Enhanced Fatigue Properties Research Articles

Enhancing tensile and fatigue properties of high-speed-extruded Mg–5Bi–3Al alloy using non-homogenized extrusion billet

This study aims to improve the tensile and high-cycle fatigue properties of a high-speed-extruded BA53 (Mg–5Bi–3Al, wt.%) alloy using a non-homogenized extrusion billet. Homogenization treatment of the BA53 billet leads to a significant reduction in the area fraction of Mg3Bi2 particles from 12.9% to 2.5% and an increase in the average grain size from 142 to 189 μm. The BA53 material produced via the high-speed extrusion of the cast billet (Cast-E) and that produced via the high-speed extrusion of the homogenized billet (Homo-E) both exhibit a fully recrystallized microstructure. However, Cast-E has a finer grain size and many more Mg3Bi2 particles than Homo-E. In addition, the tensile yield strength and elongation of Cast-E are 211 MPa and 9.4%, respectively, higher than those of Homo-E (198 MPa and 7.9%). This higher strength is attributed to the strengthening effects of the higher number of Mg3Bi2 particles, the finer grain size, and the higher internal strain energy. The fatigue limit for Cast-E is 100 MPa, which is 25% higher than that of Homo-E (80 MPa). This occurs because the numerous fine Mg3Bi2 particles in Cast-E inhibit the formation of persistent slip bands on the specimen surface and impede crack propagation. This study proposes a new manufacturing process that does not require billet homogenization, thus simultaneously improving the productivity of the extrusion process and the tensile and fatigue properties of the resulting BA53 alloy.

Compressive residual stress applied to a low-carbon steel surface alloyed with WC tool constituent elements according to friction stir processing

Friction stir processing (FSP) effectively improves the fatigue strength of arc-welded joints; however, wear of tools is inevitable in case of high-strength materials. Notably, a new benefit has been discovered: compressive residual stress is applied on the FSPed steel surface, alloyed with the WC tool constituent elements, contrasting with the tensile residual stress typically applied via conventional FSP. To elucidate the mechanism of compressive residual stress application, FSP was performed on a low-carbon steel plate at various rotational speeds. The alloyed topmost layers in the stir zone comprised martensite structures with a small amount of retained austenite grains, resulting in a hardness increase owing to the tool constituent elements. The residual stresses on the stir zone surface were influenced by the alloying contents and the corresponding martensite start temperature (Ms). Compressive residual stresses were maximized at an Ms of approximately 150 °C owing to martensitic transformation expansion near room temperature. Lowering the Ms below approximately 150 °C led to tensile residual stresses and an increased volume fraction of the retained austenite, suggesting that martensitic transformation expansion is insufficient to apply compressive residual stress. Conversely, the retained austenite can resist plastic deformation and crack propagation through deformation-induced martensitic transformation, thereby enhancing fatigue properties.

Structure, ferroelectric, and enhanced fatigue properties of sol–gel-processed new Bi-based perovskite thin films of Bi(Cu1/2Ti1/2)O3–PbTiO3

Bi-based perovskite ferroelectric thin films have wide applications in electronic devices due to their excellent ferroelectric properties. New Bi-based perovskite thin films Bi(Cu1/2Ti1/2)O3–PbTiO3 (BCT–PT) are deposited on Pt(111)/Ti/SiO2/Si substrates in the present study by the traditional sol–gel method. Their structures and related ferroelectric and fatigue characteristics are studied in-depth. The BCT–PT thin films exhibit good crystallization within the phase-pure perovskite structure, besides, they have a predominant (100) orientation together with a dense and homogeneous microstructure. The remnant polarization (2P r) values at 30 μC/cm2 and 16 μC/cm2 are observed in 0.1BCT–0.9PT and 0.2BCT–0.8PT thin films, respectively. More intriguingly, although the polarization values are not so high, 0.2BCT–0.8PT thin films show outstanding polarization fatigue properties, with a high switchable polarization of 93.6% of the starting values after 108 cycles, indicating promising applications in ferroelectric memories.

