Flexural strength and thermal expansion of 4D carbon/carbon composites after flexural fatigue loading

A four directional carbon/carbon (4D C/C) composite was fabricated by first using liquid phase impregnation carbonization (LPIC), followed by hot isostatic pressure impregnation and carbonization (HIPIC) at 75 MPa, and finally high temperature treatment. The flexural properties and fracture behavior of the composite were investigated in the through-thickness direction under static and fatigue loading. The critical fatigue limit of the composite was 80% of the static flexural strength for one million loading cycles at 10 Hz. The failure mechanism of the composite under static flexural loading was dependent on the orientation of the carbon fibers in the tested specimen. Cyclic fatigue loading decreased the interfacial bonding strength and released the inherent stresses in the composite, which increased fiber pull-out, enhanced pseudo-ductility and increased the residual static flexural strength at the expense of a decrease in the flexural modulus. The fatigue loading increased the number of noncritical matrix cracks, increased interfacial debonding, and caused the fracture of filaments in the surviving fatigued C/C composite. These features of the fatigued composite internally accommodated expansion in long direction as the temperature was increased, which resulted in a decrease in its residual thermal expansion.