Stress-controlled Fatigue Tests Research Articles

Comparative Study of High-Cycle Fatigue and Failure Mechanisms in Ultrahigh-Strength CrNiMoWMnV Low-Alloy Steels

This study provides a thorough analysis of the fatigue resistance of two low-alloy ultrahigh-strength steels (UHSSs): Steel A (fully martensitic) and Steel B (martensitic–bainitic). The investigation focused on the fatigue behaviour, damage mechanisms, and failure modes across different microstructures. Fatigue strength was determined through fully reversed tension–compression stress-controlled fatigue tests. Microstructural evolution, fracture surface characteristics, and crack-initiation mechanisms were investigated using laser scanning confocal microscopy and scanning electron microscopy. Microindentation hardness (HIT) tests were conducted to examine the cyclic hardening and softening of the steels. The experimental results revealed that Steel A exhibited superior fatigue resistance compared to Steel B, with fatigue limits of 550 and 500 MPa, respectively. Fracture surface analysis identified non-metallic inclusions (NMIs) comprising the complex MnO-SiO2 as critical sites for crack initiation during cyclic loading in both steels. The HIT results after fatigue indicated significant cyclic softening for Steel A, with HIT values decreasing from 7.7 ± 0.36 to 5.66 ± 0.26 GPa. In contrast, Steel B exhibited slight cyclic hardening, with HIT values increasing from 5.24 ± 0.23 to 5.41 ± 0.31 GPa. Furthermore, the martensitic steel demonstrated superior yield and tensile strengths of 1145 and 1870 MPa, respectively. Analysis of the fatigue behaviour revealed the superior fatigue resistance of martensitic steel. The complex morphology and shape of the NMIs, examined using the 3D microstructure characterisation technique, demonstrated their role as stress concentrators, leading to localised plastic deformation and crack initiation.

The Corrosion Fatigue Behavior and Mechanism of AerMet 100 Steel in 3.5% NaCl at Room Temperature.

AerMet 100 steel is a new type of double-hardened high-strength steel, which is often used as landing gear material in amphibious aircraft. In the present paper, the corrosion fatigue behavior and mechanism of AerMet 100 high-strength steel in a 3.5% NaCl solution was studied by stress-controlled fatigue tests and a series of subsequent characterizations of the fracture surface, microstructure, and cracks. The results indicated that the fatigue life of AerMet 100 high-strength steel decreased with a decrease in the stress level in a 3.5% NaCl solution, satisfying the relationship lgN = -2.69 × 10-3 σ + 6.49. The corrosion fatigue crack usually initiated from the corrosion pit and propagated across the martensitic flat noodles. Meanwhile, the corrosion fatigue crack tip was filled with Cr2O3, Fe2O3, and amorphous material; it propagated in the transgranular mode by a slip dissolution mechanism. This study provides some engineering significant for the fatigue performance of AerMet 100 steel in a 3.5% NaCl solution.

Fatigue Response of Additive-Manufactured 316L Stainless Steel

This study investigated the fatigue performance of 316L stainless steel fabricated via laser powder bed fusion (LPBF). Stress-controlled fatigue tests were performed at different stress amplitudes on vertically built samples using a frequency of 15 Hz and a stress ratio of 0.1. The stress amplitudes were varied to provide the cyclic response of the materials under a range of loading conditions. The average fatigue strength was determined to be 92.94 MPa, corresponding to a maximum stress of 185.87 MPa. The microstructures were observed through scanning electron microscopy (SEM) with the aid of electron backscattered diffraction (EBSD), and the average grain size of the as-built samples was determined to be 15.6 µm, with most grains having a <110> preferred crystallographic orientation. A higher kernel average misorientation value was measured on the deformed surfaces, revealing the increased misorientation of the grains. Defects were observed on the fractured surfaces acting as crack initiators while deflecting the crack propagation paths. The fatigue failure mode for the LPBF 316L samples was ductile, as illustrated by the numerous dimples on fracture surfaces and fatigue striations.

