Characterization of the mixed-mode fracture behavior of adhesively bonded metal joints for automative vehicle structures

The design of adhesively bonded joints as structural components in automotive vehicles requires the establishment of reliable testing and analytical methods for the determination of failure mechanisms, the assessment of damage tolerance and crashworthiness, and prediction of environmental degradation and long-term durability. As a result, The characterization of fracture behavior of adhesively bonded joints is an important element in all of these considerations.; The major objectives of the current research are to develop compact testing fixture/specimens and analytical models that can be used to study the fracture behavior of newly developed adhesives and different surface preparations over the full range of mode mixities from pure mode I to pure mode II, including intermediate mode mixities, and to provide design guidelines for mixed mode fracture testing of adhesively bonded metal joints.; The original Mixed Mode Bending (MMB) testing fixture which was developed by NASA for delamination tests of laminated composites was selected and successfully modified for the study of mixed-mode fracture behavior of adhesively bonded metal joints. The modifications were accomplished by using a modified load introduction scheme and specimen backing beam concept. The improved analytical models for mode I and mode II strain energy release rates including the effects of the adhesive layer and other corrections to the conventional beam theory have been developed and verified using both finite element analysis (FEA) and experiments. The fracture behavior of adhesively bonded joints, which consisted of aluminum alloy adherends bonded using either Dow Betamate 4601 toughened-epoxy or 3M-9245 structural bonding tape adhesives was studied by the use of the modified MMB testing fixture and the improved analytical models. Finally, guidelines for conducting successful fracture tests of adhesively bonded metal joints were developed.