Enhancing the fatigue property of high entropy alloy by regulating characteristics of grains and precipitates

High entropy alloys (HEAs) are a new class of alloys with many excellent mechanical properties, having the potential as structural materials. One advantage of HEAs is the vast space for designing compositions and microstructures, which provides more possibilities to develop fatigue-resistant materials. In this study, a multi-phase fine-grained microstructure was engineered through cold rolling and annealing treatment in Al0.3CoCrFeNi HEA to obtain an enhanced fatigue property. This alloy consists of FCC matrix phase with an average grain size of ∼ 1.13 ± 0.84 μm, and B2 and σ precipitates with sub-micron scale. Their phase fractions are ∼ 90 %, ∼ 9 % and ∼ 1 %, respectively. Stress-life (S-N) data indicate that fatigue endurance limit and ratio in terms of stress amplitude are ∼ 281 MPa and ∼ 0.3, based on stress ratio of 0.1. Good fatigue property is related to high ultimate tensile strength derived from good work-hardening ability and fine microstructure. In addition, activation of deformation twins, blunting of microcracks and deflection of fatigue cracks associated with precipitates are beneficial for enhancement of fatigue resistance. The above results provide theoretical guidance on how to enhance fatigue resistance of HEAs by regulating microstructures.

3D hot laser shock peening without coating based on Leidenfrost effect enables self-armored hydrophobic surface with enhanced fatigue properties

Hydrophobic metal surfaces have widespread applications in self-cleaning, ice removal, corrosion resistance, and drag reduction. The fine structure of these surfaces is crucial for their hydrophobic properties. Currently, pulse laser direct surface texturing is commonly employed to create hydrophobic surfaces, but this method primarily operates at room temperature. However, the rapid cooling rate and significant temperature gradient of the thin melted layer generated at room temperature often result in high stress concentration and cracks within the fine structure, compromising the overall integrity of the hydrophobic surface when subjected to complex external loads. To assess structural robustness by evaluating crack propagation, fatigue performance serves as a vital indicator. The thermal effect helps mitigate thermal cracking caused by direct laser irradiation and introduces an additional strengthening mechanism through dynamic strain aging. By utilizing the Leidenfrost effect, water can act as a confinement layer to achieve high temperature laser shock, which is expected to enhance the fatigue performance of hydrophobic structures. In this study, we propose a technique called 3D hot laser shock peening without coating (3HLSPwoC) as a cost-effective and efficient method to fabricate multi-scale self-armored hydrophobic surfaces with enhanced fatigue properties. We systematically discuss and analyze the wettability and mechanical properties of samples manufactured using different approaches. Furthermore, we reveal the underlying mechanism responsible for the fatigue performance improvement achieved through the 3HLSPwoC process, supported by molecular dynamics calculations and finite element simulations. Compared with the samples manufactured at room temperature, the surface of 3HLSPwoC samples exhibit greater plastic deformation, with no visible cracks on the surface. Additionally, these samples possess higher compressive residual stress and hardness. Notably, 3HLSPwoC samples demonstrate superior fatigue performance and enhanced durability of the hydrophobic properties. 3HLSPwoC has proved to be a novel process for manufacturing advanced hydrophobic surfaces with comprehensive mechanical properties.

High/very-high fatigue properties and microstructure evolutions of 9Cr3W3Co turbine rotor steel at room temperature and 650 °C

The study aimed to investigate the high and very-high fatigue properties of 9Cr3W3Co turbine rotor steel at room temperature (RT) and 650 °C. The resulting S–N curves demonstrated a continuous downward trend, without exhibiting a conventional fatigue limit. The fatigue strength corresponding to 5 × 107 cycles is approximately 53.9% of yield strength at RT, increasing to around 55.4% at 650 °C. Meanwhile, compared to the fatigue strength at 5 × 107 cycles at RT, it at 650 °C decreased by 53.3%. Surface crack initiation was found to be the main fatigue fracture failure model, observed at both RT and 650 °C. Microscopic analysis revealed that severe dislocation motion and grain deformation occurred within the material matrix during the cyclic loading process at both RT and 650 °C, resulting in the formation of abundant low-angle grain boundaries, dislocation lines and sub-grains. At RT, the presence of the M23C6 precipitated phase imposed a strong pinning effect, while the MX phase within the martensite laths effectively impeded the movement of dislocation lines, thereby enhancing fatigue properties. However, at 650 °C, the emergence of newly precipitated Cu-rich phases was observed, which can reduce the free path of dislocations, hinder slip motion of dislocations, and thus compensate for the negative effect caused by high-temperature microstructural degeneration.

