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On the application of laser shock peening for retardation of surface fatigue cracks in laser beam-welded AA6056
摘要： The present study aims to investigate the extent to which the fatigue behaviour of laser beam-welded AA6056-T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as-welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as-welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing.
关键词： fatigue crack,aluminium alloys,laser beam welding,ultrasonic crack tip diffraction,residual stress,laser shock peening
更新于2025-09-23 15:19:57
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Bionic Repair of Thermal Fatigue Cracks in Ductile Iron by Laser Melting with Different Laser Parameters
摘要： Nodular iron brake discs typically fail due to serious thermal fatigue cracking, and the presence of graphite complicates the repair of crack defects in ductile iron. This study presents a novel method for remanufacturing ductile iron brake discs based on coupled bionics to repair thermal fatigue cracks discontinuously using bio-inspired crack blocking units fabricated by laser remelting at various laser energy inputs. Then, the ultimate tensile force and thermal fatigue crack resistance of the obtained units were tested. The microhardness, microstructure, and phases of the units were characterized using a digital microhardness meter, optical microscopy, scanning electron microscopy, and X-ray di?raction. It was found that the units without defects positively impacted both the thermal fatigue resistance and tensile strength. The unit fabricated at a laser energy of 165.6+19?15 J/mm2 had su?cient depth to fully close the crack, and exhibited superior anti-cracking and tensile properties. When the unit distance is 3 mm, the sample has excellent thermal fatigue resistance. In addition, the anti-crack mechanism of the units was analysed.
关键词： bionic crack blocked unit,repair discontinuously,thermal fatigue crack,ductile iron,laser remelting
更新于2025-09-23 15:19:57
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The effect of manufacturing defects on the fatigue life of selective laser melted Ti-6Al-4V structures
摘要： The manufacturing defects introduced by selective laser melting typically lead to lower fatigue strength and a larger variation in fatigue life compared to conventionally manufactured structures. X-ray micro computed tomography (μCT) is used to characterize the porosity and lack of fusion defects in terms of population, morphology, dimension and location. The defect size and location are combined with the NASA/FLACGRO (NASGRO) fatigue crack growth model to predict the likely fatigue life, in which an effective initial crack length is defined using the cyclic plastic zone and the defect radius. An eXtended defect zone (XDZ) describing the propensity for local plasticity during fatigue around a defect has been shown through numerical analysis to be a good indicator of the ranking of the threat to fatigue caused by differently located manufacturing defects. This indicates that the effect of a defect, initial radius, r0, is likely to be pronounced when its center is within 2r0 of the surface and maximal when it lies just beneath the surface.
关键词： Additive manufacturing,High cycle fatigue (HCF),Fatigue crack initiation and growth,Digital printing,Defect tolerance method
更新于2025-09-23 15:19:57
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A new approach to correlate the defect population with the fatigue life of selective laser melted Ti-6Al-4V alloy
摘要： Microstructural features and defects arising from selective laser melting (SLM) determine the in-service performance of additively manufactured near-net-shape components. Here the grain type, shape, size and distribution were characterized using electron backscattered diffraction (EBSD). High-resolution synchrotron radiation X-ray computed tomography was used to quantify the population, morphology and dimensions of porosity and lack of fusion defects. For SLM Ti-6Al-4V alloy, the larger-sized defects in comparison with α′ grains are more important for crack initiation, typically leading to poor fatigue resistance and a pronounced variation in fatigue life. The fatigue strength was then evaluated in terms of the defect population using a combination of the statistics of extremes and the Murakami model. Finally, an extended Kitagawa-Takahashi fatigue diagram was established within the framework of defect-tolerant design, which includes a classical safe-life region and the defect-determined lifetime in the finite life region.
关键词： Defect tolerance assessment,Chapetti model,Fatigue crack initiation and propagation,Kitagawa-Takahashi diagram,Additive manufacturing
更新于2025-09-19 17:13:59
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Crack closure mechanisms in residual stress fields generated by laser shock peening: A combined experimental-numerical approach
摘要： Laser shock peening (LSP) is successfully applied to retard fatigue cracks in metallic lightweight structures by introducing specific, in particular compressive, residual stress fields. In this work, experiments and a multi-step simulation strategy are used to explain the fatigue crack retarding and accelerating mechanisms within these LSP-induced residual stress fields. Crack face contact is identified as main mechanism to retard the fatigue crack as the stress distribution changes and the stress intensity factor range decreases. Crack face contact is experimentally detected by load vs. crack opening displacement (COD) curves and scanning electron microscopy (SEM) of the crack faces, as well as during numerical simulations. The convincing agreement between experiment and simulation, especially regarding the specific crack face contact areas, allowed the proper evaluation of the stress intensity factors depending on the crack length. It is found that crack closure is indeed one of the main reasons for the efficient application of LSP for fatigue crack retardation. Furthermore, the occurrence of crack closure does not indicate a zero value stress intensity factor in complex residual stress fields, as the areas of crack face contact depend strongly on the LSP-induced compressive residual stresses.
