Improvement Of Fatigue Limit Research Articles

Effect of cold deformation on corrosion fatigue behavior of nickel-free high nitrogen austenitic stainless steel for coronary stent application

Due to the excellent mechanical properties, good corrosion resistance, high biocompatibility and nickel-free character, the high nitrogen nickel-free austenitic stainless steel (HNASS) becomes an ideally alternative material for coronary stents. Stent implantation works in harsh blood environment after a balloon dilatation, i.e., the material is used in a corrosive environment with a permanent deformation. The present study attempts to investigate effects of pre-straining on high-cycle fatigue behavior and corrosion fatigue behavior of HNASS in Hank’s solution and the relevant mechanism for coronary stents application. It is found that higher pre-straining on HNASS results in higher strength and maintains almost same corrosion resistance. Fatigue limit of 0% HNASS is 550MPa, while corrosion fatigue limit is 475MPa. And improvement in fatigue limit of 20% and 35% pre-strained HNASS is in comparison with the 0% HNASS, while corrosion would undermine the fatigue behavior of HNASS. In a suitable range, the pre-straining had a beneficial effect on corrosion fatigue strength of HNASS, such as nearly 300MPa improved with 20% cold deformation. This result provides a good reference for predicting the life of HNASS stent and as well its design.

Improvement of torsional fatigue limit and rendering surface defect harmless by shot peening for spring steel

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 residual stress was introduced into the specimens by SP. Torsional fatigue tests were carried out under a stress ratio of R = −1. The torsional fatigue limits of the shot peened specimens with Vickers hardness values of 460, 540, and 670 HV increased by 8%–67%, 33%–143%, and 36%–127%, respectively, in comparison with the non-shot peened specimens. The maximum depth of the slit that could be rendered harmless by SP was 0.15 mm for the 460 and 670 HV specimens. However, even a slit with a depth of less than 0.15 mm could not be rendered harmless by SP for the 540 HV specimen. Considering the improvement in the torsional fatigue limit and the size of the surface defect that could be rendered harmless by SP, the 670 HV specimen is optimal for practical use.

Correlation Between Microstructure and Mechanical Properties Before and After Reversion of Metastable Austenitic Stainless Steels

Reversion treatments are a way to improve the mechanical response of metastable austenitic stainless steels by means of grain refinement. To effectively apply those treatments, the steel must be previously deformed to induce a significant amount of martensitic transformation. In this work, the effect of reversion treatments was studied on a commercial AISI 301LN grade subjected to an industrial cold rolling process, with thickness reductions not higher than 40 pct. Microstructural changes and evolution of both monotonic and cyclic mechanical properties were investigated after cold rolling and upon reversion treatments. Results revealed that the finer austenitic microstructure obtained after reversion leads to an interesting combination of properties, with strong increments in hardness and yield strength, and also fatigue limit improvement, as compared to the initial annealed condition.

Effect of shot peening on metastable austenitic stainless steels

In this work, shot peening was performed in a metastable austenitic stainless steel EN 1.4318 (AISI 301LN) in order to evaluate its effect on austenite to martensite phase transformation and also the influence on the fatigue limit. Two different steel conditions were considered: annealed, i.e., with a fully austenitic microstructure, and cold rolled, consisting of a mixture of austenite and martensite. X-ray diffraction, electron back-scattered diffraction and focus ion beam, as well as nanoindentation techniques, were used to elucidate deformation mechanisms activated during shot peening and correlate with fatigue response. Results pointed out that extensive plastic deformation and phase transformation developed in annealed specimens as a consequence of shot peening. However, the increase of roughness and the generation of microcracks led to a limited fatigue limit improvement. In contrast, shot peened cold rolled specimens exhibited enhanced fatigue limit. In the latter case, the main factor that determined the influence on the fatigue response was the distance from the injector, followed successively by the exit speed of the shots and the coverage factor.

