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The paper's aim is to investigate the effects of shot peening (SP) on the bending fatigue limit of high‐strength steel containing an artificial semi‐circular slit.

SP and stress SP (SSP) were conducted on the specimens containing an artificial semi‐circular slit with a depth of a=0.1, 0.2 and 0.3 mm. Then, bending fatigue tests were conducted on the specimens.

The fatigue limit was improved by SP and SSP. In the case of SP and SSP specimens, the specimens with a semi‐circular slit under a=0.2 mm fractured outside the slit, and they had considerably high fatigue limits. Therefore, a semi‐circular slit with a depth of under a=0.2 mm could be rendered harmless by SP or SSP. It was found that the fatigue limit of specimens with a semi‐circular slit that received SP or SSP was determined by the threshold condition for non‐propagation of fatigue cracks that emanated from outside the slit. Whether the semi‐circular slit is rendered harmless or not is decided by the relationship between the stress intensity factor range of semi‐circular cracks and the threshold stress intensity factor range.

The proposed method can be applied to mechanical parts used in vehicles, aircraft and trains.

There are very few examples of evaluations of fatigue limits after SP in materials containing crack‐like surface defects. This study calcifies the effect of SP on the fatigue limit having crack‐like surface defects.

Surface defects reduce fatigue strength and may greatly reduce component reliability, particularly in pressure vessel weld regions, springs, and other applications. The fatigue strength of components, and thus their reliability, can be substantially increased by tensile overloading prior to use. The purpose of this paper is to investigate the effect of tensile overload on small cracks by applying a tensile overload to steel plates containing semicircular slits that simulate small surface cracks and by determining the degree of increase in the fatigue strength. The effect of tensile overload on the apparent fatigue threshold stress intensity factor range (ΔK_th) was also investigated.

A tensile overload stress of 1,000 or 1,200 MPa was applied once to all test pieces. Then, bending fatigue tests were conducted with a stress ratio R=0.1. The slit region was subjected to applied cyclic tensile stress by four‐point bending throughout the fatigue test. A test specimen to which no overload stress was applied was tested for comparison.

The improvement in ΔK_th by tensile overload is observed in the specimen with a small crack like surface defect. However, in the specimen with a small crack like surface defect, the improvement in ΔK_th by tensile overload is saturated as increasing tensile overload. The improvement rate of ΔK_th by tensile overload and the upper limits of improvement in ΔK_th were predicted. The predicted values of the improvement rate of K were well in agreement with the experimental results.

The proposed method can be applied to pressure vessels and springs.

The overload effects on fatigue strength are studied for large cracks. However, the effect is not understood at all for small cracks. This study focused the over load effects for small cracks. This is the original point of the present study.

The purpose of this paper is to estimate the acceptable defect size amax after needle peening (NP) and predict the fatigue limit improvement through the use of NP for an austenitic stainless steel welded joint containing an artificial semi-circular slit on a weld toe.

Residual stress and hardness distribution were measured. Microstructures around the weld toe were observed to clarify the cause for the change in hardness after NP. Finite element method analysis was used to analyze the change in the stress concentration following NP. Fracture mechanics was used to evaluate amax after NP. The fatigue limits before and after NP were predicted by determining amax for several levels of stress amplitude.

The tensile residual stress induced at the surface of the weld toe prior to NP changed to a compressive residual stress after NP. The residual stress near the surface layer after NP exceeded the yield stress prior to NP due to the increase in yield stress as a result of work hardening as well as the generation of a deformation-induced martensitic structure. The stress concentration was reduced due to the shape improvement caused by NP. The estimation value of amax after NP and the prediction results of fatigue limits were in good agreement with the fatigue test results.

The proposed method is useful in improving the reliability of welded joints used in large steel structures, transportation equipments and industrial machines.

From an engineering perspective, it is essential to estimate amax and the fatigue limit of welded joints with crack-like defects. However, it is unclear as to whether it is possible to predict amax and the effects of NP on the fatigue limit for stainless steel welded joints.