关键词： Stress intensity factor,Laser shock peening,Residual stress,Fatigue crack growth,Crack closure
更新于2025-09-16 10:30:52
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Fatigue crack growth behavior of laser-shock processed aluminum alloy 2024-T3
摘要： Laser shock processing (LSP) is a surface modification technique aimed at enhancing the resistance to wear, corrosion and fatigue of structural alloys. Recently, LSP without coating (LSPwC) has been gaining ground, using lasers with lower energies, shorter pulse duration, smaller laser spots and higher surface coverage per shot. In the present work, LSPwC treatment was performed in both sides of pre-cracked compact tension specimens of aluminum alloy 2024-T3. A pulsed (9 ns) Nd:YAG laser system operating in the second harmonic (532 nm) at 10 Hz repetition rate and with pulse energy of about 270 mJ was positioned with a 500 mm focal distance lens in order to conduct LSPwC with an estimated power density of 5.2 GW/cm2 and two distinct overlapping rates: 50% and 75%. The objective of the work was to investigate the effect of the LSPwC and cyclic load condition on the crack closure and fatigue crack growth (FCG) behavior shown by the samples. Constant amplitude FCG tests were performed with two distinct load ratios: R = 0.2 and R = 0.5. A small increase in the crack closure loads (Pcl ? 1.1-1.2 Pmin) and in the number of cycles to crack propagation was evinced for the specimens tested at R = 0.2 compared to the untreated ones, whereas negligible effect was observed in the R = 0.5 tests. Besides, the obtained results indicate that the increase in overlapping rate is not effective for the adopted LSPwC conditions.
关键词： Laser shock processing,Fatigue crack growth,Aluminum alloy 2024
更新于2025-09-12 10:27:22
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In situ tailoring microstructure in laser solid formed titanium alloy for superior fatigue crack growth resistance
摘要： For damage tolerance (DT) titanium alloy, the fatigue crack growth resistance (FCGR) is a critical properties requirement for engineering applications. However, the Ti-6Al-4V-DT parts fabricated by laser solid forming (LSF) suffer from low FCGR, because of predominant basket-wave microstructure. Here, we have explored a novel LSF fabrication design to produce full colony microstructure, via in-situ controlled growth. The creation of such microstructures leads to superior FCGR, which markedly exceed conventional additive manufactured and mill-annealed samples. The present works provide a significant guidance for LSF-fabricated titanium alloy with high DT properties.
关键词： Titanium alloy,Fatigue crack growth resistance,Laser solid forming,Colony microstructure
更新于2025-09-11 14:15:04
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Effect of Different Forms of Application of a Laser Surface Treatment on Fatigue Crack Growth of an AA6013-T4 Aluminum Alloy
摘要： This work analyzes the effect of surface-localized laser heating treatment on the fatigue crack growth (FCG) rate on region II of the sigmoidal da/dN 3 DK curve of an aerospace-grade AA6013-T4 aluminum alloy sheet with 1.3 mm thickness. The in?uence on microstructure changes is also evaluated. Aiming to improve the FCG resistance without changing the mechanical behavior of the alloy, a Yb:?ber laser beam is defocused to generate a laser spot diameter of 2 mm, using 200 W power and a laser speed of 2 mm/s. Two laser lines are applied over fatigue C(T) specimens in two different forms: on only one and on both lateral specimen surfaces. Guinier–Preston zones, dispersoids and coarse constituent particles are found on the base material. On the heat-treated material, the same precipitates and also b￠ and Q￠ precipitates are found. These microstructural variations due to the laser thermal cycle, together with the presence of induced compressive residual stresses, improved the fatigue behavior of the material. The FCG retardation is optimized when two laser lines were applied on both lateral surfaces of the specimen.
关键词： aluminum alloy,microstructure,fatigue crack growth,residual stresses,laser surface treatment
更新于2025-09-11 14:15:04
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Bolted joint integrity monitoring with second harmonic generated by guided waves
摘要： In this study, the second harmonic generation due to the contact nonlinearity caused by bolt loosening is studied experimentally and numerically using three-dimensional explicit finite element simulations. In particular, it is demonstrated that the magnitude of the second harmonic generation normally increases with the loosening of the bolted joint, and there is a reasonable agreement between the numerical simulations and experimental results. The finite element model, which was validated against the experimentally measured data, is further utilized to investigate an important practical situation when a loosened bolt is weakened by fatigue cracks located at the edge of the hole. The numerical case studies show that the contact nonlinearity and the change of the behaviour of the second harmonic generation with the tightening level are very different to the corresponding results with the fatigue cracks. This identified difference in the second harmonic generation behaviour can serve as an indicator of the bolted joint integrity and thus provide early warning for engineers to make decision on the necessity of carrying out further safety inspections. Overall, the findings of this study provide improved physical insights into second harmonic generation for bolt loosening, which can be used to further advance damage detection techniques using nonlinear guided waves.
关键词： Bolt loosening,bolted joint,second harmonic generation,fatigue crack,contact nonlinearity,torque loss,structural health monitoring,guided waves
更新于2025-09-10 09:29:36
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Microstructure evolution and crack propagation feature in thermal fatigue of laser-deposited Stellite 6 coating for brake discs
摘要： In order to reveal the mechanism of microstructure evolution and crack propagation in laser-deposited Stellite 6 alloys, a quenching thermal fatigue test was conducted. Various detection methods were applied to observe differences between the coatings as deposited and after thermal fatigue. The results showed that the γ → ε martensitic transformation occurred in the as-deposited γ-Co matrix during the thermal fatigue process, driven by a fast cooling and thermal stress. The generated ε-Co phase presented variant selection, obeying Schmidt's law. In the ε-Co phase, the slip activity derived from different slipping systems that produced stacking faults and planar defects during the phase transformation. In addition, the stacking faults on {1 1 1}γ planes promoted the precipitation of directional M7C3 fine particle carbides. The net-like eutectic structures and γ/ε interfaces acted as paths for thermal crack propagation.
关键词： Martensitic transformation,Thermal fatigue,Crack propagation,Stellite 6 coating
更新于2025-09-10 09:29:36