Rotating Bending Fatigue Tests Research Articles

Fracture Behavior of Ion-Nitrided AISI 4140 Steel in accordance with Variable Applied Current Density

In this study, the fracture behavior of AISI 4140 low-alloy steel nitrided with respect to the applied current density was investigated. A series of rotary bending fatigue tests were performed with various loads (350, 400, 450, and 500 N) at a constant rotational speed of 2000 rpm. The results show that the increase in the fatigue strength of the steel (up to 35%) strongly depends on the compound layer formed during the nitriding process. In addition, the compressive stress generated by nitrogen-ion bombardment and implantation had an advantageous effect on the substantially enhanced fatigue strength; it acted as a protective layer to secure the surface from any external impact. It was found that fatigue strength is increased with increasing fracture toughness index on the surface on the sample. It had the highest fracture toughness index and fatigue strength at about 0.85 mA/cm2 of applied current. The findings of this study will pave the way for applications in related industries.

Effect of Diamond Burnishing on Fatigue Behaviour of AISI 304 Chromium-Nickel Austenitic Stainless Steel.

The disadvantages of widely used austenitic stainless steels are their low hardness and relatively low fatigue strength. Conventional chemical-thermal surface treatments are unsuitable for these steels since they create conditions for inter-granular corrosion. An effective alternative is a low-temperature surface treatment, creating an S-phase within the surface layer, but it has a high cost/quality ratio. Austenitic steels can increase their surface micro-hardness and fatigue strength via surface cold working. When the goal is to increase the rotating bending fatigue strength of austenitic chromium-nickel steels, and the requirements for significant wear resistance are not paramount, diamond burnishing (DB) has significant potential to increase the fatigue strength and, based on the cost/quality ratio, can successfully compete with low-temperature chemical-thermal treatments. The main objective of this study is to establish the effect of DB on the rotating fatigue strength of AISI 304 L chromium-nickel austenitic steel. The influence of DB parameters on the surface integrity (SI) characteristics was studied. Optimal DB parameters under minimum roughness and maximum micro-hardness criteria were obtained. Rotating bending fatigue tests of the diamond burnished (in a different manner) and untreated specimens were performed. DB implemented via parameters providing maximum micro-hardness increased fatigue limit by 38% compared to untreated specimens.

Exploration on the fatigue behavior of low-temperature carburized 316L austenitic stainless steel at elevated temperature

The fatigue behavior of low-temperature carburized 316L austenitic stainless steel at elevated temperature was investigated. Uniaxial tensile tests and rotating bending fatigue tests were performed on untreated and carburized 316L austenitic stainless steel at 300 °C, 400 °C, 500 °C and ambient temperature. Results show that carburized case can improve the fatigue strength at both ambient and elevated temperatures significantly. The improvements of tensile strength and endurance limit by carburized case at elevated temperature are even higher than those at ambient temperature. Moreover, the higher improvement of fatigue performance at elevated temperature is mainly due to the better strengthening effect of carburized case on the strength, the brittleness reduction and the redistributions of carbon content of carburized case. Based on the observation of fracture morphologies, fatigue fracture mechanisms of untreated and carburized specimens at different temperatures were analyzed. For untreated specimens, fatigue cracks always initiate from the surface at both ambient and elevated temperatures. For carburized specimens, fatigue cracks initiate at subsurface near the boundary between carburized case and substrate at ambient temperature. At elevated temperature, fatigue cracks initiate near the boundary under the high applied stress while under low applied stress, fatigue cracks initiate from the inclusions.

Effects of micro-shot peening on the defect tolerance of Al-Si-0.6Mg alloy containing artificial defect

