Improvement Of Fatigue Strength Research Articles

Study on the high cycle fatigue behavior of titanium microalloyed high strength beam steel by magnesium treatment

Non-metallic inclusions, which was mainly modified by magnesium treatment process in this work, played a decisive role in controlling the high cycle fatigue behavior of titanium microalloyed high strength beam steel. Magnesium treatment process profoundly reduces the large-sized inclusions in experimental steels, leading to a high fatigue strength and crack growth resistance simultaneously. The magnesium treatment process improved fatigue strength by 5.1% despite a 7.3% decrease in tensile strength of 6-Mg steel. In addition, Generalized Pareto Distribution (GPD) statistical method is used to evaluate the maximum inclusion size, which decreases by 25.3% through magnesium treatment process. These findings provide a new strategy to develop a titanium microalloyed beam steel with outstanding fatigue resistance.

Improvement of Fatigue Properties of EN AW 6082 Aluminum Alloy using Different Deep Rolling Directions

Deep rolling (DR) is an effective mechanical surface treatment method to improve the fatigue properties of engineering components. In this method, the surface of the component was rolled using a roller with a predetermined force to obtain reduced roughness, hardness increases and compressive residual stresses in the surface region. These alterations allow for increasing the fatigue lives of the components in industrial applications. In the current study, DR was applied in tangential and longitudinal directions on specimens that were manufactured using EN-AW 6082-T6 aluminum. The resulting roughness, hardness and residual stresses were determined experimentally. Fatigue tests were carried out to determine the improvements in fatigue properties after DR. It was found that DR-induced compressive residual stresses depend on DR direction considerably. Due to this reason, fatigue strength improvements were found to be different for different DR direction applications. Longitudinal rolling resulted in a 23% fatigue strength increase compared to a 7% increase for tangential rolling. For both DR direction applications, fatigue cracks were shown to initiate at the sub-surface region, whereas the as-turned specimens exhibited surface crack initiation.

Study on the relationship between the root welding residual stress and the root-failure fatigue strength of Plug Welded specimens

It is difficult to examine the relation between the root notch welding residual stress (WRS) and the root-failure fatigue strength because of the difficulty in the direct root WRS measurement. The root WRS can be calculated by thermal elastic-plastic finite element analysis (TEPFEA), but its accuracy has not been fully verified yet. In this study, root-failure fatigue tests, in which direct root WRS measurement can be performed, are carried out. Plug welded (PW) specimens with backing plates are used in those tests, and the root WRS in as-welded (AW) and stress-relieved (SR = PWHT) specimens are measured using X-ray diffraction (XRD) method by cutting off the backing plate. The measured WRSs are compared with those calculated by TEPFEA. It is found that the root WRSs in the PW specimens estimated by TEPFEA become much larger than those measured when creep strain is neglected. The SR specimen’s fatigue strength improvement ratio is estimated by using the mean stress effect formulas developed for toe-failure cases (IIW Fatigue Recommendations and MIL-HDBK-5D). The estimated improvement ratio shows fair agreement with that measured.

Achieving high fatigue strength of large-scale ultrafine-grained copper fabricated by friction stir additive manufacturing

The preparation of large-scale bulk materials and the limitation of fatigue strength improvement are two crucial obstacles restricting the industrial applications of ultrafine-grained (UFG) materials. In this study, we successfully fabricated the large-scale UFG pure copper by using the water-cooling assisted friction stir additive manufacturing (FSAM) method and investigated its high cycle fatigue (HCF) properties. The microstructural characteristics before and after fatigue were almost the same, proving the high microstructure stability of FSAM Cu during the HCF deformation. Therefore, the fatigue strength of FSAM Cu was as high as 130 MPa, and the fatigue ratio (0.30) reached the same level as coarse-grained Cu. This study can provide an efficient method to fabricate large-scale bulk materials with high fatigue resistance, bringing possibility to the engineering application of UFG materials.

