Effect of Bondline Thickness on Mode I Fracture in Adhesive Joints

Abstract

Thickness of bondline in adhesive joints has been found to have a significant effect on joint strengths. In this study, mode I fracture experiments were conducted using the DCB specimen with Hysol EA-9394 paste adhesive. The trend of the failure load versus adhesive thickness followed that found by many previous authors. However, an analysis of the stress field near the crack tip indicates that the singular stress field (the K-dominance zone) is confined in a very small region near the crack tip and, thus, the stress intensity factor cannot adequately represent the fracture force in the fracture process zone. Moreover, the plastic zone was found to be much greater than the K-dominance zone implying that the small scale yielding assumption used in linear elastic fracture mechanics is violated. The CTOA is employed as an alternate fracture parameter to characterize the fracture toughness for the adhesive. Experimental results indicate that CTOA seem to be a constant value for all adhesive thicknesses as long as the crack propagates in the adhesive layer. For adhesive thickesses less than 0.8 mm, an interfacial failure occurs before crack extension. This explains why the failure load decreases as the adhesive thickness decreases below 0.8 mm.