Fatigue strength of joints with partial penetration beads

Abstract

Synopsis: Important progress has been made in fatigue fracture models for welds, since the initial propagation phase can now be described satisfactorily. When the fatigue crack reaches 1 mm, at least one third of the life has already been used up. In recent decades, metal constructions in service have seen considerable development (in shipbuilding, transport equipment, pressure vessels, etc). This development has been made possible by a constant improvement in the quality of the materials used, and by better knowledge of the mechanical behaviour of welded constructions. At present, in the case of fillet welded joints, total root penetration of the welds is stipulated ‐ often unnecessarily ‐with corresponding ultrasonic inspection. This practice has the result of considerably increasing the time and cost of producing welded structures. In order to see whether this practice is necessary, the authors propose to evaluate, for load‐carrying joints, the effect of incomplete root penetration on the fatigue life. Two earlier programmes carried out by the Institut de Soudure on T joints (of E‐36–4 steel, AFNOR standard, welded by a MIG process) subject to uniaxial tensile stresses1‐2 showed that there is: — A critical amount of incomplete penetration below which the non‐penetration has no further effect on the fatigue life (2a/t = 0.4, 2a = incomplete penetration, t = thickness of the assembled plates); ‐ A slow evolution phase of the microcracks propagating from a notch tip (at the weld toe) which can represent, according to the stress amplitude applied, 30 to 80% of the fatigue life. To pursue this study, the authors propose to evaluate more precisely the slow evolution conditions of the microc‐racks which develop from the weld toe, and to model the fatigue life of T joints as a function of the amount of incomplete root penetration, in the case of a combined tensile and bending stress.