The prediction of crack growth in bonded joints under cyclic-fatigue loading II. Analytical and finite element studies

Abstract In Part I (Int. J. Adhesion Adhesives (2003) in press) the performance of adhesively bonded joints under monotonic and cyclic-fatigue loading was investigated. The joints consisted of an epoxy-film adhesive which was employed to bond aluminium-alloy substrates. The effects of undertaking cyclic-fatigue tests in (a) a ‘dry’ environment of 55% relative humidity at 23°C, and (b) a ‘wet’ environment of immersion in distilled water at 28°C were studied. The basic fracture-mechanics data for these different joints in the two environments were measured, as well as the behaviour of single-lap joints. In the present paper, Part II, a method for predicting the lifetime of adhesively bonded joints and components has been investigated. This prediction method consists of three steps. Firstly, the fracture-mechanics data obtained under cyclic loading in the environment of interest have been modelled, resulting in an expression which relates the rate of crack growth per cycle, da/dN, to the maximum applied strain-energy release-rate, Gmax, in a fatigue cycle. Secondly, this relationship is then combined with an analytical or a computational description of the variation of Gmax with the crack length, a, and the maximum applied load per unit width, Tmax, per cycle in the joint, or component. Thirdly, these data are combined and the resulting equation is integrated to give a prediction for the cyclic-fatigue lifetime of the bonded joint or component. The theoretical predictions from the above method, using different approaches to describe the variation of Gmax with the crack length, a, and applied load, Tmax, in the single-lap joint, have been compared and contrasted with each other, and compared with the cyclic-fatigue behaviour of the lap joints as ascertained from direct experimental measurements.