Michael Keefe

A Simulation of the Draping of Bidirectional Fabrics over Arbitrary Surfaces

The development of a graphical simulation to describe the draping of bidirectional fabrics over arbitrary surfaces is reported. The simulation applies to any surface, described either analytically or numerically, and allows any number of draped configurations on a surface. A unique draped configuration results when one warp thread and one weft thread are constrained to specific paths on the surface. The warp and weft crossover-point locations are calculated by numerically solving the intersection equations of the surface and two spheres that represent all possible positions of the ends of a warp and weft segment. Fabric-wrinkling and fabric-bridging over surface depressions are extreme cases of deformation, which, as with thread orientation, can be controlled by the placement of the constrained threads on the surface to be draped. Several surfaces are draped with the simulation, which is used as a design tool for selecting suitable draped configurations for specific surfaces.

The draping and consolidation of commingled fabrics

Abstract Commingled fabric composed of yarns containing both the reinforcement and the matrix in fibre form is an innovative preform material used in the manufacture of fibre reinforced thermoplastic composites. A quality of commingled fabric is its drapability allowing it to conform to mould shapes before being consolidated into a rigid structure. This paper describes a simulation of the draping process and a mathematical model of the subsequent consolidation process which predict, for a given mould surface and processing parameters, the material characteristics necessary for determining the mechanical properties of the finished product. The draping simulation graphically depicts an arbitrary surface with a bidirectional fabric conforming to the surface. The results of the simulation determine fibre orientation at any point on the draped surface. Fibre orientations are affected by in-plane shear deformation in the fabric when it conforms to surfaces of compound curvature. Wrinkling and bridging are predicted given the deformation limits of the fabric. The simulation may be used as a design tool for selecting suitable draped configurations for specific surfaces. The consolidation model predicts, for a commingled or cowoven fabric, the laminate thickness, reinforcing fibre volume fraction and void content as functions of time during the process and the time required to reach full consolidation. The model may be used to examine the influence of various material and processing parameters on consolidation behaviour. Combining the results of the draping simulation and consolidation analysis with a laminate analysis allows the prediction, for a given set of draping constraints and processing parameters, of the mechanical properties at any point in a composite structure formed from commingled or co-woven fabric.

Global/Local Modeling of Ballistic Impact onto Woven Fabrics

This study focuses on developing a global/local three-dimensional (3D) finite element model of a Kevlar KM2® plain woven fabric applicable for examining ballistic impact from a spherical projectile. The impact event is modeled in LS-DYNA® including friction between the individual yarns as well as the projectile and fabric. When compared with the predictive capabilities of a 3D finite element model that includes the detailed undulating representation of the fabric architecture over the entire solution domain, the savings in computational effort afforded by the global/local model become especially attractive. The agreement with fully detailed 3D finite element simulations and ballistic experiments is also demonstrated.

Experimental evaluation and statistical characterization of the strength and strain energy density distribution of Kevlar KM2 yarns: exploring length-scale and weaving effects

This article experimentally investigates the tensile strength distributions of 600 den Kevlar KM2 yarns under quasi-static tensile loading. The strength distributions were best characterized using the 3-parameter Weibull and generalized Gamma distributions. In order to assess the effects of weaving on the strength distributions, Kevlar yarns were tested from a spool and then compared to yarns extracted from greige and scoured Kevlar fabrics. The weaving process and treatments caused various levels of strength degradation which shifted the strength distributions toward lower strengths. The warp yarns were degraded to a greater extent than the fill yarns. The scouring process induced further strength degradations in the woven fabric. Length-scale effects were studied by using gage lengths varying between 25.4 and 381.0 mm. The strength distributions were observed to shift toward lower strengths with increasing gage lengths. A new distribution function based on a modification to the 3-parameter Weibull distribution is proposed to account for length-scale effects. In addition to the strength distributions, the experimental load—extension plots are used to compute the strain energy density or work-to-break values normalized by the yarn volume, which are then statistically characterized and analyzed in a similar manner.

Recent advances in modeling and experiments of Kevlar ballistic fibrils, fibers, yarns and flexible woven textile fabrics – a review

Ballistic impact onto flexible woven textile fabrics is a complicated multi-scale problem given the structural hierarchy of the materials, anisotropic material behavior, projectile geometry–fabric interactions, impact velocity and boundary conditions. Although this subject has been an active area of research for decades, the fundamental mechanisms, such as material failure, dynamic response and multi-axial loading occurring at the lower length scales during impact, are not well understood. This paper reviews the recent advances in modeling and experiments of Kevlar ballistic fibrils, fibers, yarns and flexible woven textile fabrics pertinent to the deformation modes occurring during impact and serves to identify topics worthy of further investigation that will advance the basic understanding of the phenomena governing transverse impact. This review also explores aspects such as homogeneous versus heterogeneous behavior of yarns consisting of individual fibers and the inelastic transverse behavior of the fiber, which is not considered in the previous review papers on this topic.

