In the present study, electron backscattered diffraction is used to analyze the fatigue crack evolution in a high strength steel weld that was loaded cyclically in the plastic regime. Three prominent regions of a fatigue crack are investigated separately: crack tip, crack trajectory and crack initiation. Taylor and Schmid factors are mapped with respect to the defined loading matrix. Possible effective mechanisms are proposed based on the local plasticity properties like lattice rotation and misorientation. The analyses of the crack tip and trajectory regions show that although the critical resolved shear stresses in some regions are low, small deformation resistance of these regions can compromise the dislocation immobility and cause local fracture. It is shown that if the crack grows transgranularly, at least one side of the crack may show low lattice rotation or strain equivalent values, which indicates the relaxation of elastic stresses after fracture. The crack initiation is determined to be dominantly controlled by transcrystalline mechanism of initiation that takes place under plastic loading conditions. It is also shown that the secondary < 123 >11 (1) over bar type of slip systems were the most activated under such loading conditions. (C) 2015 Elsevier Ltd. All rights reserved.

In this study, Alloy 718 specimens manufactured by Electron Beam Powder Bed Fusion process are subjected to two different post-treatments to have different microstructural features. Low cycle fatigue testing has been performed both parallel and transverse to the build direction. EB-PBF Alloy 718 exhibits anisotropic fatigue behaviour; the fatigue life is better along the parallel direction compared to the transverse direction. The anisotropy in fatigue life is related to the anisotropy in the Young’s modulus. The pseudo-elastic stress vs. fatigue life approach is presented as a potential solution to handle anisotropy in fatigue life assessment of additively manufactured engineering components. © 2020 The Authors

Application of Low Transformation Temperature (LTT) consumables in welding is a recent approach to increase the fatigue strength of welds. In this paper high strength steels with yield strengths ranging from 650-1021 MPa were fillet and butt welded using different LTT and conventional consumables. The effects of weld metal chemical composition on phase transformation temperatures, residual stresses and fatigue strength were investigated. Lower transformation start temperatures and hence lower tensile or even compressive residual stresses were obtained close to the weld toe for LTT welds. Fatigue testing showed very good results for all combinations of LTT consumables and high strength steels with varying strength levels. For butt welds, the characteristic fatigue strength (FAT) of LTT welds at 2 million cycles was up to 46% higher when compared to corresponding welds made with conventional filler materials. In fillet welds, a minimum FAT improvement of 34% and a maximum improvement of 132% was achieved when using LTT wires. It is concluded that different LTT consumables can successfully be employed to increase fatigue strength of welds in high strength steels with yield strength up to 1021 MPa. Weld metals with martensite transformation start temperatures close to 200°C result in the highest fatigue strengths.

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.

High frequency mechanical impact (HFMI) is a recent post weld treatment method which can be employed to increase the fatigue strength of welded components. In this paper the fatigue strength of as-welded and HFMI treated fillet welds in a 1300 MPa yield strength steel was compared. Fatigue testing was done under fully reversed, constant amplitude bending load. Finite element analysis was used to calculate the stress distribution in the weld toe region to permit evaluation of the fatigue data with the effective notch stress approach. As-welded samples showed a mean fatigue strength of 353 MPa and a characteristic fatigue strength of 306 MPa. HFMI treatment increased the mean fatigue strength by 26% and the characteristic fatigue strengths by 3%. The weld toe radii in as-welded condition were large. HFMI only increased the weld toe radii slightly but resulted in a more uniform weld toe geometry along the weld. A depth of indentation in the base metal in the range of 0.15–0.19 mm and a width of indentation in the range of 2.5–3 mm, were achieved. Maximum compressive residual stresses of about 800 MPa in the longitudinal and 250 MPa in the transverse direction were introduced by HFMI treatment, adjacent to the weld toe. The surface hardness was increased in the entire HFMI treated region. It is concluded that the increase in fatigue strength is due to the combined effects of the weld toe geometry modification, increase in surface hardness and creation of compressive residual stresses in the treated region.

