Fracture mechanics and damage mechanics based fatigue lifetime prediction of adhesively bonded joints subjected to variable amplitude fatigue

Abstract

The first part of the paper describes an investigation into the behaviour of adhesively bonded single lap joints (SLJs) subjected to various types of variable amplitude fatigue (VAF) loading. It was seen that a small proportion of fatigue cycles at higher fatigue loads could result in a significant reduction in the fatigue life. Pamgren-Miner's damage sum tended to be less than 1, indicating damage accelerative loadd interaction effects. In the second part of the paper, fracture mechanics (FM) and damage mechanics (DM) approaches are used to predict the fatigue lifetime for these joints. Two FM based approaches were investigated, which differed with respect to the cycle counting procedure, however, both approaches were found to under-predict the fatigue lifetime for all the types of spectra used. This was attributed to the inability of the FM based models to simulate the crack initiation phase. A DM based approach was then used with a power law relationship between equivalent plastic strain and the damage rate. Good predictions were found for most of the spectra, with a tendency to over-predict the fatigue life. AbstractThe first part of the paper describes an investigation into the behaviour of adhesively bonded single lap

Fatigue damage modelling of adhesively bonded joints under variable amplitude loading using a cohesive zone model

“…In this method, a damage variable is defined which modifies the constitutive response of the adhesive. Shenoy et al [9] utilised this method for the fatigue prediction of the SLJ under VA loading. They assumed the damage rate to be a power law function of the plastic strain range and degraded the plastic behaviour of the adhesive based on this damage variable.…”

“…The approaches that follow are more generic, seeking to determine the joint response from the underlying material behaviour. A fracture mechanics based method (fatigue crack growth approach) that has been used widely for CA fatigue prediction of bonded joints has been utilised to predict VA fatigue response [8,9,14]. This method relates the fatigue crack growth rate to an appropriate facture mechanics parameter through an empirical equation.…”

“…Some of the methods employed for fatigue prediction of bonded joints under VA loading include total life, strength wearout, fracture mechanics based and damage mechanics based methods [6][7][8][9]. The total life method expresses the fatigue life as a function of a loading variable such as the maximum or the amplitude of the stress, strain or load cycle.…”

“…Some methods have been developed to predict the fatigue life of bonded joints under VA fatigue loading [6][7][8][9]. However, they are either empirical (providing limited understanding of the underlying mechanisms) or are unable to take important VA fatigue characteristics (such as load shifts) into account.…”

“…A fatigue damage model developed by the authors [10] for CA fatigue loading was adapted to predict the experimental results reported by Shenoy et al [9] for single lap joints (SLJs) subjected to VA loading. This paper is organised into three main sections: first, the main predictive models that have been used by others to predict the VA fatigue life of adhesively bonded joints are outlined.…”