High Cycle Fatigue Regime Research Articles

Very High Cycle Fatigue Properties and Damage Evolution in Al-Si Cast Alloys

Abstract The steady progress in electro mobility increases the demand for lightweight space frame and engine solutions. Advanced aluminum-silicon cast alloys with additions of magnesium, copper, or both for precipitation hardening are promising candidates for lightweight structures because of their high strength-to-weight ratio. For a reliable design, the fatigue performance in the very high cycle fatigue (VHCF) regime must be understood. Therefore, this study deals with the influence of frequency on VHCF properties as well as fatigue damage evolution in AlSi7Mg and AlSi10Mg sand-cast alloys. The VHCF tests were performed using a resonant (70 Hz), a high-frequency resonant (1 kHz), and an ultrasonic (20 kHz) fatigue testing system. Thereby, a significant frequency effect could be determined that resulted in an increased fatigue lifetime by 1 decade at 1 kHz and by 2 decades at 20 kHz, whereas no significant change in fatigue limit could be determined. Moreover, the fatigue crack initiation mechanisms change from surface crack initiation at 70 Hz and 1 kHz to volume crack initiation at 20 kHz. Using the defect-based Murakami approach, including a lightweight extension of Noguchi, a defect-induced (size, location) frequency effect could be excluded. Based on damage monitoring, the effect could be related to the crack propagation phase, which is accelerated at low frequencies (<100 Hz) because of a humidity reaction with the fatigue crack surface and the formation of atomic hydrogen resulting in a saturated hydrogen embrittlement per cycle. At high frequencies of 1 kHz and 20 kHz, no saturated hydrogen embrittlement per cycle can be reached.

Data-driven approach to very high cycle fatigue life prediction

The research on life prediction for mechanical structures in very high cycle fatigue regime is pivotal to improve structure service, but it can be costly and time-consuming to collect fatigue data. In response, the data-driven approach of machine learning emerged as a solution to data insufficiency. In this work, after extracting a small dataset of GCr15 bearing steel subjected to very high cycle fatigue tests from open literature, the Z-parameter model was applied to obtain extended datasets to establish models driven by support vector machine, artificial neural network, and Z-parameter based physics-informed neural network, respectively. With training on extended datasets and the original data as test set, fatigue life prediction for GCr15 steel was carried out and evaluated between these models. Results showed that the physics-informed neural network calibrated by Z-parameter model trained on a larger dataset featured more accurate and reliable prediction than other models did, which demonstrated effectiveness of Z-parameter in data extension and model construction as priori physics knowledge for a data-driven approach. Looking into the future, Z-parameter model deserves more attention to its employment in life prediction for more engineering materials and structures serving in the very high cycle fatigue regime.

Cyclic deformation behavior of non-oriented electrical steel sheets

The cyclic deformation behavior of a fully processed, non-oriented electrical steel sheet is investigated in dependence on loading condition and control mode. Therefore, strain- and stress-controlled fatigue tests are performed to determine cyclic deformation curves in the low- and high-cycle fatigue regimes. With symmetric strain control, continuous cyclic hardening occurs, while in asymmetric stress control with non-zero mean stress, pronounced ratcheting takes place. Depending on the loading condition, the material can exhibit a transitional period with cyclic softening. This effect is present at intermediate amplitudes in the yield point region. It emerges as the dislocation movement is initially inhibited, while at higher amplitudes, cyclic hardening occurs from the very beginning. The evolving dislocation structures are characterized by the predominantly planar slip behavior of the alloy.

Notch effect in very high-cycle fatigue behaviour of a structural steel

The presence of a notch or a detail which leads to stress concentration due to complex geometries has a considerable impact on the fatigue life of engineering structures. Therefore, on the one hand, the notch effect must be investigated and considered in the fatigue assessment of these structures. On the other hand, in the last decades, an interest in studying the very high-cycle fatigue regime (VHCF) has been perceived, due to the extension of the service life of engineering components beyond the 10 million cycles. Thus, the notch effect in the VHCF regime was analysed for the S690 structural steel. Therefore, an analytical approach to design notch specimens was developed and a numerical simulation was performed to assess it. Then, an experimental campaign with smooth and notched specimens with two different stress concentration factors was performed in an ultrasonic fatigue machine to assess the VHCF behaviour. The mechanisms of failure and fracture surfaces were also investigated. It was observed a fully notch sensitivity as well as the dominance of surface crack initiation for the S690 steel in the VHCF regime.

