Satoshi Nishijima

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.

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.

The role of corrosion in fatigue crack propagation for structural alloys in 3% NaCl solution

Abstract Corrosion fatigue tests were carried out for HT80, SM50B and SUS304 stainless steels under various frequencies in 3% NaCl solution. The results show when the crack closure effect is removed, the acceleration ratio of the crack propagation under corrosive environments against those in air is nearly constant in all tested ΔK regions except in the intergranular cracking ΔK region. In order to explain these results, a model based on the striation mechanism was proposed, and the relation between the amount of corrosion on the slip plane and the crack propagation rate was studied by the scratching electrode method.

Variation of fractographic appearance for different microstructures in welded joints having the same fatigue crack propagation properties

Abstract Fatigue fracture surfaces were examined with a scanning electron microscope to investigate the influence of the different microstructure between weld metal and heat affected zone. The specimens were centre-cracked type transverse butt welded joints. The relationship between macroscopic fatigue crack propagation rate and the stress intensity factor range is the same in spite of the difference in microstructure for both materials. It is shown that the fractographic appearance changes with microstructure even in the very low growth rate region near fatigue threshold. This suggests that fractographic appearance is not necessarily a guide to the rate of fatigue crack growth.

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.

Near-threshold fatigue crack growth properties at elevated temperature for 1Cr-1Mo-0.25V steel and 12Cr stainless steel

Near-threshold fatigue crack growth properties were investigated for a low-alloy steel 1Cr-1Mo-0.25V and a stainless steel SUS403 (13Cr) in the temperature range from 25 to 550°C. Fatigue tests were conducted at frequencies of 0.5, 5, and 50 Hz, in a manner designed to avoid crack closure. The effective value of threshold stress intensity range increased with increasing temperature and with decreasing frequency for the Cr−Mo−V steel, whereas the effective threshold stress intensity range was independent of temperature and frequency in the case of the SUS403 steel. At a given ΔK value, the fatigue crack growth rates accelerated with increasing temperature and with decreasing frequency for the Cr−Mo−V steel. However, although the rate of fatigue crack growth was independent of frequency at a given temperature for the SUS403 steel, the rate did increase with temperature. The observed threshold levels and crack growth behavior were closely related to the oxidation process of the bare surface formed at the crack tip during each load cycle.

Conservative data for fatigue crack propagation analysis

Abstract Conservative data relating to the fatigue crack propagation properties and fatigue threshold were obtained by using center-cracked-type transverse-butt-welded joints. The properties were unique in spite of the difference of the materials (four ferritic steels and an austenitic stainless steel), the welding methods (three types of are welding) and heat inputs (three levels), even under variable amplitude loading. The unique properties came from the tensile residual stresses which were always induced around the crack tip due to re-distribution on crack extension, and were the main controlling factor of fatigue crack propagation. The tensile residual stresses make the fatigue crack always open because of the increase in the stress ratio around the crack tip. The role of residual stress is revealed by examining the stress ratio effect observed in base metal and the effect of post-weld heat treatment. The advantage of inducing compressive residual stresses on the inner surface of the welded pipe to improve the fatigue crack propagation properties is pointed out.

Variable effects of stress relief on fatigue strength of butt welded joints with different plate thickness

Significant effects of stress relief on fatigue crack propagation properties of welded joints have been pointed out [i]. The stress relief effects on fatigue strength of welded joints with reinforcement have been investigated [2,3]. These investigations have revealed that stress relief could sometimes improve the fatigue crack propagation properties and the fatigue strength of welded joints containing tensile residual stresses. However, in the practical case, there are many kinds of weldments. Some of them may have compressive residual stresses around stress concentration sites at which fatigue cracks initiate, and stress relief for those weldments may deteriorate fatigue strength. This report describes the variable effects of stress relief on the fatigue properties of transverse butt welded joints with two kinds of plate thickness.

Parameter Analysis for Time-Dependent Low-Cycle Fatigue Life

Temperature and strain rate dependences of low-cycle fatigue life can be represented by a modified Larson-Miller parameter. The parameter P is written by P = T (logN[sub 25] [minus] Alog[dot [epsilon]] + B), where T is temperature, N[sub 25] is fatigue life, [dot [epsilon]] is strain rate, and A and B are constants. In the analysis, each data of several kinds of engineering materials from ferritic steels to austenitic stainless steels are used. These are the authors original data published in the documents of NRIM Fatigue Data Sheets. The result of 304 stainless steel has been compared with statistical analysis result by Diercks adopted in a design code. The fatigue life curves represented by the proposed parameter analysis fitted well test data in high-cycle region as well as ones in low-cycle region.