Improvement Of Fatigue Strength Research Articles

Fatigue behavior of additively manufactured 316L stainless steel: Competition between the effects of defects and microstructure

Laser Powder Bed Fusion (LPBF) process, is becoming more and more widespread in industry. The possibilities of microstructural control offered by this process are an opportunity to study the contribution of the different length scales of microstructure to the fatigue behavior. This paper is devoted to the understanding of this fatigue behavior resulting from the interactions between the process induced defects and the different polycrystal length scales. Two distinct defect − microstructure competition regimes have been identified. The first concerns microstructures containing large Lack of Fusion (LoF) defects. These LoFs drastically reduce the fatigue life, while microstructure has no influence on the fatigue strength. The second regime concerns microstructures containing small defects. A limited effect of the polycrystalline microstructure was revealed. Furthermore, this paper demonstrates that the ratio between damage initiation defect size and grain size, used in literature to describe the defect and microstructure sensitivity of fatigue strength, is not applicable over a wide range of defects and microstructures, such as obtained by additive manufacturing processes. Finally, the comparison between the fatigue behavior of different microstructure and defect features shows that producing a finer microstructure improves fatigue strength despite the presence of a significant defect population.

Nanotwinning grain refinement induced by micro-needle peening in arc-welded ultra-high strength steel sheet

Generally, the fatigue strength of ultra-high strength steel (UHSS) and high strength steel (HSS) arc-welded joints are comparable regardless of base metal's strength. Still, the micro-needle peening (MNP) method can improve the fatigue strength to the level of those of base metals. To understand the mechanism of this improvement, this paper investigates the microstructure of UHSS (tensile stress grade of 980 MPa) arc-welded joints treated with MNP and compares it to HSS (tensile stress grade of 440 MPa) joints. We focus on the presence of nanotwins, which exhibited a minimum thickness of 4.7 nm, observed in the UHSS joints following the MNP treatment. Importantly, these nanotwins demonstrated remarkable stability even under cyclic loading conditions (nominal stress σn = 600 MPa, N = 3 × 106 cycles). This indicates that the nanotwins contribute to the significant improvement in fatigue strength demonstrated by MNP. However, the nanotwins were not observed in the HSS joints, suggesting sufficient driving stress is necessary for their occurrence. The dislocation pileup stress at the grain boundary during twinning was estimated by the thickness of the twin, which was 8.1 GPa. This value is of the same order of magnitude as the 3.7 GPa estimated by the Hall-Petch coefficient for ferritic steel. The lower levels of C, Si, and Mn can contribute to the lower pileup stress, resulting in absence of the nanotwins in the 440 MPa joint. Overall, this study provides insights into the microstructural changes induced by MNP treatment and their impact on the fatigue strength.

Improvement of Fatigue Strength in Additively Manufactured Aluminum Alloy AlSi10Mg via Submerged Laser Peening

Improvement of Fatigue Strength in Additively Manufactured Aluminum Alloy AlSi10Mg via Submerged Laser Peening

The Influence of Shot Peening Media on Surface Properties and Fatigue Behaviour of Aluminium Alloy 6082 T6

Shot peening is generally used to improve the fatigue performance of mechanical components. However, identifying the geometrical and mechanical characteristics of the shots that improve fatigue strength is still a challenging task, as there are many variables involved in the shot peening process. The present work addresses the effect of different shot media on the fatigue behaviour of an aluminium alloy 6082 T6. Four different shot types were used: silica microspheres, alumina shots, aluminium cut wire and zinc cut wire. Axial fatigue tests were carried out to obtain the Wöhler curves corresponding to each shot peening treatment. The surface properties of the shot-peened specimens, such as grain size, hardness, residual stress and roughness were measured to determine their effect on the fatigue results. The fatigue results revealed that silica and zinc shots increased significantly the fatigue life of the alloy, whereas alumina and aluminium shots reduced its fatigue strength. Almen intensities have shown to correlate well with grain refinement and strain hardening. However, better fatigue results were obtained with the shots that generated higher surface compressive residual stresses. It is believed that small and smooth shots are preferable to sharp and irregular ones, regardless of the Almen intensity or surface hardness attained with the latter.

