads to higher stress concentrations at the interfaces between attachment and insert, insert and potting material, potting material and core. An ideal way of reducing the stress concentrations and increasing the specific strength is to use a material system for inserts with gradually varying elastic and strength properties along the radial direction. Such a material is called
12

functionally gradient. A composite insert made of functionally gradient material has through-the-thickness elastic properties nearly matching with that of the attachment along the inner circumference and through-the-thickness elastic properties matching with that of potting material along the outer circumference. Consequently, elastic properties of the insert with such a configuration would be higher along the inner circumference and lower along the outer circumference and varying radially in a functionally gradient way. For such insert materials, the stress concentrations at the interfaces would be minimum leading to higher specific strength.
V) 3D woven composites with multiple inserts
Inserts made of 3D woven composite with multiple inserts is a class of inserts made of 3D functionally gradient woven composite. In this insert, the elastic and strength properties are not varied gradually along the radius from the inner circumference to the outer circumference. Instead, the elastic and strength properties are varied in a stepped manner. This amounts to using multiple inserts instead of a single insert. If the material properties are varied in three steps, there would be three different 3D woven composites. These three different 3D woven composites are referred as material 1, material 2 and material 3.
The materials and geometry of the insert assembly is decided by mapping stress distribution for obtaining failure conditions by the following process:
A method is described to map stress distribution in sandwich structures with inserts under localized through-the-thickness tensile / compressive loading. The core is relatively thick and compressible whereas the face-plates are relatively thin. The core is analyzed using higher-order sandwich plate theory whereas the face-plates are analyzed using classical plate theory. The behavior of the sandwich structure is represented using a set of 24 governing differential equations. For the geometry of the insert assembly and the loading conditions
13

In the above method the stress distribution is mapped and failure initiation is obtained in steps comprising
♦ Establishing frame of reference for sandwich structures with inserts
♦ Applying equilibrium equations, continuity conditions and constitutive relations of the core and the face-plates to obtain 24th order partial differential equation consisting of fundamental variables such as
Mid-plane radial displacement of upper and lower face-plates in radial direction,
Mid-plane circumferential displacement of upper and lower face¬plates in circumferential direction,
Transverse displacement of upper and lower face-plates,
Derivative of transverse displacement with respect to radius of insert assembly of upper and lower face-plates,
Derivative of transverse displacement with respect to circumferential direction of upper and lower face-plates and divided by radius of insert assembly,
Bending moment resultant of upper and lower face-plates in radial direction,
Twisting moment resultant in the plane of radial and circumferential coordinates of upper and lower face-plates,
16

All the Cjj are stiffness constants and are calculated using elastic properties of the material.
The normal and shear stress distribution within the entire insert assembly can be obtained using this novel method. The induced stress state can lead to initiation of failure within insert assembly. Initiation of failure is characterized using the following through-the-thickness quadratic interaction failure criterion.

(5)

Here,

15

considered the boundary conditions are specified. In this case, there are 24 boundary conditions. Using the differential equations and the set of 24 boundary conditions, the deformation behavior and stress state within the insert assembly are calculated. Specifically, the following quantities are determined: transverse displacement of top and bottom face-plates (w1 and w2), transverse shear stress
{Trz), mid-plane radial displacement of top face-plate (ul0l), mid-plane
circumferential displacement of top face-plate (vl0l). The novel method to reliably
map stress distribution is described in the form of flow diagrams in Figures 4 and 5.
The input parameters are: geometry of the insert assembly, elastic properties of different materials used for making the insert assembly and the loading condition. This novel method enables to map the following parameters reliably:
• transverse and radial displacements of the insert assembly
• normal and shear stress components throughout the insert assembly
Equations used for mapping stress distribution for obtaining failure conditions are:
The transverse displacement of the core material,

(2)

