Fatigue threshold behavior of adhesive joints

The fatigue threshold and slow crack growth rate behavior of a highly-toughened epoxy adhesive was studied as a function of the starting condition (fatigue precrack and fillet), testing approach and interfacial bond strength. For cohesive failure, the threshold behavior was very similar when starting from a fatigue precrack or an uncracked fillet; however, the threshold was lower when interfacial cracks grew from a fatigue precrack. When compared to the time required for a crack to reach a significant length in a practical joint, the crack initiation from a fillet at fatigue threshold loads occurred relatively quickly. For loading and displacement ratios of 0.1 and 0.5, fatigue testing under force control proved to be equivalent to the faster and more convenient testing under displacement control. Mixed-mode fatigue crack growth at slow rates near the threshold became progressively interfacial as the cyclic crack speed decreased, revealing differences in bond strength that were unapparent in fracture tests and in fatigue tests at higher crack speeds. Slow crack growth fatigue tests were thus found to be exceptionally sensitive to differences in interfacial bond strength caused by changes in adhesive batch, adherend roughness and surface pretreatment.