Influence of the strain rate on the fatigue behaviour of fully ferritic high chromium steel and P91 steel at high temperatures

Fully ferritic, high-chromium HiperFer alloy presents a promising option, providing a combination of both, high corrosion resistance and material strength above 600 °C. Although these steels show improved strength at creep and thermomechanical loading compared to conventionally used 9–12 % Cr-steels, a comprehensive understanding of their fatigue behaviour and the influence of different strain rates is required. Hence, in this study the fatigue behaviour of HiperFer-17Cr2 was analysed in stress-controlled fatigue tests performed at temperatures above 600 °C and at 0.05 Hz and 5 Hz. Moreover, a martensitic steel P91 was investigated as a reference. The results obtained for HiperFer–17Cr2 reveal an increase in fatigue strength with a decrease in frequency and an increase in stress. Additionally, an influence of the frequency on the cyclic deformation behaviour of HiperFer-17Cr2 was observed, which is characterized by cyclic hardening. This cyclic hardening correlates with an increase in hardness, which is associated with the deformation induced formation of Laves phase particles. After LCF load with lower frequency a stronger hardness increase as well as a higher amount of Laves phase particles and smaller particle-free zones at the grain boundaries were observed, contributing to the higher fatigue strength obtained at lower strain rate.

Influence of the cutting method on the fatigue life and crack initiation of non-oriented electrical steel sheets

The fatigue behavior of thin electrical steel sheets under cyclic loading is investigated in dependence on the edge surface. Therefore, four different edge conditions are compared, whereas the edge is either laser cut, shear cut, wire cut, or polished. Strain- and stress-controlled fatigue tests are performed to determine S-N curves in the low cycle regime as well as in the high cycle regime. Microstructural data is collected by non-contacting (optical) Profilometry, Nanoindentation, X-Ray Diffraction, and Electron Backscatter Diffraction to understand the differences in fatigue life by considering surface roughness, residual stresses, hardness, and microstructure. Shear cut specimens achieve the lowest fatigue life, while the other edge conditions reach relatively similar values in the LCF regime. Crack initiation is mainly intergranular in the case of defect-free edges. This tendency has a considerable influence on the observed fatigue behavior.

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.

Methodology for Application of Damage Mechanics Approach to Model High Temperature Fatigue Damage Evolution in a Turbine Disc Superalloy

Aeroengine gas turbine components operate under complex loading environments. Turbine disc is one such component which experiences stresses at high temperature and accumulates life critical cyclic damage in the material during usage. This accumulation of cyclic damage results into significant deterioration in material strength which in turn may initiate the failure in these rotating components. This necessitates the need to develop an advanced lifing approach for fatigue life assessment of turbine disc. As there are no available standards for damage mechanics application, an attempt has been made in the present study to develop a methodology for application of damage mechanics approach to model high temperature fatigue damage evolution in a turbine disc Superalloy. High temperature (650oC) stress-controlled fatigue tests on turbine disc alloy have been performed to evaluate parameters for damage mechanics based models. Using this approach, damage evolution has been simulated at specimen level. A good correlation has been observed in the damage mechanics based model’s predicted damage and experimentally determined values.

An experimental study of anisotropic fatigue behavior of rolled ZK60 magnesium alloy

The strong anisotropy of wrought magnesium alloy significantly affects its application in structural components bearing fatigue loads. This study aims to explore the effects of grain orientations (RN-N versus RN-R) and sampling surfaces (RT-R versus RN-R) on the fatigue behavior of the ZK60 Mg alloy. Stress-controlled fatigue tests and quasi-in-situ EBSD tests were carried out on three different specimens. The sampling surface and grain orientation both have significant effects on the fatigue life of samples, especially at low-stress amplitude. Although the sampling methods are different, the three samples show similar cyclic evolution patterns. The consumption of available twins dominates the cyclic hardening behavior in the fatigue early stage. When loaded along ND, abundant intersecting twins induced by various variants can produce large plastic deformation, which is detrimental to the fatigue resistance of RN-N samples. When loaded along RD, the activation of specific twin variants leads to the reduction of the twinning areas, thus decreasing the plastic deformation of RN-R and RT-R samples. In addition, the constraint force along the width direction and sandwich-like metallographic structure of RN-R samples will hinder the activation and growth of twins, which results in smaller twinning areas and higher fatigue life than RT-R samples.