The effects of micro-shot peening (MSP) and artificial surface defects were investigated in relation to the defect tolerance of Al-Si-0.6Mg alloy in the framework of fracture mechanics. The results indicated that coral-shaped casting defects inside the alloy could be effectively quantified using the X-ray synchrotron radiation technique. An affected layer with a surface roughness (Ra) of 1.25 and residual stress of − 125 MPa was introduced on the MSP specimen compared to the un-peened (UP) specimen. The fatigue strengths of the UP and MSP specimens containing the artificial defects decreased as the defect size increased under rotating bending fatigue tests, except in the case of defects with the equivalent size of 395 and 490 µm, respectively. MSP could shift the crack initiation site toward the subsurface, while all the cracks of the specimens with the artificial defects originated from the surface and maintained an aspect ratio of 0.66 during the crack growth process. The relationship between the defect sizes and the fatigue strengths could be reasonably assessed by the modified Kitagawa-Takahashi (K-T) diagrams. The critical defect size of the MSP specimens was increased from 450 to 542 µm compared with that of the UP specimens, owing to the beneficial effect of the compressive residual stress. The defect tolerance of the Al-Si-0.6Mg alloy was improved by the MSP process.

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Various heat treatments are applied to improve the mechanical properties of steels depending on the intended use. In this study, the variation of fatigue strength of H13 hot work tool steel with heat treatment in a GS 550 bath was investigated. It was investigated by applying four different heat treatments as first preheating, second preheating, hardening and second hardening. Each batch of samples was subjected to rotating bending fatigue test at room temperature after heat treatment. The fatigue strength of the untreated samples was determined as 470 MPa. The fatigue strength of the second hardened untempered samples (Group 1) increased to 610 MPa. The fatigue strength of the samples (Group 2), which were annealed for two hours at 550ºC as a second heat treatment after hardening, was measured as 630 MPa. The fatigue strength of the samples (Group 3) was found to be 720 MPa by applying the third heat treatment, which was annealing at 550ºC for two hours and at 610ºC for two hours. The fatigue strength of the samples (Group 4), which were subjected to the fourth heat treatment and tempered for two hours at 550ºC and then at 635ºC for two hours after hardening, was determined as 710 MPa. It has been observed that all applied heat treatments positively affect the fatigue strength of H13 hot work tool steel. While all heat treatments resulted in increased fatigue strength, the highest fatigue strength was obtained with double annealing heat treatment (550 °C and 610 °C two hours) after initial preheating and quenching.

High-Cycle Fatigue Life and Strength Prediction for Medium-Carbon Bainitic Steels

High-cycle fatigue (HCF) behaviors of medium-carbon bainitic steels with various inclusion sizes and microstructural features were studied using the rotating–bending fatigue test. Here, the medium-carbon bainitic steels with different melting processes were treated by three heat treatment routes incorporating bainite formation, namely bainite-based quenching plus partitioning (BQ&P), bainite austempering (BAT) and “disturbed bainite austempering, DBAT”. The interior inclusion-induced crack initiation (IICI) and noninclusion-induced crack initiation (NIICI) modes were found after fatigue failure. The fracture surface of IICI is characterized by a “fish-eye” surrounding a “fine granular area, FGA” in the vicinity of an inclusion. In contrast, a microfacet, instead of an inclusion, is found at the center of FGA for the NIICI fracture surface. The predications of fatigue strength and life were performed on the two crack initiation modes based on fracture surface analysis. The results showed that a majority of fatigue life is consumed within the FGA for both the IICI and NIICI failure modes. The fatigue strength of the NIICI-fatigued samples can be conveniently predicted via the two parameters of the hardness of the sample and the size of the microfacet.

Thermomechanical instability and deformation behavior of βo(ω) phase region in a Ti-43Al-8Nb-0.2W-0.2B alloy under high-temperature rotary-bending fatigue

Phase transformation and deformation behavior of the βo(ω) phase region under high-temperature rotary-bending (HTRB) fatigue condition are studied in the current work. Rotary-bending fatigue tests of a Ti-43Al-8Nb-0.2W-0.2B (at.%) alloy were performed at 800 °C under 325, 340, 355 and 370 MPa in atmosphere condition. The quasi-fatigue limit (σ-1) was calculated as 355 MPa [4]. Microstructures were examined using scanning and transmission electron microscopy (SEM &TEM) and thermodynamic calculation was adopt to interpret the phase transition mechanisms. Under HTRB fatigue, the phase transition of ωo → βo occurred and the newly formed βo phase is completely coherent with the pre-existing βo phase. Thermodynamic calculation revealed that a tensile stress could promote the phase transition of βo → ωo. The local recrystallization was found at lamellar colony boundary and grain interior, where is consisted of γ and ω grains. The deformation mechanisms of βo phase might be the dislocation glide on the 〈111〉{123} system. The dislocations have a strong tendency for slip localization into slip bands. Disordered ω phase was observed in the residual ωo phase and it may be a transition state in the phase transformation of ωo → βo.