A data-physics integrated approach to life prediction in very high cycle fatigue regime

The defects created in metallurgical and manufacturing processes generally play a decisive role in very high cycle fatigue life of engineering structures. By taking stress level, defect size and location into account, the physical Z-parameter model on fatigue life prediction was combined with artificial neural network as a new data-physics integrated approach for fatigue life prediction of 15Cr and FV520B-I steels in this work. The original data from tests were expanded based on the Z-parameter model, and the physics-informed loss function featuring Z-parameter was integrated into artificial neural network as the constraint. Results showed that the physics-informed neural network established in this work could be applied for life prediction in the very high cycle fatigue regime, and the model came with higher predictive accuracy than the physical Z-parameter model and the Mayer’s model did.

An energy-CP-combined model for predicting the fatigue life of polycrystalline materials under high cycle and very high cycle fatigue

A fatigue life prediction model in the high to very high cycle fatigue (VHCF) regime of as-received 7075-T6 Al alloy material is proposed. The relative parameters in the life prediction model are obtained by crystal plasticity finite element (CPFE) framework. The modified Ohno-Wang constitutive model is embedded into the CPFE model considering the evolution of back stress during the cyclic loading. In the life prediction model, the range of energy efficiency factors is validated by verifying the plastic strain energy density (PSED) with fatigue experimental data. The fatigue experiments are performed using an ultrasonic fatigue system to validate the fatigue life prediction model. The predicted life is estimated using the maximum and minimum energy efficiency factors, it is found that the life prediction model can accurately predict the VHCF life. It is found that 72.8%–81.3% of the experimental fatigue life can be captured using the proposed model. Moreover, the localized deformation in the grain scale can be well-captured considering the heterogeneity of mechanical properties.

Very High Cycle Fatigue and Ultrasonic Testing in Multiaxial Loading

Abstract Fatigue damage has special relevance on the lifespan of mechanical components and structures, as it takes responsibility for the majority of registered structural failures. Most fatigue testing today still uses uniaxial loads, nonetheless it is generally recognized that multiaxial stresses occur in many full-scale structures. Nowadays, the growing need for greater lifespans forced the study of material behavior under what is nowadays very high cycle fatigue (VHCF) regime, fatigue research beyond 10E07 up to 10E10 cycles. However, most classic fatigue machines are often very costly, both in terms of time and energy, for such an elevated number of cycles. The high costs of equipment and time to conduct such experiments have seen a massive improvement with the introduction of high frequency ultrasonic machines. Presently, it is already possible to characterize materials VHCF response under axial and multiaxial loading conditions in a fraction of the time. Ultrasonic testing machines were first introduced for simpler uniaxial loading and more recently adapted for multiaxial loading conditions. The present work reviews the current background of ultrasonic fatigue testing machines working at 20-kHz frequency, with emphasis on multiaxial fatigue and VHCF. A review of a series of fatigue tests are presented, respectively, axial, shear, axial/shear, and in-plane bi-axial fatigue tests with different combining dimension specimens allowing different shear/axial stress ratios. Special attention will be put into the performance of multiaxial classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low-cost piezoelectric transducers. By comparing all so far developed and successfully reached multiaxial specimens and respective methodologies, two base approaches were possible to be established. When designing new specimens or ultrasonic machines, or both, one of the two methods must be followed. To have a well-comprehended methodology and successful design approach will undoubtedly guide future multiaxial ultrasonic method ideas.

Effect of basal texture on small crack behavior of magnesium alloy in very high cycle fatigue regime

AbstractThe behavior of small cracks in magnesium alloy is key to understanding very high cycle fatigue (VHCF), which is influenced by microstructure and deformation mechanism. This study explores how basal texture affects VHCF failure mechanisms in extruded magnesium alloys. Findings reveal that faceted and rough morphologies characterize fracture surfaces during crack initiation and early propagation, respectively. Basal texturing inhibits basal slipping during faceted‐like crack initiation while compression twinning dominates deformation during early propagation, contributing to rough zones. Stress intensity factor and plastic zone size analysis indicates significant influence of basal texture on VHCF failure mechanisms but with limited effect on stable propagation.