Kinetics-based fatigue damage investigation of asphalt mixture through residual strain analysis using indirect tensile fatigue test

Fatigue damage, one of the major distresses of asphalt pavement, has been found to have a phenomenological correlation with the accumulated residual strain (RS) of the asphalt mixture tested by stress-controlled fatigue test with excessive creep. However, it remains a challenge to quantitatively model such phenomenological correlation. This study aims to address this challenge by applying the kinetics theory to the indirect tensile fatigue test (ITFT) with excessive creep data of various asphalt mixtures. First, ITFTs of asphalt mixtures under different conditions were conducted to analyze the RS response. Then, the RS kinetics model was established based on the fast-constant rate kinetics model. Finally, two of the kinetics model parameters, the RS constant rate (kc) and activation energy, were successfully applied to characterize the fatigue life (Nf) and the fatigue damage resistance of the asphalt mixture, respectively. It was found that the established RS kinetics model can accurately describe the development of the accumulated RS determined by ITFT. The kc determined by ITFT is an effective indicator for the rate of the initial damage evolving to the failure threshold. The established kc-based fatigue equation can be used to predict the Nf of the asphalt mixture tested by ITFT from kc. The RS accumulation activation energy can effectively characterize the fatigue damage resistance of the asphalt mixture tested by ITFT.

Effect of severe thermo-oxidative aging on the mechanical behavior and fatigue durability of short glass fiber reinforced PA6/6.6

The work deals with the fatigue lifetime estimation of Short Fiber Reinforced Thermoplastics (SFRP), with a focus on conjugated effects of thermal aging. Two materials containing 35% (V35) and 50% (V50) weight ratio of short glass fibers were aged for 500 h at 200°C in air and compared to the same materials in a Dry-As-Molded (DAM) state. Monotonic and fatigue tests were performed in samples machined out of injected plates and cut along three different orientations to the injection one (0°, 45°, 90°) in order to capture the anisotropy of the skin-core microstructure, classical in injected SFRP.Monotonic tensile tests evidenced the stiffening and embrittlement of the Polyamide matrix reported in the literature, nevertheless with an acuity depending on the matrix ratio and fiber orientation. Stress-controlled fatigue tests were performed at constant amplitude, frequency (10 Hz), temperature (200°C) and stress ratio R = 0.1. The fatigue curves of V35 are more affected by aging than those of the V50 material. The combined results from the mean strain evolutions and SEM observations confirmed that the initiation approach for fatigue lifetime estimation was still valid in aged composites. Several fatigue criteria, among the most recently reported in the literature, were evaluated from this database. The effect of aging on the cyclic evolution of the different Fatigue Indicator Parameters (FIP) involved in the criteria was analyzed preliminarily. For the present fatigue conditions, the best criteria for unaged materials (i.e cyclic creep energy-based ones) were shown to still be the best for aged materials. Finally, the ability to predict fatigue lifetime for aged composites from the identification of the fatigue criterion in the unaged state was evaluated.

Dwell Fatigue Behavior of Two-Phase Ti-6Al-4V Alloy at Moderate Temperature

AbstractFatigue life of titanium and titanium alloys is frequently reduced if the load holds at the maximum stress are introduced, which is termed dwell debit. Many factors affect dwell fatigue of titanium alloys, like stress state, level and ratio, alloy chemistry, microstructure and microtexture, holding time and temperature. Dwell sensitivity of titanium alloys is usually attributed to the phenomenon of load shedding, which is a time-dependent redistribution of stress from so-called ‘weak’ to ‘strong’ grains possessing different crystallographic orientations. Stress concentration leads to crack initiation and formation of quasi-cleavage facets in ‘strong’ grains. Another factor contributing to dwell sensitivity of titanium alloys is time dependent strain accumulation, which is observed even at room temperature. In the paper, the effect of load holds and volume fraction of primary α phase on the fatigue behavior of Ti-6Al-4V alloy at the temperature of 150°C was investigated. Two variants of bi-modal microstructure of the alloy were obtained by means of the heat treatment. Stress controlled fatigue tests with and without hold time at maximum load and constant load creep test were carried out.

Stress-controlled fatigue of HfNbTaTiZr high-entropy alloy and associated deformation and fracture mechanisms

The stress-controlled fatigue tests are carried out at a stress ratio of 0.1 and a frequency of 10 Hz, and span both low-cycle and high-cycle regimes by varying the applied stress amplitudes. The high-cycle fatigue regime gives a fatigue strength of 497 MPa and a fatigue ratio of 0.44. At equivalent conditions, the alloy's fatigue strength is greater than all other high-entropy alloys (HEAs) with reported high-cycle fatigue data, dilute body-centered cubic alloys, and many structural alloys such as steels, titanium alloys, and aluminum alloys. Through in-depth analyses of crack-propagation trajectories, fracture-surface morphologies and deformation plasticity by means of various microstructural analysis techniques and theoretical frameworks, the alloy's remarkable fatigue resistance is attributed to delayed crack initiation in the high-cycle regime, which is achieved by retarding the formation of localized persistent slip bands, and its good resistance to crack propagation in the low-cycle regime, which is accomplished by intrinsic toughening backed up by extrinsic toughening. Moreover, the stochastic nature of the fatigue data is neatly captured with a 2-parameter Weibull model.