Radial displacement in the core material,
14

In-plane normal stress resultant in radial direction of upper and lower face-plates,
In-plane normal stress resultant in the plane of radial and circumferential coordinates of upper and lower face-plates,
In-plane shear stress resultant in the plane of radial and circumferential coordinates of upper and lower face-plates,
Transverse shear stress component of core in the plane of radial and circumferential coordinates,
Shear stress in circumferential direction on a plane perpendicular to through-the-thickness direction in the core,
Derivative of shear stress in circumferential direction on a plane perpendicular to through-the-thickness direction in the core with respect to radius of the insert assembly,
Derivative of transverse shear stress component of core in the plane of radial and circumferential coordinates with respect to radius of the insert assembly,
and further to obtain 24 first order coupled exact differential equations
♦ Applying boundary conditions to the 24 first order coupled exact differential equations and solving two point boundary value problem to obtain stresses (equation 2), displacements (equations 1,3,4) and failure initiation (equation 5)
wherein, frame of reference is established based on
I. The attachment is infinitely rigid
II. Insert and potting material are an integral part of the core for mathematical formulation
17

III. In-plane shear stress and in-plane normal stresses are neglected in the
core material
IV. Core material is flexible in nature
V. Effective shear modulus of the honeycomb core is considered in
modeling
VI. Insert assembly is circular in shape
VII. Interaction between two adjacent inserts is negligible
VIII. Interaction between the insert and the honeycomb core along the circumference of the insert assembly is negligible

IX. Face-plates are homogeneous and isotropic / quasi-isotropic
X. Classical plate theory is applicable for the analysis of the face-plates and
wherein, the steps to obtain 24 first order coupled exact differential equations involve
a. Representing the behavior of the sandwich structure with an insert
assembly using a set of plurality of equations based on equilibrium
equations, constitutive relations and continuity conditions
b. Combining the core and face-plate equations to obtain 24th order
governing partial differential equation with 24 unknown fundamental
variables
c. Rearranging the governing partial differential equation to 24 first order
coupled partial differential equations in terms of 24 fundamental
variables, their derivatives with respect to circumferential angle and
radius using plurality of equations
d. Eliminating the dependency of derivatives of circumferential angle in
the 24 first order coupled partial differential equations using Fourier
expansions to convert them into 24 first order coupled exact differential
equations
and
wherein, stresses, displacements and failure initiation are obtained by
18

I. Specifying 24 boundary conditions with respect to 24 first order coupled exact differential equations with 12 boundary conditions at the interface of attachment and insert and remaining at simply supported outer edge of the insert assembly.
II. Constituting a two point boundary value problem comprising 24 first order coupled exact differential equations and boundary conditions
III. Converting two point boundary value problem into a series of initial value problems by dividing the sandwich structure into a number of segments along radial direction
IV. Solving the series of initial value problems numerically using multi-segment method of direct integration for 24 fundamental variables at each segment using continuity conditions between two adjacent segments to determine the stresses and displacements throughout the insert assembly for given loading conditions
V. Obtaining the failure initiation within the insert assembly using quadratic failure criterion and the corresponding failure initiation load
VI. Obtaining transverse, radial and circumferential displacements (equations 1,3,4), through-the-thickness normal (equation 2) and shear stress components in the core, induced normal stress resultants, induced shear stress resultants and induced bending moment resultants in the face-plates and specific strength of the insert assembly based on geometrical, mechanical and physical properties of the insert assembly and loading conditions.
The method described above was used to obtain geometrical configuration of the inserts and the displacement and failure initiation were predicted and experimentally verified. The present work establishes the superiority of the inserts of the present invention over inserts of prior art.
19

