Scaling effect of crack density in GFRP laminates: Introduction to a multiscale fatigue testing scheme and finding an appropriate biaxial zone shape of the cruciform specimen

The number of off-axis cracks in a multidirectional laminate or crack density has been identified as an important fatigue parameter for laminate composites and it is related to its stiffness degradation. But for implementing crack density as a fatigue parameter in a multiscale modelling scheme, it is important to investigate its sensitivity to scale effects. In this research, rectangular coupon and cruciform specimens with variation in the in-plane dimensions are loaded in tension-tension fatigue where the crack density evolution is monitored with white light imaging. But the design of a cruciform specimen is not straightforward and is heavily dependent on the research goal. This particular work focuses on the design of the cruciform specimen through finite element simulations by comparing 4 biaxial zone geometries - circle, square, rhombus and curved square, investigating the uniformity of the induced strain state in the biaxial zone. A circular biaxial zone is found to be appropriate. The multi-scale fatigue testing and modelling scheme of the CASMaT Initiation Project at the Technical University of Denmark is presented as well.