Influence of Residual Stresses and of Mean Stress on the Fatigue Strength of Specimens with Longitudinal Non-Load-Carrying Fillet Welds

Abstract

Using results obtained in work on fatigue crack propagation in unwelded sheet specimens, an analysis has been made of fatigue test results for specimens with longitudinal non-load-carrying fillet welds tested in the as-welded and stress relieved conditions and also after treatment by local compression and by spot heating. It is shown that, by this means, it is possible to provide a quantitative explanation of the effect of residual stresses and applied stress ratio on fatigue strength, since all the results can be normalized on the basis of an effective stress range. In the course of the work it has been shown that the gross stress concentration factor for this type of joint is approximately 2·57. It has also again been demonstrated that the beneficial effect of spot heating on fatigue strength is a direct result of induced compressive residual stresses.

From a local stress approach to fracture mechanics: a comprehensive evaluation of the fatigue strength of welded joints

“…1. Metallurgical analysis performed on cruciform joints made of construction steel confirmed 24 the presence of inner technological defects at the weld root and toe. Those defects were 0.15 mm in average depth, while their maximum value was close to 0.4 mm.…”

“…Equation (1) holds true if, instead of a tunnel crack, there is a semi‐elliptic crack with the major axis 2 c much longer than the minor axis a . If these conditions do not exist, the member on the right should be divided by the second order elliptic integral Φ 0 22 , 24 . For a generic a / c ratio, Rawe's expression should be considered: 24 , 25 …”

“…In Suresh's opinion 30 (partly and elegantly acknowledged by Topper himself 31 ), the major drawback of this approach is that attempts to provide a physical reasoning for the validity of the intrinsic crack expression ‘have so far remained unconvincing’. This cannot be true for welded joints, as small cracks or, more frequently, crack‐like defects are in practice always present 3 , 4 , 24 . Moreover, the results of recent analyses performed by Ting and Lawrence 32 indicate that the intrinsic crack length ‘is the most important material property in describing the effect of the material properties on fatigue of notches at the threshold conditions’.…”

Stress Intensity Factor for Cracks at the Toe of Welded Joints

“…An alternative method for a correct evaluation of the residual fatigue life by means of traditional linear Fracture Mechanics has been proposed for different types of welded joints in refs. [9,10,11,12]. However, the total fatigue life can also be evaluated by means of artificial choice of the initial crack dimension.…”