Tensile tests of seven bias angles with a 15° increment with respect to the warp direction are conducted on a typical coated bi-axial warp knitted fabric (BWKF). By utilizing the Digital Image Correlation technique, the experimental results are processed, and detailed responses of strain contours and Poisson’s ratios are determined for some specific strain levels, and an extraordinary M-shaped relationship between Poisson’s ratio and bias angle is revealed. Then, the tested Poisson’s ratios are compared with corresponding theoretical predictions from the original theory for ordinary anisotropic materials, discrepancies and their possible causes are analyzed. Finally, the original theory used for Poisson’s ratio predication of ordinary anisotropic materials is modified for application to the BWKFs by considering the unique local valley at 45° caused by structural differences and possible yarn movements. The modified theoretical equation could exhibit the M-shaped relationship between Poisson’s ratio and bias angle, and is validated due to the high similarity between the predictions and experimental results. This investigation could provide some new insights into the orientation-dependence, Poisson’s effect and complex deformation mechanism of BWKFs.
Bi-axial warp knitted fabrics (BWKFs) Orientation-dependence Digital Image Correlation (DIC) Poisson’s ratio M-shaped relationship
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