Solid Modeling of Yarn and Fiber Assemblies

Fibers are the basic structure in yarns, and yarns are the basic structure for fabrics. Although the approach described can be applied easily and naturally to fibers, especially when dealing with composite-material fabrics, the applications presented are thought of primarily as yarn-based assemblies. Hence, when talking about the developed model, ‘yarn’ and ‘fiber’ can be thought of as interchangeable. In general, fabric models assume underlying yarns to be of zero thickness. Those who have looked at yarns as three-dimensional structures have limited themselves to simple geometric arrangements of the yarns or have used assumptions which are non-geometric in order to simplify the model. A basic model is proposed which treats a yarn as a true three-dimensional solid object. This approach is first applied to three yarns twisted together to form a rope-like bundle. Next, the technique is used to model a woven fabric; the Peirce geometry is analyzed, and then the model is extended to look at fabric shear. An a...

Solid Modeling Applied to Fibrous Assemblies. Part I: Twisted Yarns

In our earlier work, we developed a model for fibrous assemblies by treating each yarn as a true three-dimensional structure. We then were able to predict various orientation limits by manipulating the model until geometric self-intersection would occur. However, that model treated the yarns as inextensible and incompressible. In this paper, we propose an abstraction to visualize the effects of yarn compressibility and then study the results of applying the new model on a final object that is primarily one-dimensional in nature - a rope-like structure composed of a twisted bundle of individual yarns.

Modeling the fiber length-scale response of Kevlar KM2 yarn during transverse impact

In this study, transverse impact of a cylindrical projectile onto a 600 denier Kevlar KM2 yarn (400 individual fibers) is studied using a fiber length-scale three-dimensional finite element model to better understand projectile–fiber and fiber–fiber contact interactions on wave propagation and fiber failure within the yarn. A short time scale response indicates significant transverse compressive deformation in the fiber that increases with impact velocity. Fiber-level modeling predicts a flexural wave that induces curvatures in the fibers significant enough to induce compressive fiber kinking and fibrillation. A spreading wave normal to the direction of projectile impact develops and spreads the fibers at high velocity. The models predict bounce velocities of the individual fibers within the yarn that varies based on spatial location. These mechanisms result in non-uniform loading and progressive failure of fibers within the yarn. In addition, the models show a gradient in the axial tensile stress in the fiber cross-section at the location of failure. Current state-of-the-art experimental capabilities in yarn/fabric impact testing do not have the spatial resolution to track individual single-fiber micron length-scale deformations in real time. These fiber-level mechanisms may explain the experimentally observed lower breaking speed for yarns better the classic Smith solution, which assumes yarns are homogenous (i.e. individual fibers and their interactions are not considered) and loaded uniformly in tension (multi-axial loading and stress gradients are neglected).

Effect of multiple non-coincident impacts on residual properties of glass/epoxy laminates:

A series of low-velocity drop-weight tests were conducted on glass/epoxy laminates to study the effect of multiple non-coincident impacts on residual compression and flexural properties. The impact characteristics and residual properties were recorded to determine the damage tolerance to various impact energies and impact separation distances. A flexure after impact method was also developed and compared to the compression after impact test. Experimental results were combined with a finite element model of the test. The damaged region was modeled as an elastic inclusion of reduced stiffness resulting in a stress concentration that was used to predict residual strength. Post-impact tests and stress analyses revealed that damage size, inclusion stiffness, and strength reduction were sensitive to impact energy and impact separation distances. Multiple impacts at larger distances create distinct, non-interacting damage regions that cause no more reduction in residual properties than the damage regions from a single impact. However, with a small separation distance, the damage regions from multiple impacts overlap and display increased energy absorption and reduction in the residual mechanical properties.

Simulation of injection molding into rapid‐prototyped molds

Injection molding is a very mature technology, but the growth of layer‐build, additive, manufacturing technologies (rapid prototypying) has the potential of expanding injection molding into areas not commercially feasible with traditional molds and molding techniques. This integration of injection molding with rapid prototyping has undergone many demonstrations of potential. What is missing is the fundamental understanding of how the modifications to the mold material and mold manufacturing process impact both the mold design and the injection molding process. This work expanded on an approach to utilize current numerical simulation programs and created a tool for optimizing the creation and use of non‐metal molds for injection molding. Verification and validation work is presented. The model was exercised by studying the effect of varying the thermal conductivity on final‐part distortions. This work clearly showed that one could not obtain reasonable results by simply changing a few input parameters in the current simulations. Although the approach did produce more realistic results, more work will be required for a tool capable of accurate, quantitative predictions.