Estimation of the fatigue strength of welded joints by an equivalent notch stress analysis

In this work the well known local approach to predict the fatigue strength of sharply notched components, based on the analytic expressions of the local stress field as proposed in literature, is applied to welded joints in aluminium alloys in a simplified form oriented to practical applications. A particular value of the general expression of the local stress field parameter is taken into account, which the fatigue strength depends on. Then a simple model is proposed by the authors in order to estimate such a parameter, based on the calculation of a geometric (or structural) contribution to the local stress field, depending on the overall joint geometry, and a local contribution evaluated by considering a specimen with lateral V-notches characterised by the same weld toe profile and a depth proportional to the weld throat thickness. Doing so, the estimation of the fatigue strength of a welded joint can be reduced to the estimation of the fatigue strength of the equivalent V-notch subjected to a remote stress equal to the structural stress (that can be regarded as a ‘hot spot’ stress). Finally a simple fatigue strength diagram, in the form recently proposed by Atzori and Lazzarin and calibrated on experimental fatigue test results, is proposed, so that one can estimate the fatigue strength of a welded joint, in terms of structural stress at a given number of cycles, as a function of the equivalent V-notch depth. By considering this diagram, the scale effect and the effectiveness of the methods to improve the fatigue strength by smoothing the weld toe radius are also taken into account.