13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Tables 1 summarizes the tensile properties of both specimen sizes in both warp and fill directions. There is no apparent size effect in terms of Young’s modulus, toughness and ultimate strain, but the average tensile strength of 50 mm wide specimens is slightly lower (2~6%) than those of 25 mm wide specimens in both directions. For longer specimens (25 × 280 mm and 25 × 355 mm), the tests were stopped once the load on the specimen fell below 222N (corresponding to approximately 10% maximum strain). Toughness in these samples is underestimated as much as 16~20% lower than those of 200 mm long specimens. However, the toughness remains almost same when the gage length increases from 280 mm to 355 mm, so does the tensile strength, Young’s modulus, and ultimate strain, indicating that gage length effect on the fabric’s mechanical properties is negligible. 0 0.2 0.4 0.6 0.8 1 Cumulative Failure Probability 1400 1600 1800 2000 2200 2400 Tensile Strength, MPa 50 mm 125 mm 200 mm 275 mm 350 mm 425 mm Kevlar 49 Single Yarn 1600 1800 2000 2200 2400 2600 Tensile Strength, MPa 0 0.2 0.4 0.6 0.8 1 Cumulative Failure Probability Warp (50x200 mm) Warp (25x200 mm) Fill (50x200 mm) Fill (25x200 mm) Kevlar 49 Fabric (a) (b) Figure 4. Comparison of cumulative failure probability versus tensile strength: (a) fabric and (b) single yarn under quasi-static loading Weibull parameters were obtained using a 2-parameter Weibull equation: 0 ( ) 1 exp m fP σ σ σ ⎡ ⎤ ⎛ ⎞ ⎢ ⎥ = − − ⎜ ⎟ ⎢ ⎥ ⎣ ⎝ ⎠ ⎦ (1) where σ0 is the characteristic yarn strength and m is the shape parameter, characterizing the spread
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