Akihiko Ohta

Superior fatigue crack growth properties in newly developed weld metal

The fatigue threshold and high growth rate region properties of conventional welded joints were improved by using newly developed low transformation temperature welding wire. The developed weld metal which contains 10 wt% nickel and 10 wt% chromium begins to transform from austenite to martensite at about 180°C and finishes it at room temperature. During the transformation the weld metal expands. This expansion induces a compressive residual stress around the welded part. The stress ratio effect due to this compressive residual stress makes the fatigue crack growth properties of the developed weld metal superior by intensifying the fatigue crack closure.

FATIGUE CRACK PROPAGATION RATES AND THRESHOLD STRESS INTENSITY FACTORS FOR WELDED JOINTS OF HT80 STEEL AT SEVERAL STRESS RATIOS

Abstract Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.

Fatigue strength evaluation of welded joints containing high tensile residual stresses

Abstract An important limitation of laboratory-scale welded specimens is that they do not contain the high tensile residual stresses, approaching yield strength in magnitude, which are present in real welded structures. A new test method is proposed which aims to simulate the situation in real structures by cycling the specimen from yield strength downwards. Tests performed on transverse butt joints in structural steel under these conditions and under fixed stress ratios of −1, 0 and +0.5 showed that the proposed method gave the lowest fatigue lives.

Statistical evaluation of fatigue crack propagation properties including threshold stress intensity factor

Abstract The relationship between fatigue crack propagation rate, da dn , and range of stress intensity factor, ΔK, including threshold stress intensity factor, ΔKth, is analyzed statistically. A non-linear equation, da dn = C{(ΔK) m -(ΔK th ) m } , is fitted to the data by regression method to evaluate the 99% confidence intervals. Several experimental results on fatigue crack propagation properties of welded joints are compared by using these confidence intervals.

Repair of fatigue cracks initiated around box welds using low transformation temperature welding material

Large-sized welded structures generally sustain stress concentration due to the weld toe shape and tensile residual stress fields due to thermal shrinkage of the weld metal during the cooling process. For this reason, the fatigue strength of these structures is substantially reduced by the stress ratio effect, stress concentration, etc. The fatigue strength of welded joints located within a range susceptible to the effect of such tensile residual stress fields and stress concentration due to the weld toe shape shows a virtually identical value per joint type without being positively correlated with the yield strength in the same way as the base metal (see Fig. 1). When large-sized welded structures sustain fatigue cracking, they fall into the categories of those to be decommissioned for subsequent dismantling and those that continue to be used after repair. Dismantling of largesized welded structures is an extremely high-cost process, and it is therefore advantageous to prolong the life of structures by repair. Repair welding involving the use of conventional welding materials, however, generates large tensile residual stress fields which reduce the fatigue strength. To improve the fatigue strength of welded structures, it is effective to reduce the tensile residual stress as one key factor. To improve the fatigue strength, it is conventional practice to arrange for a shot peening process to follow welding, to introduce compressive residual stress into fatigue cracking-affected zones, and to improve the fatigue strength by the stress ratio effect conferred. To introduce compressive residual stress into fatigue cracking-affected zones, efforts have recently been made to form compressive residual stress fields by transformation expansion exploiting the transformation properties of the weld metal, to introduce compressive residual stress without the inclusion of any follow-on process at completion of welding, and to improve the fatigue strength of welded joints by the stress ratio effect thus conferred. To induce compressive residual stress in joints, recourse has been had to low transformation temperature welding material exploiting the expansion due to martensitic transformation developed in the weld metal. Based on recent materials research, this article describes an investigation of fatigue strength improvement by repair of fatigue cracks initiated around box welds using low transformation temperature welding material and reports a fatigue limit improvement compared with conventional welded joints.

Fatigue crack propagation in tensile residual stress fields of welded joints under fully compressive cycling

Abstract Fatigue crack propagation rates in centre-crack-typed transverse butt-welded joints were measured at a constant stress intensity factor range obtained by decreasing the applied and mean loads on test specimens. The propagation rate was maintained constant except at extremely compressed stress ratios. Fatigue crack propagation properties under compressive loading were found to be similar to those under tensile loading. Only under highly compressive cycling did crack propagation rates decrease.

Significance of residual stress on fatigue properties of welded pipes

Abstract The mean stress effect on the fatigue properties of two kinds of welded pipes was investigated in cantilever bending. The fatigue strength changed with the mean stress on fillet welded pipes, but did not change on butt welded pipes. The fatigue crack initiated from the toe of weld on the outer surface of fillet welded pipes and from the undercut on the inner surface of butt welded pipes. The measurement of the fatigue crack propagation rate and the residual stress distribution through the thickness of pipe revealed that the difference in the fatigue properties between fillet and butt welded pipes arose from the weld-induced residual stress, tension on the inner surface and compression on the outer surface. It is suggested that the production of compressive residual stress along the inner surface would be an effective means for improving the fatigue strength of butt welded pipes.

Random loading fatigue life assessments for notched plates

Fatigue tests were performed on double edge notched specimens made of JIS SM570Q steel. Four types of notched specimens were used and stress concentration factors were 1, 2, 3 and 4. Cyclic and random loading was applied to the specimens. Random loading was corresponding to Rayleigh distribution of stress ranges, and it was applied as randomized loading blocks. During the loading the maximum stress was kept constant and equal to yield strength while the minimum stress is changed randomly. This test condition is considered to avoid the complex residual stresses at the notch root during variation of stresses. It is believed that by these tests it is possible to simulate the fatigue behavior of welded joints. Suggested equivalent stress was used to correlate the fatigue lives for the variable amplitude histories.

Near-threshold fatigue crack propagation of welded joint under varying loading

Abstract The influence of varying loading on the fatigue crack propagation properties of HT80 steel welded joint and base metal was investigated by using center cracked specimens under two-step programmed test. The higher stress intensity range was slightly above the threshold level obtained by constant amplitude test and the lower one was 70% of the higher one. The fatigue crack propagated below the threshold level for the base metal at the stress ratio of 0 and 0.4. However, the fatigue crack did not propagate below the threshold level either for the base metal at the stress ratio of 0.9 or for the welded joint at the stress ratio of 0. These results mean that the use of the threshold level obtained under the constant amplitude test would be dangerous for assessing the fatigue performance of the base metal under varying loading. The use of the threshold level obtained for the center cracked welded joint specimens would be conservative even under the varying loading.

Doubled Fatigue Strength of Box Welds by Using Low Transformation Temperature Welding Material

The low transformation temperature welding material is developed to improve the fatigue strength by introducing compressive residual stress around weld. The developed welding material which contains 10% chromium and 10% nickel, begins to transform from austenite to martensite at about 180°C and finishes it at room temperature. During the transformation, the weld metal expands. This expansion induces the compressive residual stress around the welded part of 20 mm thick JIS SM49O steel plate. The magnitude of welding residual stress is estimated to be about -100 MPa for the developed joint, while that is about 500 MPa for the conventional one. The stress ratio effect due to the compressive residual stress makes the fatigue strength doubled. The fatigue limit for conventional box welds is 65 MPa, while that for developed one is about 130 MPa.