Mechanical properties and energy absorption capability of woven fabric composites under ±45° off-axis tension Tim Bergmann a,⇑ , Sebastian Heimbs a , Martin Maier b a Airbus Group Innovations, 81663 Munich, Germany b IVW, Institute of Composite Materials, Kaiserslautern University of Technology, 67663 Kaiserslautern, Germany article info Article history: Available online 4 February 2015 Keywords: Fabrics/textiles Mechanical properties Energy absorption Mechanical testing Strain rate abstract Fibre-reinforced polymer composites are generally known for their brittle failure behaviour. Ductility of composites, in contrast, which may be of relevance for specific applications like for energy-absorbing structures, can typically be obtained under ±45° off-axis tension using the in-plane shear effect. In order to provide an extensive database for the in-plane shear behaviour, a comprehensive experimental study of woven fabric composites under quasi-static and high strain-rate ±45° off-axis tensile loading is pre- sented, assessing the non-linear stress–strain behaviour and weight-specific energy absorption capability under different loading rates. The test campaign aims at characterising the influence of fibre material (carbon, glass, aramid, Vectran Ò and Dyneema Ò ), matrix material (untoughened epoxy resin, toughened epoxy resin and thermoplastic PEEK), weave pattern (plain weave, twill weave, satin weave and braid) and fibre areal weight on the ±45° off-axis tensile mechanical properties. The results reveal failure strain values of up to 28% and significant strain rate effects, influencing stiffness, strength, strain-to-failure and energy absorption. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Fibre-reinforced composite materials are well-known for their high weight-specific mechanical properties such as stiffness and strength. However, these materials usually suffer from their brittle mechanical behaviour under tensile loading, showing almost no to very limited ductility compared to metals and conventional plas- tics. There are applications in which only stiffness and strength are the key design parameters, e.g. for static sizing of a structure. But in case of a highly dynamic crash or collision event, which is a common design load case in the transportation sector, ductility and energy absorption capability play a major role as well. Composite materials show a remarkable performance as energy absorbers under compression and fragmentation, for example in crush tubes for automotive structures. The weight-specific energy absorption capability (SEA) of those structures can reach values of up to 225 kJ/kg, depending on numerous parameters like fibre type, matrix type, fibre architecture, specimen geometry, manufac- turing and processing conditions, fibre volume fraction and loading velocity [1]. However, the energy absorption capability of compos- ite materials under tensile loading is negligible, because of the almost ideal elastic material response and lack of plasticity up to failure. For energy absorption purposes under tension, they cannot compete with metals and plastics. High manganese austenitic steels, e.g. TWIP steels (twinning induced plasticity) like Outo- kumpu H500, admittedly suffer from their high density of 7.71 g/ cm 3 , but with a strength of almost 1000 MPa and a strain-to-failure of about 70%, resulting in SEA values of 60–75 kJ/kg on material level, they are superior for energy absorption under tensile loading [2,3]. Within this context, several researchers tried to improve the ductility and energy absorption capability of composite materials by adding ductile steel fibres and wires into the composite structure [4–6]. Others were looking for highly ductile composite materials by adapting the overall structural appearance from a flat to a sinusoidal wave structure for the case of application as tensile energy absorbers [7–9]. All these efforts still do not provide ductile performance comparable to metals, though. Nevertheless, it is well-known that woven fabric composites show a very nonlinear stress–strain behaviour under ±45° off-axis tension, which is almost similar to the plastic deformation beha- viour of metals and hence, could be used for energy absorption purposes (Fig. 1). The so-called shear effect is a matrix dominated failure mechanism initiated by microcracks and matrix failure, allowing the fibre rovings to rotate towards the loading direction. The reorientation of the interlocked fibre rovings results in a http://dx.doi.org/10.1016/j.compstruct.2015.01.040 0263-8223/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +49 89 607 28735; fax: +49 89 607 23067. E-mail address: tim.bergmann@airbus.com (T. Bergmann). Composite Structures 125 (2015) 362–373 Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct