Koji Takahashi

Yokohama National University · Department of Material Science and Engineering

The effect of shot peening (SP) on pre‐fatigue tested carburized steel was investigated. Plane‐bending fatigue tests were conducted on carburized CrMo steel that was pre‐fatigue tested and treated with SP. Fatigue cracks were induced by pre‐fatigue tests with a stress amplitude of σ a = 1000 MPa, and the number of the pre‐fatigue test cycles was 2....

Ball burnishing (BB) is a surface finishing process that involves moving the tool while pressing it against the workpiece under rolling contact. We investigated the effects of BB on the fatigue strength of an aluminum alloy with a semicircular slit simulating an initial surface defect. After performing BB under a typical condition, compressive resi...

The objective of this study was to find the shot peening (SP) conditions for to increase the size of defects that can be is rendered harmless in vacuum-carburized steel. After introducing semi-circular slits of 0.1, 0.2, and 0.3 mm depth into vacuum-carburized SCM420H steel, SP was performed under two different conditions to vary the depth of compr...

Remanufacturing has become popular as a system for reducing CO2 emissions caused by the life cycle of products. Therefore, producing more components via remanufacturing is important. Shot peening can be used to render surface defects harmless owing to the compressive residual stress effects. This study investigated the effects of shot peening as a...

The effects of shot peening (SP) and laser peening (LP) on the fatigue strength of additively manufactured (AM) aluminum alloy (AlSi12) specimens with rough surfaces (labeled as-built specimens) were examined. Plane bending fatigue tests were conducted, and the fracture surface was observed. Distributions of the residual stress and hardness from th...

This study aimed to improve the bending strength and reliability of ceramics using laser peening (LP). In the experiment, LP without coating (LPwC) and with coating (LPC) were applied to silicon nitride (Si3N4) under various conditions. The surface roughness, residual stress, and bending strength were then measured for the non-LP, LPwC, and LPC spe...

Additive-manufactured metals have a low fatigue limit due to the defects formed during the manufacturing process. Surface defects, in particular, considerably degrade the fatigue limit. In order to expand the application range of additive-manufactured metals, it is necessary to improve the fatigue limit and render the surface defects harmless. This...

In this study, a 0.4–1.6 mm deep semicircular crack was machined in DNV F690 offshore structural steel with an aspect ratio (As) of 1.0. After needle peening (NP), four-point bending fatigue tests were conducted. The fatigue limit estimated using the equation proposed by Ando et al. decreased with increasing crack size, but agreed with that obtaine...

The purpose of this study is to estimate the acceptable defect size αmax after needle peening (NP) and laser peening (LP) for a high tensile steel welded joint containing a surface defect on a weld toe. The magnitude of the relationship between the positive value of the stress intensity factor range ΔKT and the threshold stress intensity factor ran...

Recently, surface crack nondamaging technology has been developed to render harmless surface cracks that can reduce the fatigue limit of metals by 20–60%. This is due to the effects of compressive residual stress induced by shot peening. To improve the reliability of high‐strength steel equipment designed against fatigue limit using surface crack n...

Surface defects decrease the fatigue limit of metals. In this study, the effects of surface defects on the fatigue limit of high-strength spring steel specimens were investigated. Several equations to predict the fatigue limit of specimens with surface defects were evaluated. Specimens with a semicircular slit with depths ranging from 30 to 400 µm...

The effects of laser peening (LP) on fatigue limit improvement and acceptable defect size of a high strength steel HT780 (JIS-SHY685) welded joint were investigated. Fatigue tests were performed, following the optimization of the LP conditions. The fatigue limit of the welded specimen without the defect increased by 45% via LP. The specimen contain...

In this study, the rolling contact fatigue (RCF) life of nitrided Cr-Mo steel, which has three types of different nitride compound layers, was investigated under contaminated lubrication. Specimen-γ’ has nitride compound layer which crystal structure is adjusted to γ’-Fe4N. Specimen-ε and specimen-εpor have the ε-Fe2-3N type nitride compound layer,...