High-cycle fatigue behavior and chemical composition empirical relationship of low carbon three-sheet spot-welded joint: An application in automotive industry

In this paper, the authors have attempted to provide an empirical relationship between the fatigue behavior of three-sheet spot-welded joint and the chemical composition of low carbon steel. To this end, the application of this joint in the automotive industry was considered and laboratory samples were prepared based on the actual specifications in the industry, including the raw material (i.e., material and thickness of the primary sheets), the resistance spot welding (RSW) process parameters, and other factors. The results of tensile and quantometric tests along with microscopic observations were utilized to evaluate the raw material and to study the compliance of the steel grade with the required standards. Next, axial cyclic test was performed in order to extract the high-cycle fatigue (HCF) properties of three-sheet spot-welded joint. Finally, a relationship between the number of cycles to failure of the spot-welded joint, repetitive load level, and the percentage of constituent elements was presented by multiple linear regression (MLR) technique. The results showed that the greatest effect of the constituent elements is when we are in the regime of low-cycle fatigue (LCF) and by moving towards the HCF regime, its importance decreases until it becomes almost ineffective in the very-high-cycle fatigue (VHCF) area. In addition, the presented relationship is able to predict the fatigue behavior of three-sheet spot-welded joint via the chemical composition of the primary sheet and the cyclic force with a maximum error of 13.8% compared to the experimental results.

Influence of trace rare earth elements on very high cycle fatigue behavior of hot rolled bearing steel

In this study, the effect of trace rare earth (RE) elements on the fatigue property of bearing steel in very high cycle fatigue regime was investigated. For this purpose, ultrasonic tension–compression fatigue tests were conducted on specimens extracted from hot rolled bearing steels with and without RE addition. Crack propagation behavior within the characteristic “fish-eye” (FiE) area and the non-inclusion induced fatigue failure were analyzed after the fatigue test. Results showed that the addition of trace RE can modify inclusions to varying degrees. Further, it significantly reduces the volume fraction of inclusions as well as the number of large inclusions. Consequently, the fatigue life of bearing steel is more than doubled based on the 10% and 50% failure probabilities. During the propagation of cracks within the FiE area, the microstructure and intrinsic defects in bearing steel exhibit high sensitivity to the instantaneous driving force for crack propagation. The discontinuity in their size may result in abnormally slow crack propagation. For the non-inclusion induced fatigue failure of bearing steel, microcracks preferentially form at the junction of lath interfaces or at the enclosed area of martensite laths/plates which tend to grow along the interfaces perpendicular to the loading direction.

Crack initiation mechanisms and life prediction of GH4169 superalloy in the high cycle and very high cycle fatigue regime

In this study, the high cycle and very high cycle fatigue behavior of the GH4169 superalloy was investigated. The results of the fatigue tests demonstrated that failures in the high cycle fatigue regime were initiated from the material surface, whereas failures in the very high cycle fatigue regime had two competing modes of crack initiation: surface and internal. The fatigue fracture analysis revealed a quasi-cleavage fracture mode with facets for internal crack initiation. Additionally, the electron backscatter diffraction (EBSD) results indicated that the dislocations located at the internal twin boundaries of the material were preferentially activated under fatigue cyclic loading, resulting in significant strain localization at the twin boundaries. The {111} <110> slip systems with high Schmid factors had a higher probability of activation. The stress intensity factor (SIF) estimated based on rough area (RA) and facet sizes correspond to the fatigue long crack and short crack growth thresholds of the material, respectively. Finally, a modified FIP fatigue life prediction model was proposed based on the microstructure of crystallographic facets, and the results from experiment and the model prediction were found to be in good agreement.

A novel evaluation method for high cycle and very high cycle fatigue strength

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.