On the fatigue ductile-brittle transition of EA4T steel for high-speed railway axle

EA4T is a widely used high-speed railway axle steel with BCC crystal structure. It has the characteristics of ductile-brittle transition under single impact load, but whether it has the ductile-brittle transition under cyclic load has not been investigated. For this reason, the rotating bending fatigue tests (stress ratio of R = −1) for EA4T steel were carried out at RT, −40 °C, −80 °C and −120 °C, respectively. For all temperatures tested, except −120 °C, the fatigue strength increases with the drop of temperature. When the temperature is −120 °C, the fatigue strength at lower stress level was higher than that at −80 °C, whereas, the fatigue strength at higher stress level was lower than that at −80 °C. Based on the above, the mechanism of low temperature strengthening of fatigue performance was explained. When the temperature was higher than TFDBT (fatigue ductile-brittle transition temperature), lower temperature leads to increased flow stress, and then, the higher fatigue performance. When the temperature was lower than TFDBT, the longitudinal tensile stress near the crack-tip increased and the location of the maximum normal stress (σyymax) was closer to the crack tip with the drop of temperature, resulting in σyy⩾σf was satisfied at the characteristic distance, and then, cleavage fracture occurred. In addition, the FATT50 (Fracture Appearance Transition Temperature) of EA4T was −35.46 °C determined by impact test, which was higher than the TFDBT. Because the strain rate under impact load is higher, which results in higher yield stress and strain hardening exponent under impact load.

Maximum Inclusion Size Evaluation and Fatigue Strength Analysis of 40Cr Structural Steel

Statistics of extreme values (SEV) and generalized Pareto distribution (GPD) are adopted to predict the maximum inclusion size in 40Cr structural steel, and the fatigue strength was estimated according to the obtained maximum inclusion size. The estimated results were compared with the experimental results obtained in rotating bending fatigue testing, where all failure-relevant inclusions of the present study were quantitatively analyzed with respect to area (square root of the projected inclusion area). Both the estimation results are consistent with the experimental results. Furthermore, a suitable maximum inclusion size equal to the prior austenite grain size is proposed for the material manufacturing process.

Evaluation of Hardness Distributions around Fracture Surface in Induction-Heated SUJ2 Steel after Rotating Bending Fatigue Test

Induction-heated carbon steel is used in various mechanical parts and the fracture mechanism from internal cracks is still being investigated. In order to explain the fracture mechanism of an induction-heated SUJ2 steel bar specimen, we investigated Vickers hardness distributions around the fracture surface after a rotating bending fatigue test. We found an area which is defined as the transition area near the fracture surface whose HV values decreased about 150 HV and this area was formed by the fatigue crack during the rotating bending fatigue test.

Fatigue limit estimation for carburized steels with surface compressive residual stress considering residual stress relaxation

The mechanism of residual stress relaxation for carburized steels was elucidated and their fatigue limit was estimated. The compressive residual stress in the carburized steel after rotating bending fatigue tests is mainly influenced by the relaxation during the compressive loading phase, although it increases at the early stage. The fatigue limit of carburized steels can be estimated according to the modified Goodman diagram considering the residual stress relaxation. The proposed equation shows the maximum value of the fatigue limit (1013 MPa) of carburized steel and the maximum value of the compressive residual stress contributing to the increased fatigue limit.

