Journal of the Japan Society for Composite Materials

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Polymer films are expected to be feasible candidates for the liner of cryogenic composite tank structures. This study investigates the damage behaviors of polymer-lined CFRP laminates under tensile loading at room and cryogenic temperatures. Because matrix cracks in CFRP laminates are considered to be more susceptible to formation than polymer film fracture, damage behaviors of polymer films beyond the onset of matrix cracks in layers adjacent to polymer film are the major concerns herein. Specimens consisting of polymer films and cross-ply laminates were prepared by co-curing and subjected to tension load. Penetration of matrix cracks in composite layers into polymer films was observed in the case of thermoplastic polyimide-lined CFRP laminates, while crack deflection into interfaces between CFRP and polymer films was observed in the case of liquid crystalline polymer and polyimide films. Finally, mechanical properties of polymer films and adhesive properties between CFRP and polymer films were measured in order to discuss the crack penetration/deflection behaviors in polymer-lined CFRP laminates. An energetic criterion on crack penetration/deflection including the effects of residual thermal stress was applied to the present experimental results, and suitable polymer characteristics were clarified.

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This study experimentally and numerically investigated the tensile damage progress in stitched CFRP laminate with an open-hole. Detailed damage observation in the tensile tests revealed that stitching had almost no effect on the onset of the ply cracks. Delamination also appeared at the all ply interfaces near the hole, and was connected by neighboring ply cracks. We then performed damage extension simulation for holed stitched-laminate using a layer-wise finite element model with cohesive elements for the damage (ply cracks and delamination) and beam elements as stitch threads. Simulation results confirmed that the delamination extended toward the area surrounded by the ply cracks in the neighboring two layers and the edge of the open-hole. Moreover, we demonstrated that stitching could suppress the delamination progress from the hole edge.

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