13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- provided in the references [3, 13]. The fabric samples of two different gage lengths (25mm and 50 mm) were tested at strain rates ranging from 25 to 170 s-1. The Young’s modulus, tensile strength, maximum strain, and toughness were investigated at these strain rates. Single yarn samples were tested at strain rates of 30, 50 and 100 s-1 for the 25 mm gage length, and at strain rates of 20 and 60 s-1 for the 50 mm gage length. It should be noted that the strain rates listed above are the average values and the actual strain rate of an individual test may be slightly different. 3. Results and Discussion 3.1. Quasi-static response The typical stress versus strain relationship and the fabric deformation under uniaxial tension are shown in Fig. 3. There are four distinct regions in the stress-strain behavior of both warp and fill directions: crimp region, linear pre-peak region, linear post-peak region and non-linear post-peak region. In the undeformed, but undulated state (strain ε=0), the warp and fill yarns are orthogonal to each other and free of any stretch. In the crimp region, the stress increase is relative low due to the straightening of the undulated yarns in loading direction with limited yarn stretching. The maximum strain in crimp region in warp and fill directions is only 0.0065 mm/mm and 0.0025 mm/mm, respectively. These strain values are negligible when compared with the strain at failure. As the strain increases, the yarns in the loading direction are extended, and when fully straightened ( ε=0.01 to 0.02), the fabric exhibits a linear response with no visible failure signs. The Young’s modulus (elastic stiffness) is defined by the slope of the stress-strain curve in this region. Both the crimp and pre-peak regions are fitted by linear curves to obtain corresponding stiffnesses [9]. As the stress level reaches the strength of the constituent yarns, the yarns in the loading direction start to fail, resulting in a dramatic decrease in the fabric load-carrying capacity until reaching a transition point at about 200 MPa (the end of linear post-peak region) where the yarns/fibers are broken. After that the stress decreases gradually to zero when the strain increases up to about 0.2 mm/mm, representing the nonlinear post-peak region where the failed yarns/fibers slip out of the fabric and the load is carried by the friction between sliding fibers ( ε=0.02 to 0.04). The toughness of the fabric is defined by the area under the entire stress-strain curve. Analysis of the stress versus strain curves in both warp and fill directions for both specimen sizes (50 mm × 200 mm and 25 mm × 200 mm), indicates that the pre-peak elastic stiffness (Young’s modulus) of warp direction is almost identical to that of fill direction, and the stiffness in the crimp region for warp and fill directions is as much as 6% and 20% of the elastic stiffness (Young’s modulus) in pre-peak region, respectively. The absolute value of the stiffness in linear post-peak region of warp and fill directions is 2.2 and 5.6 times of the elastic stiffness in pre-peak region. Comparing the stress-strain behavior between warp and fill directions, the major difference is the crimp strain, tensile strength (peak stress) and the ultimate strain (strain at peak stress) in both directions. The crimp strain in warp direction is about 2.6 times larger than that of fill direction. The tensile strength in warp direction is approximately 10~15% lower than that in fill direction, while the ultimate strain in warp direction is approximately 7~12% higher than that in fill direction. This
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