Efficient critical distance approach to predict the fatigue lifetime of structural adhesive joints

The development of accurate, robust, and reliable methods for fatigue lifetime prediction is a key aspect to safe and efficient design of adhesively bonded structures. Here, a method for lifetime prediction of structural adhesive joints is proposed. The method is based on the stress-life approach and the Theory of Critical Distances (TCD) to account for the effects of stress concentrations. Two types of structural adhesives (epoxy-based and PU-based) are considered for the investigations. Stress assessment is carried out by standard 2D-FEA using continuum elements and elastoplastic material behaviour. A novel process for the determination of the critical distance for fatigue of adhesives is used and validated. The robustness of the method is evaluated by predicting the fatigue lifetime of three notched bulk specimens (2.52 < k(T) < 3.90) and a single-lap joint using the same reference SN-curve. The efficiency of the proposed method, considering a pressure-dependent Drucker-Prager equivalent stress and the Line Method of the TCD, is evidenced by the good accuracy of lifetime predictions for both adhesives with predictions lying within a factor of 2.