It has been reported that the fatigue limit of shot peened high strength steels with a semicircular slit of 0.2mm or less in depth was improved to the same value as that of defect-free shot peened specimens. In this study ultrasonic shot peening (UP) which can introduce deeper compressive residual stress was noticed in order to investigate the effe...

The effects of laser peening (LP) on the bending fatigue strength of the 7075‐T651 aluminum alloy were investigated. Accordingly, the defect tolerance of the aluminum alloy subjected to LP is discussed on the basis of fracture mechanics. The results indicate that a deeper compressive residual stress was induced by LP compared with the case of shot...

Laser peening (LP) is a process that has been mainly applied to metals. However, few studies have applied LP to ceramics. In this study, laser peening without coating (LPwC) was applied to silicon nitride reinforced with silicon carbide (Si 3 N 4 /SiC). Smooth, laser-peened, and shot-peened specimens were prepared, and the surface conditions, surfa...

When the crack length is larger than a certain size, the maximum stress intensity factor (K max) is used to explain the occurrence condition of brittle crack, and the limit condition of hydrogen assisted and stress corrosion crack propagation. Also, the fatigue crack can be explained by the stress intensity factor range (ΔK ). However, since the st...

The effects of needle peening (NP) on the bending fatigue limit of a high tensile steel (HTS) HT780 (JIS-SHY685)-welded joint containing an artificial semicircular slit on the weld toe were investigated. Three-point bending fatigue tests were conducted at a stress ratio of R = 0.05 for NP-treated welded specimens with and without a slit. The fatigu...

The effects of laser peening on the fatigue strength of A2024-T3 were investigated. Plane bending fatigue tests were conducted using test specimens treated with laser peening (LP) and shot peening (SP). In addition, surface roughness and residual stress distribution were measured and fracture surface was observed to evaluate the experimental result...

The effects of shot-peening (SP) and crack-healing on the rolling contact fatigue (RCF) strengths of Al2O3/SiC composite ceramics were investigated. Non-shot-peened, shot- peened, and shot-peened + crack-healed specimens were prepared. SP was performed using ZrO2 beads. The shot-peened + crack-healed specimen was crack-healed after SP. X-ray diffra...

The aim of this study was to evaluate the influence of surface crack on the bending fatigue limit of nitrocarburized steel. Semicircular slits with depths of 0.040, 0.075, and 0.100 mm were introduced on the surface of nitrocarburized medium carbon steel specimens to simulate the presence of small cracks. Bending fatigue tests were performed at roo...

To investigate the influence of surface defect with arbitrary shape on fracture strength of ceramics, three-point bending tests on alumina-silicon carbide (Al2O3/SiC) ceramic composites were carried out. A standard pre-crack, a tilted pre-crack and multiple pre-cracks were introduced on the specimen surface using a Knoop indenter. The specimens wit...

The aim of this study was to clarify the effects of micro surface defects introduced by the focused ion beam (FIB) technique on the fracture strength of ceramics. Three-point bending tests on alumina-silicon carbide (Al2O3/SiC) ceramic composites containing crack-like surface defects introduced by FIB were carried out. A surface defect with area in...

The effects of shot peening (SP) and cavitation peening (CP) on the bending fatigue strength of the 7075 aluminum alloy with a crack-like surface defect were investigated. Test specimens containing a small semicircular slit were subjected to SP and CP. Bending fatigue tests were conducted on the specimens to determine the fatigue strength and criti...

The effects of laser peening (LP) on the bending fatigue limit of an austenitic stainless steel JIS-SUS316 welded joint containing an artificial semi-circular slit on the weld toe were investigated. Three-point bending fatigue tests were carried out at a stress ratio of R = 0.05 for both LP treated welded specimens with the slit on the weld toe and...

Purpose The purpose of this study involved estimating acceptable defect size amax after needle peening (NP) and predicting 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. Design/methodology/approach Residual stress and hardness distribution...

In this study, the effect of shot peening on rolling contact fatigue life of bearing steel was investigated under contaminated lubrication. Shot peening was carried out on bearing steel SUJ2 specimens. Peened specimens were polished to remove the dimples generated by shot peening. The distribution of Vickers hardness and residual stress of the spec...

