•VHCF strength decreases with building orientation.•Fatigue crack origin is always from an internal defect.•Rough area and fisheye have been observed on fracture surfaces.
The effect of building ...orientation on the very-high-cycle fatigue (VHCF) response of Ti-6Al-4V specimens produced through selective laser melting (SLM) process with three different building orientations (0°, 45° and 90°) has been experimentally assessed. The fatigue performance decreases with different building orientations from 0° to 90°. The fatigue crack origin has been found to be always an internal defect both at high-cycle fatigue and VHCF regime independent of building orientations. Size of defects induced fatigue failures and the stress intensity factor range decrease with the number of cycles to failure. By considering the VHCF strength at 109 cycles, the median value decreases from 217 MPa (0°) to 201 MPa (45°) and finally to 155 MPa (90°), with a 40% reduction from 0° to 90°. The building orientation significantly influences both the defect size and the resulting VHCF response.
This paper proposes a new method to investigate the effect of specimen size on fatigue life of metallic materials in high‐cycle fatigue and very‐high‐cycle fatigue regimes in the light of statistical ...analysis. The performance of large specimen is correlated with that of small specimen via control volume (a high stress domain where potential fatigue crack initiation sites are located). The simulation results are in good agreement with a series of experimental data in high‐cycle fatigue and very‐high‐cycle fatigue regimes from literature. The method is helpful for a quantitative estimation of fatigue life of large specimens from the experimental data of small specimens.
The paper brings results of high‐cycle fatigue and very‐high‐cycle fatigue tests of the steel S355 tested during low‐ and high‐frequency loading. Two grades of structural steel (S355J0 and S355J2) ...were tested to gain fatigue properties that serve as inputs to reliable finite element calculations of cyclically loaded structures. Experimental measurements of both steel grades were performed on low‐frequency hydraulic and high‐frequency ultrasonic fatigue testing systems. Both low‐ and high‐frequency loading showed higher lifetime for the grade S355J2. The difference between the two studied steel grades was more apparent during low‐frequency loading. The significant difference between the results of low‐ and high‐frequency loading tests can be explained by the fact that the resistance to plastic deformation increases with an increasing rate of deformation. Fracture surfaces were studied by electron microscopy, where they exhibited both surface and internal crack initiation.
Highlights
The fatigue life of the studied steels exhibited strong dependence on loading frequency.
The life under fatigue ultrasonic loading is much higher than the life under low frequency.
The purity of the S355 steels has a significant impact on their fatigue life.
The fatigue life of the steels S355J0 and S355J2 is in correlation to their ultimate strength.
•Uniform gage section specimens additively manufactured via SLM.•Low and high cycle fatigue characterization of the as-built condition.•Effect of post-process treatments on the high-cycle fatigue ...response is investigated.•Size and spatial distribution of defects is determined through metallographic and CT analyses.•A area method is devised for interpreting the experimental results.
Selective laser melting (SLM) is a net-shape AM technology to produce metal parts also for load-carrying applications. The present work is aimed at investigating the fatigue performance of the biomedical titanium Grade 23 (aka Ti-6Al-4V ELI) AMed via SLM. Low and high cycle fatigue tests are carried out on samples that received a low temperature stress-relief treatment. In addition, the effect of selected post processing treatments on the high cycle fatigue response is assessed. Material characterization is complemented with residual stress and microhardness measurements, computed tomography scans, metallographic and fractographic inspections. These experimental analyses served to elaborate an interpretative model accounting for the modifications produced by the post-processing treatments. The results denote the important role exerted by mean and residual stresses as well as defects on the fatigue performance. The relatively low fatigue strength of SLM manufactured parts indicates that further developments in this fabrication route are still necessary to make their mechanical properties competitive with those of traditionally processed components.
In this paper, a comparative study of high-cycle fatigue tests (T=650°C, f=110Hz, R=0.1, Kt=1) were carried out with wrought Inconel 718 and LAMed Inconel 718. The results show that the influences of ...the Laves phases on high-cycle fatigue properties are based on the applied stress amplitudes. At a low stress amplitude, most of the Laves phases held their original morphologies. The fatigue cracks stopped in front or detoured around them, which means that the unbroken Laves phases play an important role in hindering crack propagation. In this way, the high-cycle fatigue life of LAMed Inconel 718 was superior to that of wrought Inconel 718. However, at a high stress amplitude almost all of the Laves phases in the crack propagation region splintered into smaller fragments, parts of which separated from the austenite matrix. Microscopic holes or cracks were formed at the interface, which provided passages for the fatigue cracks to propagate. In this case, the Laves phases were harmful, leading to the degradation of fatigue performance in LAMed Inconel 718 compared with wrought Inconel 718.
