Measuring of damage parameters in adhesive bonding

In this paper, the characterisation of damage in an epoxy adhesive has been investigated. Bulk adhesive samples were used in this study for two reasons; firstly the stress distribution in the bulk adhesive sample is simpler than that in a joint, secondly, the specimen's dimensions meet fatigue test specimen standards. Low cycle fatigue (LCF) tests with a load ratio of 0.1 and a frequency of 5 Hz were performed on bulk adhesive dumbbell specimens. Damage curves, relating damage in the specimen to number of cycles to failure, were plotted using an isotropic damage equation in which damage is a function of stress, which decreases as damage progresses. The damage curves were then fitted using a LCF damage evolution law. This equation was derived from a dissipation potential function using Continuum Damage Mechanics (CDM) theory. Curve fitting was performed using a Robust Least Square technique rather than ordinary linear least square because the damage curve has extreme points (usually at the breaking point). Two damage parameters alpha and beta were found from the curve fitting process. This process resulted in different values of alpha and beta for different stress levels. The logarithmic alpha and beta points were then plotted respect against stress level and linear regression was used to determine alpha and beta as a function of stress. With this function, damage parameters for other stress level can be predicted.