The purpose of this study is to clarify the influence of stress ratio (R) on the effects of shot peening (SP) on the fatigue limit of high-strength steel containing an artificial small defect.

SP was subjected on the specimens with a semi-circular slit with a depth of a=0.1, 0.2 and 0.3 mm. Then, bending fatigue tests were carried out under R=0.4.

The fatigue limits of specimens with a semi-circular slit were improved by SP under R=0.4. The fatigue limits of the SP specimens with a semi-circular slit under a=0.2 mm fractured outside the slit, and they had considerably high fatigue limits equal to specimens without a slit. Therefore, a semi-circular slit with a depth of under a=0.2 mm could be rendered harmless by SP under R=0.4. Compared to the results of R=0, the increasing ratios of fatigue limits under R=0.4 were lower than those under R=0. However, the size of semi-circular slit that could be rendered harmless by SP was same. In addition, it was found that whether the semi-circular slit is rendered harmless or not is decided by the relationship between the stress intensity factor range of semi-circular cracks and the threshold stress intensity factor regardless of stress ratio.

The proposed method can be applied to mechanical parts used in vehicles, aircraft and trains.

This is the first paper to investigate the fatigue limits after SP in materials containing a surface defect under positive stress ratio. In this study, the authors investigated the influence of stress ratio on the effects of SP on the fatigue limit containing a surface defects.

The purpose of this paper is to clarify the effects of shot peening (SP) on the torsional fatigue limit of high‐strength steel specimens containing an artificial small defect.

Specimens containing a drilled hole 0.1‐0.4 mm deep or a semi‐circular slit 0.15 or 0.3 mm deep were subjected to SP. Torsional fatigue tests were then carried out.

The torsional fatigue limits of specimens containing a drilled hole and those with a semi‐circular slit were increased 25‐64 per cent and 156‐186 per cent by SP, respectively. The torsional fatigue limits of the specimens subjected to SP and containing a drilled hole less than 0.1 mm in depth or a semi‐circular slit less than 0.15 mm in depth were almost equal to those of SP specimens without a defect. Based on these results, it can be concluded that a drilled hole less than 0.1 mm in depth and a semi‐circular slit less than 0.15 mm in depth could be rendered harmless by SP.

The proposed method can be applied to mechanical parts subjected to cyclic torsion, such as coil springs, crank shafts and drive shafts.

This is the first paper to investigate the torsional fatigue limits after SP in materials containing a surface defect. In this paper, the effect of SP on the torsional fatigue limit having a surface defect is investigated.

Even though modern welding technology has improved, initial defects on weld notches cannot be avoided. Assuming the existence of crack-like flaws after the welding process, the stage of a fatigue crack nucleation becomes insignificant and the threshold for the initial crack propagation can be used as a criterion for very high cycle fatigue whereas crack growth analysis can be applied for the lifetime estimation at lower number of cycles. The purpose of this paper is to present a mechanism based approach for lifetime estimation of welded joints, subjected to a multiaxial non-proportional loading.

The proposed method, which is based on the welding process simulation, thermophysical material modeling and fracture mechanics, considers the most important aspects for fatigue of welds. Applying worst-case assumptions, fatigue limits derived by the weight function method can be then used for the fatigue assessment of complex welded structures.

An accurate mechanism based method for the fatigue life assessment of welded joints has been presented and validated.

Compared to the fatigue limits provided by design codes, the proposed method offers more accurate lifetime estimation, a better understanding of interactions between welding process and fatigue behavior. It gives more possibilities to optimize the welding process specifically for the considered material, weld type and loading in order to achieve the full cost and weight optimization potential for industrial applications.

The purpose of this paper is to investigate the effect of shot peening (SP) on the fatigue limit of high‐tensile‐strength steel containing a crack in the stress concentration zone.

An artificial semi‐circular slit was introduced into the bottom of notch, and SP was performed. Bending fatigue tests were then carried out.

