Evaluation of Fatigue Damage in Adhesive Bonding: Part 1: Bulk adhesive

The measurement of damage constants in adhesive bonding has been investigated. Bulk adhesive was used in this study for two reasons: a) stress distribution in bulk adhesive is simpler than adhesive in joints and b) specimen dimensions met fatigue test standard. Bulk adhesive was made from film-form of epoxy resin. In general, the characteristics and the behaviour of bulk adhesive may differ from adhesive in joint because of the presence of voids and the constrains imposed by the substrates. Low cycle fatigue tests with load amplitude ratio of 0.1 at frequency 5 Hz were performed to determine the damage variable as a function of number of cycles. Damage curves, i.e. evolution of damage variable as a function of number of cycles, were derived and plotted using isotropic damage equation. Damage was evaluated using the decrease of stress range during lifecycles of a constant displacement amplitude test. It was found that damage curves were fitted well by low cycle fatigue damage evolution law equation. This equation was derived from a dissipation potential function. Curve fitting was performed using Robust Least Square rather than ordinary linear least square because damage curves have extreme points (usually near failure point). It was found that fitting process could not converge for adhesive fracture at higher cycles (Nf > 9000). Two damage constants A