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, d a/d N, to the maximum applied strain-energy release-rate, G max, in a fatigue cycle. Secondly, this relationship is then combined with an analytical or a computational description of the variation of G max with the crack length, a, and the maximum applied load per unit width, T max, 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 G max with the crack length, a, and applied load, T max, 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.