First, the fatigue limits of specimens containing a slit of 0.2 or 0.3 mm in depth were improved up to approximately twice their original values. Second, in the case of shot‐peened specimens with a crack of 0.2 mm in depth, the fractures occurred from outside the slit. Moreover, the specimens recovered to fatigue limits up to those of non‐slit specimens. Finally, the effect of stress concentration (Kt=1.9) on the slit size could be rendered harmless by SP was not found in the fatigue test.

There are very few examples of evaluations of the fatigue limit of materials containing crack‐like surface defects after SP has been performed in the stress concentration zone. The study elucidated the effect of SP on the fatigue limit in such materials, compared with that of a smooth zone.

The purpose of this study was to investigate the effect of overload (bending moment with plastic deformation: Mp) on three point bending specimen at the fatigue limit of high-tensile-strength steel containing a crack in the stress concentration zone.

An artificial semi-circular slit was introduced and Mp was applied after which bending fatigue tests were carried out.

The relationship between the level of Mp and the fatigue limit (σ_w) was proportional; the fatigue limits of specimens containing 0.2- and 0.3-mm-deep slits are improved by the Mp process as much as twice the original values; the slit size that can be rendered harmless by the Mp process is a=0.05 mm in depth; and all non-propagating cracks appeared around the artificial slit.

Very few studies have been conducted on the fatigue limit of materials containing crack-like surface defects after overload in the stress concentration zone. This study elucidated the effect of Mp on the fatigue limit.

The present work aims to deal with a very high cycle fatigue (n=10⁹ cycles) of gas metal arc welded joints, subjected to a multiaxial and non‐proportional loading. Different design codes and recommendations can greatly reduce the analysis effort in the design of welded structures providing a suitable balance between computational accuracy and ease of use for many industrial applications. However, various assumptions have to be made in a conservative way making this approach less accurate. This paper deals with a refined fatigue assessment, which considers the most important aspects for welded joints and provides an accurate lifetime prediction of welded structures.

For an accurate prediction of the total lifetime of welded components the information about the material state and the welding induced residual stresses on weld toes is essential. If the surface condition after welding is poor in this area, which is usually the case, the presence of defects can be assumed and the fatigue crack nucleation process can be neglected. The microstructural threshold for initial crack propagation can be therefore used as a lower bound for the fatigue limit prediction.

Based on the results from the simulation of a welding process and a post‐weld heat treatment in combination with a fracture mechanics approach, this work successfully attempts to reproduce a fatigue behavior, which was observed at the fatigue tests of the multi‐pass single bevel butt weld.

The proposed approach is able to predict accurately the fatigue strength of welded structures and to achieve the full cost and weight optimization potential for industrial applications.

The purpose of this paper is to employ analytical and numerical techniques to generate modal displacement data of damaged beams containing very small crack-like surface flaws or slots and to use the data in the development of damage detection methodology. The detection method involves the use of double differentiation of the modal data for identification of the flaw location and magnitude.

The modal displacements of damaged beams are simulated analytically using the Bernoulli-Euler theory and numerically using the finite element method. The principle used in the analytical approach is based on changes in the transverse displacement due to the localized reduction of the flexural rigidity of the beam. Curvature analysis is employed to identify and locate the structural flaws from the modal data. The curvature mode shapes are calculated using a central difference approximation. The effects of random noise on the detectability of the structural flaws are also computed.

The analytical approach is much more robust in simulating modal displacement data for beams with crack-like surface flaws or slots than the finite element analysis (FEA) approach especially for crack-like surface flaws or slots of very small depths. The structural flaws are detectable in the presence of random noise of up to 5 per cent.

Simulating the effects of small crack-like surface flaws is important because it is essential to develop techniques to detect cracks at an early stage of their development. The FEA approach can only simulate the effects of crack-like surface flaws or slots with depth ratio greater than 10 per cent. On the other hand, the analytical approach using the Bernoulli-Euler theory can simulate the effects of crack-like surface flaws or slots with depth ratio as small as 2 per cent.