A Local Fatigue Strength Concept for Evaluation of Post Weld Treatments of Fatigue Loaded Welds

摘要

Many investigations have revealed the beneficial effect of mechanical surface hardening in combination with induced compressive residual stresses which lead to an improved fatigue strength as more, as the higher the quality of the weld is. On the other hand methods like shot peening are useless, if the weld toe profile is poor and the connected notch effect strong. Then an increased fatigue strength is only possible after a post weld treatment which produces a smoother shape of the weld toe. Experiments reveal that methods which are working in order to increase the intensity of the penetration depth must not necessarily produce the best fatigue performance. The paper introduces a model which enables the estimation of the resulting fatigue strength of a welded joint as a result of a post weld treatment with help of a mechanical or thermal surface treatment like the described peening or toe geometry improvement methods. Based on a model which has been developed for the application in quenched and tempered steels for automotive parts after mechanical surface treatments it can be shown that the model is principally applicable as well for welded joints. A reliable estimation of the benefit of post weld treatments can be given in relation to the as-welded condition if the weld toe geometry and the near surface residual stress and hardness profiles of the treated weld are known. The results show, that the model gives an explanation, that not necessarily the highest treatment intensity must produce the best fatigue performance. The amount of the fatigue strength improvement however depends on the agreement between the local load stress profile and the profile of the local fatigue strength.