Example 1
Experimental studies
♦ Fabrication of Through-the-Thickness Insert Assembly
The insert assembly comprises of six constituents. They are: insert, potting material, foam core, lower face-plate, upper face-plate and the attachment. Lower face-plate and the upper face-plate are made of woven fabric E glass and epoxy resin using matched-die molding technique. The core is made of polyurethane foam. The attachment is made of mild steel. The material used for composite inserts is glass. The potting material is epoxy resin. Three insert assemblies were constructed using the above with aluminum, 2D woven composite and 3D woven composite as insert materials.
• Measurement of load, displacements and failure initiation
• The insert assembly was placed on a support ring and then located on Hounsfield Test Equipment-450 KS, 50 KN UTM.
• Compressive load was applied through the attachment on to the insert assembly.
• The displacement of the attachment, lower face-plate, upper face-plate and the corresponding load were measured at loading rate of 0.25 mm/min.
• Failure initiation of the insert assembly is obtained from sudden change in the load-displacement plot.
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Experimental Results
Transverse displacement as a function of compressive load for through-the-thickness inserts is presented in Figures 6-8. For the same geometrical configurations and material properties (Tables 1 and 2), analytically obtained transverse displacement plots, compressive load at failure initiation and specific strength of inserts are presented in Figures 6-8 and Tables 3 and 4. The compressive loading was applied until the failure initiation took place.
Failure functions (equation 5) are plotted as a function of compressive load in Figure 9. It is observed from Tables 3 and 4 that the compressive load at failure is higher for the case of 3D woven composite compared to the aluminum as insert material. The specific strength of insert is significantly higher for the case of 3D woven composite compared to the aluminum insert case. For the case of 2D woven composite insert, it is in between aluminum and 3D woven composite.
Table 1. Geometrical configuration of the insert assembly for the experimental studies.

Configuration Diameter, mm Thickness, mm

Attachment, Da Insert, D, Potting
material,
DD Support, Dh Upper plate,
ti Lower
plate,
t2 Core, c
1 5 10 40 120 2 2 10
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Table 2. Material properties of foam core sandwich structure with inserts: used for experimental studies.

Material Young"s modulus, Ez (GPa) Shear modulus, Grz (GPa)
aluminum 70 27
2D woven composite 6 2.5
3D woven composite 10 4.5
Epoxy resin 2.5 0.93
Foam core 0.025 0.009
Face-plate 1 10* 4.2*
Face-plate 2 10 4.2*
Table 3. Specific strength of through-the-thickness inserts with different materials:experimental studies. Volume of insert, V=1.22x 10-6 m3 Density of aluminum = 2800 Kg /m3 Density of 2D woven composite = 1700 Kg/m3 Density of 3D woven composite = 1700 Kg/m3

S.No. Insert At failure initiation Mass of insert, m(Kg) Specific
strength of
insert =
(Max.
compressive
load/
weight of
insert) %
increase
in specific
strength
of insert %
decrease
in mass of
insert

Compre
ssive
load,
Qc(N) Displace
ment, w (mm)

1 Aluminum 5610 5.9 3.416xl0"3 0.1642xl06 Reference Reference
2 2D woven composite 5420 5.2 2.074 xlO-3 0.2613xl06 59.10 39.29
3 3D woven composite 5695 7.7 2.074 xlO-3 0.2746 xlO6 67.23 39.29
22

Table 4. Specific strength of through-the-thickness inserts with different materials: analytical predictions for experimental configurations. Volume of insert, V= 1.22x10"6 m3 3D woven composite insert: Zt = 45 MPa, Srz = 36 MPa 2D woven composite insert: Zt = 27 MPa, Srz = 36 MPa Aluminum insert: Zt = 150 MPa, Srz = 30 MPa

S.No. Insert At failure initiation Specific
strength of
insert =
(Max.
compressive
load / weight
of insert) %
increase
in specific
strength of
insert %
decrease
in mass of
insert

Compressive
load, Qc
(N) Displacement, w (mm)