Study the Mechanical Properties and Fatigue Effect of Multilayer Woven E-Glass/Epoxy Composite under Constant and Variable Loading

The present work studies the effect of adding woven E-glass fibers [0°/90°], 16 layers with 50% weight fraction to pure epoxy (matrix) on fatigue behavior under constant and variable loadings. The CNC water jet cutting machine was used to cut the composite samples with five fiber direction angles, such as (0°, 5°, 15°, 30°, and 45°). The tensile test was used to determine the composite material’s mechanical properties. The results showed that the composite material with 5° of fiber direction had the highest ultimate tensile stress, i.e., 353 MPa, and the highest Young's Modulus, i.e., 11940 MPa, compared to other samples with angles of (0°, 15°, 30°, 45°) of fiber direction. The sample with 5° angle was adopted in constant and variable fatigue loading tests. The fatigue test results under constant loading showed a 24.8 times fatigue strength improvement for composite material at 107 cycles compared to pure epoxy. The fatigue test under variable loading was conducted with two types of sequence loading program tests at a constant number of cycles at each stress level: high-low sequence loading with stress of 170-130 MPa with 10,000 and 20,000 cycles for each stress level and low-high sequence loading with stress 130-170 MPa with 10,000 and 20,000 cycle for each stress level loading and so on to failure. The results showed that the fatigue life under high-low sequence loading for both 10,000 and 20,000 cycles was less than that of the low-high sequence loading. Also, the results showed that Miner's rules were safe to calculate the damage of composite material used in this work where the damage was above one (D > 1).

Observation of Internal Fatigue Cracks in Repeatedly Induction-Heated S45C Bearing Steel under Rotating Bending Fatigue

Induction-heated steel has hard and soft layers. These layers can cause an internal fatigue crack originating from the boundary of these layers when cyclic stress is applied. Repeated heating is known as a method for improving fatigue strength, and it was applied to induction heating method. Repeatedly induction-heated steel had high fatigue strength compared to single quenching. We performed rotating bending fatigue tests of low carbon steel (JIS-S45C) induction-heated three times, and observed the fracture surfaces and the microstructures of internal fatigue cracks. The internal fatigue cracks originated from the area around the boundary between soft and hard layers surrounding crack origin. Some pearlite and ferrite can be seen. There were pearlite and dimples on the soft layer of internal fatigue crack and clear grains on the hard layer of the crack. From chase-up observation, we revealed that internal fatigue crack originated from soft layer.

Effects of high-energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness, and fatigue life and strength of single-crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser-peened single-crystal nickel superalloy specimens compared to unpeened and shot-peened specimens following hot corrosion exposure and then fatigue testing. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. The latter treatment supports the benefit of a unique process involving application of layers of laser peening using high energy with large footprint spots combined with interspersed cyclic annealing. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Unpeened and peened specimens were exposed to sulphate corrosives at 700 °C for 300 h and then fatigue tested. Tests of five non-laser-peened specimens all failed in low-cycle fatigue regime, whereas three identically tested laser-peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests also showed fatigue strength improvement of 2:1 by laser peening. Residual stress measurements post hot-corrosion exposure and fatigue testing showed notable 5 mm depth retention of residual eigenstress in a laser-peened specimen.

Characterization of wear and fatigue behavior of aluminum piston alloy using alumina nanoparticles

Abstract Due to their excellent thermal conductivity, lightweight, and ease of processing, aluminum alloys are the material of choice for piston manufacture in internal combustion engines. Nanoparticles (NPs) of alumina (Al2O3) with a size of 25 nm were incorporated into an aluminum piston alloy to examine the effect of the NP addition on wear resistance and fatigue behavior. The stir casting method has been utilized to manufacture experimental samples of the composite material by altering the particle weight ratio of aluminum to the matrix alloy to 2, 4, and 6 wt%. The surface morphology of the samples has been examined using an electronic scanning microscope. The results of the wear and fatigue tests indicate that the addition of Al2O3 to the composite enhanced its fatigue resistance and wear strength, with the exception of 6 wt% weight ratio. The best improvement in wear resistance and fatigue strength occurs at 4 wt% Al2O3 particles, which are 12.13 and 67.5%, respectively, more significant than the pure metal and other composites. The mechanical properties of the alloy samples have been enhanced by adding Al2O3 NPs of 25 nm size into the piston’s aluminum matrix alloy. Stir casting was employed to produce the needed composites by incorporating Al2O3 NPs at varied weight percentage ratios of 0, 2, 4, and 6 wt% into the master alloy. Before the composite alloy reached 6 wt%, including Al2O3 NPs, the alloy’s hardness and tensile strength improved, according to the experiment results.