ショットピーニングによる高強度鋼における疲労強度向上と表面欠陥の無害化 (応力比の影響)

ショットピーニングによる高強度鋼における疲労強度向上と表面欠陥の無害化 (応力比の影響)

OS8-5 Improvement in Fatigue Limit By Shot Peening for Highstrength Steel Containing Cracklike Surface Defect : Influence of Stress Ratio(Shot peening,OS8 Fatigue and fracture mechanics,STRENGTH OF MATERIALS)

OS8-5 Improvement in Fatigue Limit By Shot Peening for Highstrength Steel Containing Cracklike Surface Defect : Influence of Stress Ratio(Shot peening,OS8 Fatigue and fracture mechanics,STRENGTH OF MATERIALS)

Improvement of Fatigue Limit and Rendering Crack Harmless by Peening for Stainless Steel Containing a Crack at the Weld Toe Zone

Improvement of Fatigue Limit and Rendering Crack Harmless by Peening for Stainless Steel Containing a Crack at the Weld Toe Zone

Improvement of fatigue limit by shot peening for high-strength steel containing a crack-like surface defect

Purpose – 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. Design/methodology/approach – 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. Findings – 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. Practical implications – The proposed method can be applied to mechanical parts used in vehicles, aircraft and trains. Originality/value – 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.

ショットピーニングによる高強度鋼における表面欠陥の無害化 (き裂形状の影響)

ショットピーニングによる高強度鋼における表面欠陥の無害化 (き裂形状の影響)

Improvement of fatigue limit by overload for high-tensile strength steel containing a crack in the stress concentration zone

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 applied after which bending fatigue tests were carried out. Findings – The relationship between the level of Mp and the fatigue limit (σw) 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. Originality/value – 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.

Fatigue behavior of functionally graded steel produced by electro-slag remelting

The present study attempts to investigate the fatigue behavior of functionally graded steel (FGS) produced through electroslag remelting (ESR) process. To produce FGSs, two different slices of plain carbon steel and austenitic stainless steel were welded and used as ESR's electrode. Some of alloying elements in austenitic stainless steel, such as Nickel and Chromium, as well as carbon in plain carbon steel may be replaced during remelting stage; graded layers (austenite and martensite layers) may also be fabricated. Vickers micro-hardness test and scanning electron microscopy (SEM) of FGS were performed and variations in hardness and microstructure were observed. SEM images exhibited multi-phase graded steel. The rotating bending fatigue test was performed on specimens. The fatigue test results showed improvement in fatigue limit of FGS in comparison with that of its ingredients. SEM's images of fatigue fracture surfaces in FGS showed deviation and branching in crack propagation when crack propagates from graded austenite to graded martensite phase.

Improvement of fatigue limit by shot peening for high‐tensile strength steel containing a crack in the stress concentration zone

PurposeThe 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.Design/methodology/approachAn artificial semi‐circular slit was introduced into the bottom of notch, and SP was performed. Bending fatigue tests were then carried out.FindingsFirst, 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.Originality/valueThere 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.

Improvement of Fatigue Limit and Rendering Crack Harmless by Peening for Rolled Steel Containing a Crack at the Weld Toe Zone

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 specimens, bending fatigue tests were carried out. The following results were obtained. (1) The fatigue limits of specimens containing a slit improved considerably by peening. (2) The fracture occurred from outside the slit. Moreover, the specimens recovered to fatigue limits up to that of non-slit specimens. (3) A slit of 0.4- 1.2 mm in depth was rendered harmless by the peening.

Improvement of Fatigue Limit and Rendering Crack Harmless by Over-Moment Loading for Spring Steel Containing a Crack in the Stress Concentration Zone

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, bending fatigue tests were carried out. The following results were obtained. (1) The fatigue limits of specimens increased with increasing value of Mp. (2) In case of specimens containing a slit of 0.2 and 0.3mm in depth, the fatigue limits of the Mp-loaded specimens were improved up to around twice those of the specimens without Mp loading. (3) A slit of 0.05mm in depth was rendered harmless by the loading of Mp, because the Mp-loaded specimens with a slit of 0.05 mm in depth showed the fatigue limits about the same as that of specimens without slit. (4) Results of calculation by FEM showed that the loaded Mp induces a large compressive residual stress in the stress concentration zone.

