Progressive matrix cracking in carbon/epoxy cross-ply laminates under static and fatigue loading

•The crack density is used as the mechanistic variable for mode I cracking.•A master curve for mode I static cracking is derived.•A Paris law master curve for mode I fatigue cracking is obtained.•Diffuse damage cannot be used to explain the multiplication of mode I fatigue cracks. This paper presents an experimental investigation into the growth of matrix cracks in carbon/epoxy cross-ply laminates under static and fatigue loading. Particular emphasis is placed on how to address fatigue damage. Two models are proposed to describe the evolution of fatigue cracks, where the crack density is used as the mechanistic damage variable. The first model defines the evolution of cracks according to a Paris law function. The second assumes that fatigue degradation is mainly caused by diffuse damage, resulting in an apparent decrease in the fracture energy, so that the cracking process is controlled by the static function. Both models are analysed in detail and compared against experimental data for different transverse plies and loading levels. The presence of cracks is monitored by enhanced X-ray radiography and edge camera images. Two crack density master curves are derived, one based on fracture mechanics for static and one based on a Paris law for fatigue cracking. The predictions show good correlation with the experiments.