Enhancing fatigue performance of TiN-coated Ti-6Al-4V by N/Ti ratio modulation

In this paper, we propose an approach to enhance fatigue properties of TiN-coated Ti-6Al-4V for aero-engine compressor blades by N/Ti ratio modulation of TiN. TiN coatings with N/Ti ratios of 0.70, 0.85, and 0.92 were prepared by modifying the nitrogen flow during coating deposition. The resulting coatings were evaluated for surface roughness, phase structure, microstructure, hardness and residual stress. Furthermore, the rotating bending fatigue test indicated that TiN coatings at different N/Ti ratios negatively affected the fatigue properties of Ti-6Al-4V. Specifically, the fatigue limit of the substrate dropped from 531 MPa to 480 MPa, 510 MPa, and 440 MPa when coated with TiN coatings at N/Ti ratios of 0.70, 0.85, and 0.92, respectively. However, the high-temperature insulation process during coating preparation enhanced the substrate's fatigue performance. The reduced fatigue performance of TiN coated Ti-6Al-4V was attributed to the cracking of TiN, led to the fatigue crack initiation on the substrate surface and accelerated fatigue failure. In the fatigue crack initiation stage, the TiN coating's higher stress concentration factor accelerated crack initiation by inducing stress concentration, reducing initiation cycles. During fatigue crack growth, the coating's residual compressive stress hindered propagation and reduced crack length caused by coating fracture in the substrate.

Fracture mechanism of aluminum/steel laser-arc welded-brazed joints during quasi-static tensile and dynamic fatigue tests

The primary focus of this study was the fracture mechanism of aluminum/steel laser-arc welded-brazed joints under quasi-static tensile and dynamic fatigue conditions. Digital image correlation (DIC) was applied to record the tensile and fatigue fracture process, while finite element method (FEM) was employed to analyze stress distributions during fatigue tests. The tensile results indicated that the weld reinforcement changed the strain evolution and significantly improved mechanical properties of aluminum/steel joints. During fatigue tests, the weld reinforcement enhanced fatigue properties and changed the joint geometry of aluminum/steel joints, leading to the emergence of distinct failure behaviors. At larger stress loads, the maximum cyclic stress on the interface surpassed the interfacial bonding strength, which resulted in a gradual shift of strain concentration from the weld toe to the interface. At smaller stress loads, the interfacial region exhibited tremendous crack resistance, and strain gradually accumulated in the weld toe, which led to fatigue damage. Interestingly, the strain in the interfacial region became more significant during the fatigue process after removing the weld reinforcement. Overall, this study shed light on the fracture mechanism of aluminum/steel joints under different loading conditions. Knowledge of this mechanism is essential for developing reliable aluminum/steel structures.

Enhanced fatigue properties of AA5086 friction stir weld joints by Cu-reinforcement

AA5086-H116 plates were welded, with and without copper (Cu)-reinforcement, using friction stir welding. Cu-reinforcement during the solid state welding process developed a localized composite (AA5086/Cu) which was processed using single-pass and double-pass, both. A significant improvement of ∼15%, ∼13% in the microhardness, and ∼95%, ∼98% in the weld joint efficiency was achieved during the single-pass and double-pass of the Cu-reinforced joints, respectively. The uniaxial load-controlled cyclic tests under fully-reversed conditions were performed in ambient environment to study the fatigue response of the welded joints. The fatigue strength of single-pass (=65MPa) and double-pass (=65MPa) Cu-reinforced weld joints was improved by ∼18%, as compared to the simple weld joints (=55MPa) without any reinforcement. The location of failures in the samples tested under fatigue loading was shifted from thermo-mechanical affected zone in simple weld to base material in the double-pass Cu-reinforced welded joints. A deeper analysis of broken specimens revealed the role of surface irregularities, residual stresses and microstructural inhomogeneity in the improved fatigue properties and the associated damage mechanisms, which are shown and discussed in this study.

Rotary bending fatigue behavior of a rare earth addition bearing steel: The effects of a gradient nanostructured surface layer formed by surface mechanical rolling treatment

A 700 μm-thick gradient nanostructured surface layer with enhanced hardness and compressive residual stress was produced on a rare earth addition bearing steel by surface mechanical rolling treatment under fine lubrication and cooling. Rotary bending fatigue tests showed that fatigue properties are significantly enhanced in the surface modified samples under the internal-induced failure mode, with the fatigue strength increasing from 1050 MPa in the as-received counterparts to 1200 MPa in them at a fatigue life of 107 cycles. Analyses revealed that the enhanced fatigue properties are related with the transfer of crack initiation site to a deeper region in the surface layer. And the formation of a thick gradient nanostructured surface layer with enhanced hardness and compressive residual stress contributes to the enhancement of fatigue properties of bearing steel.