An experimental setup for fatigue testing of thin electrical steel sheets

In this study, the fatigue behavior of thin electrical steel sheets under cyclic loading is investigated. Results from strain-controlled and stress-controlled fatigue tests with different specimen geometries and different test setups are presented and compared with conventional testing methods. The results imply that conventional testing methods should be adjusted for testing thin electrical steel sheets because the fatigue life depends significantly on the test setup as well as the specimen geometry. Therefore, this study proposes an improved specimen geometry and test setup for stress- and strain-controlled fatigue tests of thin electrical steel sheets depending on the desired testing parameters.

Investigations of fatigue crack propagation in ER8 railway wheel steel with varying microstructures

In this study, fatigue crack occurrences and propagations of high-speed railway wheel steel grade ER8 were investigated through both experiments and FE simulations. Three varying microstructures of the wheel steel including as-received, ferrite- and bainite-containing microstructures, which were obtained by specific heat treatment routes, were characterized. Stress-controlled fatigue tests were carried out for determining relationships between crack propagation rate and stress intensity factor of the steels. In addition, FE simulations of fatigue crack developments on both macro- and micro-scale using representative volume element (RVE) 2D models were conducted with consideration of microstructural features. The extended finite element method (XFEM) coupled with the Paris law was applied in the cyclic simulations for predicting crack propagation. For the microstructure level, flow stress-strain curves based on micromechanics approach and corresponding fatigue damage parameters of different phases in steels were identified and afterwards calibrated with experimental results. Finally, influences of ferrite, pearlite and bainite on fatigue crack resistance in the wheel steel were precisely examined by the RVE model. It was found that fatigue micro-cracks mostly initiated close to interface regions between soft and hard phases. The bainitic phase could obviously retard fatigue crack propagation of steel, whereas the ferritic phase contrarily exhibited deteriorated crack resistance. Moreover, the proposed framework could be acceptably employed for predicting fatigue crack propagation behavior and its rate in railway wheel subjected to microstructure changes occurred.

Formation process of fatigue slip bands with unique configurations of ultrafine-grained high-purity Cu fabricated by severe plastic deformation

Fatigue-induced grain growth was observed in ultrafine-grained (UFG) metals processed by the severe plastic deformation technique, and the slip bands (SLBs) formed on coarse grains served as potential crack initiation sites. The SLBs in conventional grain-sized materials are characterized as parallel linear-like configuration along with primary slip orientation. By contrast, four typical configurations of SLBs were commonly observed in UFG materials, including granular, square lattice-like, triangular lattice-like, and parallel linear-like configurations. In the present study, stress-controlled fatigue tests were conducted on oxygen-free copper processed by equal-channel angular pressing under constant stress amplitudes. In addition, two-step block loading fatigue tests were carried out to observe the formation behavior of SLBs in a large dynamically recrystallized grain subjected to a higher cyclic stress. The objective of this study was to investigate the formation process of SLBs with a variety of configurations in UFG high-purity copper based on the microstructural evolution and the change in surface morphology because of cyclic stressing.

Effects of Surface Softening on the Mechanical Properties of an AISI 316L Stainless Steel under Cyclic Loading

The effects of surface softening on fatigue behavior of AISI 316L stainless steel were investigated. Using cold-rolling and electromagnetic induction heating treatment, a gradient structure was fabricated on AISI 316L stainless steel within which the grain size decreased exponentially from micrometers to nanometers to mimic the surface softening. Stress-controlled fatigue tests were applied to both the gradient and homogeneous structures. Compared with the homogeneous sample, surface softening had no evident effect on fatigue behavior when the stress amplitude was greater than 400 MPa, but significantly deteriorated the fatigue behavior at stress amplitude ≤400 MPa. At high-stress amplitude, fatigue behavior is dominated by crack propagation. When the stress amplitude is lowered, strength reduction and stress concentration caused by surface softening accelerate crack initiation and propagation, resulting in an inferior fatigue behavior.

