Mechanical property degradation of fine z-pin reinforced polymer composites under substitute ocean water environment

This study investigates the mechanical properties and failure behavior of carbon fiber reinforced polymer (CFRP) with implanted fine z-pins exposed to a substitute ocean water environment. The results indicate that water uptake remained comparable between the control and z-pinned composites at thicknesses of 1 mm and 2 mm, whereas at 4 mm, the z-pinned specimens exhibited quicker absorption, owing to the reduced effect of osmotic pressure. The stress concentrations around the z-pins initiate cracking during tensile testing, causing dry z-pinned composites to fracture into numerous needle-like fragments, while water-saturated composites fail into wider strips. Under compressive loading, dry z-pinned composites exhibited a sudden, explosive failure, whereas the water-saturated specimens failed via an angled fracture within the gauge section. During flexural testing, the fracture surface propagated through rows of z-pins, and water absorption rendered the z-pins more susceptible to shear failure. The retention rates of tensile and compressive strength for CFRP_(0.1pin) and CFRP_(0.2pin) were lower than those of control sample, whereas the flexural strength retention rate was comparable to that of the control sample. Notably, the initiation G_IC values for both CFRP_(0.1pin) and CFRP_(0.2pin) increased following moisture absorption. The effect of hygrothermal aging on propagation G_IC was governed by the prevailing failure mode. In cases where pin fracture dominated, the degradation of pin mechanical properties and interfacial weakening were primarily responsible for the observed decrease. Conversely, when pull-out failure prevailed, swelling of the resin-rich zones enhanced pin traction load, yielding an increase in propagation toughness.