Effect of Translaminar Reinforcements and Hybridization on Damage Resistance and Tolerance of Composite Laminates

It was shown that the damage resistance and tolerance of laminated composites can be enhanced by the employment of translaminar reinforcements (TLR) such as stitching, z-pinning and 3D weaving and also by hybrid composites. A non-dimensional analytical model focused on Mode I delamination was developed to understand the role of the TLR on delamination behavior. An explicit formula for the apparent interlaminar fracture toughness was derived in terms of the inherent fracture toughness of original materials and the bridging force due to z-pins. This model is capable of estimating the apparent fracture toughness, the bridging length and allowable bridging force thus can be useful in the design of TLR for composite laminates. Along with understanding advantages of TLR in increasing the damage tolerance of laminated composites, the damage behavior of laminated composites subjected to low velocity impact loading was studied. Based on the similarity in damage development between quasi-static and dynamic loadings observed through the short beam shear (SBS) tests, the FE analyses of the SBS specimens for quasi-static indentation and at several rates of low-velocity impact loadings were performed. The results reveal that inertia effects in the typical velocity range of the striker in SHPB, around 10m/s, can be negligible, and hence the quasi-static analysis is useful and valid in the study of damage in composite specimens under low velocity impact loading. The delamination behavior of 3D woven composites was investigated focusing on the effect of z-yarn. The 3D woven composites containing both single and double z102 yarns were chosen and compared with the 2D plain woven laminate. The double z-yarn woven composite exhibited enhanced damage tolerance compared to the single z-yarn and the plain woven laminate. The relative sliding motion between two layers is constrained by z-yarns thus the crack propagation of delamination is suppressed.
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