Fatigue Properties of Weakened and Non-weakened Roots Restored with CAD-CAM Milled Fiber Post, Prefabricated Fiber Post, or Cast Metal Post.

Customized glass fiber posts using CAD-CAM technology have been suggested for restoring endodontically treated teeth. However, how weakened or non-weakened roots restored with anatomical CAD-CAM posts behave under cyclic fatigue is not clear. To evaluate the load-bearing capacity under fatigue (fatigue failure load [FFL], the number of cycles for failure [CFF], and survival probabilities) and fracture pattern of weakened and non-weakened roots restored with CAD-CAM fiber post and cores, metal cast-post-core, and prefabricated fiber post and resin core. A total of 60 crack-free bovine incisor roots (13 mm in length) with standard geometry were obtained and randomly allocated considering the factor "root condition" in two levels (weakened and non-weakened). Thus, half of the roots were weakened to obtain a wall thickness of 0.5 mm. After that, the endodontic treatment was executed, all roots embedded with acrylic resin and the specimens randomly allocated (n=10) considering the factor "post system" in three levels (CAD-CAM: CAD-CAM milled glass-fiber post and core; MBC: metallic-based post and core; and FRC: prefabricated glass-fiber post and composite resin core). The posts were luted with a dual-cure self-adhesive luting agent. Then, all teeth received a metallic crown. An initial load of 100 N at 20 Hz for 5000 cycles was applied for the step-stress fatigue test, followed by incremental steps of 50 N for 20,000 cycles each step, up to failure. A fracture pattern analysis was performed. CAD-CAM fiber post (FFL: 865 N; CFF: 311,000 cycles) presented similar fatigue performance (p>0.05) to FRC (FFL: 925 N; CFF: 335,000 cycles), with 100% of repairable fractures for non-weakened roots; however, both groups presented worse performance than MBC (p<0.05; FFL: 1265 N; CFF: 471,000 cycles) which led to 100% of catastrophic failures. No statistical difference was found in fatigue performance among the three systems for weakened roots (p>0.05; FFL: 1035-1170 N; CFF: 379,000-433,000 cycles), with a high rate of catastrophic failures. CAD-CAM fiber post presented similar fatigue performance to MBC and FRC approaches when restoring weakened roots. CADCAM was similar to FRC when restoring non-weakened roots, while MBC enhanced fatigue properties in this scenario.

A critical comparative review of cavitation peening and other surface peening methods

Mechanical surface modification such as shot peening offer powerful enhancement of fatigue properties of metals and other materials. Cavitation usually causes surface damage in hydraulic machineries. However, careful selection of process parameters allowed developing an approach known as “cavitation peening.” Its advantage is surface roughness increase is lower than in conventional shot peening as there are no solid collisions involved. As cavitation is a hydrodynamic phenomenon, an understanding of both fluid dynamics and materials science is required. Cavitation peening is distinguished from “water jet peening,” in which water column impulse is used. Another flavor is “Submerged laser peening” that involves the use of a pulsed laser and can be considered a type of cavitation peening where cavitating bubbles are generated due to laser ablation. Ultrasound vibration, a popular method for generating cavitation for cleaning, has also been adapted for cavitation peening. The present comparative review presents key insights and achievements and addresses future directions that are required for advancing cavitation peening technology by considering the mechanisms of cavitation peening based on the reported data for water jet, pulsed laser, and ultrasonic cavitation peening. The data and methods are critically considered and summarized in comparison with shot peening. Strategic view of future challenges is presented. • Fatigue resistance enhancement using water jet, pulsed laser and ultrasonic cavitation peening was quantified • Cavitation peening using jet, pulsed laser & ultrasonic enhanced fatigue properties • Cavitation peening clearly distinguished from water jet peening • Cavitation peening benefits shown to exceed those by shot peening • Dislocation density induced by cavitation peening was lower than by shot peening • Importance of vortex cavitation is shown for experimental and numerical future work

Enhanced fatigue property by fabricating a gradient nanostructured surface layer in a reduced-activation steel

