High Cycle Fatigue Loading Research Articles

Crack initiation mechanism in ultrafine-grained copper fabricated by severe plastic deformation in the high-cycle fatigue regime

Ultrafine grained (UFG) metals processed by the severe plastic deformation technique have an unusual microstructure of ‘high energy/non-equilibrium’ state. It has been shown that fatigue cracks in UFG metals under strain-controlled low-cycle fatigue loading are initiated from shear bands that are the area of severe cyclic strain localization. On the other hand, current research lacks the required data to verify the crack initiation mechanism of UFG metals under high-cycle fatigue (HCF) loading. In this study, HCF tests under stress control were conducted on UFG copper to examine the behavior of crack initiation. The morphological change close to the damage traces, which evolved to a fatigue crack during ensuing cyclic stressing, was monitored. Inner fatigue damage was also analyzed by using a focused ion beam technique. Four typical sites of surface-crack initiation were commonly observed: i) persistent slip bands inside coarse dynamically recovered/recrystallized grains, ii) line-shaped damage traces in UFG structure, iii) near coarse grain boundaries, and iv) protruded surface inside the near-by oriented grains. The initiation mechanism of HCF crack in UFG copper was discussed in light of the morphological change in surface damage during cyclic stressing and the formation of embryonic cracks in high energy state microstructure.

Examining metrics for fatigue life predictions of additively manufactured IN718 via crystal plasticity modeling including the role of simulation volume and microstructural constraints

There is a growing demand for the development of precise, accurate, and industrially applicable predictive models for evaluating the fatigue performance of additively manufactured (AM) materials to accelerate their qualification. Six fatigue metrics based on microstructure-sensitive crystal plasticity finite element (CPFE) simulations are utilized to assess the fatigue performance of Inconel 718 produced via selective laser melting (an AM technique). Each metric is used to predict the probable locations of failure and the scatter in fatigue life under high cycle fatigue loading conditions. The predicted scatter from all the fatigue metrics was in good agreement with the experimental test data. The predictions locations of failure using two of the metrics, plastic strain accumulation, and plastic strain energy density, were found to correlate in a statistical sense with the post mortem fractography results. Moreover, an additional set of CPFE simulations were performed with varying volumes of the input microstructures, with the largest microstructure having a volume close to that of the test specimen's gauge section. This additional analysis was intended to provide informed guidelines for simulation volume that is both computationally tractable, and results in consistent scatter predictions, which was estimated to be a simulation volume consisting of ~200 grains. Lastly, fatigue life predictions obtained from traction-free and more realistic microstructure constraints were compared to understand the role of boundary conditions (BCs) in fatigue life predictions. This work is beneficial in determining the statistical minimum for safe-life analyses, thereby reducing the overall number of experimental tests and accelerating the qualification process.

Numerical comparison of several composite bonded repairs under fatigue loading

In this work, a numerical analysis of typical composite bonded repairs is performed aiming to establish a comparison between their performance under quasi-static and high-cycle fatigue loading. The studies include progressive damage analysis employing cohesive zone models suitable for quasi-static and high-cycle fatigue under mixed-mode I+II loading. Five repair geometries, e.g., Single-Strap, Double-Strap, Scarf, One-Stepped and Two-Stepped were analysed under three-point bending loading considering three different overlap lengths. In the quasi-static analysis, the focus was the initial stiffness, peak load and the corresponding displacement. Fatigue studies were executed under load control and under displacement control aiming to determine the fatigue lives. A final research addressed two of the embedded repairs, Scarf and Two-Stepped, reinforced with a bonded patch, which was verified to improve significantly fatigue resistance, especially in the case of the Scarf repair.

Evaluation of transverse crack initiation of cross-ply CFRP laminates subjected to very high cycle fatigue loading

Evaluation of transverse crack initiation of cross-ply CFRP laminates subjected to very high cycle fatigue loading

Experimental Procedure for Energy Dissipation Estimation during High-Cycle Fatigue Loading of Metallic Material

The fatigue failure process of metal is a complex energy exchange process accompanied by temperature evolution due to energy dissipation and heat transfer, which can be well recorded by infrared thermography. With the development and popularization of infrared thermography, many energy dissipation estimation methods have been proposed. In this work, three types of energy dissipation estimation methods in different time periods during the high-cycle fatigue experiment are derived and improved. The energy dissipations of FV520B stainless steel specimens (which are subjected to different heat treatments) during fatigue cyclic loading are estimated with the different derived methods and compared. The assumptions for thermal loss may be the main reason for error in the different methods. The energy dissipations estimated by the different methods have basically the same overall trend. Finally, some suggestions on high-cycle fatigue experimental procedures for energy dissipation estimation are proposed.