The Cross-Scale Life Prediction for the High-Speed Train Gearbox Shell Based on the Three-Interval Method

The gearbox shell is a key component of class A of high-speed trains. In engineering applications, the fatigue life prediction of the gearbox shell is a critical issue to be addressed. It is not feasible to obtain fatigue life results for the gearbox shell experimentally because of its long design life, lack of actual failure data, complex structure, high test cost, and material dispersion. Therefore, the cross-scale method was introduced to accurately predict the fatigue life of the gearbox shell. In this study, the entire gearbox shell is divided into two scales of “material structure.” Firstly, the S-N curve is plotted within the material layer, based on the data from the rotating bending fatigue test. Secondly, the finite element model of the gearbox shell is established within the structural layer via the simulation platform. The characteristics and random vibration of the established model are analyzed and presented. Additionally, the first ten-order frequency of modal analysis and power spectral density responses of the gearbox are obtained. The fatigue life of the gearbox shell and the safe running distance of the train are calculated by using the three-interval method and the linear cumulative damage rule, respectively, by combining the fatigue analysis from the material layer with the simulation analysis from the structure layer. Finally, to illustrate the application of the proposed method, a group of small-scale test examples is provided. The proposed method can be used in fatigue life prediction more effectively than the single finite element simulation method.

Effect of Heat-Treating on Microstructure and High Cycle Bending Fatigue Behavior of AZ91 and AZE911 Magnesium Alloys

This article investigates the influences of heat-treating on the microstructures and the high cycle fatigue (HCF) properties of AZ91 and AZE911 (AZ91 + 1%RE) magnesium alloys. For such an objective, AZ91 and AZE911 alloys were used after solution treatment at 415°C for 5 hours aged at 215°C for 3 hours and at 215°C for 5 hours, respectively. To investigate the HCF behavior, a rotational bending fatigue test was performed with stress ratio (R) −1 and frequency of 100 Hz at room temperature. Optical microscopy (OM) study demonstrates that heat treatment leads to a reduction in grain size and transformation of continuous and discontinuous precipitations into needle-shaped precipitations, which are located on the grains of the alpha phase. Scanning electron microscopy (SEM) showed both marks of quasicleavage and cleavage on the fracture surface of specimens. These planes indicated the brittle behavior of the fracture. Moreover, in heat-treated specimens, the size of cleavage patterns was smaller, and microcracks were shorter. These behaviors affected the strength of the material and the fatigue lifetime. The results of mechanical tests show a negligible influence of heat-treating on the HCF behaviors of AZ91-T6 and AZE911-T6 alloys. Stress-lifetime curves (S-N) show an increase in fatigue strength in 3.8 × 105 fatigue cycles, from 95 MPa to 125 MPa for the AZ91-T6 alloy and from 125 MPa to 155 MPa for the AZE911-T6 alloy, after heat treatment.

Residual stress relaxation in the carburized case of austenitic stainless steel under alternating loading

Low-temperature gaseous carburization is an effective thermochemical surface modification process for austenitic stainless steels. The treatment results in the development of a carbon-enriched case with high hardness, improved localized corrosion performance and high compressive residual stress, that in principle improves the fatigue performance. Residual stress relaxation occurs during cyclic loading and the residual stress relaxation in carburized case on 316L austenitic stainless steel under cyclic loading was investigated with rotating bending fatigue testing. Two stages of residual stress relaxation in the surface region were identified: an instantaneous and large relaxation in the first cycle followed by gradual relaxation with the number of fatigue cycles. Depth-resolved analysis of residual stress showed that relaxation mainly occurs in the surface-near region of the carburized case. The rate of stress relaxations depends on the amplitude of the alternating load. The occurrence of plastic accommodation provoked by lattice expansion and the additional plastic deformation imposed during loading is decisive for stress relaxation. A model of surface residual stress evolution for carburized case under cyclic loading was established, which could accurately describe the relaxation.

Study of the Rotary Bending Fatigue Resistance of 30MnB5, 41CrS4 and 30MnVS6 Steels

In this study, a comparative analysis of the fatigue behavior of four types of steels, three of quenching and tempering (30MnB5 subjected to two different heat treatments and 41CrS4) and one microalloyed (30MnVS6), was carried out. The objective of the study is to determine if it is feasible to replace the quenching and tempering steel traditionally used in the manufacture of commercial vehicle axles (30MnB5) with alternative ones with the same composition but with modifications in their microstructure that improve their mechanical properties; a quenched and tempered chromium steel (41CrS4) and one that is microalloyed (30MnVS6). For this, rotary-bending fatigue tests have been carried out on the four types of steels with different stress levels. The fatigue resistance of quenched and tempered steels and microalloyed steel was evaluated using the fit of Basquin’s experimental data. Where possible, the fatigue limit was determined using the maximum likelihood method. It was concluded that, in general, the fatigue resistance of chromium-alloyed steel is higher than that of the reference steel, while the rest have lower fatigue resistance. On the other hand, it was determined that the fatigue limit of microalloyed steel is higher than the reference one and that of the reference steel is higher than that of the other two steels.

