Fatigue Design of Cruciform Joints including V-notch Effect at the Weld Toe

The present paper proposes a new and more accurate fatigue life prediction model for fillet welded joints in steel subjected to constant amplitude loading. With the traditional fracture mechanics approach, the greatest difficulty when computing the fatigue life of a welded detail is to determine the initial crack size a0. The classical way to determine the stress intensity factor K (SIF) is by using the following formula Where σ is the applied stress, a is the crack size and g(a/T) the geometrical correction factor which has been determined by Gurney function or similar solutions. This approach is not accurate for short crack because of the singular V-notch behaviour close to the crack tip is not taken into account. In fact, the singular behaviour at this point has a great influence on the SIF calculation. The present work proposes an alternative approach based on the determination of the Energy Release Rate (G) in the V-notch region at the weld toe in order to calculate the stress intensity factor as follows E being the Young's modulus. Once the ΔK is determined for varying stresses, we can calculate the life fatigue based on the Paris crack propagation law. The size of the weld toe region influenced by the singular V-notch behaviour is based on experimental work concerning a cruciform steel joint subjected to an applied axial stress of 150 MPa and a weld angle of 45°. Based the same conditions as in these experimental tests, we compared the results from the classical Gurney's approach with the results from the present methodology. The experimental results in terms of fatigue life and crack length were used as reference. When the crack growth rate is plotted against ΔK we can see that, in our approach two slopes appear. The first slope that corresponds to the short crack propagation in the weld toe region and the second slope that corresponds to the stable crack propagation far from the weld toe region. Also, for short cracks, our approach is closer to the experimental results than the Gurney's approach. Moreover, with our approach, we do not need to introduce the real initial crack size at the weld toe. It is demonstrated how the time consumed for short crack propagation becomes the dominant part of the entire fatigue life at low stress levels. The short crack propagation is more accurately described by the present model than in traditionally approaches. Future extensions of the present work will deal with the introduction of the influence of the weld toe radius and the material characteristics of the HAZ on the fatigue life duration.