It is a well known fact that the fatigue strength and the life of welded steel components are affected, to a considerable extent, by residual stresses distributed around their weldments. When externally applied load is superimposed on residual stresses, unexpected deformations and failure of the components can occur. These residual stresses are not constant, but are relaxed or redistributed during in-service. Under monotonic load relaxation takes place when the sum of external and residual stress locally exceeds the yield stress of material used. It is noteworthy that under cyclic load the residual stress is considerably relieved by the first or the early cycles of load, and then is gradually relaxed with increasing loading cycles. Although many investigations in this field have been carried out, the phenomenon of and mechanism of stress relaxation are still not clear, and only a few comprehensive models have been proposed to predict amount of relaxed residual stress. In this study, the characteristics of residual stress relaxation under monotonic and cyclic load were investigated, and a model for quantitatively predicting the amount of residual stress relaxation is proposed.

Residual stress relaxation of welded steel components under cyclic load

Typically, for a fatigue strength assessment of a welded structure, the influence of welding residual stresses has to be considered. An easy applicable approach is given within the framework of the IIW recommendations; the design S-N curves can be upgraded in case of medium or low residual stresses. A relaxation or redistribution of tensile residual stresses, which is often accompanied by an increase in fatigue life, cannot be considered. To obtain a better understanding of the residual stresses and their influence on the fatigue strength, fatigue tests have been performed on longitudinal stiffeners in as-welded and stress relieved state with both stress ratios R = −1 and R = 0. Local strains and stresses were measured using strain gauges and X-ray diffraction technique to get insight into the local material’s response due to global loading close to the weld toe. The crack initiation and propagation phase was detected by a camera. FE models were set up to reproduce the observed material behavior in the notch for the crack initiation phase. By numerical analysis it could be shown that, due to sharp notches at the weld toe with an average toe radius of r = 0.05 mm, a stress redistribution occurs during the first load cycle. This leads to a fatigue strength, which is, particularly at higher load amplitudes, independent from mean as well as from initial residual stresses. A comparison of fatigue data derived on longitudinal stiffeners from literature confirms this effect. For lower load amplitudes instead, mean and residual stresses gain more influence. In this investigation a reliable assessment of the crack initiation period could be performed with strain-based concepts using damage parameter PSWT for a mean stress assessment. All fatigue tests could be transformed into a single damage parameter PSWT-N curve with a low scatter if residual stresses as well as the distortion of the specimens are regarded.

Influence of weld geometry and residual stresses on the fatigue strength of longitudinal stiffeners

Fatigue crack initiation assessment of welded joints accounting for residual stress

A two-phase approach to estimate fatigue crack initiation and propagation lives of notched structural components

For the improvement of the fatigue strength for welded structures, mechanical posttreat- ments have been applied in various industrial fields and in most cases have been found to give substantial increases in their fatigue lives. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In mechanical surface treatments, for example, PHP (pneumatic hammer peening) and UNSM (ultrasonic nanocrystal surface modification), the weld profile is modified due to removed or reduced minute crack-like flaws, and compressive residual stresses are also induced. In this study, a PHP procedure and a UNSM device were introduced, and a quantitative measure of fatigue strength improvement was performed. The fatigue strength at 2 × 10 6 cycles of hammer-peened and UNSM treated on a non-load-carrying cruciform welded joint shows 220 and 260 MPa, respectively, which are more than two times higher than that of as-welded specimen. Especially, the surface layer in the vicinity weld toe treated by the UNSM provides nanocrystal structure created by an ultrasonic cold forging and introduces very high welding residual stress in compression.

Fatigue life improvement for cruciform welded joint by mechanical surface treatment using hammer peening and ultrasonic nanocrystal surface modification

The fatigue life of welded joints is associated with crack initiation and propagation life. Theses cannot be easily separated, since the definition of crack initiation is vague due to the initiation of multiple cracks that are distributed randomly along the weld toes. In this paper a method involving a notch strain and fracture mechanical approach, which considers the characteristics of welded joints, e.g. welding residual stress and statistical characteristics of multiple cracks, is proposed, in an attempt to reasonably estimate these fatigue lives. The fatigue crack initiation life was evaluated statistically, e.g. the probability of occurrence in 2.3, 50 and 97.7%, in which the cyclic response of the local stress/strain in the vicinity of the weld toes and notch factors derived by the irregular shape of the weld bead are taken into account. The fatigue crack propagation life was simulated in consideration of the Mk-factor and the mechanical behavior of mutual interaction/coalescence between two adjacent cracks. The estimated total fatigue life as a sum of crack initiation and propagation life was found to be in good agreement with the experimental results.

Fatigue Crack Initiation and Propagation Life of Welded Joints

A New Model For the Fatigue Life Prediction Considering Residual Stress Relaxation

Evaluation of Fatigue Life Based On a Modified Notch Strain Approach Considering Welding Residual Stress Relaxation

In this paper fatigue strength reduction of butt weld with penetration defect, which can be seen frequently in the steel bridge, was assessed quantitatively. S-N curves were derived and investigated through the constant amplitude fatigue test of fully or partially penetrated welded specimen made of SWS490 steel. The fracture mechanical method was applied in order to calculate the remaining fatigue life of the partially penetrated butt welds. The fatigue limit of the fully penetrated butt welds was higher than that of category A in AASHTO's fatigue design curves, and the slope of S-N curves with 5.57 was stiffer than that of other result for welded part generally accepted as 3. The fatigue strength of the partially Penetrated butt weld was strongly influenced by the size of lack of penetration, D. It decreased drastically with increasing D from 3.9 to 14.7mm. Fracture behaviour of the partially penetrated butt weld is able to be explained obviously from the beach mark test that a semi-elliptical surface crack with small a/c ratio initiates at a internal weld root and propagates through the weld metal. To estimate the fatigue life of the partially penetrated butt weld with fracture mechanics, stress intensity factors K of 3-dimensional semi-elliptical crack were calculated by appling finite elements method and fracture mechanics parameters such as C and m were derived through the fatigue test of CT-specimen. As a result, the fatigue lives obtained by using the fracture mechanical method agreed well with the experimental results. The results were applied to Sung-Su bridge collapsed due to penetration defects in butt weld of vertical member.

Byung Chun Shin

Papers

Fatigue Crack Initiation and Propagation Life of Welded Joints

A New Model For the Fatigue Life Prediction Considering Residual Stress Relaxation

Evaluation of Fatigue Life Based On a Modified Notch Strain Approach Considering Welding Residual Stress Relaxation

Investigation of Fatigue Strength and Prediction of Remaining Life in the Butt Welds Containing Penetration Defects

Fatigue Life Estimation of Cruciform Welded Joint Considering Multiple Collinear Surface Cracks