Fractography of Adhesively Bonded SMC-Metal Joints

Failure surfaces of glass fiber-reinforced sheet molding compounds (SMC) adhesively bonded to metal brackets were examined by scanning electron microscopy. The adhesive was shown to fail by mixed modes of tear and brittle fracture, probably due to an unusual microstructural morphology which also contributed to a decrease in its fracture toughness at elevated temperatures. The SMC failure mode of the substrate was shown to be delamination by tear. For the debonded joints, a range of failure modes were observed, from simple adhesive failure to substrate delamination beneath the bondline. Wetting of the SMC substrate was found to be inadequate, probably due to the high viscosity of the adhesive, leading to inadequate interdiffusion between the adhesive and the adherend. In some samples, the SMC substrate was found to have delaminated directly beneath the bondline, to a depth of one to two times the diameter of the glass fiber. The results demonstrated that an interdisciplinary program to study the chemical, interfacial and mechanical properties of the various components in adhesive joints is needed.