Influence of plasticity on the fatigue lifetime prediction of adhesively bonded joints using the stress-life approach

The use of adhesive bonding for designing lightweight load-bearing components has increased in recent decades. In this paper the influence of plasticity on the lifetime prediction of bonded joints using the stress-life approach was investigated. The adhesive was a toughened epoxy for structural applications. Stress calculations were performed using finite element analysis. Three material models were employed, a linear-elastic model and two elastoplastic models: Von Mises (pressure independent) and Drucker-Prager (pressure dependent). Effective stress was calculated using the theory of critical distances. Lifetime predictions were based on SN curves from literature for scarf and single-lap joints at four different temperatures (−35, −10 °C, RT, +50 °C). The material properties were acquired from uniaxial tensile quasi-static experiments on bulk adhesive specimens. These experiments showed a reduction in the values of Young's modulus and yield stress with increasing temperature. A model was proposed based on an Arrhenius-type equation in order to fit the yield stress as a function of temperature. The model showed good agreement to the experimental findings. Regarding lifetime predictions (a) the influence of critical distance was higher for single-lap joints than scarf joints and (b) the prediction errors were lower for elastoplastic modelling than linear-elastic modelling, especially for single-lap joints.