Transient delamination growth in GFRP laminates with fibre bridging under variable amplitude loading in G-control

To predict fatigue-driven delamination growth in real structures, it is inevitable to understand delamination growth under real load spectra, also known as in-service loads, which most commonly are variable amplitude load patterns. In this work, delamination growth in glass fibre-reinforced polymer laminates subjected to two-level block amplitude loading is investigated using a pure moment loaded DCB test configuration for G-controlled cyclic testing. The variable amplitude load patterns consist of single- and periodic repeated load amplitude changes. The method utilises a digital image-based technique that allows for precise tracking of delamination fronts in translucent materials. The work characterises transient crack growth phenomena following load amplitude changes with emphasis on the actual fatigue crack growth rate. The load amplitude changes may increase the crack growth rate by factors of 5–6 in comparison to the crack growth rate in constant amplitude base-line tests. Periodic repeated load amplitude changes particularly increase the fatigue crack growth. For example, after just 10 periodic repeated low- and high-load blocks, the delamination has extended more than a factor of 2 times the delamination extension predicted from a non-interaction model based on constant amplitude data. The effect of the frequency of load amplitude changes on the fatigue crack growth is also investigated. Frequent load amplitude changes are proven to increase the fatigue crack growth significantly because of the transient crack growth responses following the load amplitude changes.