Effect of laser beam welding on microstructure, tensile strength and fatigue behaviour of duplex stainless steel 2205

This study focuses on the microstructure, tensile and fatigue properties of laser beam welded butt joints in 4 mm thick sheets of duplex stainless steel 2205. The microstructural characteristics of the joints were investigated via optical microscopy and electron backscattered diffraction. A 300 μm wide heat affect zone with increased ferrite content and a nearly fully ferritic 800 μm wide fusion zone were found. No porosity could be found with X-ray radiography. Microhardness measurements revealed increased strength in the fusion and heat affect zones of the weldments. Uniaxial tensile and stress controlled fatigue tests were performed in order to characterize the mechanical properties of specimens containing laser beam welded joints and the base material. The specimens containing weldments were stronger, but less ductile than the base material, due to the weld metal restricting deformation. The as-welded joints exhibited worse fatigue performance than the base material due to the notch at the excess weld metal. The fatigue properties of the specimens containing joints could be elevated to the base material level by a laser surface remelting treatment.

In situ neutron diffraction study of fatigue behavior of CrFeCoNiMo0.2 high entropy alloy

In situ neutron diffraction measurement was applied to study the low-cycle fatigue behavior of CrFeCoNiMo0.2 high entropy alloy. A two-step stress-controlled fatigue test with a stress range of 100–460 and 20–500 MPa was employed. The as-cast sample was cycled 129,000 and 61,000 times, respectively, before fracture. The evolution of the lattice strain and peak intensity demonstrates that the main deformation mechanism is dislocation slip, while no evidence of stacking fault was found. A three-stage ratcheting was clearly observed. Under the stress-controlled mode, the experimentally determined dislocation density increases linearly with the ratcheting strain. The increase of the dislocation density results in a decay of the ratcheting strain rate, which stabilizes the structure and is beneficial for fatigue resistance.

Understanding the influence of temperature and frequency on the fatigue resistance of bitumen

Fatigue performance of bitumen holds great importance in the design and durability of asphalt pavements. However, there are still gaps in knowledge on the influence of specific experimental conditions on the degradation of bitumen under cyclic loading. Stress-controlled fatigue tests at three temperatures (-10 °C, 15 °C and 20 °C) and two loading frequencies (10 Hz and 20 Hz) were performed on cylindrical specimens of a 50/70 grade bitumen, using a recently developed fatigue testing protocol in the Dynamic Shear Rheometer (DSR) equipment. Experimental data were analysed through multiple analysis approaches, including a novel ‘failure lines’ method that uses the dissipated energy (DE) of the material and the number of cycles to failure (Nf). Experimental results corroborated the impact of temperature and frequency testing conditions on the fatigue response of the bitumen. For example, at 200 kPa and 20 °C, the bitumen dissipated 3.07 times more energy at failure at 10 Hz than at 20 Hz; while at 400 kPa and 10 °C, the bitumen only dissipated 1.07 times more energy at 10 Hz than at 20 Hz. Also, depending on the applied stress, the bitumen tested at 10 Hz dissipated between 2.45 and 4.02 times more energy at 20 °C than at 15 °C, and between 4.11 and 6.52 times more energy at 15 °C than at 10 °C. Additionally, 15 °C was identified as a ‘frequency-free transition temperature’ for this bitumen, as the ‘failure lines’ at both frequencies overlapped. Finally, a new temperature and frequency independent fatigue representation was identified when analysing the initial DE and Nf data.

Mechanical properties and microscopic analysis of sintered rhenium subjected to monotonic tension and uniaxial fatigue

Some reports on the mechanical testing of pure rhenium can be found in the literature. However, very few studies have been published regarding the fatigue properties of rhenium. To gain new knowledge on this subject, the present research aimed to determine mechanical properties in monotonic tension, hardness, and stress-controlled fatigue tests of sintered rhenium. The results of the basic mechanical tests are in agreement with data in the literature. The determined exploratory S–N fatigue curve for pure, sintered rhenium is a unique result, and it cannot be compared with others. Based on the monotonic tests data, estimated S–N curves were also determined. The comparison of the experimental curve and the estimated curves showed the usefulness of approximation methods to estimate the fatigue properties of rhenium. Considering the high cost of conducting research on rhenium, this is especially important for engineering design. Subsequently, microscopic fracture analysis and microstructure evolution observation were performed. Microscopic observations were made using a light optical microscope and a scanning electron microscope. Based on the performed metallographic analyses, it was found that under fatigue and monotonic loadings, cracks propagated along the grain boundaries, and the pores present in the material did not affect this process. It was confirmed that the basic deformation mechanism in sintered rhenium is twinning. To sum up, the present work provides new data on the strength properties of rhenium and helps to better understand the deformation and damage processes of this metal.