The stress states of elbow and tee pipes are complex and different from those of straight pipes. The low-cycle fatigue lives of elbows and tees cannot be predicted by Manson's universal slope method; however, a revised universal method proposed by Takahashi et al. was able to predict with high accuracy the low-cycle fatigue lives of elbows under co...

The effects of shot peening (SP) on the torsional fatigue limit of spring steel (SUP7) were investigated for specimens with Vickers hardness values of 460, 540, and 670 HV containing a semicircular surface slit. SP was conducted on smooth specimens and specimens containing a semicircular surface slit with a depth of 0.15 or 0.3 mm. Compressive resi...

In this paper, the sliding contact fatigue wear performance of shot-peened zirconia-silicon carbide composite (ZrO2/SiC) plates in contact with silicon nitride balls under compressive residual stress in dry conditions was investigated in order to improve the wear resistance of ZrO2/SiC friction parts. The wear resistance of ZrO2/SiC plates after sh...

The effects of shot peening (SP) on the fatigue strength of partially stabilised zirconia were studied. Smooth specimens and specimens containing a surface pre-crack with depths in the range 35–110 μm were subjected to SP. The cyclic fatigue tests were performed using a three-point bending setup. SP introduced compressive residual stresses on the s...

The effects of needle-peening on the bending fatigue limit of an austenitic stainless steel JIS-SUS316 welded joint containing an artificial semi-circular slit on the weld toe were investigated. Peening was applied to specimens with a semi-circular slit at depths of a = 1.0 mm and 1.5 mm. Then, plane bending fatigue tests were carried out at a stre...

The stress states of elbow and tee pipes are complex and different from those of straight pipes. Several researchers have reported the low-cycle fatigue lives of elbows and tees under cyclic bending with internal pressure conditions. In this work, finite element analyses were carried out to simulate the reported experimental results of elbows and t...

The effect of shot peening on the wear resistance of partially stabilized zirconia (PSZ) was investigated. PSZ plate specimens with shot peening (SP), without SP (Non-SP), and polished specimens after SP (SP+Polished) were prepared. Ball on plate tests were carried out using the specimens in air under dry condition. The wear volume of SP specimens...

The effects of shot peening (SP) on the wear performance of silicon nitride were investigated. Silicon nitride plate specimens with and without SP were prepared. Then, ball on plate tests were carried out using the silicon nitride plate specimens in air under dry conditions. The wear volume of the silicon nitride plate specimens with SP was lower t...

The wear performance between a shot-peened zircinia-silicon carbide composite (ZrO_2/SiC) plate and a silicon nitride ball under dry conditions was investigated, in order to improve the wear resistance of ZrO_2/SiC frictional parts. The wear resistance of a ZrO_2/SiC plate with shot peening was higher than that of a ZrO_2/SiC plate without shot pee...

The mechanisms of improvement of fatigue limit by portable pneumatic needle-peening (PPP) were investigated for a welded joint specimen of austenitic stainless steel (JIS-SUS316) containing a semi-circular slit at the weld toe. The residual stress near the weld toe was measured before and after PPP. Moreover, finite element analysis was conducted i...

The effect of shot peening (SP) on rolling contact fatigue life of bearing steel JIS-SUJ2 under contaminated lubrication was investigated. Vickers hardness of the specimen were measured before and after SP. Resistance to indentations was compared by measuring the profiles of the Vickers indentation. Then, rolling contact fatigue tests were carried...

The effects of material hardness on the maximum size of a slit, which was rendered harmless by shot peening, were investigated. Specimens of two types of hardness were made of spring steel (SUP9), and a semi-circular slit was introduced by using an end mill. After the shot-peening process, bending fatigue tests with a stress ratio of R = 0.05 were...

Low cycle fatigue tests and finite element (FEM) analysis were conducted using 100A pipe bend specimens made of STPT410 carbon steel with and without local wall thinning local wall thinning was machined on the inside of the elbow and was prepared at extrados, crown, and intrados. The parameters of the wall thinning were same (the wall thinning rati...

The effects of shot peening (SP) on the strength of partially stabilized zirconia (PSZ) were studied. The compressive residual stress, apparent fracture toughness (K C), and bending strength values of specimens subjected to SP were investigated. Results of x-ray diffraction analyses showed that SP introduced large compressive residual stress in spe...