Effect of peening treatment on fatigue strength of welded joints of aged steel

This study experimentally investigates the effect of peening treatment on the fatigue strength of welded joints of aged steel. Fatigue tests were conducted on T-shaped fillet welded joints made from three different types of aged steel under three-point bending loading conditions. To reveal fatigue strength improvement mechanism, residual stress measurement, hardness measurement, and metallographic analysis were performed at the weld toe for both as-weld and peened specimens. Fractured surfaces were examined to identify crack initiation sites and propagation. The findings reveal that peening treatment introduces compressive residual stress on the peened surface of welded joints of aged steel equal to or higher than conventional steel welded joints. Higher improvement of hardness was observed in the near surface of the peened welded joints of aged steel, and the improvement continued up to around the zero crossing points of the residual stress distribution. Additionally, peening treatment might refine near-surface grains of aged steel welded toe similar to conventional steel. The fatigue strength improvement effect of the peening treatment was the same for aged steel as conventional one. Cracks were observed on the peened surface of the aged steel due to over-peening treatment. However, it doesn't affect the fatigue strength improvement of the welded joints of aged steel. In summary, it can be said that peening treatment can improve fatigue strength of welded joints of aged steel in the same manner of conventional steel.

Effects of laminate stacking sequence on the strength properties of aluminum alloy–carbon fiber-reinforced plastic dissimilar single-lap adhesive joints

Here, two single-lap adhesive joints (SLJs) of dissimilar materials were subjected to static and cyclic loading tests. A2017 aluminum alloy was used as an adherend for one, and carbon fiber reinforced plastic (CFRP) was used as the adherend for the other. Four types of orthogonal laminated CFRPs with different laminate stacking sequences were used as adherends to investigate the effect of adherend stiffness on the strength properties of the joints. Furthermore, the results of the finite element analysis of the dissimilar SLJs revealed that when the tensile load was applied to them, the out-of-plane deformation asymmetry increased with increasing difference in stiffness between both adherends. This asymmetry affected the peel and shear stress distributions. Furthermore, the experiments revealed that the static tensile strength of the SLJs increased with increasing stiffness of the CFRP adherend. Additionally, fracture simulation using cohesive-zone modeling (CZM) revealed that the SLJs with higher CFRP stiffness exhibited higher strength, qualitatively agreeing with the experimental results. CZM analysis and adhesion strain measurements during the tests indicated that failure occurred at the A2017 adherend–adhesive interface. In contrast, no differences were observed between the fatigue strengths of the different types of adherends in the short-life region, with a number of cycles to failure (Nf) being ≤ 2 × 105. However, in the long-life region, beyond Nf = 2 × 105, the SLJ bearing the unidirectional CFRP adherend exhibited lower fatigue strength than the others. The anodizing process on A2017 was found to improve fatigue strength by a factor of two or more.

Fatigue Strength Improvement of Laser-Directed Energy Deposition 316L Stainless Steel with In Situ Ultrasonic Rolling by Preliminary Investigation.

In this study, to improve the fatigue strength of the LDED (laser-directed energy deposition) 316L stainless steel, an in situ ultrasonic rolling technology is developed to assist the laser-directed energy deposition process (LDED-UR). The microstructural characteristics and fatigue behavior are comprehensively discussed. The results show that the average size of pores of the LDED-UR alloy is about 10.2 μm, which is much smaller than that of the LDED alloy (34.1 μm). Meanwhile, the density of the LDED alloy is also enhanced from 98.26% to 99.27% via the in situ ultrasonic rolling. With the application of the in situ ultrasonic rolling, the grains are transformed into fully equiaxed grains, and their average grain size is greatly reduced from 84.56 μm to 26.93 μm. The fatigue limit of the LDED-UR alloy is increased by 29% from 210 MPa (LDED alloy) to 270 MPa, which can be ascribed to the decreased porosity and the fine grains. In particular, the crack initiation site of the LDED alloy is located at the surfaces, while it is nucleated from the sub-surface for the LDED-UR alloy. This is mainly attributed to the compression residual stress induced by the in situ ultrasonic rolling. This research offers a valuable understanding of the failure mechanisms in additively manufactured metals, guiding the development of effective strategies to improve their fatigue threshold under severe operating conditions.