Fatigue performance is crucial for gas turbine components, and it is greatly affected by the manufacturing processes. Ability to predict the expected fatigue life of a component based on surface integrity has been the objective in this work, enabling new processing methods. Alloy 718 samples were prepared by different machining setups, evaluated in fatigue testing and surface integrity investigations. These results generated two predictive statistical multi-variate regression models. The fatigue correlated well with roughness, residual stresses and deformation. The two models showed great potential, which encourages further exploration to fine-tune the procedure for the particular case.

Abstract The use of specimen loss factor as fatigue damage indicator of Hastelloy-X substrates with different surface treatments was investigated together with other fatigue damage indicators, namely resonance frequency and crack mouth length. The tested surface treatments included grit-blasting and plasma spraying of NiCoCrAlY bond coat and yttria stabilized zirconia (YSZ) top coat. The loss factors of fatigue test specimens were measured repeatedly during the resonance bending fatigue test using the conventional free decay method. The analysis of the damping spectra, i.e. the model describing the relation of loss factor to maximum macroscopic specimen strain εyy was drafted. The model is based on the combination of defect models developed by Göken and Riehemann (2004) and classical dislocation model of Granato and LÌcke (1956). It appears, that the damping spectra can be well approximated as a combination of two defect peaks (C1 and C2) and one dislocation peak (D1). The low strain defect peak (peak C1) is sensitive to the presence of fatigue cracks. The second defect peak (peak C2) can be attributed to the remaining substrate and coating defects such as embedded grit particles, coating porosity, surface roughness and sliding in the sample clamping area. The fatigue damage detection using the C1 peak magnitude was performed and its results were related to the crack length obtained by digital image correlation (DIC) method. In the crack initiation stage I., the C1 peak height shows different behavior than the resonance frequency and therefore provides new information. The underlying processes causing C1 peak changes need to be found yet, however. In the crack growth stage II., both resonance frequency and peak height C1 correlate with the measured fatigue crack size.

Abstract The use of specimen loss factor as fatigue damage indicator of Hastelloy-X substrates with different surface treatments was investigated together with other fatigue damage indicators, namely resonance frequency and crack mouth length. The tested surface treatments included grit-blasting and plasma spraying of NiCoCrAlY bond coat and yttria stabilized zirconia (YSZ) top coat. The loss factors of fatigue test specimens were measured repeatedly during the resonance bending fatigue test using the conventional free decay method. The analysis of the damping spectra, i.e. the model describing the relation of loss factor to maximum macroscopic specimen strain εyy was drafted. The model is based on the combination of defect models developed by Göken and Riehemann [1] and classical dislocation model of Granato and LÌcke [2]. It appears, that the damping spectra can be well approximated as a combination of two defect peaks (C1 and C2) and one dislocation peak (D1). The low strain defect peak (peak C1) is sensitive to the presence of fatigue cracks. The second defect peak (peak C2) can be attributed to the remaining substrate and coating defects such as embedded grit particles, coating porosity, surface roughness and sliding in the sample clamping area. The fatigue damage detection using the C1 peak magnitude was performed and its results were related to the crack length obtained by digital image correlation (DIC) method. In the crack initiation stage I., the C1 peak height shows different behavior than the resonance frequency and therefore provides new information. The underlying processes causing C1 peak changes need to be found yet, however. In the crack growth stage II., both resonance frequency and peak height C1 correlate with the measured fatigue crack size.

Uniaxial plain fatigue and fretting fatigue tests were carried out on detonation gun sprayed Cu-Ni-In coating on Al-Mg-Si alloy samples, The samples in three conditions were considered: uncoated, as- coated and ground after coating. Ground coated specimens exhibited superior plain fatigue and fretting fatigue lives compared with uncoated and as-coated specimens. The life enhancement has been discussed in terms of surface finish and residual compressive stresses at the surface. (C) 2008 Elsevier Ltd. All rights reserved.