1 Aluminum 6355 6.1 0.186x106 Reference Reference
2 2D woven composite 5950 5.4 0.287x106 54.30 39.29
3 3D woven composite 7040 7.3 0.339* 106 82.25 39.29
Example 2
Comparison of through-the-thickness inserts of present invention with inserts of prior art
Using the method of the present invention, compressive load at failure initiation, failure function and specific strength of inserts are mapped for the prior art disclosed in US Patent 50532285 (corrugated aluminum insert) and for the insert (3D woven composite insert) of the present invention with the same geometry as used for the prior art (R = 30 mm). The results are given in Figure 10 and Table 5. Further, the geometry of the insert of the present invention was modified (R = 10 mm) to reduce the insert assembly weight. With such a modified configuration, compressive load at failure initiation, failure function and specific strength of inserts are mapped. From Figure 10 and Table 5, it is established that the inserts of the present invention are having higher specific strength compared to the insert of prior art.
23

Table 5. Specific strength of through-the-thickness inserts with corrugated aluminum and 3D woven composite: analytical studies. Density of Corrugated aluminum = 459 Kg / m3 Density of 3D woven composites = 1700 Kg / m3

Insert Compressive
load at failure
initiation,
Qc (KN) Mass of insert, m (Kg) Specific
strength of
insert =
(Max.
compressive
load / weight
of insert) %
increase
in specific
strength of
insert %
decrease
in mass of
insert
Corrugated aluminum 8.27 0.038 21.62x103 Reference Reference
3D woven
composite
with radius of
30 mm 15.85 0.065 24.23 x103 12.1 -71.0
3D woven
composite
with radius of
10 mm 14.08 0.0068 206.77x103 856.6 82.2
24

Example 3
Analytical studies with different insert materials
Using the experimentally validated method for mapping of stresses, displacements and failure initiation, the results for aluminum, 2D woven composite, 3D thermoelastic isotropic woven composite, 3D woven composite, 3D woven composite with multiple inserts and 3D functionally gradient woven composite are obtained for the geometrical configuration of the insert assembly as given in Table 6. Material properties of the insert assembly are presented in Tables 7 and 8.
Maximum displacement, maximum normal stress and maximum shear stress corresponding to maximum compressive load at failure initiation are presented in Table 9. Specific strength of insert for different materials is also presented in Table 9. Failure function as a function of compressive load for different insert materials is presented in Figure 11.
Percentage increase in specific strength of insert and percentage decrease in mass of insert for the composite inserts compared to the aluminum insert is presented in Table 9. It is observed that there is significant increase in specific strength and decrease in mass of insert for the composite inserts. The maximum gain is for the case of 3D functionally gradient woven composite inserts.
Table 6. Geometrical configuration of the insert assembly for the analytical studies.

Configuration Diameter, mm Thickness, mm

Attachment, Da Insert, Di Potting
material
,DD Support ,Dh Upper
plate,
Lower
plate,
t2 Core, c
1 10 20 50 120 2 2 10
25

Table 7. Material properties of the insert assembly. Volume of insert, V= 4.006 x10"6 m3 3D woven composite insert: Zt = 60 MPa, Srz = 36 MPa, 2D woven composite insert: Zt = 27 MPa, Srz = 36 MPa Aluminum insert: Zt = 150 MPa, Srz = 30 MPa, 3D thermoelastic isotropic woven composite insert: Zt = 55 MPa, Srz= 36 MPa
Epoxy : Zt = 38 MPa, Zc = 85 MPa, S - 42 MPa 3D woven composite with multiple inserts: Zt = 60 -> 38 MPa, Srz = 36 MPa
3D functionally gradient woven composite insert: Zt = 55 -> 38 MPa, Srz = 36 MPa

Material Young"s modulus, Ez (GPa) Shear modulus, Gz (GPa)
Aluminum 70 27
2D woven composite 9 4
3D thermoelastic isotropic woven composite 37.7 3.71
3D woven composite 43 3.55
Insert material 1 37.7 3.71
Insert material 2 22.0 2.5
Insert material 3 8 1.5
3D functionally gradient woven composite 37.1 -- 2.5 3.71 - 0.93
Honeycomb 0.310 0.138
Epoxy resin 2.5 0.93
Face-plate 1 15*
4.5
Face-plate 2 15* 4.5*
* in-plane properties
26

