| Summary: | Adhesives can enable significant weight reductions in automotive constructions. However, the performance of adhesive joints and the energy absorption of bonded structures needs to be well understood. This thesis describes the perfonnance of a number of automotive adhesive joints using a fracture mechanics approach. The fracture energy, Gc ' was used to characterise the joint perfonnance during different modes of loading at low and high r,ates. A detailed analysis strategy was developed to account for the different types of crack propagatidn observed and the high rate effects encountered. This analysis also included the use of load-independent equations and incorporated the effects of kinetic energy. A high-speed video system was used to study the fracture behaviour and to measure accurately the deformation ofthe joint and the crack growth. During mode I tests the values of Gc decreased as the test rate was increased. The increase in test rate did not induce significant variations in the initiation values of Gc in mode II and mixed-mode VII when load independent methods were used. Delamination of the composite substrates in some tests complicated the interpretation of the results and made some mixed-mode failure criteria difficult to apply. However, an analytical model was introduced to predict the composite delamination observed in the different tests and this showed an excellent agreement with the observed failure paths. The results obtained for mode I demonstrated that neither test velocity nor crack speed was the parameter controlling the toughness. Instead the time parameter rl/2 appeared to describe the variation in G/c more closely. This parameter relates to the adiabatic heating at the crack tip, which causes a material softening. Thermographic measurements were performed to support this proposal. Finally, structural tests on composite crush tubes showed that adhesively bonded structures can indeed be used with confidence in automotive applications.
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