Reduced activation ferritic/martensitic (RAFM) steels have been selected as candidate structural materials for future advanced nuclear power systems. In the present work, the influence of a gradient nanograined surface layer on the fatigue properties of RAFM steels was studied. A gradient nanostructured (GNS) surface layer with a thickness of ∼85 ​μm was prepared on RAFM steel utilizing surface mechanical rolling treatment (SMRT). The mean grain size was approximate 43 ​nm at the topmost surface and increased gradually with depth. The results of the stress-controlled tension-compression fatigue experiments showed that the fatigue life enhanced approximately 6 times in the SMRT samples compared to the corresponding base metal counterparts. The relationship between the applied stress amplitude and the fatigue lifetime, and the fracture morphology showed that the surface strengthening and strain delocalization were caused by GNS, which suppressed surface crack initiation process, and hence the fatigue properties of RAFM steels improved. In addition, the deformation compatibility in GNS and coarse-grained boundaries leading to more dislocation interactions and accumulation during the cyclic process, also plays a crucial role in enhancing the fatigue properties of RAFM steel.

Controllable Martensite Transformation and Strain-Controlled Fatigue Behavior of a Gradient Nanostructured Austenite Stainless Steel.

Gradient nanostructured (GNS) surface layer with a controllable martensite fraction has been synthesized on 316L austenitic stainless steel by means of surface mechanical rolling treatment (SMRT) with temperature being controlled. The mean grain size is in the nanometer scale in the near-surface layer and increases gradually with depth. In addition, the volume fraction of martensite decreases from ~85% to 0 in the near-surface layer while the SMRT temperature increases from room temperature to 175 °C. Fatigue experiments showed that the strain-controlled fatigue properties of the GNS samples are significantly enhanced at total strain amplitudes ≥0.5%, especially in those with a dual-phase surface layer of austenite and pre-formed martensite. Analyses on fatigue mechanisms illustrated that the GNS surface layer enhances the strength-ductility synergy and suppresses the formation of surface fatigue defects during fatigue. In addition, the dual-phase structure promotes the formation of martensite and stacking faults, further enhancing fatigue properties at high strain amplitudes.

Influence of heat treatment on the high-cycle fatigue properties and fatigue damage mechanism of selective laser melted AlSi10Mg alloy

The high-cycle fatigue properties and fatigue damage mechanism of AlSi10Mg alloy manufactured by selective laser melting (SLM) were investigated. The influence of the post-heat treatment on the microstructure and high-cycle fatigue properties was also examined. The SLM-manufactured AlSi10Mg alloy was heat-treated under two different conditions, namely T6 and direct aging (DA). The results of the high-cycle fatigue tests, which were carried out at room temperature, imply that, under all three conditions (as-built, T6, DA), the fatigue strength decreases as the number of cycles increases along with the estimated endurance limit. Predominantly, the fatigue properties were influenced by the morphology and size of the Si particles, both controlled by adjusting the heat treatment. The alloy that was processed using DA heat treatment had superior fatigue properties due to its cellular structural morphology characterized by a significant amount of fine Si precipitates. Compared with the as-built and T6 specimens, the superior fatigue limit of the DA alloy is attributable to its outstanding strength characteristics. The reliability of the fatigue life of the DA alloy is relatively high. Fatigue fracture samples revealed that the DA alloy had evenly distributed dislocations along the cellular boundaries. The fine Si particles were found to play an important role in preventing fatigue crack propagation, thereby enhancing fatigue properties. The effect of the heat treatment on the microstructure and mechanical properties of the SLM AlSi10Mg specimens was investigated with a specific focus on fatigue strength. The results highlighted that an appropriate DA process could significantly enhance the fatigue performance of the AlSi10Mg alloy under investigation. The resulting characteristics are highly advantageous and are superior to those obtained with other conventional heat treatment processes.

Effect of laser shock peening on high cycle fatigue properties of aluminized AISI 321 stainless steel

In this work, the effect of laser shock peening (LSP) treatment on fatigue property of aluminized AISI 321 stainless steel was studied. The high fraction twinning and compressive residual stress (CRS) induced by LSP at the severe plastic deformation (SPD) layer were found to be responsible for the enhanced fatigue property. De-twinning was conspicuously noticed in LSP sample. Moreover, the dynamic recovery (DRV) was only noticed at SPD layer during cyclic deformation related to CRS. In sum, the aluminized stainless steel achieved an enhanced fatigue property by LSP treatment.