Crack Initiation in Bulk Matrix of Austenitic Stainless Steel during Very High Cycle Fatigue

Abstract In the very high cycle fatigue regime, fatigue crack initiation in high-strength steels is usually correlated to a subsurface inclusion with a fine granular area (FGA). Localized stress–strain concentration at the subsurface inclusion is a critical factor. Fatigue crack initiation with an FGA in the bulk matrix without any defect has rarely been reported. In this paper, a fundamental study on the formation of FGAs in the bulk matrix of an austenitic stainless steel has been carried out using a progressive stepwise load-increasing test with a cycle step of about 108 cycles. FGA formation in the subsurface bulk matrix has been observed. The microstructural damage in the fatigue-tested specimens has been studied using the electron channeling contrast imaging electron microscopy technique. Strain localization and grain fragmentation are the main processes for the formation of FGAs. Local plasticity exhaustion leads to crack initiation due to local stress concentrations. This method can also be used to predict the fatigue damage process, especially the damage rate in individual specimens.

Influence of Transformation Temperature on the High‐Cycle Fatigue Performance of Carbide‐Bearing and Carbide‐Free Bainite

This study investigates the high‐cycle‐fatigue (HCF) behavior of carbide‐bearing bainite (CBB) and carbide‐free bainite (CFB) fabricated at different transformation temperatures. The fatigue limit of each material is determined via staircase method using a 1 kHz resonant testing machine. A new load increase test is proposed as an efficient alternative to estimate the fatigue limit in HCF regimes. The assessment of the fatigue behavior is accompanied by data‐driven microstructural analyses via state‐of‐the‐art computer vision tools. The analyses reveal that the finer carbide distribution, which is obtained at lower transformation temperature, enhances the overall performance of CBB. Electron backscatter diffraction (EBSD) measurements of CFB before and after tensile testing evidence the transformation of retained austenite (RA) to martensite during deformation. The finer film‐like and stable RAs, which are promoted via reduction in transformation temperature, enhance the HCF properties by absorbing the energy required for fatigue crack propagation through improved transformation‐induced plasticity. However, blocky unstable RA and/or martensite‐austenite (MA) islands at prior austenite grain boundaries deteriorate the HCF properties of high‐temperature CFB. Furthermore, unindexed regions in EBSD maps are effectively used to differentiate the MA islands of CFB, as validated by scanning electron microscopy (SEM) images and deep learning‐based MA island segmentation.

Fatigue-induced HCP-to-FCC phase transformation assisting crack nucleation of pure zirconium in high cycle fatigue regime

This paper investigates the high cycle fatigue behavior of industrial pure zirconium at room temperature under a stress ratio of R = 0.1, focusing on the HCP-to-FCC phase transformation and its role in fatigue crack nucleation. The results show the presence of fatigue damage in the form of the HCP-to-FCC phase transformation, which is responsible for the crack nucleation. The fatigue-induced phase transformation is triggered by the activation and slip of partial dislocations. This phase transformation results in lattice expansion, local strain redistribution, and macroscopic strain. Notably, the strain concentration along the HCP/FCC phase interfaces is identified as a critical factor contributing to fatigue crack nucleation. These findings provide new insights into the mechanisms underlying fatigue-induced phase transformations and the nucleation of fatigue crack.

Effects of an enhanced fibre–matrix adhesion on the fatigue behaviour of composite materials under very high cycle fatigue

The fatigue behaviour of two glass fibre-reinforced epoxy composite materials with a cross-ply layup is investigated in the very high cycle fatigue (VHCF) regime with up to 10^8 load cycles. In addition to the influence on the fatigue lifetime, different materials can also cause a different damage phenomenology. One reference fibre (OC111A) and one fibre with increased fibre–matrix adhesion (SE2020) are tested. The experimental and numerical investigations show that the damage threshold is shifted to higher loads with increased fibre–matrix adhesion, but the damage mode of delaminations becomes more important.

