Mixed-mode fracture behavior of epoxy-based impact adhesives

Abstract

The focus of our study was to determine the mixed-mode fracture behavior and the governing fracture (failure) criteria for different impact-resistant/toughened structural adhesives. Four of the five model adhesives used contained organic tougheners in the form of carboxyl terminated butadiene acrylonitrile (CTBN) copolymer, as well as polyurethane adducts. The main crystalline inorganic phases were calcite (CaCO 3 ), wollastonite (CaSiO 3 ) or calcium silicate (CaSiO 3 ), talc (Mg 3 Si 4 O 10 (OH) 2 ), zeolite which is an alumina silicate based mineral and has many different elements in its composition (M 2 /nO.Al 2 O 3 .xSiO 2 .yH 2 O., M can be Mg, Na, Ca, K, Li). The total amount of inorganic fillers was different in each adhesive. Independently Loaded Mixed-Mode Specimen (ILMMS) was used for independent measurement of P I (opening) and P II (in-plane shear) loads which were necessary to determine the strain energy release rate values, G I and G II without using a finite element analysis. Two versions of the ILMMS specimen geometry (a, b) were used: monotonic in opening and static in shear loads, and monotonic in shear and static in opening loads, respectively. Elliptical energy balance failure criterion and parabolic principal stress criterion were used for curve fitting. The experimental data revealed that all but one of the adhesives were more accurately represented by the energy balance criterion except for one that was more accurately represented by the principal stress criterion indicating more brittle behavior. Results obtained from ILLM specimen a showed that ranking varied as related with the G Ic and G IIc values and, also, the applied static load. For example, while Adhesive 2 was in the first rank based on G Ic value, it became third in the ranking based on G II c value. Thus, it can be suggested that ranking as well as the fracture behavior (i.e., cohesive vs. adhesive failure; interfacial separations) change with the extent of the mode mixity present in loading of toughened epoxy adhesives.