High-cycle fatigue analysis of adhesively bonded composite scarf repairs

This work addresses the fracture characterization of scarf adhesive repairs on carbon epoxy composites under high-cycle fatigue loading. Scarf repairs were tested under static and fatigue loading in three-point bending considering a scarf angle of 10°, aiming to determine its static strength and fatigue life. Static and fatigue finite element analyses including a cohesive mixed-mode I+II model were performed. The high-cycle fatigue model is based on the modified Paris law to simulate material softening as function of time, considering a unique damage parameter that englobes quasi-static and fatigue degradation. The numerical static load-displacement curve, fatigue life prediction and evolution of normalized compliance in function of the number of fatigue cycles are in good agreement with the average experimental results. Furthermore, several additional numerical analyses were performed to assess the influence of the scarf angle on the static strength and fatigue life predictions of these repairs.

The fatigue limit prediction of notched components – A critical review and modified stress gradient based approach

Classical fatigue limit prediction models and material-dependent characteristic length parameters for notched components are critically reviewed before a modified stress gradient based approach is proposed to integrate the non-propagation behavior of microstructurally small crack. Assuming fatigue limit (strength) as a general function of root surface stress and stress gradient defined over a characteristic length, an effective stress parameter is derived to characterize the resistance of a notch to fatigue limit (high cycle fatigue) loading. The model is validated by comparing with reviewed models through a large amount of test data. Physical interpretation and perspectives on the model are also discussed.

Inclusion initiated fracture in spring steel under axial and torsion very high cycle fatigue loading at different load ratios

Inclusion initiated fracture under axial and torsion very high cycle fatigue loading at load ratios R = −1, 0.1 and 0.35 is compared. High strength spring steel with intentionally increased density of aluminate inclusions was tested at ultrasonic frequency. Failures under axial loading started at internal aluminate inclusions at all load ratios, and at aluminate and MnS inclusions in cyclic torsion at positive load ratios. Considering crack initiation at same size aluminate inclusions, a cyclic torsion stress amplitude at the inclusion that is a factor 0.86 ± 0.05 smaller than the cyclic tension stress amplitude has an equally detrimental effect on cyclic strength.

Investigation of crack initiation mechanism of a precipitation hardened TC11 titanium alloy under very high cycle fatigue loading

TC11 alpha-beta titanium alloy (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) is widely used in the construction of aero-engine components. In service, these components are subjected to cyclic loading due to vibrations. This work aims to assess the fatigue properties of a TC11 alloy and to reveal the effect of silicide precipitates and α2 phase (Ti3Al) on fatigue behavior including failure mechanism in very high cycle fatigue (VHCF) regime. For this purpose, tension-compression fatigue experiments were carried out with an ultrasonic fatigue testing system. Fatigue crack initiation and early propagation mechanisms were finely investigated using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). The results show that this alloy exhibits a high fatigue strength at 109 cycles with a fatigue limit up to 618 MPa. Fracture surface observation indicates that the fatigue failure can be categorized into three modes: (a) surface failure in short life regime, (b) subsurface failure and (c) internal failure with the presence of a fish-eye pattern for long life regime. The internal and subsurface crack initiation regions are characterized by microvoids, dimples and peaks without the presence of crystallographic facets in the VHCF regime. This signifies that ductile damage mechanism instead of brittle one is the dominant failure mode for this alloy in the VHCF regime. Detailed observation of fracture surfaces seems to indicate that this ductile damage mechanism can be attributed to the combined effect of the silicide precipitates and α2 phase. Moreover, the results show that the cuboid micro-silicide precipitates significantly deteriorate the fatigue life of this studied alloy.

Improving surface integrity of additively manufactured GP1 stainless steel by roller burnishing

Additive manufacturing can rapidly fabricate the desired components by selectively melting and solidifying feedstock, rather than conventional subtractive machining. However, the difference between the two routes in terms of surface integrity of the final component is relevant. This paper presents a strategy to control the surface characteristics of additively manufactured stainless steel by roller burnishing. In particular, process parameters have been carefully selected to improve the surface integrity of the worked material. The quality of the surface has been analyzed in terms of roughness, hardness, microstructure and residual stresses. The overall product endurance under high cycle fatigue load has been evaluated.

Lifetime estimation for 316 stainless steel specimens by using a critical plane approach

The lifetimes of 316 stainless steel specimens are herein evaluated using the multiaxial critical plane-based criterion by Carpinteri et al. for high-cycle fatigue (HCF) loading with constant amplitude. In the present paper, the stress-based version of such a criterion is applied to assess the results of experimental tests under HCF triaxial loading, the hollow cylindrical dog-bone specimens being subjected to a quenching heat treatment. The analytical results of fatigue life and initial crack plane orientation are compared with the experimental data.

