Prasad Potluri

Novel Stitch-bonded Sandwich Composite Structures

Abstract Sandwich construction with composite skins and a variety of core materials is getting popular for a range of structural applications. These structures are subjected to various impact loads, such as accidental dropping of objects, vehicular collisions, drilling for traditional fastening or routing electrical cables etc. For example, lateral penetration of a blunt tool results in severe debonding of the bottom skin, which is not always visible/accessible and hence not easy to repair. In the present work, stitch-bonded sandwich structures have been developed using commercial close-cellular core and woven broadcloth. Traditional sewing machines are not suitable for relatively hard and thick core materials. The authors, using tufting and weft insertion techniques, derived from the textile industry, developed a novel stitch-bonding technique. A number of sandwich samples were prepared and subjected to quasi-static indentation tests, for optimising the stitch geometry.

Geometrical modelling and control of a triaxial braiding machine for producing 3D preforms

Abstract Braiding is a relatively less explored textile process for producing composite preforms. Biaxial braids can be produced as hoses and subsequently be draped over different three-dimensional surfaces. However, triaxial braids are relatively stable structures and should be produced to the desired shape during the braiding process. This is achieved by over-braiding on mandrels that either form part of the finished composite or removed before the moulding process. Triaxial braided composites have superior mechanical properties due to fibre orientations along three directions. Geometry of a braided structure depends on the number of yarn carriers, rotational speed of the carriers, take-up speed and the effective perimeter of the cross-section of a mandrel. In the present work, a VRML based geometrical visualisation tool has been developed to simulate a braid structure on any predefined mandrel geometry, and using a predefined yarn cross-section. Braid angles, cover factors and yarn volume fractions can be computed from these simulations. A triaxial braiding machine has been developed with an independent servo control of the carrier movement and the take-up mechanism; geometrical simulation is used as an input to the control system to continuously vary the braid structure along the length of a mandrel. Flexible tooling is important for rapid product development. A flexible mandrel has been developed that can be mechanically adjusted to change the cross-section and the taper. This system enables rapid development of braided preforms.

Mesoscale modelling of interlaced fibre assemblies using energy method

The knowledge of mesoscale behaviour of interlaced fibrous structures is necessary to predict their macroscale behaviour. The aim of this study is to highlight the advantages of energy-based approach to solve fabric mechanics problems with out the necessity of complex 3D finite element analysis. A mechanical model to predict the tensile response of plain-woven fabric unit cell under in-plane uniaxial/biaxial loads is presented here. The model incorporates non-linear properties of constituent yarns, rather than idealised linear behaviour. All possible mechanisms of deformation including elongation, bending and compression of yarns have been considered. A modified geometry of yarn path based on a polynomial has been proposed. The predictions are compared with experimental data reported in literature as well as the finite element analysis. The computational aspects of implementation of the model are also discussed briefly.

Geometrical Modeling of the Yarn Paths in Three-dimensional Braided Structures:

A circular braiding process is adapted to produce three-dimensional shapes by braiding over a contoured mandrel. The present work deals with the simulation of various braided structures. The simulated structures are based on geometrical models developed for yarns on different shapes of mandrels. The geometry of a braided structure depends upon the machine parameters, such as number of yarn carriers, rotational speed of the carriers, take-up speed, and the effective perimeter of the mandrel cross section. Virtual reality modeling language (VRML) has been employed as a visualization tool to simulate these geometrical models on a predefined mandrel geometry. Models relate to the machine speeds and a number of machine settings. The path of the yarn on the mandrel is dependent upon the shape of the mandrel, i.e., if it is circular then it will be in the form of a helix or if it is a cubical body then it will be in the form of straight lines. An expert system has been generated to take into account all the parameters interfacing with VRML to simulate the braid geometry. A comparison is also made between the experimental and simulated structures.

Modelling Fabric Mechanics

Mechanics of textile fabrics by modelling of equilibrium of forces is difficult to apply broadly in practical applications. An alternative, which offers more promise for industrial utility in computer-aided design, is the energy-based approach described by Hearle and Shanahan (1978 a, b). The paper reviews the basic principles and considers the ways of introducing appropriate energy terms to cover yarn extension, yarn bending, yarn flattening, and friction at crossovers. The main discussion is given for the elastic response of simple, plain-weave fabrics, based on several different geometric models, but the ways to deal with other fabrics and conditions are also suggested. The paper provides a protocol for advancing the subject from academic research to commercial use.

