Void content and displacement ratio effects on fatigue crack growth in thick adhesively bonded composite joints under constant amplitude loading

This article investigates the influence of displacement ratio (R d-ratio) and void content on the fatigue behavior of Double Cantilever Beam (DCB) joints with thick adhesive layer (10 mm). Two R d-ratios and two different void contents were studied. To achieve cohesive failure, grooved DCB geometry was utilized to guide the crack propagation path into the adhesive layer, and the experimental conditions were adjusted to maintain stable propagation. The results showed that the crack propagation path under fatigue loading was mainly determined by the void size and distribution. Secondary cracks were also observed from digital image correlation (DIC) and X-ray tomography, which could lead to crack path switching on the lateral surface and influence the crack measurement. Data analysis using the total fatigue life model showed that the fatigue crack growth (FCG) depends on the R d-ratio, and void content.