1209 応力集中部にき裂を有するSUP9のショットピーニングによる疲労限度向上(G02-4 疲労の特性および評価,G02 機械材料・材料加工部門)

1209 応力集中部にき裂を有するSUP9のショットピーニングによる疲労限度向上(G02-4 疲労の特性および評価,G02 機械材料・材料加工部門)

Improvement of fatigue limit by shot peening for high‐strength steel containing a crack‐like surface defect

PurposeThe 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.Design/methodology/approachSP 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.FindingsThe 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.Practical implicationsThe proposed method can be applied to mechanical parts used in vehicles, aircraft and trains.Originality/valueThere 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.

Analysis on fatigue property of microshot peened railway axle steel

In this paper alloy railway axle steel and medium carbon railway axle steel were shot peened by steel balls with a diameter of 100μm and the 109 cycles fatigue tests with the specimens unpeened and peened were carried out. The fatigue property and mechanism of surface compress residual stress relaxation were investigated. The results of fracture surface observation show that all the cracks initiate from the surface and the specimens both peened and unpeened have the conventional fatigue limit in the very high cycle fatigue regime. The fatigue limits of peened alloy steel and medium carbon steel increase by 10% and 35%, respectively. The improvement of fatigue limit is related to the residual stress relaxation while the degree of residual stress relaxation depends on the yield strength of surface hardening layer, the yield strength of original material and the applied stress amplitude. For the alloy steel which has the high yield-tensile ratio it relaxes quickly at the first cycles and then remains stable, while for the medium carbon steel which has the low yield-tensile ratio the residual stress relaxes quickly at the initial cycles and relaxes slowly afterwards.

Effect of Combined Shot Peening Process for Surface Nanocrystallization Method on Fatigue Strength of TC4

In order to enhance the fatigue limit of titanium (TC4), the combined shot peening were employed. Firstly, nano-crystallization in surface layer of TC4 were carried out by high-energy shot peening, and then the surface repair shot peening with small balls were introduced for repairing the surface damage by high-energy shot peening, and reducing the surface roughness, thus to enhance the quality of nanocrystalline surface layer of TC4. The fatigue tests were performed using the specimens treated by the combined shot peening. In addition, the influence factors of fatigue limit, the behavior of fatigue crack initiation and propagation etc. are discussed. The results showed that the fatigue limit of TC4 specimen treated by high-energy shot peening is increased by 13%, while the fatigue limit of TC4 specimen treated by the compound shot peening process is increased by 34% (165MPa). The main reason of fatigue improvement is considered to be that the combined shot peening process may effectively repair the surface damage, and also there is the certain relationship between the fatigue limit improvement and the surface roughness reducing.

Fatigue Behaviour of Friction Stir Processed Cast Aluminium and Magnesium Alloys

Friction stir processing (FSP) was applied to a cast aluminium alloy, A356-T6, and a cast magnesium alloy, AZ91-T5, and fully reversed axial fatigue tests have been performed using FSPed specimens. It was indicated that FCP exerted different influence on fatigue behaviour depending on alloy system. In A356-T6, the fatigue strengths of the FSPed specimens were lower than those of the as-cast ones in the finite life region, but the fatigue limit was significantly increased by FSP. The enhanced crack initiation resistance due to the elimination of casting defects by FCP resulted in the improvement of fatigue limit of the FSPed specimens, while the matrix softening due to the dissolution of precipitates by the heat input during FSP caused faster crack growth rates in the FSPed specimen, leading to the inferior fatigue strength of the FSPed specimens to the as-cast one in the finite life region. In AZ91-T5, both the fatigue strengths in the finite life region and fatigue limit were improved by FSP, because the hardness was increased and both the crack initiation and crack growth resistances were enhanced.

き裂状表面欠陥を有する高強度鋼のショットピーニングによる疲労限度向上と表面欠陥の無害化

き裂状表面欠陥を有する高強度鋼のショットピーニングによる疲労限度向上と表面欠陥の無害化