Improvement Of Fatigue Limit Research Articles

Evaluation of Fatigue Limit Improvement and Harmless Crack Size of Maraging Steel Using Shot Peening

The fatigue strength of maraging steel, which is an ultra-high-strength steel, is relatively low, compared to that of conventional high-strength steel. The fatigue life of a structure is highly dependent on the surface conditions, because fatigue cracks generally start at the surface of the material. In particular, surface cracks considerably degrade the fatigue limit. To expand the application range of maraging steel, it is necessary to improve the fatigue limit, and render the surface cracks harmless. This study aims to investigate the effect of shot peening (SP) on the fatigue strength of maraging steel with surface cracks. The SP application introduced a compressive residual stress from the specimen surface to a depth of 170 μm, and increased the fatigue limit by 77 %. The estimated crack size that can be rendered harmless, based on fracture mechanics, is (0.170 − 0.202) μm in the range As = (1.0 − 0.1). The intersections of the harmless crack sizes were determined at depth. A semicircular surface crack below this value is harmless in terms of fatigue limit. The usefulness of non-destructive inspection (NDI) and non-damaging technology was evaluated in relation to ahml, aNDI, a25,50, and As. Thus, the SP process can improve the reliability of the maraging steel. Compressive residual stress is the dominant factor to improve fatigue strength and render the surface crack harmless.

Effect of Cryogenic Treatment on the Properties of Low Carbon A858 Steel

This study is concerned with the effect of Deep Cryogenic Treatment (DCT) at liquid nitrogen temperature (-196 o C) on the mechanical properties and performance of low carbon steel (A858). The tests specimens were divided in to two groups, the first group was subjected to the conventional heat treatment of normalizing, and the second group was also normalized then subjected to (DCT). The results have shown that after (DCT), the Hardness, Tensile properties and the impact energy absorbed were all slightly increased. However the fatigue test showed some positive improvement in fatigue limit by 20(N/mm2 ), and the volume wear rates at different loads were significantly decreased after (DCT). The changes in microstructure due to (DCT) were clearly noticeable, the grain boundaries were no longer visible, and the Pearlite isles globalization was obvious.

Influence of Nonuniform Microstructure on High‐Cycle Fatigue Properties of the Electron‐Beam‐Welded Joint of High‐Strength Steel

The relatively weak fatigue property of electron beam (EB)‐welded joints is one of the key issues restricting their wide application. Herein, the impact of accelerating voltage, specifically the process‐induced nonuniform microstructure, is investigated on the high‐cycle fatigue properties of 15CrMnMoVA steel EB‐welded joints. It is indicated in the results that increasing the accelerating voltage from 120 to 150 kV (150‐specimen) results in a longer fatigue life and an 11.8% improvement in fatigue limit for the welded joint with the locking bottom structure. However, even after removing the locking bottom structure, the fatigue life of the 150‐specimen is still inferior to that of the base metal. The shorter fatigue life is primarily due to stress concentration and premature fatigue failure caused by the nonuniform distribution of grain size and crystallographic orientation across the EB‐welded joint. Thus, further optimization of the welding process is necessary to improve the fatigue reliability of EB‐welded joints by obtaining a more uniform microstructure.

Fatigue Limit Improvement and Rendering Surface Defects Harmless by Shot Peening for Carburized Steel

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 means of remanufacturing gears. In this study, carburized steel specimens containing artificial defects were used to investigate the effects of shot peening on the fatigue strength; the defect size was rendered harmless by shot peening. Shot peening was conducted after inducing semicircular slits with depths of a = 0.15, 0.20, and 0.30 mm. Subsequently, plane bending fatigue tests were carried out. A maximum compressive residual stress of 1400 MPa was induced after shot peening. The fatigue limit of the smooth specimen increased by approximately 31% after shot peening. A semicircular slit of at least 0.20 mm deep could be rendered harmless by shot peening (SP). The defect size reduced by SP was evaluated on the basis of fracture mechanics. The estimated results are consistent with the experimental results. On the basis of the results, the feasibility of shot peening as a remanufacturing method for gears is discussed.

