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.
A comprehensive review is conducted on the very high cycle (VHC) and gigacycle (GC) fatigue of fiber-reinforced composite materials. The main objective was to conduct a benchmarking survey of ...high-frequency fatigue test methods that have been introduced by different research groups for testing beyond 107 loading cycles. Therefore, test methods are classified into two major groups, i.e. axial and bending tests. Axial fatigue tests were performed whether by conventional servo-hydraulic or by ultrasonic testing machines. For bending tests, three approaches have been taken into account. Firstly, conducting cantilever bending fatigue tests using a shaker-based actuator. Secondly, testing by an ultrasonically actuated three-point bending test set-up, and finally, using an electro-dynamically actuated four-point bending test device. The measures that are taken to mitigate challenges of testing at high-frequencies are put forward. The effect of loading frequency and temperature rise on fatigue behavior is investigated. VHC fatigue damage mechanisms are scrutinized for different composite materials, layup configurations, and testing approaches. Considering the importance of modeling in the design stage, the models that have been developed so far for predicting fatigue behavior of composite materials in the VHC and GC fatigue regimes, are presented. Finally, by performing a gap analysis, new perspectives required to be investigated more deeply are nominated.
This article predicts bending very high cycle fatigue (VHCF) life of three typical nickel‐based alloys SM2550, BG2532, and G3 used for completion strings. Fatigue tests were conducted on the three ...alloys using an ultrasonic fatigue system at a frequency of 20 kHz. The results showed that the fatigue strength ranges of the three alloys were markedly different, reflecting their different sensitivities to fatigue loading. Scanning electron microscope observations revealed numerous fatigue crack origins with internal decohesion in the fatigue source region. To achieve unified prediction of the fatigue life for the three alloys, a prediction model based on deep learning was built with inputs including fatigue initiation quantity, cleavage facet size, and other fatigue fracture characteristics. It was found that single source feature was insufficient to obtain satisfactory prediction accuracy for all alloys, while multifeature coupling integration could significantly improve the prediction precision, enabling reliable prediction of alloy fatigue life. This study provides new insights into bending VHCF life prediction.
Highlights
This article predicts bending VHCF life for three completion strings.
Bending VHCF life model utilizing deep wise separable convolution was established.
Deep learning can effectively integrate with bending VHCF analyses.
•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.
•Fatigue tests using AM Ti-6Al-4V were conducted at elevated temperature.•Maximum defect size due to AM process was statistically investigated.•The applicability of Murakami’s model for AM-Ti at ET ...was experimentally shown.
Rotating bending fatigue tests were conducted at room temperature (RT) and at elevated temperature (ET) of 250 °C using a Ti-6Al-4V alloy fabricated by an additive manufacturing (AM) process, and the effect of defects on fatigue strength was investigated. Particularly, the applicability of Murakami’s model for predicting the fatigue limit at ET was examined. Microstructure observations revealed that the AM Ti-6Al-4V alloy had acicular α + β microstructures while the conventionally manufactured (CM) Ti-6Al-4V alloy had equiaxed α phases in β matrix. Round-shaped defects and crevice-liked sharp defects with the size of up to 50 μm were observed in the AM Ti-6Al-4V, which were not observed in the CM Ti-6Al-4V. The fatigue strengths at 107 cycles of the CM Ti-6Al-4V were 625 MPa at RT and 475 MPa at ET, and those of the AM Ti-6Al-4V were 300 MPa at RT and 250 MPa at ET. Scanning electron microscopy (SEM) on the fracture surfaces revealed that fatigue cracks in the AM Ti-6Al-4V specimens initiated from defects at ET as well as they did at RT. The fatigue strengths of the AM Ti-6Al-4V at RT and at ET were evaluated by Murakami’s model and the predicted value agreed well with the experimental value.
•The √area parameter model is applicable to defects with a maximum size of ∼100 µm.•The long-crack threshold, ΔKth,lc, is applicable to defects larger than ∼100 µm.•Models based on the relative ...stress gradient can be used for blunt defects.•An equation to predict the critical size of detrimental defects is introduced.•The critical defect size increases with increasing load ratio.
The defect tolerance of three different martensitic stainless steels (17-4PH, X20Cr13 and AISI403) under cyclic loading was investigated. Fatigue tests were performed with specimens containing diverse artificial defects (corrosion pits, single and multiple drilled holes, sharp circumferential notches and pre-cracked holes). The measured cyclic strengths are compared to predictions based on the area parameter model proposed by Murakami and Endo. This model is well applicable to small sharp defects if the threshold condition for crack propagation determines the fatigue limit. The transition size from small to large defects, areatrans, – above which the threshold stress intensity factor range, ΔKth, becomes a constant value – is between 80 and 166 µm for the investigated steels. For larger defects, the threshold for long cracks, ΔKth,lc, serves well to predict the fatigue strength under fully reversed loading, i.e. at a load ratio of R = −1. Comprehensive tests were performed to study the influence of mean stress, and threshold conditions were determined that enable to predict the fatigue limit at different load ratios.
A simple equation is introduced to estimate the critical defect size, areacritical, which is the size above which defects become detrimental under cyclic loading. It has been found that areacritical increases with increasing load ratio while areatrans is independent of R. Furthermore, higher values of the threshold stress intensity factor range were determined for large defects (with sizes larger than areatrans) compared to long cracks if the load ratio is higher than R = −1. This could be explained by the plane stress condition at the material surface which is prevalent for surface defects, while the condition for long cracks can be assumed as plane strain.
It is further demonstrated that the effect of small defects on the fatigue strength of stainless steels is highly sensitive to the notch root radius, ρ. Small holes with diameters of 100 µm (ρ = 50 µm) or larger are less detrimental for the fatigue strength than defects with high stress concentrations – such as corrosion pits.
•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).
•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.
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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