Tables 2, 7 and 8 are based on the following references.
3D functionally gradient woven composite inserts are analyzed for the range of
properties given in Table 7.
References:
1. Naik, N. K. and E. Sridevi. 2002. An analytical method for thermoelastic analysis of 3D orthogonal interlock woven composites, Journal of Reinforced Plastics and Composites, Vol. 21, pp. 1149-1191.
2. Naik N. K. et al. 2001. Stress and failure analysis of 3D orthogonal interlock woven composites, Journal of Reinforced Plastics and Composites, Vol. 20, pp. 1485-1523.
3. Naik, N. K. and V. K. Ganesh. 1996. Failure behavior of plain weave fabric laminates under on-axis uniaxial tensile loading: II - analytical predictions, Journal of Composite Materials, Vol. 30, pp. 1779-1822.
4. Shembekar, P. S. and N. K. Naik. 1992. Elastic behavior of woven fabric composites: II - laminate analysis, Journal of Composite Materials, Vol. 26, pp. 2226-2246.
5. Engineered Materials Handbook, Vol. 1, Composites, 1989, ASM International, Materials Park, OH.
Table 8. Elastic properties of orthotropic composite inserts (at 0=0).

Young"s modulus Shear modulus Poisson"s ratio
Material Er (GPa) Ee (GPa) Ez (GPa) Gr6
GPa) Grz (GPa) G9z
(GPa) »rB »rz v6z
3D
thermoelastic
isotropic
woven
composite 37.1 37.5 37.7 3.71 3.71 3.71 0.111 0.106 0.106
3D woven composite 46.7 16.4 43.1 3.57 3.55 3.55 0.088 0.222 0.225
27

Table 9. Specific strength of through-the-thickness inserts with different
materials: analytical studies. Volume of insert, V= 4.006x 10"6 m3
Density of Aluminum = 2800 Kg / m3
Density of 2D and 3D woven composites = 1700 Kg / m3
Density of 3D functionally gradient woven composite = 1700 -> 1100 Kg / m

S.No. Insert At failure initiation Mass of
insert,
m (Kg)
10~3 Specific
strength of
insert =
(Max.
compressive
load / weight
of insert) % increase
in specific strength of insert %
decrease
in mass
of insert

Compre ssive load,
Qc (KN) Displace
ment, w
(mm)

1 Aluminum 13.15 1.40 11.217 0.117x106 Reference Reference
2 2D woven composite 14.65 1.48 6.810 0.215x106 83.93 39.29
3 3D
thermoelastic
isotropic
woven
composite 14.45 1.42 6.810 0.212x106 81.19 39.29
4 3D woven composite 14.90 1.49 6.810 0.219x106 87.18 39.29
5 3D woven
composite
with multiple
inserts 15.90 1.32 6.009 0.265x106 126.50 46.43
6 3D
functionally
gradient
woven
composite 16.35 1.62 5.176 0.316x106 170.09 53.86
28

We Claim:
1. An insert assembly of high specific strength for sandwich structures comprising a core with an opening, sandwiched between an upper face¬plate and a lower face-plate; an insert-attachment assembly consisting of a flanged tubular insert and an attachment means positioned inside the said flanged tubular insert; the said insert-attachment assembly positioned inside the said opening of the core, extending from the upper face-plate to the lower face-plate; and potting material bonded to the core and the said insert-attachment assembly; wherein materials for making the tubular insert are selected from
• 2D composites
• 3D thermoelastic isotropic woven composites
• 3D woven composites
• 3D functionally gradient woven composites
and their combination thereof, and the geometrical configurations of the insert assembly selected from:
• fully potted having potting material thickness equal to the thickness of the core and the length of the tubular insert equal to or less than the thickness of the core; or
• partially potted having insert-attachment assembly extending from the upper face-plate and terminating above the lower face-plate; potting material thickness being less than the thickness of the core; or
• through-the-thickness having insert-attachment assembly extending from the upper face-plate to the lower face-plate; length of the tubular being equal to the total thickness of the core and the two face-plates;
and wherein the said materials and geometry of the said insert assembly are decided by mapping stress distribution for obtaining failure conditions, as described herein.
30