The objective of this work was to investigate the performance of microarc oxide coatings of two different thicknesses (40 and 100 mu m) on Al-Mg-Si alloy samples under plain fatigue and fretting Fatigue loadings. Tensile residual stress present in the substrate of 40 mu m thick coated samples induced early crack initiation in the substrate and so their plain fatigue lives were shorter than those of untreated specimens. Presence of more pores and tensile surface residual stress in 100 mu m thick coated samples caused early crack initiation at the surface leading to their inferior plain fatigue lives compared with 40 mu m thick coated samples. While the differences between the lives of coated and uncoated specimens were significant under plain fatigue loading, this was not the case under fretting fatigue loading. This may be attributed to relatively higher surface hardness of coated specimens. The performance of 40 mu m thick coated samples was better than that of 100 mu m thick coated specimens under both plain fatigue and fretting fatigue loadings. (C) 2007 Elsevier Ltd. All rights reserved.

The fatigue behavior of additively manufactured (AM) structural parts is sensitive to the surface and near-surface material conditions. Chemical post-processing surface treatments can be used to improve the surface condition of AM components, including complex geometries with surfaces difficult to access. In this work, surfaces of electron beam powder bed fusion (EB-PBF) produced Ti–6Al–4V were subject to two different chemical post-processing surface treatments, chemical milling and Hirtisation. As-built and machined surfaces, as well as hot isostatic pressing (HIP), treated conditions were also investigated. Fatigue testing was carried out in four-point bending. The investigation focused on the relationship between fracture mechanisms and fatigue life through fractographic study. It was found that a majority of fractures were initiated at internal surface-near defects or defects on the surface. Chemical post-processing was found to smoothen the surface but to leave a surface waviness. Material removal during post-processing could open up internal defects to the treated surface. In HIP-treated specimens, fractures initiated at defects open to the surface. Despite post-processing increasing the mean life of fatigue specimens, no significant improvements in the lowest tested life were observed for any specimen condition.

Surface asperities play a leading role in determining the fatigue life of as-built Ti-6Al-4 V components manufactured by electron beam powder bed fusion (PBF-EB). Several roughness parameters are available to characterize the surface asperities This study focuses on identifying the surface roughness parameter that correlates best with fatigue life. To this end, several fatigue test specimens were manufactured using the PBF-EB process and utilizing different contour melting strategies, thus producing as-built surfaces with varying roughness. The focus variation microscopy technique was employed to obtain surface roughness parameters for the as-built surfaces. Selected specimens were characterized using x-ray computed tomography (XCT). Tomography can detect surface-connected features obscured by other parts of the surface that are not visible through optical microscopy. The fatigue life of all specimens was determined using four-point bend testing. Through regression model analysis, maximum pit height (Sv) was identified as the statistically significant roughness parameter with the best fit affecting fatigue life. The fracture zone was closely inspected based on the data collected through XCT prior to fatigue tests. This led to another estimate of the worst-case value for the statistically significant roughness parameter Sv. The Sv parameter values obtained from optical microscopy and XCT were used as the initial crack size in a crack growth model to predict fatigue life. It is observed that life estimates based solely on optical measurements of Sv can be overly optimistic, a situation that must be avoided in predictive design calculations.

Fatigue crack initiation in engineering components is predominantly associated with tensile surface residual stresses or stress raisers caused by discontinuities or mechanical notches. During welding, surface residual stresses arising from plastic mismatch due to the temperature gradient can accelerate the initiation of cracks during cyclic fatigue testing. Direct energy surface treatment is currently considered a viable post-processing methodology for modifying surfaces within the weld zone to improve fatigue life. This paper discusses results obtained during experimental research, investigating the effect of direct energy surface treatment on surface residual stresses, microstructure, and fatigue life of cylindrical rotary friction welded Ti-6Al-4 V samples. Direct energy post-processing technique aided in improving the fatigue life of friction welded specimens by modifying the surface microstructure, introducing a uniform refined and homogenous structure in the welded region. This near-surface refined microstructure inhibited crack initiation in the welded region, shifting initiation sites to an area adjacent to the modified surface region. © 2022 Elsevier Ltd