Static and fatigue behavior of basalt fiber-reinforced thermoplastic epoxy composites

The static, fatigue properties and their damage mechanism of basalt fiber-reinforced thermoplastic epoxy polymer composites are investigated. The stress–life curves and stiffness degradation under long-term cyclic loading were tested for basalt fiber-reinforced thermoplastic epoxy polymer. An advanced fatigue loading equipment combined with in situ scanning electron microscopy was used in the tests to track the damage propagations and analysis the fracture surfaces of all specimens. Results were also compared with those of thermosetting epoxy-based basalt fiber-reinforced polymer composites. The results show that the basalt fiber-reinforced thermoplastic epoxy polymer has good interface properties between the fiber and new thermoplastic epoxy, which results in high tensile strength and ductility. Different degradation rates of low- and high-cycle fatigue loads are observed for the basalt fiber-reinforced thermoplastic epoxy polymer composites. Under high fatigue stress levels, a high degradation rate of the fatigue life is found because the dominating damage pattern showed fiber fractures. At low and medium fatigue stress levels, the damage pattern is dominated by matrix cracking and interface debonding, which results in a low degradation rate of the fatigue life. A bilinear phenomenological fatigue model has a higher accuracy for fitting the stress–life data than linear fatigue models. In addition, 80–90% stiffness degradations are observed before failure for all stress levels at a stress ratio of 0.8. Furthermore, compared with thermosetting epoxy-based basalt fiber-reinforced thermoplastic epoxy polymer, the basalt fiber-reinforced thermoplastic epoxy polymer has similar static strength and similar fatigue life at high-stress levels. However, at low-stress levels, the fatigue life of the this polymer is much higher than that of thermosetting one.

Microstructural Control of Fatigue Behaviour in a Novel α + β Titanium Alloy

The novel titanium alloy TIMETAL® 407 (Ti-407) has been developed as an alternative to Ti-6Al-4V (Ti-6-4), for applications that demand relatively high ductility and energy absorption. Demonstrating a combination of lower strength and greater ductility, the alloy introduces a variety of cost reduction opportunities, including improved machinability. Thermo-mechanical processing and its effects on microstructure and subsequent mechanical performance are characterised, including a detailed assessment of the fatigue and crack propagation properties. Demonstrating relatively strong behaviour under high-cycle fatigue loading, Ti-407 is nevertheless susceptible to time-dependent fatigue effects. Its sensitivity to dwell loading is quantified, and the associated deformation and fracture mechanisms responsible for controlling fatigue life are explored. The intimate relationship between thermo-mechanical processing, micro-texture and fatigue crack initiation through the generation of quasi-cleavage facets is highlighted. Consistent fatigue crack growth kinetics are demonstrated, independent of local microstructure.

Microplasticity behavior study of equiaxed near-β titanium alloy under high-cycle fatigue loading: crystal plasticity simulations and experiments

A crystal plasticity finite element (CPFE) model has been developed to study the microplasticity behavior of Ti-5553 alloy with equiaxed dual-phase microstructure under high-cycle fatigue (HCF) loading process. The microstructure and density distributions of geometrically necessary dislocations (GNDs) after HCF loading were characterized by electron backscattered diffraction (EBSD). Experimental results show that the microplasticity accumulation behavior happens in α phase during the HCF loading process. The developed CPFE model captures the essential physics of microplasticity accumulation in the primary α phase and stress concentration at the softer α/β interface. Besides, the effect of stress levels and volume fraction of primary α phase on microplasticity behavior was discussed. Results indicate that the stress concentration can be taken as fatigue indicator parameter (FIP) for high stress level HCF loading conditions, while the cumulative shear strain can be taken as FIP for low volume fraction of primary α phase conditions. This work provides a strategy not only contributing to understand the microplasticity behavior during HCF loading process but also choosing a suitable FIP for different loading conditions and microstructure states.

Effect of Ultrasonic Peening Treatment on VHCF Behavior of Friction Stir Welded Joints in Aluminum Alloys

Studying the mechanical properties of aluminum alloy 6061 friction stir welded (FSW) joints after ultrasonic peening treatment (UPT) under very high cycle fatigue (VHCF) loading. The effect of joints ultrasonic peening treatment was explored. It can be seen from the results that the ultrasonic peening treatment can form a reinforced layer on the surface of joints, and the fatigue crack source is transferred from the surface of the specimen to the inside. However, the change of fatigue crack initiation site does not improve the fatigue strength of specimen through ultrasonic peening treatment. The fatigue strength of the aluminum alloy 6061 joint after ultrasonic peening treatment is even lower than as a received condition. The characteristics of fatigue crack initiation and the mechanism of surface strengthened layer on fatigue strength were analyzed and discussed.