2. An insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein potting material is selected from resins such as epoxy, polyester, polyimide and their like.
3. An insert assembly of high specific strength for sandwich structures as claimed in claims 1-2 wherein upper face-plate is selected from material with density of 1700 - 7800 Kg / m3, Young"s modulus of 5 - 200 GPa, Shear modulus of 2 - 77 GPa and Poisson"s ratio of 0.108 - 0.35.
4. An insert assembly of high specific strength for sandwich structures as claimed in claims 1-2 wherein lower face-plate is selected from material with density of 1700 - 7800 Kg / m3, Young"s modulus of 5 - 200 GPa, Shear modulus of 2 - 77 GPa and Poisson"s ratio of 0.108 - 0.35.
5. An insert assembly of high specific strength for sandwich structures as claimed in claims 1 & 3 wherein the profile of the upper face-plate is flat or curved or a combination thereof.
6. An insert assembly of high specific strength for sandwich structures as claimed in claims 1 & 4 wherein the profile of lower face-plate is flat or curved or a combination thereof.
7. An insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein specific strength of the insert is enhanced by about 200 % with respect to aluminum and about 500 % with respect to other metals.
8. An insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein upper face-plate is selected from materials such as aluminum, aluminum alloys and fiber reinforced plastic composites.
31

9. Insert assembly of high specific strength for sandwich structures as claimed in claim 1 wherein lower face-plate is selected from materials such as aluminum, aluminum alloys and fiber reinforced plastic composites.
Dated 2nd day of December 2004 Dr. Prabuddha Ganguli
Agent on behalf of Applicant
32

Documents:

496-mum-2004-cancelled pages(19-10-2005).pdf

496-mum-2004-claims(granted)-(19-10-2005).pdf

496-mum-2004-correspondence-(ipo)-(05-09-2005).pdf

496-mum-2004-correspondence-1-(25-11-2005).pdf

496-mum-2004-correspondence-2-(19-10-2005).pdf

496-mum-2004-drawing(19-10-2005).pdf

496-mum-2004-form 1(15-07-2004).pdf

496-mum-2004-form 1(30-04-2004).pdf

496-mum-2004-form 19(15-07-2004).pdf

496-mum-2004-form 2(granted)-(19-10-2005).pdf

496-mum-2004-form 3(08-10-2004).pdf

496-mum-2004-form 3(15-07-2004).pdf

496-mum-2004-form 3(30-04-2004).pdf

496-mum-2004-form 5(15-07-2004).pdf

496-mum-2004-power of attorney(15-07-2004).pdf

496-mum-2004-power of attorney(30-04-2004).pdf

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Patent Number

211354

Indian Patent Application Number

496/MUM/2004

PG Journal Number

45/2007

Publication Date

09-Nov-2007

Grant Date

26-Oct-2007

Date of Filing

30-Apr-2004

Name of Patentee

INDIAN INSTITUTE OF TECHNOLOGY BOMBAY

Applicant Address

Administrative Office at Powai, Mumbai-400 076

Inventors:

#	Inventor's Name	Inventor's Address
1	NIRAJAN KRISHNA NAIK	Aerospace Engineering Department, Indian Institute of Technology-Bombay, Powai,Mumbai-400 076
2	NAGESWARA RAO GANJI	S/o Amaralingeswara Rao G, Kesanapalli(post),Dachepalli(Mandal), Guntur(District),Andra Pradesh 522 414

PCT International Classification Number

F16B 5/01

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA