•Solution-annealed and precipitation-hardened Ni-based superalloy 718.•Uniaxial, pure-torsion and torsion under a static tension fatigue tests.•Inability of precipitation-hardening to improve ...shear-mode fatigue crack threshold.•Little effect of crack-opening and -closing stresses on the shear-mode threshold.•A possibility of shear-mode-threshold-based comprehensive fatigue limit evaluation.
Fatigue strength of Ni-based superalloy 718 was investigated relative to shear-mode crack-growth thresholds, to evaluate the influence of crack-opening/-closing stresses perpendicular to shear-mode crack-planes. The impact of precipitation-hardening was also examined. Using solution-treated and precipitation-hardened materials, three different types of fatigue tests were performed: (i) push–pull; (ii) pure-torsion; (iii) torsion with superposed static tension. All tests revealed non-propagation of small, shear-mode cracks, while confirming the negligible impact of precipitation-hardening on shear-mode crack-growth threshold. Additionally, crack-opening/-closing stresses were determined to only imperceptibly alter shear-mode resistance. Based on these findings, a novel strategy was proposed to quantify the alloy’s fatigue strength.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•As-fabricated horizontally orientated parts show greater fatigue life than vertical.•Machining improves overall fatigue life and reduces anisotropic fatigue behavior.•Post EBM-fabrication HIP alone ...does not universally improve fatigue life.•Machined and HIPed EBM parts show comparable fatigue life to conventional Ti-6Al-4V.
Electron beam melting (EBM) is a rapidly-developing metal additive manufacturing process that holds significant interest in the aerospace and biomedical industries for the high-strength titanium alloy, Ti-6Al-4V. These industries have fatigue-limited applications, but a lack of understanding of the EBM process-microstructure-fatigue relationships limit widespread use and presents challenges for certification. In this review, uniaxial fatigue data was collected and the effects of build orientation, surface roughness, and hot-isostatic pressing are linked to the fatigue properties highlighting microstructure, defects, and failure mechanisms. The observations and conclusions are supported by statistical analysis using the mean fatigue life obtained by the Statistical Fatigue-Limit Model. Both EBM-process and post-process structure relationships are discussed in order to identify the best-practice for fatigue-resistant design. The performance of the EBM material is compared to conventionally manufactured Ti-6Al-4V and possible methods to increase the fatigue resistance are discussed. Anisotropic fatigue behavior was observed in as-fabricated parts and no statistical distinction was found in the fatigue performance of HIPed and as-fabricated material provided the same as-fabricated rough surface condition. However, comparable fatigue life to traditionally manufactured lamellar Ti-6Al-4V is achieved when both post-process HIP and machining are applied to EBM-fabricated parts.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Static and fatigue cracking behavior of R-UHPC overlay in UHPC-OSD systems are tested.•Cracking behavior of R-UHPC overlay in transverse and longitudinal directions are studied.•Nominal cracking ...strength of R-UHPC overlay of composite bridge decks is evaluated.•Fatigue resistance of R-UHPC overlay of composite bridge decks is determined.
Ultra-high performance concrete (UHPC) has been used to enhance the fatigue resistance of orthotropic steel decks, but the enhancement effect is compromised by UHPC cracks in negative moment regions. This research investigates the flexural cracking behavior of reinforced UHPC (R-UHPC) overlay in composite bridge deck under static and fatigue loads. The investigated behavior included the load-carrying capacity of composite deck, and the cracking process, pattern and resistance of the R-UHPC overlay. Ten composite specimens were tested, including five slabs for the transverse behavior and five beams for the longitudinal behavior. Static tests investigated the load-carrying capacity, strain distribution and nominal cracking strength. Fatigue tests investigated the cracking process and fatigue resistance of R-UHPC overlay. The fatigue process included an internal force redistribution stage (less than10% of fatigue life) and a stable stage (greater than90% of fatigue life). The fatigue resistance of R-UHPC overlay was evaluated with a modified Goodman diagram.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Mean stress effect on the fatigue strength is similar to that for metals without twinning.•EBSD analysis showed no twinning at the crack initiation site.•Fatigue crack were initiated from large ...grains with large Schmid factor of the basal slip system.•Twinning is not involved in the fatigue crack initiation of high cycle fatigue regime.
Plane bending fatigue tests were conducted under stress ratios of −1, −0.1, 0.1, and 0.5 at room temperature in a laboratory atmosphere to elucidate the fatigue crack initiation mechanism of an extruded AZ31 magnesium alloy. The fatigue life can be expressed as a unique function of the equivalent stress amplitude based on Smith–Watson–Topper theory independent of the stress ratio, and the dependence of the fatigue limit on the mean stress can be expressed by Morrow’s equation, both of which were proposed for conventional metals without twinning. In addition, the specimen surface was observed by optical microscopy and scanning electron microscopy (SEM), and the surface near the crack initiation site was analyzed by electron backscatter diffraction (EBSD) analysis to discuss the fatigue crack initiation mechanism. On the basis of the results of EBSD analysis, it is concluded that the existence of large grain with large Schmid factor of the basal slip system is essential for crack initiation, and the crack initiation mechanism is based on irreversible slipping and unrelated to twinning under the alternating stress condition (R = −1).
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•Fatigue/fracture under mixed-mode of composite bonded joints was performed.•The ADCB and ASLB mixed-mode I+II fatigue/fracture tests were employed.•The evolution of the Paris law coefficients in ...function of mode ratio was obtained.•A high-cycle fatigue CZM under mixed-mode loading was used for procedure validation.
This work addresses a simple methodology to perform fatigue/fracture characterisation under mixed-mode I+II of composite bonded joints. The asymmetrical double cantilever beam and asymmetrical single-leg bending fatigue/fracture tests were considered to address mode-mixities with predominance of mode I and II, respectively. The modified Paris laws coefficients were correlated with the ones of other tests defining suitable relations describing the evolution of these coefficients as function of mode-mixity. These functions were input in a cohesive zone model appropriate for high-cycle fatigue under mixed-mode loading. It was observed that numerical fatigue lives and Paris laws are in accordance with the experimental results.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A new method for predicting high-cycle fatigue life under constant multiaxial amplitude loading is proposed using a unified equivalent shear stress amplitude proposed by the author. This criterion is ...an extension of the multiaxial fatigue limit criterion proposed previously by the author. This criterion uses the axial loading S-N curve and the true fracture strength as commonly available material characteristic values. The fatigue test data from the literature such as the bending and torsion combined loadings, the biaxial loadings and the biaxial loadings with sinusoidal, triangular and trapezoidal waveforms, were used to verify the criterion. The fatigue lives estimation error and fatigue limit estimation error of the proposed criterion were better than that of the other criteria used for comparison.
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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.
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Electron beam melting (EBM) and Selective Laser Melting (SLM) are powder bed based additive manufacturing (AM) processes. These, relatively new, processes offer advantages such as near net shaping, ...manufacturing complex geometries with a design space that was previously not accessible with conventional manufacturing processes, part consolidation to reduce number of assemblies, shorter time to market etc. The aerospace and gas turbine industries have shown interest in the EBM and the SLM processes to enable topology-optimized designs, parts with lattice structures and part consolidation. However, to realize such advantages, factors affecting the mechanical properties must be well understood – especially the fatigue properties. In the context of fatigue performance, apart from the effect of different phases in the material, the effect of defects in terms of both the amount and distribution and the effect of “rough” as-built surface must be studied in detail. Fatigue properties of Alloy 718, a Ni-Fe based superalloy widely used in the aerospace engines is investigated in this study. Four point bending fatigue tests have been performed at 20 Hz in room temperature at different stress ranges to compare the performance of the EBM and the SLM material to the wrought material. The experiment aims to assess the differences in fatigue properties between the two powder bed AM processes as well as assess the effect of two post-treatment methods namely – machining and hot isostatic pressing (HIP). Fractography and metallography have been performed to explain the observed properties. Both HIPing and machining improve the fatigue performance; however, a large scatter is observed for machined specimens. Fatigue properties of SLM material approach that of wrought material while in EBM material defects severely affect the fatigue life.
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•Additively manufactured steel shows similar strength to its wrought counterpart.•Lower ductility is attributed to pores in the additively manufactured steel.•Pores in the additively manufactured ...stainless steel decrease its fatigue strength.•A kink point is observed in the strain-life curves of the 17–4 PH stainless steel.•Both austenite and martensite phases are found in the 17–4 PH stainless steel.
Static and fatigue experiments were conducted on conventionally manufactured (CM) and additively manufactured (AM) 17–4 PH stainless steels in ambient air. The fatigue experiments were tested under the fully reversed strain-controlled condition with the strain amplitudes ranging from 0.15% to 1.0%. The two manufacturing conditions result in similar tensile strengths but the AM 17–4 PH stainless steel has a significantly lower ductility than that of the CM steel. The fatigue limit of the CM 17–4 PH stainless steel is approximately 640 MPa and the fatigue limit of the AM steel is approximately 300 MPa. The strain-life fatigue curves of both materials display a kink point from low cycle fatigue regime to high cycle fatigue regime. Defects and porosity from the additive manufacturing process contribute to the weakened fatigue properties and ductility of the AM stainless steel.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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