R-curve characteristics of adhesives modified with high rubber content under mode I loading

For rubber-modified epoxy adhesives with high rubber content, stable crack propagation is observed under mode I loading. In such a situation, a further increase in rubber content not only extends the plastic zone in front of the crack tip, but also promotes the formation and growth of voids in the adhesive layer. To clarify the effect of a further increase in rubber content on crack initiation and propagation behavior, fracture toughness tests were conducted under mode I loading using adhesively bonded double cantilever beam (DCB) and compact tension (CT) specimens: these specimens were used for evaluating the crack growth resistance and fracture toughness in the initial stage of crack propagation, respectively. Stable behavior in crack growth was observed for adhesives with rubber contents of both 10 and 14 wt%. The crack growth resistance of the adhesive with 14 wt% rubber was higher than that with 10 wt%, whereas in the initial stage of crack propagation the energy release rate for 10 wt% rubber was higher than that for 14 wt%. The crack growth behavior of the DCB specimen was discussed on the basis of Gurson’s model. The estimated R-curve agreed well with the experimental result in the initial stage of crack propagation, although the difference between estimated and experimental plots increased with crack growth.