Microstructure‐Informed Fatigue Delamination Behavior of CFRP Composites Under Variable Amplitude Fatigue Loading

ABSTRACT This study investigates Mode I fatigue delamination in carbon fiber reinforced polymer composites under two‐level block variable amplitude (VA) loading, with particular emphasis on correlating delamination behavior with microstructural damage evolution. Double cantilever beam (DCB) specimens were subjected to controlled variations in peak load, load ratio, and loading transition rate, using constant amplitude (CA) loading as a baseline reference. A piecewise fitting methodology was developed for compliance‐cycle curves to accurately determine delamination onset life ( N _onset ), yielding more conservative estimates than standard methods. Experimental results identify peak load, load ratio, and loading transition rate as decisive factors governing fatigue damage accumulation. Fractographic analysis reveals two competing failure mechanisms with distinct delamination behaviors: fiber/matrix interfacial debonding, dominant at higher load levels and faster transition rates, produces relatively clear crack paths and accelerated damage growth; whereas mixed failure mode, combining interfacial debonding with matrix shearing damage and prevailing at lower load levels and slower transition rates, generates tortuous crack paths that significantly retard delamination progression. The transition between these mechanisms is governed by the synergistic effects of load level and loading transition rate, with higher transition rates promoting earlier transitions to interfacial‐dominated failure. Hackle length measurements provide quantitative evidence of this transition, exhibiting distinct characteristics under different failure mechanisms. These findings establish comprehensive relationships between VA loading parameters and damage mechanisms, providing critical insights for developing microstructure‐informed fatigue life prediction models.