Development of nail-anchor strengthening system with iron-based shape memory alloy (Fe-SMA) strips

This work discusses the development process of a direct fastening technique for strengthening steel members using iron-based shape memory alloy (Fe-SMA) strips. First, several lap-shear tests were performed to examine the efficiency of five nail-anchor systems in terms of load-carrying capacity, slippage, and convenience of installation. To this end, 3D optical digital image correlation (DIC) measurements (using an ARAMIS system) and numerical simulation were conducted. Different failure modes including tensile rupture of the strips, shear rupture of the nails, and/or sliding of the strips were observed and discussed. The optimum nail-anchor system was then determined. This system has a very short anchorage length of approximately 50 mm and only 12 nails (i.e., 3 × 4). Second, to demonstrate the efficiency of the proposed system, a steel I-beam was strengthened with the selected nail-anchor system and activated using an infrared heating (IRH) technique. The SMA-strengthened beam was subjected to static and then high-cycle fatigue loadings with a loading ratio of R=0.2 for 2.5 million cycles without any failure or damage, indicating reliable performance of the system. Owing to the larger coefficient of thermal expansion of the Fe-SMA material compared to that of steel, it was observed that a decrease and increase in the environmental temperature resulted in an increase and decrease of the pre-stress level in the Fe-SMA strip, respectively (i.e., an inverse correlation).

Multiscale Analysis of Concrete Damage and Crack Propagation Under High Cycle Loading

Concrete is a typical multiphase composite material in which the initiation and propagation of cracks under fatigue load are mainly determined by its mesoscopic structure. In this paper, a concrete multiphase mesoscopic model which considers thickness of interfacial transition zone (ITZ) varying with aggregate’s size is established by the integrated scripting method. The model comprehensively contains the stochastic characteristics of concrete mesoscopic structure. According to the fatigue crack propagation characteristics in different stages, a multiscale method is proposed by establishing the interactive mesoscopic and macroscopic models to targetedly analyze the whole process of fatigue crack initiation, propagation and failure of concrete specimen. Based on the technique of cycle block, concurrent simulation of concrete damage and crack propagation under high-cycle fatigue load is realized. The analysis results show that fatigue cracks in concrete mainly born near the ITZ and gradually enter into the cement mortar matrix to formulate the cracks with finite size. These cracks influence each other until the dominant crack appears and insatiably propagates to result in the failure of specimen.

The effects of grain size, dendritic structure and crystallographic orientation on fatigue crack propagation in IN713C nickel-based superalloy

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendritic structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting pores and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendritic areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendritic areas. Here, we propose a concept of ‘Crack Propagation Unit (CPU)’ for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendritic structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pile-up at the grain boundary or in-between the γ/γ′ channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters.

Microstructure evolution of 400 MPa class rebar produced by QST and VNM technology under the high strain and low cycle fatigue

The 400 MPa grade hot rolled rebar are widely used as the construction and building materials. The main produced technologies for this rebar are V-N Microalloyed (VNM) and Quenched and Self-Tempered (QST). Under the high strain and low cycle fatigue (HSLCF) loading, the microstructure evolution and performance of above two types of rebars were investigated systematically. In order to evaluate the rate of energy-absorption under HSLCF, a new model named cyclic toughness was proposed. The results show that QST rebar exhibit greater rate of cyclic softening over ±1% strain amplitude, which caused by the secondary cracks in the quenched and self-tempered martensite microstructure of surface layer and the formation of larger dislocation cell structure. Additionally, VNM rebar had higher cyclic toughness (15.4 J/cm3) due to higher dislocation density and smaller dislocation cell structure in the cyclic hardening stage, which indicate that the HSLCF performance of VNM rebar are much better than QST rebar.

Iron-based shape memory alloy (Fe-SMA) for fatigue strengthening of cracked steel bridge connections

Smart iron-based shape memory alloys (Fe-SMAs) are used in this study to retrofit fatigue-cracked riveted connections in steel bridges. The prestressed strengthening technique is found to be an effective approach to overcome fatigue-related damage in riveted connections. Because of the property of Fe-SMAs known as shape memory effect, these alloys can be prestressed without difficulty. The activated (i.e., prestressed) Fe-SMA strips (two 50-mm wide × 1.5-mm thick) are anchored to the flanges of a steel I-beam in either side of the connection. Thereafter, a test setup is specifically designed to examine the SMA-strengthened cracked double-angle connections. First, a static test is performed on the unstrengthened connection without any crack. Subsequently, two high-cycle fatigue (HCF) tests are conducted on a pre-cracked connection. The pre-cracked connection with no strengthening is subjected to fatigue loading with a load ratio of R = 0.1. After practically N=2×106 loading cycles, the crack propagates up to 50% of the connection depth, whereas the fatigue crack growth rate gradually decreases because of the reduction in connection rigidity. Finally, the SMA-strengthened connection is subjected to the HCF loading. It is observed that the fatigue life is substantially enhanced, and the fatigue crack is arrested by the activated Fe-SMA strips.