The Influence of Sample Thickness on the Bending Fatigue Performance of PBF-Lb 316L Material

Additive manufacturing, specifically Laser Powder Bed Fusion (PBF-LB), has gained prominence for its capability to produce complex near-net-shaped components. While PBF-LB offers advantages such as lightweight construction and cost-effectiveness, post-processing remains crucial to meet specific design requirements. This study investigates the post-processing technique of severe shot peening (SSP) on PBF-LB-manufactured 316L stainless steel, a material widely used for its favorable mechanical properties and corrosion resistance. The research focuses on the enhancement of bending fatigue properties through SSP treatment, examining the influence of material thickness on fatigue behavior. Comparative analysis reveals the effectiveness of SSP in significantly improving fatigue strength irrespective of variations in material thickness. Mechanical properties are explored for different thicknesses subjected to SSP treatment. Electron Backscatter Diffraction (EBSD) is employed to scrutinize the surface properties of the samples, providing knowledge on the microstructural changes induced by SSP. The study contributes to the understanding of the role of material thickness in the context of SSP treatment, offering a comprehensive exploration of the mechanical and fatigue characteristics of PBF-LB-manufactured 316L stainless steel.

Fatigue strength improvement of aluminum alloy with surface defect by ball burnishing

AbstractBall 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 residual stress was induced to a depth of 0.4 mm, with a reduction in surface roughness by 81%. Consequently, the fatigue strength increased by 22%. BB prevented the fatigue strength reduction caused by the 0.1 mm deep slit. The result indicated that BB could render the below 0.1 mm deep surface defect harmless in terms of fatigue strength. The maximum depth of surface defect that can be rendered harmless is 0.18 mm according to fracture mechanics, which is in agreement with the experimental results.

Penetration of oils into hair.

The objective of this work was to understand how triglyceride plant oils can deliver strength and softness benefits to hair by their penetration. These plant oils are complex mixtures of TAGs, so the initial studies performed were with pure TAGs and then these data compared to plant oils and their measured TAG compositions. LC-MS was used to identify the di and triglycerides in coconut oil, Camellia oleifera oil and safflower seed oil. Penetration of these plant oils and pure individual triglycerides was measured by a differential extraction method. Cross-sections of oils treated with 13C-labelled triolein were studied by NanoSIMS to visualize location of triglyceride inside hair. Fatigue strength was measured using constant stress to generate a survival distribution. Models of the lipid-rich cell membrane complex (CMC) were created with the equimolar ratio of 18-methyl-eicosanoic acid (MEAS), palmitic acid (C16:0) and oleic acid (C18:1). Penetration of the individual pure TAGs was confirmed for all chain lengths and degree of unsaturation tested with higher penetration for shorter chain lengths and unsaturated fatty acids. Detailed compositional analysis of selected plant oils showed a wide variety of TAGs and penetration was also demonstrated for these oils. NanoSIMS and modelling confirmed these TAGs are penetrating the lipid-rich CMC of hair and are interacting with the fatty acids that make up the CMC. All plant oils delivered a fatigue strength improvement by penetration into the CMC and it is proposed that these oils prevent formation and/or propagation of flaws in the CMC network that leads to breakage. Many plant oils with a wide range of triglyceride compositions can penetrate into hair and NanoSIMS data confirmed these oils partition into the lipid-rich cell membrane complex. Penetration studies of individual TAGs shown to be present in these oils confirmed TAGs of varying chain length can penetrate and there is a correlation between increased penetration efficacy and shorter chain lengths and presence of unsaturation in the fatty acid chains. All the oils studied delivered single fibre fatigue strength benefits.

Strengthening mechanism of abnormally enhanced fatigue ductility of gradient nanostructured 316L stainless steel

Structural materials with a good combination of high fatigue strength and high fatigue ductility are highly desirable in engineering applications. However, it remains a challenge to “break” the exclusive relationship between fatigue strength and fatigue ductility. The cylindrical 316L stainless steel specimens with a gradient nanostructured surface layer were prepared using surface mechanical rolling treatment (SMRT). Fully reversed strain-controlled tension–compression fatigue experiments indicated that the SMRT processed 316L specimens achieved simultaneous improvement of fatigue strength and fatigue ductility. The abnormal increase in fatigue ductility is attributed to the significant secondary cyclic hardening caused by the martensitic phase transformation and the transition in fatigue crack initiation mode. The martensitic phase transformation enhances the fatigue strength while reducing the proportion of plastic deformation in the controlled strain amplitude. The competition between the driving force and resistance of fatigue nucleation in different structural units results in the migration of fatigue crack initiation sites from the surface to subsurface, thereby prolonging the fatigue crack initiation life. This work reveals the synergistic strengthening mechanism of fatigue strength and fatigue ductility of a gradient nanostructured 316L stainless steel.

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.