Comprehensive drape modelling for moulding 3D textile preforms

A comprehensive drape model has been developed to deal with a range of 3D surfaces, from simple open surfaces to closed tubular sections with 3D bends. Existing drape algorithms, developed mainly for broadcloth composites, cannot cope with closed sections. These algorithms consider the woven fabric as a network of linkages with pin joints and perform kinematic mapping by solving a set of sphere-intersection equations. This method of kinematic drape assumes only in-plane shear deformation and hence cannot be readily applied to a number of 3D shapes, involving other modes of deformation. In the present work, a kinematic mapping algorithm was implemented at first and subsequently modified to drape two-layer tapered preforms to open surfaces. Following this work, a more general algorithm was developed to drape closed preforms on bent tubes, which the authors believe to be the first such attempt.

Automated manufacture of composites: handling, measurement of properties and lay-up simulations,

This paper presents some issues relevant to the automated manufacture of low-cost composites. The process of cutting fabric panels from broadcloth and assembling individual pieces had its origins in the clothing industry. The paper investigates the advances made, and the problems encountered in clothing automation projects during the 1980s, with a view to adopting some of the relevant techniques for composites manufacture. Laminate materials, either in the form of prepregs or dry fabrics, exhibit highly non-linear and large deformation behaviour. Hence, a key step towards full-scale automation is the accurate prediction of handling and subsequent processing behaviour of the preforms, with the aid of numerical simulations. The paper describes some automated test methods for measuring the non-linear material properties, required as constitutive properties for simulations. Because of the variety and variability of the preform mechanical properties, rapid and accurate measurements are necessary. A commercial robot has been adopted to conduct a series of tests on dry laminate materials. The tests include intra-ply shear, transverse compression, tension, bending and inter-ply shear. Process simulations such as laying-up, fabric folding and draping have been conducted, based on the measured properties.

Thermo-Mechanical Behavior of Textile Heating Fabric Based on Silver Coated Polymeric Yarn

This paper presents a study conducted on the thermo-mechanical properties of knitted structures, the methods of manufacture, effect of contact pressure at the structural binding points, on the degree of heating. The test results also present the level of heating produced as a function of the separation between the supply terminals. The study further investigates the rate of heating and cooling of the knitted structures. The work also presents the decay of heating properties of the yarn due to overheating. Thermal images were taken to study the heat distribution over the surface of the knitted fabric. A tensile tester having constant rate of extension was used to stretch the fabric. The behavior of temperature profile of stretched fabric was observed. A comparison of heat generation by plain, rib and interlock structures was studied. It was observed from the series of experiments that there is a minimum threshold force of contact at binding points of a knitted structure is required to pass the electricity. Once this force is achieved, stretching the fabric does not affect the amount of heat produced.

Influence of Fibre Architecture on Impact Damage Tolerance in 3D Woven Composites

Abstract3D woven composites, due to the presence of through-thickness fibre-bridging, have the potential to improve damage tolerance and at the same time to reduce the manufacturing costs. However, ability to withstand damage depends on weave topology as well as geometry of individual tows. There is an extensive literature on damage tolerance of 2D prepreg laminates but limited work is reported on the damage tolerance of 3D weaves. In view of the recent interest in 3D woven composites from aerospace as well as non-aerospace sectors, this paper aims to provide an understanding of the impact damage resistance as well as damage tolerance of 3D woven composites. Four different 3D woven architectures, orthogonal, angle interlocked, layer-to-layer and modified layer-to-layer structures, have been produced under identical weaving conditions. Two additional structures, Unidirectional (UD) cross-ply and 2D plain weave, have been developed for comparison with 3D weaves. All the four 3D woven laminates have similar order of magnitude of damage area and damage width, but significantly lower than UD and 2D woven laminates. Damage Resistance, calculated as impact energy per unit damage area, has been shown to be significantly higher for 3D woven laminates. Rate of change of CAI strength with impact energy appears to be similar for all four 3D woven laminates as well as UD laminate; 2D woven laminate has higher rate of degradation with respect to impact energy. Undamaged compression strength has been shown to be a function of average tow waviness angle. Additionally, 3D weaves exhibit a critical damage size; below this size there is no appreciable reduction in compression strength. 3D woven laminates have also exhibited a degree of plasticity during compression whereas UD laminates fail instantly. The experimental work reported in this paper forms a foundation for systematic development of computational models for 3D woven architectures for damage tolerance.

Large Deformation Modelling of Flexible Materials

This paper presents a practical numerical solution to predicting large deformations of flexible materials such as fabrics. Previous researchers have employed various other approaches such as pure analytical solutions or computer-based finite difference or finite element approaches. They made simplifications in the analysis in terms of the object geometry, load conditions, boundary conditions and material behaviour. However, the present model can predict the deformations based on the exact geometry and a non-linear moment-curvature relation. The modelling technique reported in this paper is verified by comparing deformation results with those published elsewhere. In addition, the present authors have obtained data from their own experimental work in order to verify the non-linear capability of their modelling technique. The non-linear large deformation modelling technique gives closer agreement with the practical tests than previously published methods.