Selective laser melted 316L stainless steel: Influence of surface and inner defects on fatigue behavior

In additive manufacturing, despite its several indisputable advantages, detrimental variables to fatigue strength still remain poor surface finishing and porosities. However, because the majority of defects locate close to the surface, their mechanical removal is expected to appreciably improve fatigue strength. Considering SLMed 316L, fully-reversed rotating-bending fatigue tests in both as-built and machined conditions are performed. Fatigue failures are discussed using the Kitagawa-Takahashi diagram. In each condition, the fatigue stress is related to the equivalent micro-notch length of the killer defect. Then, this work analyses the possibility of predicting fatigue limits at 50 % probability considering the equivalent micro-notch length.

Improvement of Fatigue Strength and Wear Resistance of Molybdenum Diffusion-Bonded Alloyed Steel by Sinter-cold-forging and Vacuum Carbonitriding Processes

This study aims to obtain the superior mechanical properties of Mo diffusion-bonded alloyed steel by the optimum combination of processes: primary-sintering (PS), cold-forging (CF), heat treatment (HT) and shotblast. In our previous research, it was found that the fracture of sintering connections, namely, micro-cracks occurred on the surface layer of high-density specimens in addition to the work hardening on materials, which decreased both the impact energy and the bending strength. Also, since the optimum heat treatment leads the diffusion bonding of the pressurized cracks in the surface layer, the microstructure could be reformed. In this study, it was found that the 0.4 mass%Mo-steel powder had both the highest densification and deformability among the powders of 0.4, 1, 2 and 4 mass%Mo without precipitation of the unneeded ternary molybdenum carbide Fe3Mo3C. Also, when sintered specimens were cold-forged, the impact waveform of those showed clearly effects of both the work hardening and the damage of the microstructure. Finally, both the superior fatigue strength and the wear resistance were implemented with the proper processes to 0.4 mass%Mo sintered steel such as a second sintering, vacuum carbonitriding heat treatment and shotblast. Obtained properties were equal to or more than those of wrought steel SCr420H.

Numerical simulation for the effect of scanning speed and in situ laser shock peening on molten pool and solidification characteristics

The unique thermal cycle of selective laser melting (SLM) significantly affects the undesirable formability and mechanical properties of the deposited parts, especially for materials with complex compositions. Laser shock peening (LSP) is a strengthening technology that can refine grain, convert tensile stress to compressive stress, and improve fatigue strength. In situ LSP is a technology that combines LSP and SLM without ablative coating. The combination can strengthen the additive manufacturing microstructure layer by layer. Some literature has verified the feasibility of no absorption layer and pressure confining layer LSP. However, little research reported the effects of the in situ combination on the molten pool. In this work, the finite element method (FEM) has systematically investigated the impact of scanning speed and in situ LSP on fluid flow behavior, heat transfer, and the solidification process of the molten pool. The flow velocity and the size of the molten pool decrease as the scanning speed increases. The solidification rate shows an increasing-decreasing-increasing process at low scanning speed during the solidification process. Moreover, the value of R is consistently stable at high scanning speeds. The temperature gradient increases gradually and decreases sharply with the scanning speed increase. The in situ LSP reduces the temperature and the fluid flow of the molten pool, which decreases the heat convection and the value of Peclet number, but has little effect on the solidification process of the molten pool.

