A micromechanical framework to study load history effects in variable amplitude fatigue: The role of fiber bridging in delamination

The load history influences fatigue-driven delamination growth in FRP composites, for example the elevated crack growth rate at load amplitude step changes, yet the underlying mechanisms remain unknown. This work develops a mechanistic understanding of the underlying mechanisms and physical processes governing load history effects. A computational framework is developed to simulate the behaviour of discrete bridging fibers in a fiber bridging zone of a DCB specimen undergoing fatigue crack growth and load amplitude step changes. The simulation is based on a novel micro-mechanical bridging model. This study presents a qualitative analysis of bridging force distributions, the crack tip shielding effect, and the crack tip energy release rate when subjecting the DCB specimen to amplitude step changes. The model captures a transient phase of elevated crack tip energy release rates following high-to-low amplitude step changes, caused by significant redistribution of bridging forces. The predicted crack tip energy release rates align very well with the observed transient crack growth rates in variable amplitude fatigue experiments.