Effects of inter-ply angles on the failure mechanisms in bioinspired helicoidal laminates

Following encouraging findings on the potential of helicoidal laminates under transverse loads to significantly outperform cross-ply laminates, a more extensive study was undertaken to include thicker helicoidal laminates with the greater range of variation in inter-ply angles. Previous research has shown that thin helicoidal laminates outperform cross-ply laminates when the inter-ply angle is less than 10 degrees. It was reported that the small inter-ply angles in helicoidal laminates make them more resistant to delamination. Consequently, less delamination occurs and they are deeper inside the laminates. This delays catastrophic failure, which occurs when transverse cracks propagating from the surface of the laminates merges with the delamination. The current study shows that thick helicoidal laminates with small inter-ply angles promote a different damage mechanism not apparent in thin laminates. Hence reducing inter-ply angles does not always lead to higher transverse load bearing capability. Observations from CT scan images of thick helicoidal laminates suggest that higher delamination resistance offered by small inter-ply angles is offset by the ease with which cracks between fibers propagates transversely when the angle is too small. Hence, helicoidal laminates comprising 73 plies of unidirectional carbon fiber reinforced laminas with 2.5 degrees inter-ply angle could not achieve the peak loads of 73-ply laminates with 10 degrees inter-ply angle. The optimal inter-ply angle to achieve high peak load appears to be between 5 degrees to 10 degrees. This study shows that the peak transverse load for helicoidal laminates can be up to 73% higher than that of cross-ply laminates when they are optimized.