Keywords: stab resistance, knife penetration, yarn failure, contact simulation, finite element method, woven fabric.

Stab resistant garments are required for police, soldiers, correction officers and security personnel to prevent injuries from knife stabs or slashes. Textile composite materials have recently received considerable attention for body armour, due to their structural advantages of high specific-strength and high specific-stiffness as well as improved resistance to stabbing. As the penetration processes and fabric architectures are very complex, homogenization techniques are often used to create a simple model to represent fabric properties. The constitutive models commonly describe a woven fabric as a nonlinear material with a standard elastic-plastic response. Although the homogenization methods have been demonstrated to be effective in predicting material properties, and successfully used in modelling the ballistic impact on traditional laminated textile composites [1,2], the simple homogenization techniques cannot be used to effectively study the mechanism of fabric structural failure. In particular, they cannot be used to study the process of knife penetration through woven fabrics, due to the transverse impact of the knife on adjacent fibres and the slide between yarns.

To fully understand how knives or other edged weapons penetrate stab resistant fabrics, in this paper, the penetration of a knife through a plain-weave fabric is computationally simulated with the finite element method at a yarn level. A commercially available explicit nonlinear finite element analysis code, LS-Dyna [3], is used to model the transverse knife stabbing process

The structure of the woven fabric is described by a unit cell, which is the smallest repeat geometric unit of the fabric. The cross-section of yarns is of a similar ellipse. The pattern of the yarns in longitudinal direction is of sinusoidal wave form. The woven fabric is composed of 8 warp yarns and 8 weft yarns. The knife used has two sharp edges of 0.02 mm in thickness and is defined as a rigid body.

The material model assumes that the yarn is an elastic-plastic material, the same as its constituent fibres until one of the fibres breaks. The breakage of fibres occurs between strain values of 0.280 to 0.335. The yarn damage occurs at the point where all fibres lose strength. Thus, the stress in the yarn is a function of the strain and the number of broken fibres. In the simulation, the multilinear elastic-plastic material model named piecewise linear plasticity model in Ansys LS-Dyna [4] is used to describe the material properties of the fibre yarns, as the model allows the total true stress vs. the effective plastic strain curve input and strain rate dependency and has a capability to model the failure of material.

The knife penetration of woven fabrics is an energy transfer process through knife-yarn contacts, which make the modelling of the knife stabbing process very complex and time consuming. In addition to the contacts between the knife and the yarns, the contacts between the yarns are also taken into account in the simulation, because the yarns in the woven fabric are modelled individually and there is a relative slide between the yarns during the knife stabbing process. All possible yarn-yarn contacts, including the contacts between the warp yarns and the waft yarns, the contacts between the neighbouring warp yarns, and the contacts between the neighbouring weft yarns, are taken into account in the simulation. The general surface-to-surface contact algorithm (STS) [3] is used to model the above contacts.

The results show that the response of the woven fabric resistance force to the knife penetration depth is nonlinear. This implies that the penetration depth does not linearly increase with the knife incident energy. The nonlinear response of the resistance force to the penetration depth is attributed to the slide and friction between the yarns. If the slide is not taken into consideration, the initial response of a woven fabric to a stab is largely linear-elastic with the incident energy [4].

The results also show that the resistance force sharply reduces to a very small value after the two yarns in contact with the knife edges breaks. Although the breakage of a yarn is due to the failure of individual fibres, the breakage of the two yarns takes place in a very short time. For the fibres with a Young's modulus of MPa, the breakage of fibre yarns occurs at the penetration depth of 7.0 - 9.5 mm. The duration of the breakage is about 0.033 ms.

1

P. Boisse, et al., "Finite Element Simulations of Textile Composite Forming Including the Biaxial Fabric Behavior", Composites Part B-Engineering, 28(4), 453-464, 1997. doi:10.1016/S1359-8368(96)00067-4

2

Y. Gowayed and L. Yi, "Mechanical behavior of textile composite materials using a hybrid finite element approach", Polymer Composites, 18(3), 313-319, 1997. doi:10.1002/pc.10284

3

Ansys Inc., ANSYS LS-DYNA User's Guide.

4

S. Abrate, "Impact on Composite Structures", Cambridge University Press, Cambridge, 1998.

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