In this study, the effects of portable pneumatic needle-peening (PPP) on the bending fatigue limit of a low-carbon steel SM490A welded joint containing a semi-circular slit on the weld toe were investigated. PPP was applied to the specimens with a semi-circular slit with depths of a = 0.4, 0.8, 1.2, and 1.6 mm. Then, three-point bending fatigue tes...

Nanocelluloses, including nanocrystalline cellulose, nanofibrillated cellulose and bacterial cellulose nanofibers, have become fascinating building blocks for the design of new biomaterials. Derived from the must abundant and renewable biopolymer, they are drawing a tremendous level of attention, which certainly will continue to grow in the future...

Self-crack-healing is one of the most valuable phenomena to overcome the decrease in reliability of ceramics caused by cracking. This is because self-healing is triggered by crack initiation and gives complete strength recovery through the passive oxidation of SiC. Thus, self-healing ceramics are candidates for high-temperature materials for gas tu...

Structural ceramics are leading candidate materials for high-temperature applications. However, ceramics exhibit low fracture toughness and are very sensitive to flaws such as pores and cracks. Thus they have low reliability. To overcome the problem, a new concept of self-crack-healing was developed by the authors. A material design with self-crack...

Purpose – 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. Design/methodology/approach – An artificial semi-circular slit was created in the weld toe, and peening was conducted. Then, bending fatigue tests were carried out. Findings – First, ow...

Low-cycle fatigue tests and finite element analysis were conducted using 100A elbow pipes made of STPT410 carbon steel or SUS304 stainless steel with local wall thinning. Local wall thinning was machined on the inside of the elbows in order to simulate local wall thinning. The local wall thinning located in two different areas, called extrados and...

We propose a model for overload effects on the fatigue crack growth (FCG) threshold of work-hardening elastic-plastic materials at various stress ratios R ≥ 0 by evaluating the effective residual stress intensity factor K_ at crack-tip, which combines elastic-plastic analysis using the finite element method and a theoretical-model-based calculation...

The purpose of this study 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 semicircular slit was created in the weld toe, and peening was conducted. Then, bending fatigue tests were carried out. Owing to the shot peening, the maximum slit depths that can be...

The wear performance between Partially Stabilized Zirconia (PSZ) plate and silicon nitride ball under dry conditions was investigated. The wear durability of PSZ with shot peening was higher than that of PSZ without shot peening in the sliding wear test under low Hertzian contact pressure. Due to shot peening, the transformation from tetragonal to...

The wear performance between Si_3N_4/ SiC composite plate and Si_3N_4 ball under dry conditions was investigated. The wear depth of Si_3N_4/SiC with shot-peening was shallower than that of Si_3N_4/ SiC without shot-peening in the sliding wear test under low Hertzian contact pressure. This result is achieved by the compressive residual stress which...

The maximum crack size rendered harmless on the weld toe of type 316 stainless steel welded joint by conducting Portable Pneumatic needle-Peening (PPP) was estimated focusing on stress intensity factor of crack tip. Stress intensity factor of external force and that of residual stress introduced by conducting PPP were calculated by FEM analysis. Th...

In this paper the wear performance between Partially Stabilized Zirconia (PSZ) plate and silicon nitride ball under dry conditions was investigaed. The wear durability of PSZ with shot peening was higher than that of PSZ without shot peening in the sliding wear test under low Hertzian contact pressure. By shot peening, the transformation from tetra...

It is well known that shot peening (SP) prevents the initiation and propagation of fatigue cracks and improves the fatigue limit of metals. It was recently reported that a surface crack can be rendered harmless by SP. Actual pressure vessels are generally operated under a positive stress ratio (R > 0). However, the effects of SP on the improvement...

Self-crack-healing is one of the most valuable phenomena to overcome the decrease in reliability of ceramics caused by cracking. This is because self-healing is triggered by crack initiation and gives complete strength recovery through the passive oxidation of SiC. Thus, self-healing ceramics are candidates for high-temperature materials for gas tu...