The role of microstructural evolution on the fatigue behavior of additively manufactured Ti–6Al–4V alloy

The fatigue behaviour of additively manufactured Ti–6Al–4V via Laser Powder Bed Fusion (L-PBF) was evaluated in three different conditions, as-built, heat-treated and hot isostatically pressed (HIP'ed). Fractography analysis interpreted together with the S–N curves indicates that fatigue failure in as-built and heat-treated conditions where <0.2% porosity was present, was mainly driven by early-stage crack growth. However, crack initiation was determined to be the main controlling factor for fatigue deformation of HIP'ed samples. Moreover, a strong correlation between the impact energy and fatigue limit was found. The findings were based on detailed microstructural and crystallographic characterization, as well as mechanical testing. The as-built and heat-treated conditions exhibited poor fatigue response in comparison to HIP'ed which is largely attributed to the lower levels of porosity identified. Even though similar levels of porosity are present in as-built and heat-treated samples, improvement in fatigue limit was determined in the heat-treated condition due to phase transformation and microstructural coarsening leading to reduction in micro-strain.

The influence of carbon potential after gas-carbonitriding on the microstructure and fatigue behavior of low alloyed steel

In this paper, the effects of gas-carbonitriding time on fatigue limit improvement of low alloy steel specimens were investigated by experimental tests of three points fatigue flexion. Besides, metallurgical evaluations and micro-Vickers hardness tests were performed employing metallographic techniques, optical, scanning electron microscopy and x-ray diffraction techniques. Test findings showed that this amount remarkably improved the components fatigue resistance. The fatigue life of carbonitrided specimens was prolonged between 16%, for CN11 (i.e. austenization-870°/7 h, atmosphere with 1% carbon potential, quenching-oil medium and tempering-200 °C/1 h), and 32%, for CN21(i.e. austenization-870°/8 h, atmosphere with 1, 2% carbon potential, quenching-oil medium and tempering-200 °C/1 h), compared to the untreated state. It is obvious, from the fractography analysis, utilizing (SEM), that fatigue cracks appeared first at the carbonitrided specimens surface.

Improving high cycle fatigue performance of gas tungsten arc welded Ti6Al4V titanium alloy by warm laser shock peening

Welded components of titanium alloy used in aviation and aerospace industries were generally vulnerable to alternate cycles of vibrations stress during their service, which were weakest part and highly prone to failure and fracture. To improve the resistance towards vibration failure of weldments, warm laser shock peening (WLSP) technology was successfully applied as a post weld treatment in present work and significant 42.3% increase in high cycle vibration fatigue limit was achieved (from 399 ± 5 MPa to 568 ± 19 MPa). The notable effects of WLSP on the residual stress, microhardness, roughness and microstructure of weldments was investigated to clarify its strengthening mechanism. It is analyzed that the large improvement of fatigue limit was attributed to high amplitude compressive residual stress, high stability of compressive residual stress and surface nanostructure induced by WLSP. Our work indicated that WLSP could be an effective approach for promoting weld quality and fatigue performance over conventional methods of post weld treatment.

Fatigue performance improvement of laser shock peened hole on powder metallurgy Ni-based superalloy labyrinth disc

It is well understood that the components with holes used in aviation and aerospace industries are generally vulnerable to alternate cycles of working stress during their service, which are weakest parts and highly prone to failure and fracture due to the stress concentration at the edge of hole. To improve the resistance towards fatigue failure of the hole, laser shock peening (LSP) technology is successfully applied to the hole on powder metallurgy (P/M) Ni-based superalloy labyrinth disc in present work and significant 18% increase in high cycle fatigue limit is achieved (from 224 ± 4 MPa to 265 ± 6 MPa). The notable effects of LSP on the residual stress, microhardness and microstructure of the hole with and without LSP is investigated based on experimental and simulation method. It is found the large improvement of fatigue limit is attributed to the comprehensive action of high amplitude compressive residual stress and microhardness, grain refinement and the removement of inclusions caused by LSP. The finding here in promoting the fatigue resistance of hole by LSP are general and flexible, thereby exhibiting a potential application to a wide spectrum of engineering components with hole structures.

Improving the fatigue limit and rendering a defect harmless by laser peening for a high strength steel welded joint

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 containing 0.2 mm deep semicircular defect at the weld toe reached almost the same level of fatigue limit as that of the defect-free, LP-treated specimen. Therefore, a semicircular defect of less than 0.2 mm deep could be rendered harmless by LP. From the results, the fatigue strength and the reliability of HSS-welded joints can be significantly improved by LP. The dominant factor that contributed to fatigue limit improvement was the introduction of compressive residual stress by LP.

