Abstract
This thesis has identified the failure and damage processes in a particle filled epoxy which is typical of adhesives used industrially. Micromechanical analyses have been carried out to predict the material properties of damaged adhesive, and to investigate the applicability of different failure criteria. The general body of evidence suggests that there is no direct method of predicting the failure load of adhesive joints from the strength and toughness of the adhesive used. Therefore, a favoured approach has been to postulate a failure criterion, and to implement it in the constitutive equation for the adhesive. In contrast, this work has begun from the microstructural modelling of damage, and derived credible failure criteria from this model. The experimental program quantified the adhesive morphology and identified the damage processes that occur in the adhesive prior to failure. Bulk and joint specimens were tested both in-situ in a scanning electron microscope, and on a conventional tensile testing machine. The tests showed that the mechanisms for damage and failure in both joint and bulk form are particle debonding followed by cracking in the matrix. The concept of a representative unit cell of material was used to determine the effects of particle cracking and debonding. In a regular' array of cracked particles, the stiffness remained relatively unchanged in the plane of the cracks, but perpendicular to it, a significant reduction was found. Modelling debonded particles is more complex, because partial contact must be considered in addition to the fully bonded and fully debonded conditions. The unit cell was used to define the elasticity matrix for adhesive containing debonded particles as a function of strain state. The unit cell concept was extended further by including material that obeyed a modified (i.e. hydrostatically sensitive) Von Mises yield criterion. Particle debonding was found to contribute significantly to the hydrostatic sensitivity and to the softening of the adhesive. The unit cell concept was used to implement a strain at a distance failure criteria, using both elastic and plastic material properties. New types of failure criteria also based on the unit cell have been proposed. The criteria relate the strain state in an adhesive joint to the likelihood of shear banding or tensile plastic flow. The regions in a joint that experience one or the other of the mechanisms were identified. Hence the nature and extent of the adhesive failure in joints with varying joint geometry and loading may be predicted.