This study describes an investigation of the effect of peening on the fatigue limit of rolled steel containing a crack at the weld toe. 3-point bending specimens with a weld zone were prepared for fatigue test. An artificial semi-circular slit was made into the weld toe, and subsequently the weld toe was peened by needle-peening. Using those specim...

Effect of shot peening and self-crack-healing on rolling contact fatigue of Al_2O_3/SiC composite ceramics was investigated. The specimens were shot-peened using zirconium dioxide shots, and subsequently crack-healed at 950°C for 100 hours. The X-ray result clearly showed that the compressive residual stress was introduced at subsurface by shot-pee...

The effect of shot-peening crack initiation and propagation of partially stabilized zirconia, PSZ, was investigated. The X-ray results showed that shot-peening could introduce large compressive residual stress at surface. The residual stress were -1400 MPa and -1800 MPa at depth from the surface of Oum and 20μm, respectively. The X-ray results also...

This study describes an investigation of the effect of peening on the fatigue limit of rolled steel containing a crack at the weld toe.3-point bending specimens with a weld zone were prepared for fatigue test. An artificial semi-circular slit was made into the weld toe, and subsequently the weld toe was peened by needle-peening. Next, bending fatig...

Low cycle fatigue tests and finite element analysis were conducted using 100A elbow specimens made of STPT 410 steel with local wall thinning in order to investigate theinfluences of local wall thinning on the low cycle fatigue behavior of elbows with internal pressure. Local wall thinning was machined on the inside of the elbow and was prepared at...

One of the concerned technical issues in the nuclear piping under operation is pipe wall thinning caused by flow accelerated corrosion. This paper focuses on influence of internal pressure on low cycle fatigue life of pipe bends with local wall thinning and evaluation of safety margin against seismic loading in order to apply the obtained knowledge...

Low-cycle fatigue tests and finite element analysis were conducted using 100A elbow specimens made of STPT410 with local wall thinning in order to investigate the influences of local wall thinning on the low-cycle fatigue behaviors of elbows with internal pressure. Local wall thinning was machined on the inside of the elbow in order to simulate met...

The effect of shot peening (SP) on the bending fatigue limit of high-strength steel (SUP9A) containing a semi-elliptical surface slit was investigated. SP was conducted on specimens containing a semi-elliptical surface slit with an aspect ratio (a/c) of 0.4, where a was the crack depth (a = 0.1, 0.2, and 0.3 mm) and c was half the surface length of...

A model of overload effect for hardening elastic-plastic solids is proposed to evaluate the stress intensity factor for compressive residual stress Krs at fatigue crack-tip fields. The residual stress σrs introduced at the tip in SUS316 by overload Kov = 6, 15, 30 and 45 MPa·m1/2 can be estimated using Finite Element Method (FEM). The Krs values as...

The effect of the shot-peening on rolling contact fatigue of Si3N4 was investigated. Additionally we tried to clarify damage mechanism of rolling contact fatigue. By shot-peening, high compressive residual stress can be introduced near the surface. The residual stress can improve apparent fracture toughness KIC and Vickers hardness HV of Si3N4 by a...

Effect of shot peening on the critical stress for crack-healing of Si3N4/SiC composite ceramics was investigated. Shot peening was carried out on the Si3N4/SiC composite with a surface crack by Yittorium stabilized Zirconia beads. Peend specimens were healed at 1100 °C for 5 hours in air under constant tensile stress σapp ranging from 0 to 500 MPa....

Structural ceramics are superior to metallic materials in terms of their high-temperature strengths and critical heat proof temperatures. However, compared to metallic materials, ceramics exhibit lower fracture toughness, so they are more sensitive to flaws such as pores and cracks. The shortness considerably decreases the component reliability. To...

Ceramics have been used as bearing and cutting tool components, which are subjected to contact loading during their operation. The presence of surface cracks on these components decreases their contact strength. Thus, the reliability of ceramic components can be increased by improving their contact strength through crack healing. In the present stu...

Purpose – 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. Design/methodology/approach – An artificial semi-circular slit was introduced and Mp was...

