Double cantilever beam tests on a viscoelastic adhesive: effects of the loading rate

Abstract

Adhesive bonding technology is a promising alternative to traditional joining techniques. Indeed, bonded joint shows higher strength and fatigue life than bolted or riveted joints having identical weight. However, bonded joints are sometime reputed to be little reliable since significant dispersion could be observed while measuring their strength but also due to strong sensitivity to adhesion defect and poor surface preparation. Damage tolerance philosophy is now recommended for more reliable design of critical bonded parts by precise prediction of decohesion initiation and propagation along the bondline.Double cantilever beam (DCB) test is the most popular method to characterize the decohesion resistance of bonded interface by measuring their fracture energy or their R-curve in case significant nonlinear behaviour is observed. These past years, several efficient analysis techniques have been proposed to evaluate the fracture energy but also some optimization techniques to identify more complex interface behaviour. However, most of these techniques consider non time dependent behaviour while thermoset adhesives are known to be viscoelastic and in some condition can also show viscoplastic behaviour. Such effects are important to evaluate when bonded joint sustain stationary loads since they could lead to delayed fracture and slow crack growth.In the present work, we evidence some strain rate sensitivity at the bondline scale by performing DCB test under different opening rate conditions. At first, the viscoelastic behaviour of the adhesive is studied by performing creep test in a Dynamic Mechanical Analyser. The DCB tests results are interpreted with several methods including the Simple Beam Theory. It is shown that fracture energy is not an appropriate quantity to evaluate the crack propagation condition.