Microcrack induced delamination growth in fiber composites is a damage mechanism of major concern, especially when the structure has to sustain load reversals or compression loadings, due to stability issues.
To increase the understanding of damage interaction, an experimental campaign was conducted, focusing on the initiation and growth of fatigue cracks and crack induced delaminations. Therefore, GFRP cross-ply laminates were subjected to tension-tension, tension-compression and compression-compression fatigue loading. Damage observation of cracks and delaminations was performed by use of a camera with close-up lens and back as well as front illumination, respectively. Damage quantification was performed by in-house MATLAB codes. Additionally, the specimen’s dynamic stiffness was monitored regularly. It was found, that the presence of load reversals significantly increases the rate of crack initiation and growth and therefore leads to a more pronounced delamination growth, compared to pure tension or compression loading. The photographic images further prove, that crack initiation is inevitably linked with the initiation of minor crack tip delaminations, that subsequently extend in both directions from the crack. Despite the different loading conditions, no significant difference in the initial crack tip delamination lengths has been observed. Nevertheless, subsequent delamination growth strongly depends on the applied load level and load ratio. The strain energy release rates for microcracking and delamination fracture are calculated by existing modeling techniques, e.g. VCCT and analytical methods [1], to estimate delamination initiation and derive fatigue delamination growth relations for embedded offaxis plies in composite laminates in terms of Paris-like curves.