Thermoplastic adhesive bonding and its fracture analysis

A thermoplastic adhesive bonding process was developed for galvanized steel to polypropylene (PP) composite using a high-density polyethylene (HDPE) based adhesive with a total process cycle of less than 120 seconds. The processing temperature significantly affected the strength of adhesive bonded lap joints. Cataplasma and cyclic moisture/temperature aging tests demonstrated that the hot-dipped galvanized steel with a polyester melamine based primer did not improve the joint durability of galvanized steel to PP composite bonded with this HDPE based adhesive. The significant loss in the adhesion of the primer to galvanized steel under Cataplasma environment was due to the blistering of the primer. The blistering was essentially a process of the cathodic disbondment. The fracture performance of HDPE based adhesive bonded double cantilever beam (DCB) aluminum joints was investigated. The fracture toughness, 1.77 kJ/m2, of the joints with a bondline thickness of 0.2 mm at a testing crosshead speed of 5.08 mm/minute, derived from beam theory equation, was significantly different from fracture toughness, 2.54 kJ/m2, obtained by cohesive zone model (CZM) approach, in which Needleman\u27s modified exponential traction-separation law was employed. The large amount of deformation occurring in adhesive ahead of the crack tip significantly contributes the fracture toughness of joints. A modified equation was proposed to compute the fracture toughness of the adhesive bonded DCB joints. The mechanical properties of the adhesive bonded DCB joints are strongly rate-dependent due to the viscoelasticity of the adhesive layer. A new rate-dependent cohesive zone model was developed to analyze the rate-dependent fracture behavior. The model consists of a rate-independent cohesive zone model in parallel to a Maxwell element. The numerical simulations were in good agreement with the experimental data for various testing crosshead speeds and for a relaxation test. A generalized rate-dependent CZM was suggested