Al2O3/SiC composite ceramics with high crack-healing ability were subjected to shot peening (SP) using zirconium oxide shots with several peening pressures and shot diameters. Specimens subjected to SP were heat-treated in air to heal the surface cracks induced by SP. The residual stress, the apparent fracture toughness, and the Weibull distributio...

This study describes an investigation of the effect of over bending moment (Mp) on the fatigue limit of spring steel containing a crack in the stress concentration zone. 3-point bending specimens with a notch were prepared for fatigue test. An artificial semi-circular slit was made into the bottom of the notch, arid subsequently Mp was loaded. Next...

抄録 This study describes an investigation of the effect of peening on the fatigue limit of rolled steel containing a crack at the weld toe. 3-point bending specimens with a weld zone were prepared for fatigue test. An artificial semi-circular slit was made into the weld toe, and subsequently the weld toe was peened by needle-peening. Using those spe...

Effect of shot peening on the fatigue limit of high tensile strength steel containing a crack on stress concentration zone was investigated. A pre-crack was introduced, subsequently shot peening was introduced. And the bending fatigue tests were carried out. Obtained results were as followings; (1) the fatigue limit of specimens containing a crack...

The effect of the shot-peening on rolling contact fatigue of Si_3N_4 was investigated. Additionally we tried to clarify damage mechanism of rolling contact fatigue. By shot-peening, high compressive residual stress -8500MPa can be introduced near the surface. The residual stress can improve apparent fracture toughness K_ and Vickers hardness HV of...

Effect of shot peening on the critical stress for crack-healing of Si_3N_4/SiC composite ceramics was investigated. Shot peening was carried out on the Si_3N_4/SiC composite with a surface crack by Yittorium stabilized Zirconia beads. Peend specimens were healed at 1100 ℃ for 5 hours under tensile stress σ_ ≤ ranging from 0 to 500 MPa. The result c...

Low cycle fatigue tests and finite element analysis were conducted using 100A elbow specimens made of STPT410 steel. The elbows were subjected to cyclic in-plane bending under displacement control without or with inner pressure of 9 MPa. Initially, fatigue tests were conducted under constant cyclic displacements. Then, two step cyclic fatigue tests...

A model of overload effect for hardening elastic-plastic solids is proposed to evaluate the stress intensity factor for compressive residual stress K_ at fatigue crack-tip fields. The residual stress σ_ introduced at the tip in SUS316 by overload K_ = 6, 15, 30 and 45 MPa・m^<1/2> can be estimated using Finite Element Method (FEM). The K_ values as...

Effect of shot peening on the fatigue limit of high tensile strength steel containing a crack on stress concentration zone was investigated. A pre-crack was introduced, subsequently shot peening was introduced. And the bending fatigue tests were carried out. Obtained results were as followings; (1) the fatigue limit of specimens containing a crack...

Purpose 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. Design/methodology/approach 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 t...

Low-cycle fatigue tests for STPT410 elbow pipes were conducted under displacement control with and without an internal pressure of 9 MPa. First, preliminary fatigue tests were conducted under constant displacements of ±15, ±20 and ±30 mm. Next, two-step fatigue tests were carried out in which the elbows were first subjected to cyclic displacements...

Elbow pipes are commonly used in the piping systems of power plants and chemical plants. The stress states at the elbow part are complex and quite different from those of the straight pipes. It is well known that the fatigue lives of metals under simple push-pull conditions can be successfully predicted by Manson’s universal slope method. However,...

Purpose – 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. Design/methodology/approach – Tensile overload was applied to a compact tension specimen, and fatigue tests were p...

The combination effects of shot peening (SP) and crack-healing on the contact strength of ceramics were investigated. Si3N4/SiC composite ceramics with high crack-healing ability were subjected to SP using zirconium oxide shots with several peening pressures and shot diameters. Then, some of the specimens subjected to SP were heat-treated in air to...

This study describes an investigation of the effect of over bending moment (Mp) on the fatigue limit of spring steel containing a crack in the stress concentration zone. 3-point bending specimens with a notch were prepared for fatigue test. An artificial semi-circular slit was made into the bottom of the notch, and subsequently Mp was loaded. Next,...