Fatigue strength of shot-peened as-welded joints and post-weld-treated and subsequently clean-blasted fillet weld joints

Surface treatment methods, such as shot peening and clean blasting, can improve the fatigue strength of welded joints. The aim of the current work is to experimentally evaluate the effects of clean blasting on the fatigue performance of post-weld-treated joints made of normal, high-strength, and ultra-high-strength steels, as well as obtain the fatigue strength improvement gained by the shot peening processes in welded connections made of different steel grades using literature data. A literature review was carried out to extract fatigue test data of welded joints, considering both clean blasting and shot peening processes. This data was statistically re-evaluated and used for a comparison to the results obtained in the current experimental work which focuses on evaluating the improvement gained by the clean blasting after post-weld treatments. Experimental fatigue tests were carried out on non-load-carrying transverse attachment joints prepared with gas metal arc welding, and made of S355, S700, and S1100 structural steels. TIG dressing and HFMI treatment post-weld treatments were implemented and, subsequently, the specimens were blast-cleaned using two different abrasives: corundum and sand. The re-analysis of existing fatigue test data indicated higher improvement by shot peening, i.e., average improvements of 1.71 and 1.52 in the fatigue strength for butt weld and fillet weld joints, respectively, than by clean blasting process for which an average improvement of 1.19 in fatigue strength was obtained. The enhancement factors, however, highly varied among different data sets indicating a clear impact of processing parameters on the improvement level. The statistical re-analyses considering all data sets of shot-peened specimens (butt weld and fillet weld joints) showed that one fatigue (FAT) class higher fatigue strength could be recommended for shot-peened joints compared to the as-welded condition with weld toe failures. The experimental work on the post-weld-treated joints indicated that the fatigue strength of clean-blasted joints was similar to that of non-blasted joints, and thus showing no major advantages or disadvantages by the clean blasting post-weld-treated joints with corundum and sand.

Achieving superior fatigue strength in a powder-metallurgy titanium alloy via in-situ globularization during hot isostatic pressing

Powder-metallurgy (PM) titanium alloys exhibit outstanding quasistatic-mechanical properties, but suffer from low fatigue performance, which severely limits their applications in aerospace. Here, we achieve a superior fatigue strength of 600 MPa in a near-α PM titanium alloy, using a two-step hot-isostatic-pressing scheme, during which more than 80 vol.% (volume fraction) randomly orientated equiaxed grains was obtained. The largely improved fatigue strength (∼ 25%) is mainly attributed to the in-situ globularization of the lamella-like microstructure, leading to higher crack nucleation resistance and lower growth rates of short cracks. The present findings offer a useful route for fabricating PM titanium alloys with high fatigue strengths.

Verification of the Maximum Stresses in Enhanced Welded Details via High-Frequency Mechanical Impact in Road Bridges

High-frequency mechanical impact (HFMI) is an efficient post-weld treatment technique that enhances fatigue strength in metallic welded structures. Steel or steel-concrete composite road bridges, where the fatigue limit state often governs the design, compose one category of structures that can benefit from the application of this technology. To assert an improvement in fatigue strength using HFMI, the induced compressive residual stresses must be stable. Therefore, the maximum service stresses that can be allowed on HFMI-treated joints should be controlled to avoid the relaxation of the induced beneficial compressive stresses by HFMI treatment. Using statistical analysis of recorded traffic, this paper compares the measured maximum traffic loads to those generated by a load model. More than 870,000 and 470,000 recorded vehicles from traffic measurements in Sweden and the Netherlands are used in this analysis. To capture the characteristic bending moment, the daily maxima of the resulting measured load effect are combined with the extreme value distribution of the bending moment. In addition, it is found that the characteristic load combination is the best-studied option to assess the maximum stress in HFMI-treated weldments in road bridges.

Fatigue strength improvement method for thin sheet GMA welding joints by weaving

Fatigue strength improvement method for thin sheet GMA welding joints by weaving

Dual precipitate simultaneous enhancement of tensile and fatigue strength in (FeCoNi)86Al7Ti7 high-entropy alloy fabricated using selective laser melting

Recent studies have indicated that precipitation-strengthened high-entropy alloys (HEAs) show superior mechanical performance and have been successfully fabricated by additive manufacturing. However, the lack of fatigue and fracture research has limited the engineering applications of additive manufacturing HEAs. This work explored a dual precipitation-strengthened (FeCoNi)86Al7Ti7 HEA with excellent tensile and fatigue strength, prepared through selective laser melting and heat treatment. Compared with the as-built samples, the tensile properties and fatigue endurance limit improved through aging by 48.7% and 30%, respectively. The strengthening mechanism and enhanced fatigue performance were clarified in detail. The improvement in fatigue strength was attributed to the improved resistance of the L12 and L21 precipitates. During deformation, the dislocation shear coherent L12 precipitates reduced slip band energy and inhibited slip band expansion, while the L21 particles acted as obstructions for further slip band propagation, severely limiting the rapid formation and propagation of crack growth. In-situ TEM cyclic tensile-tensile testing also clarified the fatigue crack growth behavior, demonstrating that crack deflection due to L21 precipitate obstruction slowed down the crack growth rate and efficiently promoted the closure of the microcrack tips. This work offers implications for a new strategy to develop additive manufacturing HEAs.

