A modified TDCB specimen was used to compare crack resistance in pure mode I and mixed mode I–III loading. Static and fatigue tests were conducted on two commercial, modified epoxy film adhesives (FM 73 and FM 300K) having a wide open knit polyester carrier. The fatigue tests were conducted at 10 Hz in a low humidity and at 3 Hz in a high humidity.
The fracture toughness of both adhesive systems, in terms of Gtotal, was 1 ⅔ to 2 times higher for the mixed-mode specimens than for the pure mode I specimens. For the dry fatigue tests, ΔGT for a fixed crack growth rate was also much higher for mixed mode than for Mode I cracking. A fracture law based on these data would be quite different from one based on the large amount of mixed mode I–II data now available. These differences appear to be the result of differences in fracture behavior produced by different types of carrier cloths. For Mode I cracking in the present study, separation occurred between the adhesive and the carrier cloth, which lowered both the cyclic and static debond resistance to values less than those reported earlier for FM 300 with a tricot knit carrier. For the mixed-mode specimens, on the other hand, the carrier cloth, rather than separating from the adhesive, shredded, which increased the crack resistance above the mixed-mode values reported previously for FM 300. This behavior suggests that the carrier cloth in film adhesives has a greater influence on crack resistance than generally thought. Thus fracture laws for adhesive joints may also be more complex than previously thought.