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Zirconia ceramics exhibit high strength and fracture toughness. The purpose of this paper is to research a possibility of crack healing in zirconia ceramics.

ZrO₂/SiC composite ceramics are sintered and subjected to three‐point bending. A surface crack of 100 μm in diameter is formed on each specimen. The cracks are healed and the specimens are tested under bending.

The paper finds that ZrO₂/SiC composite ceramic material had a high crack‐healing ability at a considerably low temperature. For example, a crack of 100 μm in diameter is healed even at 600°C.

The paper provides a low temperature healing and a new mechanism of crack healing.

The paper shows the healing temperature and the minimum time required to heal showed a good proportional relation on the Arrhenius plot at temperatures of 600‐800°C. Moreover, the crack healing is caused by SiO₂ cristobalite produced during the healing.

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 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 paper is to describe the effects of stress ratio (R) on the threshold stress intensity factor range (ΔKth) by applied overload and to conduct an analytical investigation of the effect of the stress ratio.

Tensile overload was applied to a compact tension specimen, and fatigue tests were performed at R=0.1 or 0.5.

The value of ΔK_th increased as the tensile overload was increased, and the nominal threshold values were given by the equation Δ^NK_th,R = C+ DK_ov, where C represents ΔK_th, and D is a proportional constant. Experimental results showed that the value of D showed good agreement with theoretical value.

The paper proposes a new model that arrests crack growth or makes cracks harmless by utilizing the overload effect.

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.

The purpose of this paper is to study the effects of overload on the threshold stress intensity factor range (ΔK_th) in SUS316.

The fatigue tests are carried out to determine the resultant threshold stress intensity factor range (ΔK_th). The mechanism of the improvement of ΔK_th by the tensile overloading is analyzed using the Dugdale model.

It is clarified that the value of ΔK_th increases as increasing the overloading.

The apparent value of ΔK_th of stainless steel can be improved by a tensile overload, the fatigue strength of structural members that have a surface crack can be increased by a tensile overload.

As a result, the reliability and safety of structures, such as energy plants, can also be improved.

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 investigate the effect of peening on the fatigue limit of steels for welded structure with a crack in the weld toe zone.

An artificial semi-circular slit was created in the weld toe, and peening was conducted. Then, bending fatigue tests were carried out.

First, owing to the shot peening, the maximum slit depths that can be rendered harmless were 1.0 and 1.2 mm in SUS316 and SM490, respectively. Second, during the fatigue test, the fracture of a peened specimen originated outside the slit, which indicated that peening eliminated the effect of the slit on the fatigue limit. Third, the fatigue limit of a slit specimen was improved by the enhanced residual stress distribution and the decreased stress concentration due to plastic deformation at the weld toe.

There are very few studies about which a fatigue crack is rendered harmless by residual compressive stress, as a result the structures could be continued to use. Moreover, the study defining the concept about rendering crack harmless and systematic investigation was not able to be found.