Fatigue enhancement of welded thin-walled tubular joints made of lean duplex steel

In the present work, the fatigue strength and enhancement techniques of welded thin-walled (wall thickness of t = 2 mm) square hollow section (SHS) joints made of lean duplex stainless steel (grade EN 1.4162) are experimentally and numerically studied. Experimental fatigue tests were carried out on gas metal arc-welded tubular X-joints subjected to fully reversed (applied stress ratio of R = −1) constant amplitude in-plane bending moment. Geometrical improvement techniques were investigated by applying fully penetrating fillet welds and using structural reinforcements. The applications of high-frequency mechanical impact (HFMI) treatment were examined in the context of further enhancing the fatigue strength of the geometrically improved joints. In addition to the experimental work carried out in this study, the fatigue test data of welded thin-walled SHS and rectangular hollow section (RHS) joints were extracted from literature. Structural stress concentration factors were numerically and analytically obtained to validate the fatigue assessment model for welded thin-walled tubular joints using the structural hot-spot stress method. In addition, the effective notch stress (ENS) concept with the reference radius of rref = 0.05 mm was implemented. Compared to the joints in the as-welded state, fatigue strength improvements of 1.53–2.7 were found in the geometrically improved joints. On the other hand, the HFMI treatment did not lead to an additional increase in the fatigue strength capacity of the studied AW joints. The CIDECT guidelines for the structural hot-spot stress method were found conservative for thin-walled joints (t < 3 mm) and could be recommended for fatigue design.

Optimization of Mechanical Properties and Evaluation of Fatigue Behavior of Selective Laser Sintered Polyamide-12 Components.

In this paper, a comprehensive study of the mechanical properties of selective laser sintered polyamide components is presented, for various different process parameters as well as environmental testing conditions. For the optimization of the static and dynamic mechanical load behavior, different process parameters, e.g., laser power, scan speed, and build temperature, were varied, defining an optimal parameter combination. First, the influence of the different process parameters was tested, leading to a constant energy density for different combinations. Due to similarities in mechanical load behavior, the energy density was identified as a decisive factor, mostly independent of the input parameters. Thus, secondly, the energy density was varied by the different parameters, exhibiting large differences for all levels of fatigue behavior. An optimal parameter combination of 18 W for the laser power and a scan speed of 2666 mm/s was determined, as a higher energy density led to the best results in static and dynamic testing. According to this, the variation in build temperature was investigated, leading to improvements in tensile strength and fatigue strength at higher build temperatures. Furthermore, different ambient temperatures during testing were evaluated, as the temperature-dependent behavior of polymers is of high importance for industrial applications. An increased ambient temperature as well as active cooling during testing was examined, having a significant impact on the high cycle fatigue regime and on the endurance limit.

Improved Equivalent Strain Method for Fatigue Life of Automobile Aluminum Alloy

Improved Equivalent Strain Method for Fatigue Life of Automobile Aluminum Alloy

The effects of submerged laser peening, cavitation peening, and shot peening on the improvement of the torsional fatigue strength of powder bed fused Ti6Al4V produced through laser sintering

This study demonstrates the improvement in the fatigue strength of additive manufacturing (AM) metals such as laser-based powder bed fusion of metals by post-processing. Titanium alloy samples manufactured by powder bed fused (PBF) Ti6Al4V produced through laser sintering (LS), treated by submerged laser peening (SLP), cavitation peening (CP), and shot peening accelerated via a water jet (SPwj), were subjected to torsional fatigue testing and compared with the as-built specimen. At SLP, the samples were treated by laser ablation (LA) and laser cavitation (LC) which was developed following LA. A cavitating jet was used for CP. For comparison, conventional post-processing using SPwj was also performed. To characterize the microstructural modification caused by the three post-processing methods, the cross-section of the treated surface was observed by electron backscatter diffraction. The fatigue strengths at 107 cycles were found to be 217, 361, 313, and 285 MPa for the as-built, SLP, CP, and SPwj specimens, respectively. The primary factors contributing to fatigue strength improvement by post-processing were surface smoothing and the introduction of compressive residual stress. The experimental observations were used to derive correlation formulas to estimate the fatigue life improvement due to post-processing as the function of the surface roughness and surface residual stress.