Fatigue life improvement of similar and dissimilar aluminum friction stir welds by deep rolling

At the moment, high production costs prevent the friction stir welding (FSW) process from further industrial applications even if comparable high fatigue strength of the joints can be reached. Higher welding speed may reduce the production costs but decrease the fatigue strength of the manufactured FSW joints. A potential solution is a mechanical post weld treatment directly after welding to increase the fatigue strength of the FSW joint again. In this study, hydrostatic deep rolling was applied for fatigue strength improvement of similar and dissimilar FSW joints made of EN AW 5083 and EN AW 6082 alloys manufactured with different welding speed. Additionally, the fatigue strength was directly compared to conventional joints manufactured by metal inert gas (MIG) welding and to base material specimen made of EN AW 5083. The surface state in as-welded condition was characterized by surface roughness and residual stress measurements. Fatigue tests were performed to quantify the fatigue strength of the FSW joints. Similar compressive residual stresses were determined after deep rolling for similar and dissimilar joints. No fatigue life improvement was determined for deep-rolled similar joints made of EN AW 5083. However, in this condition, the fatigue life of the specimen was within the range of the base material. Thus, significant lower fatigue life and a high fatigue life improvement by deep rolling were reached for dissimilar joints. An increase of the welding speed from 300 to 800 mm/min strongly decreased the fatigue strength of dissimilar welded joints in the investigated case.

Effects of building direction and loading mode on the high cycle fatigue strength of the laser powder bed fusion 316L

The present study aims to investigate the high cycle fatigue (HCF) performance of steel 316L fabricated by the laser powder bed fusion (LPBF) process. Bending and torsional fatigue test specimens built horizontally (0°), inclined (45°), and vertically (90°) have been prepared and tested. Stress-relieving heat treatment was carried out to reduce the residual stresses. The 90°, 45°, and 0° built near-net-shape and polished specimens have fatigue strengths between 140 and 250 MPa under bending loading and 150 and 170 MPa under torsion loading. This difference illustrates a more pronounced defect sensitivity in bending compared to torsion. Fractographical analyses revealed the fatigue failure mechanisms. As the building direction inclines from vertical to horizontal, the effective areas of inherent defects diminish contributing to improving fatigue strength under bending loading whilst the differences from building directions in torsional fatigue strengths are minor. Microstructural features are seen to compete with inherent defects to affect fatigue performance in the condition that the effective defect sizes are close to the critical fatigue crack size.

切削摩擦加工材の疲労特性に及ぼす負荷形式の影響

Aiming for a result to improve fatigue strength, “cutting and rubbing process” which is a kind of surface treatment was used for forming a round-bar smooth-specimen, showing drastically enhanced rotating-bending fatigue strength. On the other hand, actual machine components and structures invariably contain geometrical irregularities, such as fillets, keyways, screw threads, and holes. Fatigue cracks initiated from such stress raisers, indicating that the validity of the cutting and rubbing process on fatigue strength of specimen with a stress raiser should be examined. In this study, fatigue tests of smooth and notched specimens finished by the cutting and rubbing process were conducted under some loading types including rotating-bending, tension-compression and torsion loading. Results showed that fatigue strength of notched specimens was significantly enhanced compared to that of smooth specimens under rotating-bending and tension-compression loads. In other words, the cutting and rubbing processing was more effective in stress raisers than the smooth surface. The physical background on the effect of loading type on the fatigue strength of the cutting and rubbing processed specimen was discussed based on the fracture origin, microstructure in the surface layer, stress gradient caused by both loading type and notch.

Study on fatigue strength improvement of nuclear turbine by laser peening

Study on fatigue strength improvement of nuclear turbine by laser peening