Ratcheting in structural adhesives

When adhesively bonded joints are subjected to a cyclic load, they will primarily behave viscoelastically. Under certain loading conditions, they can also experience permanent deformation, or ratcheting. This research considered loading conditions that result in ratcheting for a toughened and standard adhesive in scarf joint coupons. Adhesive strain was measured using edge mounted resistance strain gages that was verified with digital image correlation. At low stress levels both adhesives exhibited a purely viscoelastic response. At high stress levels, the standard adhesive exhibited a viscoelastic response in tension-tension and a ratcheting response under reversed loading. At high stress levels the toughened adhesive exhibited ratcheting during tension-tension and reversed loading with higher ratcheting strain in tests that involved reversed loading. Enhanced ratcheting during reversed loading is consistent with a kinematic hardening response, which these adhesives have been shown to follow.