Influence of residual thermal stress in carbon fiber-reinforced thermoplastic composites on interfacial fracture toughness evaluated by cyclic single-fiber push-out tests

Cyclic thermal expansion measurements and single-fiber push-out tests performed with a flat-end indenter tip have been conducted on unidirectional carbon fiber-reinforced thermoplastic composites. The cyclic thermal expansion measurements revealed process-induced residual thermal stress. By annealing the composite above the glass transition temperature of the Polyphenylene sulfide matrix, the residual thermal stress can be reduced. This is attributed to structural relaxation processes. The influence of residual thermal stress on the interfacial fracture toughness was investigated by push-out measurements. Using a cyclic loading schedule consisting of subsequent unloading–reloading cycles, the dissipative and non-dissipative energy contributions during push-out test can be evaluated separately. An adapted energy-based method allows the evaluation of the interfacial fracture toughness. Reducing the amount of residual thermal stress causes a change in failure behavior from brittle to quasi-ductile failure. The altered failure behavior leads to an increase in interfacial fracture toughness by a factor of 2.4.