Characterization of flexural fatigue behaviour of additively manufactured (PBF–LB) gyroid structures

Additive manufacturing (AM) holds remarkable potential for producing cellular materials with intricate structures and tailored mechanical properties. The study investigates the flexural fatigue behaviour of additively manufactured triply periodic minimal surface (TPMS) gyroid structures using laser powder bed fusion (PBF–LB) technique. The fatigue properties, especially the bending fatigue properties, of additively manufactured cellular structures are not well understood to date. The research aims to enhance understanding of bending fatigue in complex cellular geometries and assess the suitability of rotating bending tests. The PBF–LB process parameters were modified to study their impact on the specimen’s fatigue properties. The modified parameters led to increased surface roughness but significantly improved fatigue behaviour. This enhancement is attributed to a reduction in build defects, namely pores and finer grain size in thin-walled structures. The study also includes analysis of microstructure, hardness, surface roughness, and porosity of the specimens. The results indicate that optimizing process parameters for thin walled cellular structures can lead to substantial improvements in fatigue strength, at the expense of increased surface roughness. This finding offers practical insights for applications in which a rough surface finish may not be critical or even intentionally desired by the application. The research contributes to the understanding of additive manufacturing, cellular structures, and material testing, with potential implications for materials science and engineering applications.

Investigations on mechanical features and mechanical performance of rotary friction welded Al-Cu joint at optimized conditions

In this experimental investigation, rotary friction welding (RFW) was employed to fabricate dissimilar Al-Cu joints, and the RFW variables were rotated against the ultimate tensile strength (UTS) in an effort to strengthen the dissimilar AA1100 and pure copper (Al-Cu) joint using response surface methodology (RSM) with a three-factorial design. Following this, micro-hardness, metallurgical characterization, and fatigue properties studies were done on the dissimilar joints fabricated under the optimized conditions. As a result of this investigation, the UTS of 208 MPa was attained as a maximum as the rotation speed was set at 1800 rpm, the friction time was 10 s, and the forging load was 5 kN. The dissimilar joint exhibits a better fatigue strength of 98.5 MPa when compared to the AA1100 base metal. At optimized conditions, the feature of the fracture image was found to be brittle due to the development of new compounds in the weld interface. A significant dip in micro hardness (52 Hv) was noticed in the region that connects TMAZ and HAZ of the AA1100 side of the dissimilar joint, which was concluded to be the weakest zone as the dissimilar joints were fractured in the same location during the tensile test. The observed results benefit the automotive sector, especially when fabricating components that require effective thermal management and weight reduction, like electrical connectors, heat dissipation systems, and radiators. For heat exchanger applications, improved fatigue strength in dissimilar Al-Cu joints is important because it guarantees longer structural integrity and improved durability.

Analysis of the effect of deep-profile compressive residual stress induced by laser peening on fatigue strength of 7075 aluminum alloy

The effect of laser peening-induced compressive residual stress on the fatigue strength improvement of A7075 specimens was investigated. Rotary bending fatigue tests and residual stress distribution measurements using the sequential polishing method were conducted on untreated, shot-peened, and laser-peened specimens. The results revealed that laser peening provides a deeper compressive stress and a deeper fracture origin, resulting in higher fatigue strength. Furthermore, controlling the laser parameters could extend the depth of the compressive stress layer by approximately 5 mm, indicating the possibility of applying laser peening to thicker materials to improve the fatigue strength.

Fatigue performance of beta titanium alloy topological porous structures fabricated by laser powder bed fusion

The performance of porous β-titanium alloys is crucial for their diverse applications, with fatigue characteristics playing a pivotal role in shaping the design of these porous structures. While topological configurations have exhibited exceptional property, a systematic understanding of their fatigue performance is lacking in recent literature. This study reveals that at a porosity level of ∼75%, designed topologically optimized (TO) structures surpass rhombic dodecahedron (RD) structures in compressive yield strength by ∼4 times (149 ± 3 MPa vs. 38 ± 1 MPa). Furthermore, post-heat treatment further enhances the yield strength of TO structures by ∼15%, reaching 172 ± 3 MPa due to grain refinement and β phase stabilization. Moreover, the as-printed TO structure exhibits exceptional compressive fatigue endurance, enduring stress approximately ∼5 times higher than the as-printed RD structure at a cycle count of 106. Microstructure analysis after fatigue testing reveals a reduction in the α" martensitic phase and an increase in stress-induced ω phase in the heat-treated TO structure, contributing to a slight improvement in fatigue strength compared to the as-printed TO structure. The exceptional fatigue performance observed in LPBF-built beta-type titanium alloy within TO structures highlights the complex relationship between porous structure design, microstructure, compressive strength, and fatigue performance.