13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- yarn specimens under quasi-static and dynamic tensile loadings and the results of Weibull statistical analysis. Images captured during loading process are used to study the deformation and failure mechanisms of the fabric and yarn. 2. Experimental Program 2.1. Specimen preparation The plain-woven Kevlar® 49, a high performance fabric for ballistic protection application, made by EI du Pont de Nemours & Co., is used in this study. The fabric is manufactured using a plain-weave of 17×17 yarns (per linear inch) each consisting of hundreds of filaments. The bulk density (mass per unit of volume) and linear density (mass per unit of length) are 1.44 g/cm3 and 1.656 × 10-3 g/cm, respectively. The cross-sectional area of each yarn was calculated as 1.15 × 10-3 cm2 by dividing the linear density of the material by its bulk density [13]. This value is then taken as the total c/s area of individual fibers within the yarn. To make a strip specimen for quasi-static tests, the fabric was first cut into an oversized rectangular strip, and then a number of yarns along the fabric length were removed from both sides, thereby producing a sample without yarn crossovers along the edges. This step is necessary to ensure that the edge defects are minimized and that the loaded yarns will not slip out of the cross yarns during the test. The final sample dimension had a length of 250 mm with two alternate widths of 30 mm and 60 mm. In each set, yarns are removed from both sides of the strip such that the samples are left with 17 and 34 longitudinal yarns, respectively. The initial gage length was 200 mm. The total cross-sectional area of a specimen was defined as the cross-sectional area per yarn multiplied by the number of yarns within the width. In order to investigate the effect of gage length on the mechanical properties of the fabric, additional specimen sizes of 25 × 280 mm and 25 × 355 mm were also tested in warp direction under the same loading rate. Smaller specimens were used for dynamic tensile tests. The fabric was cut to the width using an electric scissor allowing eight yarns in the section of gage length. Thin aluminum sheets were glued using high-strength epoxy to the ends of the test specimen to reduce the stress concentration and improve load transfer in grips so as to prevent any slippage, as shown in Fig. 1. The single yarn specimen was constructed to contain one yarn in the central position by removing the rest of the yarns as shown in Fig. 2. Two different gage lengths of 25 mm and 50 mm were tested for both fabric and single yarn specimens. Figure 1. Fabric specimens with (a) 25mm and (b) 50mm gage length for dynamic tensile tests (a) (b)
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