Effects of mean stress and fibre volume content on the fatigue-induced damage mechanisms in CFRP

• Carbon/epoxy with 30% and 55% fibre volume fraction and epoxy resin are tested.
• Fracture surfaces after R0.1 and R-1 fatigue tests are studied by using SEM.
• Damage mechanisms depend on applied load and on fibre content.
• Matrix cracking and fibre pull out occur under tensile loads.
• Fibre crushing was monitored under tensile–compressive loads.

The effect of the load type (tension and compression) in quasi-static and of the applied mean stress in fatigue tests on the mechanical behaviour and on the damage mechanisms in unidirectional (UD) carbon/epoxy laminates has been studied in combination with the influence of fibre volume content. Results show that the fibre volume content increases the mechanical properties in tension–tension fatigue tests for all tested angles 0°, 45° and 90°. The tensile damage mechanisms of off-axis specimens depend on the fibre volume content and change from matrix cracking and matrix–fibre debonding to fibre-pull out with an increasing amount of fibres as investigated in detail in a previous work. In tension–compression tests, higher fibre volume contents are only beneficial in fatigue tests at angles of 0° and 45°. Fatigue strengths of UD 90° specimens in tension–compression tests are not significantly improved by the fibre volume content which can be ascribed to breakage of entire fibre bundles and crushed fibres on the fracture surfaces.