The potential of mechanical twinning in ultrafine retained austenite to enhance high cycle fatigue property of advanced bainitic steels

We have studied here the high cycle fatigue properties and sub-surface fatigue crack initiation mechanism in advanced bainitic steel subjected to conventional bainitic austempering (BAT) and novel “disturbed” bainitic austempering (DBAT) processes. The retained austenite in DBAT steel mainly exhibited ultrafine film-like morphology, whereas a considerable amount of coarse blocky retained austenite was present in the BAT steel. The study indicated that the DBAT steel had higher fatigue strength and endurance, compared to the BAT steel. The enhanced fatigue properties are attributed to mechanical twinning that occurred in film-like retained austenite at the sub-surface fatigue crack initiation area.

Enhanced Fatigue Property of Welded S355J2W Steel by Forming a Gradient Nanostructured Surface Layer

Welded joints are usually characterized by microstructural and compositional inhomogeneities, which may significantly degrade their fatigue properties and result in unpredictable failures. The present work demonstrates a novel and simple method to effectively optimize the microstructure in the surface layer and promote the fatigue properties of welded specimens. By a recently developed approach—surface mechanical rolling treatment (SMRT), a gradient nanostructured surface layer is formed on welded S355J2W steel specimens. The mean grain size is refined to nanometer scale, and the hardness is significantly enhanced in the SMRT surface layer. Independent of the initially inhomogeneous microstructure and hardness distributions, the microstructure and hardness distributions in the surface layers are comparable on different zones of a welded specimen after SMRT with the same procedure. Consequently, fatigue property of the SMRT specimens is significantly enhanced relative to that of the as-welded specimens within the high cycle fatigue regime.

Characterization of High-Cycle Bending Fatigue Behaviors for Piston Aluminum Matrix SiO2 Nano-composites in Comparison with Aluminum–Silicon Alloys

In automotive industries, one failure mechanism in engine pistons is due to the fatigue phenomenon. Therefore, to enhance fatigue properties of piston aluminum alloys is a major concern for designers. One reinforcement method could be the addition of nano-particles in the aluminum matrix. In this article, high-cycle fatigue properties of the aluminum matrix nano-composite were characterized under bending loadings and then compared to those of the aluminum–silicon alloy. For this objective, fully reversed bending fatigue tests were performed on standard specimens, based on the ISO-1143:2010 standard. Before testing, nano-composite samples were stir-casted by the addition of 1 wt% SiO2 nano-particles, and aluminum specimens were gravity-casted in a cast-iron mold. The microstructure of materials and the distribution of nano-particles in the aluminum matrix were evaluated by the optical microscopy and the field emission scanning electron microscopy. Experimental data indicated that nano-particles had a significant effect on the high-cycle fatigue lifetime. The reason for this improvement in high-cycle fatigue properties could be finer grains, higher hardness, the proper distribution of nano-particles in the aluminum matrix and stronger bonding strength at the Al/Si interface. However, based on fracture surfaces, all samples had the brittle behavior due to cleavage and quasi-cleavage marks.

Quarter Century Development of Laser Peening without Coating

This article summarizes the development of laser peening without coating (LPwC) during the recent quarter century. In the mid-1990s, the study of LPwC was initiated in Japan. The objective at that time was to mitigate stress corrosion cracking (SCC) of structural components in operating nuclear power reactors (NPRs) by inducing compressive residual stresses (RSs) on the surface of susceptible components. Since the components in NPRs are radioactive and cooled underwater, full-remote operation must be attained by using lasers of water-penetrable wavelength without any surface preparation. Compressive RS was obtained on the top-surface by reducing pulse energy less than 300 mJ and pulse duration less than 10 ns, and increasing pulse density (number of pulses irradiated on unit area). Since 1999, LPwC has been applied in NPRs as preventive maintenance against SCC using frequency-doubled Q-switched Nd:YAG lasers (λ = 532 nm). To extend the applicability, fiber-delivery of intense laser pulses was developed in parallel and has been used in NPRs since 2002. Early first decade of the 2000s, the effect extending fatigue life was demonstrated even if LPwC increased surface roughness of the components. Several years ago, it was confirmed that 10 to 20 mJ pulse energy is enough to enhance fatigue properties of weld joints of a structural steel. Considering such advances, the development of 20 mJ-class palmtop-sized handheld lasers was initiated in 2014 in a five-year national program, ImPACT under the cabinet office of the Japanese government. Such efforts would pave further applications of LPwC, for example maintenance of infrastructure in the field, beyond the horizons of the present laser systems.