Experimental study of cracks at interfaces with voids

Heterogeneities are inherent parts of adhesively bonded joints. In order to take the full advantage of the adhesive bonding, it is commonly accepted that the bondline and the interface should be homogenous. Flaws and voids present at surfaces of the adherents or trapped inside the bondline are expected to lower the resistance to fracture. Indeed, with a simple inspection of the force vs. displacement curves, as obtained from mode I double cantilever beam experiments for assumed homogenous bond lines, some fluctuations were observed. These fluctuations are due to the aforementioned voids. A set of specimens were designed with strong/weak adhesion zones perpendicular to the crack propagation direction. Specifically, we address the problem of crack propagation along such interfaces with focus on the relation between the process zone size and the size of the void. In this paper, experimental results are presented followed by a fundamental analytical model. This is sufficient to gain phenomenological insight into the process of crack propagation along adhesively discontinuous interfaces.