Experimental investigation of the transverse mechanical properties of a single Kevlar® KM2 fiber

https://doi.org/10.1016/j.ijsolstr.2004.05.016Get rights and content

Abstract

A new experimental setup is developed to investigate the transverse mechanical properties of Kevlar® KM2 fibers, which has been widely used in ballistic impact applications. Experimental results for large deformation reveal that the Kevlar® KM2 fibers possess nonlinear, pseudo-elastic transverse mechanical properties. A phenomenon similar to the Mullins effect (stress softening) in rubbers exists for the Kevlar® KM2 fibers. Large transverse deformation does not significantly reduce the longitudinal tensile load-bearing capacity of the fibers. In addition, longitudinal tensile loads stiffen the fibers' transverse nominal stress–strain behaviors at large transverse deformation. Loading rates have insignificant effects on their transverse mechanical properties even in the finite deformation range. An analytical relationship between transverse compressive force and displacement is derived at infinitesimal strain level. This relation is used to estimate the transverse elastic modulus of the Kevlar® KM2 fibers, which is 1.34 ± 0.35 GPa.

Introduction

The protection of military and law enforcement personnel from injury by high-velocity-object impact has created new challenges for fundamental scientific research since World War II and even more so recently. In particular, impact-energy-absorbing material development and characterization has become an imminent task for the scientific research community. As an important class of shock-absorbing materials, fabrics and flexible fibrous composites have been widely used in the bullet-proof vests and other body armor systems. Recent use of these materials in personnel protection applications creates an urgent need to develop a better scientific understanding of the mechanical response of these materials and the components made of these materials.

Kevlar® KM2 fabrics are widely used to produce personnel protection system because of their high stiffness, lightweight, and high strength. To understand the deformation process of a fabric armor system, many aspects such as material properties, fabric structure, projectile geometry, impact velocity, multiple ply interaction, far field boundary conditions and friction must be studied (Cheeseman and Bogetti, 2003). Among these factors, material properties are critical in determining the armor effectiveness.

In order to accurately simulate the ballistic performance of fabric armors for design purposes, many efforts have been focused on computational models to predict the resistance of fabrics to a high-velocity impact. For example, Taylor and Vinson (1990) proposed a ballistic fabric model that simulates the transverse impact by treating the fabric as a homogeneous, isotropic, elastic plate, which deforms into the shape of a straight-sided conical shell. However, this simple isotropic material model precludes the possibility of accurate simulation.

The earliest efforts in the attempt to predict ballistic impact effects on fabrics were those of Roylance et al. (1973) on biaxial fabrics. Fabrics are composite materials that offer significant computational challenges. The stress–strain behavior of these materials is generally nonlinear as well as anisotropic. Since there are few experimental investigations on these materials, many models assume a simple linear stress–strain relationship (Parga-Landa and Hernandez-Olivares, 1995; Roylance et al., 1995). In order to overcome this problem, Lim et al. (2003) employed a three-element spring-dashpot model to describe a rate-dependent nonlinear stress–strain behavior of Twaron fabric in their finite-element modeling study. They also accepted a failure criterion (Shim et al., 2001) that specifies the rate-dependent failure strain.

During impact, fibers are subjected to transverse compressive loading while they are being extended (tensile loading). Therefore, it is essential to obtain tensile behaviors of the fibers with transverse compressive loading superimposed on the fibers. However, few numerical models include the transverse behaviors of the fabric materials into account because of the lack of experimental investigations on this issue. Cunniff and Ting (1999) recognized this problem. They developed a numerical model to characterize the ballistic behavior of fabrics with warp and fill yarn elements modeled independently as elastic rod elements. Coupling between these elements was modeled with transverse spring elements corresponding to physical crossover. A nonlinear model with three empirical constants was used to describe the transverse behavior of the yarns.

Kevlar® KM2 fabrics and their ballistic application have recently been a research focus for many investigators (Johnson et al., 1999; Johnson et al., 2002). In order to obtain accurate information about the transverse behavior of yarns that will feed into these types of simulations, the behavior of their constituent, single fiber, is necessary to be examined first. As a first step to fulfill an accurate finite element simulation for ballistic impact, this paper discusses the development of a new experimental technique to investigate the transverse behavior of a Kevlar® KM2 single fiber. With this experimental technique, experiments were conducted with different loading arrangements on the Kevlar® KM2 single fiber to study the effects on the transverse compressive behavior under various conditions such as different loading rates, pre-tension in longitudinal direction, and cyclic transverse loading with increasing strain amplitudes. Finally, the experimental results in a small strain range are used to estimate the transverse elastic Young's modulus through the development of a relation between transverse compressive load and displacement.

Section snippets

Experiments

Because of the highly oriented chain molecules or crystals along the fiber axis, high-performance fibers, such as Kevlar® KM2 fibers, exhibit strong anisotropy. Since the properties have no significant deviation among directions perpendicular to the fiber axis, high-performance fibers are usually considered transversely isotropic. The Young's modulus in the longitudinal direction (fiber axis direction) is much higher than that in the transverse direction (Kawabata, 1990). In addition, the

Estimation of elastic modulus in transverse direction

The experimental results at small nominal strains can be used to estimate the elastic modulus (Young's modulus) in the transverse direction.

Hadley et al. (1965) are the pioneers in the experimental determination of transverse Young's modulus of fibers. They compressed a fiber between two rigid parallel transparent flats. The amount of flattening of the contact area of the fiber was measured by a microscope as a function of compressive load. The transverse Young's modulus was then estimated from

Conclusions

A new experimental facility is developed to measure the transverse compressive behavior of Kevlar® KM2 fibers, which is transversely isotropic to the fiber axis with a diameter of 12 μm. A byproduct of this experimental facility is the capacity of estimating one of the Poisson's ratios associated with longitudinal loading.

The transverse compressive behavior of the Kevlar® KM2 fibers is nonlinear and pseudo-elastic. The original loading path is totally different from its unloading path and

Acknowledgements

This research was supported by US Army Research Laboratory (ARL) and US Army Natick Soldier Center through a cooperative agreement (DAAD19-02-2-0006) with The University of Arizona. The authors wish to thank Professor Emeritus J.W.S. Hearle of University of Manchester Institute of Science and Technology, University of Glasgow, UK and Dr. Philip Cunniff of US Army Natick Soldier Center for insightful email and oral discussions, respectively.

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