Influence of Cryogenic Temperature (CT) on Tensile Properties and Fatigue Behavior of 2024-Al2O3 Nanocomposites

The stir casting process used to manufacture the AL/Al2O3. AA2024 with matrix material alumina Al2O3 at different weight percentage ratio 0.3, 0.5, 0.7 and 0.9%. Were fabricated by using cryogenic temperature (CT). It was observed that the maximum enhancement is occurred at nanocomposite containing 0.9% Al2O3 which shows improvement percentage of 9.9%, 14.63% and 14.28% for ultimate strength, yield stress and Brinell hardness respectively and the ductility is reduced by 34.57% compared to 0% by weight AA2024. It clearly shows that all the mechanical properties are improved by increasing weight percentage of Al2O3 but the best properties are obtained at 0.9% Al2O3 nanocomposite. The fatigue S-N curve tests were done for all the specimens having average of roughness between 0.2 to 0.5Mm.The fatigue endurance limit of the 0.9% wt. Al2O3 composite into two cases without (CT) and with (CT) is taken at 107 cycles. The fatigue limit at 107 cycles increases from 83 MPa for without (CT) to 89.6 MPa with (CT) resulted in 7.95% fatigue limit improvement.

Effect of Carbide Density on Fatigue Limit of H21 Die Steel

In the present work, the effect of carbide density on microstructure, hardness, wear rate, and fatigue limit of AISI H21 die steel has been studied. The specimens of H21 die steel have been employed hardening at 1195 °C, double tempering at 540 °C, cryotreatment at − 185 °C for 24 h, and soft tempering at 100 °C. The microstructure, hardness, wear rate, and fatigue limit of the conventionally treated and cryotreated specimens have been examined. The pin-on-disk machine has been used for the dry sliding wear test. The fatigue test has been carried out using constant amplitude rotating bending fatigue test machine which indicates enhanced fatigue limit for cryotreated specimens. The improvement in hardness, wear behavior, and fatigue limit of cryotreated specimens as a result of increased carbide density with reduced carbide size and retained austenite has been observed. The carbides Fe6W6C, Cr23C6 and Cr7C3 played an important role in increasing the hardness, wear behavior, and fatigue limit of the material. The Basquin’s equation was used for the analysis of the fatigue test. The minimum value of the fatigue strength exponent of Basquin’s equation shows the enhanced fatigue limit of cryotreated specimens.

Very High-Cycle Fatigue Properties and Residual Stress Relaxation of Micro-shot-Peened EA4T Axle Steel

In this paper, EA4T alloy railway axle steel was shot peened with steel balls and ceramic balls with a diameter of 50 μm, and 109-cycle fatigue tests of unpeened and peened specimens were carried out. The fatigue properties and the surface compression residual stress relaxation processes were investigated. The results of surface fracture observation indicated that all the cracks initiated from the surface, and both peened and unpeened specimens have conventional fatigue limits in the very high-cycle fatigue regime. Moreover, compared with unpeened (UP) specimens, the fatigue limits were improved by 31 and 28% for ceramic shot-peened (CSP) and steel shot-peened (SSP) specimens, respectively. The improvement of fatigue limit is related to the surface compressive residual stresses (SCRS) and the surface-hardened layer. The SCRS and full-width half-maximum (FWHM) values during the relaxation processes have been analyzed. At stress amplitudes lower than the cyclic yield strength, the residual stress of SSP specimens relaxed quickly in the first loading cycle and then remained stable, while the residual stress of CSP specimens relaxed quickly in the first loading cycle and relaxed slowly afterward, then remained stable. For all SSP and CSP specimens with applied stress amplitudes larger than the cyclic yield strength, the residual stress relaxed quickly in the initial cycles, and then remained stable after the first loading cycle.

Effect of micro-shot peening, conventional shot peening and their combination on fatigue property of EA4T axle steel

In this study, EA4T axle steel specimens were subjected to three shot peening treatments: conventional shot peening (CSP), micro-shot peening (MSP) and their combination, i.e. dual shot peening (DSP). The residual stress, surface roughness, morphology, microhardness, were used to characterize the affected surface layer. The results indicated that MSP achieved maximum surface compressive residual stress (SCRS), minimum surface roughness, high surface microhardness. Moreover, compared to CSP, DSP improved the SCRS and microhardness while reducing surface roughness. However, scanning electron microscopy (SEM) observations indicated the presence of intensive defects of the DSP-treated specimens, which hindered the fatigue strength of the specimens. Fatigue tests were conducted to evaluate the effect of different treatments on fatigue life, the median S-N curve demonstrated that the fatigue limit at 107 cycles was improved by 24, 32, and 27% for the CSP, MSP, and DSP specimens, respectively. Furthermore, SEM observation was conducted to analyze the fracture mechanisms, the results indicated that the failure of all specimens originated at the surface and the fracture mechanisms were not changed by the shot peening treatments. The SCRS during the cyclic loading have also been analyzed. Moreover, the factors influencing fatigue limit improvement were discussed.