Review of Fatigue Crack Initiation Mechanisms Development and Monitoring in the Very High Cycle Fatigue Regime

Abstract Fatigue devices working at high frequency (20 kHz) allow now to explore the fatigue domain with a total number of cycles much higher than 107 cycles in reasonable times. In that case, lifetime belongs to the very high cycle fatigue (VHCF) domain. As for low and high cycle fatigue (LCF and HCF), the fracture begins by microstructural damage because of microplasticity, before leading to the initiation stage (stage I), and then the short and long crack propagation stages (stage II). It is now admitted that in VHCF the initiation stage is higher than 90 % of the total life. The main difference with LCF and HCF is the crack initiation location. For materials without internal defects (type I), the initiation is always on the specimen surface, whereas for materials with internal defects (inclusions, porosities, etc.) named type II, the microstructure appears in subsurface on defects leading to a typical “fish eye” on fractographies. If the crack initiation mechanisms on surface are well accepted to be related to dislocations gliding in well-oriented grains, the mechanisms in subsurface are not very clear and many hypotheses on the fish-eye development are suggested by different authors. In the fish-eye center, it appears a fine granular area more or less large, the origin of which is debated. In the first part, this article proposes a review on these crack initiation mechanisms (surface and subsurface). Microplasticity produces a temperature increase in fatigue tests. It has been demonstrated that the temperature recording during tests is a mean to determine the number of cycles corresponding to crack initiation and crack propagation. In the second part, through two examples on Armco iron (surface initiation) and a low alloy steel (subsurface crack initiation), the method to determine the number of cycles during the initiation and propagation stages is exposed.

Experimental investigation and crystal plasticity simulation for the fatigue crack initiation of the equiaxed Ti-6Al-4V alloy in the very high cycle regime

This work investigated the fatigue crack initiation mechanism of the equiaxed Ti-6Al-4V alloy in the very high cycle fatigue (VCHF) regime. Ultrasonic fatigue experiments were carried out on Ti-6Al-4V specimens and a detailed analysis of the failure mechanism was conducted by means of scanning electron microscopy (SEM). The localized cyclic plastic response of the alloy was investigated using the crystal plasticity finite element (CPFE) method to further clarify the fatigue crack initiation process in the VHCF regime, and the CPFE models with varying transformed β(βt) phase volume fractions were presented to investigate the impact of βt phase on the VHCF behavior. The simulation results indicate that the proposed CPFE model can effectively captures the microstructural plastic deformation mechanisms, such as stress concentration near the phase boundaries and plastic accumulation. And the βt phase significantly affects the VHCF behavior of the alloy.

An Investigation Into the Synergistic Effect of CAPVD Cr/CrN Nanolayered Coating Deposition and Hard-Anodizing Methods on Fretting Fatigue Life of 7075-T6 Aluminum Alloy

The influence of duplex surface treatment including a Cr/CrN nanolayered coating deposited by cathodic arc physical vapor deposition (CAPVD) and an alumina layer coated by hard-anodizing method on the fretting fatigue strength of Al7075-T6 alloy is studied in this investigation. Although fretting fatigue life of both single Cr/CrN coated alloy and Cr/CrN coating deposition onto hard-anodized alloy was increased in high cycle fatigue (HCF) regime, employing an alumina layer as a pretreatment for the Cr/CrN deposition enhanced the life improvement in the HCF regime. In low cycle fatigue (LCF) regime, despite a decrease in fretting fatigue life of both Al7075-T6-coated alloys, using the alumina layer beneath the Cr/CrN coating resulted in mitigation of fretting fatigue life deterioration. Interestingly, a single-hard-anodized specimen represented a completely different variation trend of fretting fatigue life in HCF and LCF regimes. Unlike the as-received and single-hard-anodized specimens, Cr/CrN-coated specimens did not bear the maximum axial stress, 260 MPa. Partial slip regime was mostly seen at the contacting area.

Failure mechanism and life correlation of Inconel 718 in high and very high cycle fatigue regimes

High and very high cycle fatigue (HCF and VHCF) behaviours and failure mechanism of Inconel 718 were investigated via ultrasonic fatigue tests and microscopic characterization techniques. It is shown that the S-N curve continuously decreases in the HCF regime, while forming a plateau for VHCF. Cracks initiate from a single site on the surface or subsurface. Site-specific focused ion beam sampling and transmission electron microscopy characterization reveal that the crack initiation is governed by slip-induced localized plastic deformation. Microcracks initiate near non-metallic inclusions or slip band-grain boundary site. Plus, a life correlation model considering the size of micro-defect was proposed to better correlate the fatigue life.