•Three models are provided to assess the HCF and VHCF properties of AlSi10Mg.•The building orientation has a great effect on the fatigue performance of AlSi10Mg.•Defects can obviously reduce the HCF ...and VHCF performances of AlSi10Mg.
The effects of defects and building directions in additive manufactured AlSi10Mg on its high-cycle fatigue and very-high-cycle fatigue performance are studied based on crystal plasticity finite element (CPFE). Among the three models provided in this work, the results of model 2 (Np2) are in good agreement with the experimental ones. It is found the fatigue life of the sample with defects are much lower than that without defects. 0° specimens have a better fatigue performance than 90° ones. This work is beneficial in determining the fatigue lifetime and helps to improve the fatigue behavior of AlSi10Mg.
•Two shot peening intensities (0.15 mmA and 0.3 mmA) were used to shot peened GH4169 superalloy and HCF and VHCF properties were investigated.•Shot peening improves the fatigue properties of GH4169 ...superalloy.•Compressive residual stress plays a paramount role in improving HCF and VHCF of GH4169.•High magnitude tensile residual stresses reduce the beneficial effect of shot peening on improving the fatigue properties of GH4169 superalloy.
In this work, the GH4169 superalloy specimens were shot-peened at two different intensities (0.15 mmA and 0.3 mmA) to improve the fatigue properties, and the fatigue behavior of the shot-peen-treated specimens was investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. The results of the HCF and VHCF tests indicated that shot peening improved the fatigue properties of the material. In addition, it has shown that the tensile residual stresses introduced by excessive shot peening intensity reduce the benefits of shot peening in terms of improving fatigue properties.
This paper investigated the microstructure and fatigue behavior of PM-HIPed Inconel 625. The microstructure was composed of γ phase and (Mo, Nb) carbonitrides located mostly on prior particle ...boundaries. Despite the presence of these carbonitrides, the samples showed good tensile properties with high elongation. Two different surface conditions, pickled and machined, were used for high cycle fatigue testing under a four-point bending test. The results indicated that pickled samples had 6% lower fatigue strength (at 106 cycles) with three times higher standard deviation compared to the machined ones. Fatigue failure mechanisms were found to be dependent on surface conditions and showed different failure modes due to non-metallic oxide inclusions and surface defects in samples with machined and pickled surfaces, respectively. The effect of type, size, and location of defects, multiplicity of crack initiations, as well as surface roughness were analyzed and discussed.
•Investigating fatigue behavior of Inconel 625: Machined and pickled surfaces.•Two fatigue failure mechanisms of inclusions and surface defects were identified.•Regardless of the type of defects, their size had a noticeable relation with fatigue strength.•Most detrimental failure reason in pickled samples: Multiple close initiation points.•A clear correlation between Ssk roughness parameter and fatigue life was determined.
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•A continuous runout method (CRM) is proposed to evaluate the fatigue strength.•CRM could greatly reduce the testing period compared with up-and-down method (UDM).•The testing period ...could be 1/5–1/3 of that by the UDM for 16 samples.•CRM is validated by the experimental data of different metallic materials.
In this paper, a continuous runout method (CRM) is proposed to evaluate the fatigue strength in high cycle and very high cycle fatigue regimes based on the probability and statistics theory. The CRM features the simultaneous testing of multiple samples such that the testing period could be 1/5–1/3 of that by the common up-and-down method for 16 samples. It is validated by the experimental data for G20Mn5QT steel, 40 Cr steel, and Ti-6Al-4V alloy. The predicted lower limits of fatigue strength by CRM are a little more conservative than those by the up-and-down method.
•The separate S-N curves are firstly detected in additively manufactured metals.•Fatigue cracks originate from gas pores, lack of fusions, and columnar grains.•ΔK at the border of rough area ...corresponds to propagation threshold for long cracks.•Fatigue sensitivity of maximal micro-crack type, size, and location increase successively.
Ultrasonic fatigue tests were performed to investigate high and very high cycle fatigue behaviors of a laser additively manufactured Inconel 718 (IN718) alloy in the as-deposited condition. The results indicate that the competition failure behavior between the surface and interior crack initiation results in the separate S-N curve. Both manufacturing defects (e.g., gas pore, lack of fusion) and columnar grains (matrix) observed in the microstructure could act as the original fatigue micro-cracks due to the effective restriction on manufacturing defects. The fatigue sensitivity levels increase successively in terms of the type, size, and location of the maximal micro-crack.
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