Fatigue Limit Improvement and Rendering Defects Harmless by Needle Peening for High Tensile Steel Welded Joint

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 fatigue limits of all specimens increased by 9–200% due to the NP treatment. Furthermore, NP-treated specimens with slit depths of a = 1.0 mm exhibited high fatigue limits that were equal to those of NP-treated specimens without a slit. Therefore, a semicircular slit of less than a = 1.0 mm could be rendered harmless through NP treatment. This result indicates that the reliability of HTS-welded joints can be significantly improved via NP for surface defects with depths that are less than 1 mm, which are not detected through non-destructive inspection (NDI). Therefore, the problem regarding the reliability of HTS-welded joints that restricts the industrial utilization of HTS can be solved by performing both NDI and NP. The dominant factor that contributed to the improvement of the fatigue limit and increase in the acceptable defect size was the introduction of large and deep compressive residual stress with non-propagating cracks.

Fatigue Limit Improvement and Rendering a Defect Harmless for Stainless Steel Welded Joint Containing a Crack-like Surface Defect

Fatigue Limit Improvement and Rendering a Defect Harmless for Stainless Steel Welded Joint Containing a Crack-like Surface Defect

Mechanical Properties of Burnished Steel AISI 1008

Burnishing improves fatigue strength, surface hardness and decrease surface roughness of metal because this process transforms tensile residual stresses into compressive residual stresses. Roller burnishing tool is used in the present work on low carbon steel (AISI 1008) specimens. In this work, different experiments were used to study the influence of feed parameter and speed parameter in burnishing process on fatigue strength, surface roughness and surface hardness of low carbon steel (AISI 1008) specimens. The first parameter used is feed values which were (0.6, 0.8, and 1) mm at constant speed (370) rpm, while the second parameter used is speed at values (540, 800 and 1200) rpm and at constant feed (1) mm. The results of the fatigue test showed that improvement in fatigue limit, where the highest fatigue limit was obtained at (1mm feed, 1200rpm speed) in burnishing process which was (169 Mpa). The hardness results, showed increasing feed and speed values lead to increasing the hardness. The burnishing process reduces surface roughness by producing accurate and better surface finish. The best surface fineness of metal at (1mm feed and 1200 rpm speed) was 0.11 μm.

Improvement of Fatigue Limit and Rendering a Surface Defect Harmless for Stainless Steel Welded Joint Containing a Crack-like Defect

Improvement of Fatigue Limit and Rendering a Surface Defect Harmless for Stainless Steel Welded Joint Containing a Crack-like Defect

Very high cycle fatigue behaviour of 42CrMo4 steel with plate-like alumina inclusions

Recent progress in steel refining shows significant reduction of non-metallic inclusions (NMIs) of which alumina (Al2O3) is one of the most problematic. Among other refining methods, metal melt filtration by ceramic foam filters shows promising results in steel cleaning. In the present work the influence of alumina inclusions on the fatigue behavior is investigated after the reaction of steel melt with filters. Different batches are compared where carbon-bonded alumina foam filters with different coatings were introduced into the steel melt of 42CrMo4 for 10 s (so called “finger test”). Fatigue tests were performed using ultrasonic fatigue testing (USFT) up to 109 cycles. Specimens were nitrided in order to prevent crack initiation from the surface and to study internal failure on NMIs. Surface hardening of quenched steel increased fatigue limit significantly. Metallographic sections were analyzed using optical and scanning electron microscopy (SEM) for the estimation of NMIs distribution properties. NMI size distribution analysis based on maximum Feret diameter (instead of area) is found to be an effective method for detecting plate-shaped inclusions. Fracture surfaces after fatigue tests were investigated by methods of SEM and confocal laser scanning microscopy (CLSM), revealing that plate-like NMIs initiate crack with all their area even being inclined to the crack plane. Properties of crack initiating NMIs – alumina plates and MnS dendrites – are compared and analyzed. Formation of alumina NMI as plate lead to significant enlargement of its stress-concentrating area in comparison to the spherical shape of the same volume. Thus, total NMI content reduction in steel could give no fatigue limit improvement if NMI morphology changes from spherical to plate-like.

Does fatigue limit improvement require high-concentration carbon?

Does fatigue limit improvement require high-concentration carbon?

Prediction of fatigue limit improvement in needle peened welded joints containing crack-like defects

PurposeThe 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.Design/methodology/approachResidual 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.FindingsThe 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.Practical implicationsThe proposed method is useful in improving the reliability of welded joints used in large steel structures, transportation equipments and industrial machines.Originality/valueFrom 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.