Grain growth and fatigue behaviors of GH4169 superalloy subjected to excessive ultrasonic surface rolling process

In this study, the fatigue behaviors of GH4169 superalloy, which possessed a gradient nanostructure layer introduced by an ultrasonic surface rolling process (USRP) and characterized using scanning electron microscopy and transmission electron microscopy, were investigated. Rotary bending fatigue test results verified that the fatigue resistance of the alloy was enhanced due to the improved surface integrity, such as refined grains, improved microhardness and the great compressive residual stress field. However, excessive USRP treatment induced grain growth (from 58.23 to 280.7 nm) at the top surface of the sample accompanied by residual stress relaxation and decreased microhardness, leading to the degradation of fatigue resistance. In addition, the mechanism of the grain growth observed in the alloy subjected to excessive USRP treatment was revealed. It was found that the collective migration of neighboring grain boundaries caused by excessive cyclic plastic deformation promoted the formation of nanotwins, and such process played a key role in accelerating grain growth.

Application of machine learning algorithms for the optimization of the fabrication process of steel springs to improve their fatigue performance

Machine Learning algorithms are aimed at building generalizable models to provide accurate predictions or to find patterns from noisy data. These characteristics are potentially beneficial for the fabrication of steel products. In this research, 529 rotating bending fatigue tests (R = -1 and σa = 400 MPa) were carried out on steel suspension spring bars fabricated using different combinations of manufacturing parameters. A reliable regression model (R2 = 0.877 on the test dataset) based on the Gradient Boosting algorithm was obtained. The interpretation of the model was carried out through the Permutation Importance algorithm, revealing the relevance of the temperature in the tempering treatment applied after quenching on the fatigue lifespan. This pattern was quantitatively described by means of the Partial Dependence Plot of this feature. Besides, a specific study was carried out to obtain a reliable interpretation of the results derived from the Machine Learning analysis. In this sense, it has been observed that specimens subjected to high temperature tempering display a lower surface hardness that provokes a higher surface roughness after shot peening; this, in turn, facilitates the initiation of surface cracks during the fatigue tests reducing the fatigue lifespan. This study provides a reliable framework to optimize the suspension spring manufacturing conditions to increase their fatigue lifespan as well as an example, generalizable to other manufacturing processes, of the potential benefits of Machine Learning.

Investigations on the adhesion and fatigue characteristics of hybrid surface-treated titanium alloy

This study is critical for aiding the development of a hybrid treatment consisting of a tungsten-doped diamond-like carbon coating (WC/C) deposited atop shot-peened Ti-6Al-4V surfaces. Residual stresses, considered a key indicator of fatigue performance, were measured by X-ray diffraction (XRD). Rotating bending fatigue tests were carried out while microscopy allowed for fractographic analysis of resultant surfaces. Results show that despite the pronounced surface roughness produced by the shot peening treatment, the high residual compressive stresses, together with the refined and cold-worked microstructure were capable of marginally improving the fatigue performance of the shot peened Ti-6Al-4V compared to the untreated alloy. Conversely, following fatigue testing, the WC/C coating displayed extensive cracking which propagated prematurely into the substrate and yielded a reduction in fatigue limit. On the other hand, despite some annealing of the compressive residual stresses during the coating deposition treatment, the hybrid treatment yielded a reasonable improvement in the fatigue limit over the untreated condition. This result, coupled with the high coating hardness and adhesion, confirm this hybrid treatment's capability in retaining both the adhesive and fatigue performance of the Ti-6Al-4V alloy.

Application of Deep Neural Network to Predict the High-Cycle Fatigue Life of AISI 1045 Steel Coated by Industrial Coatings

In this study, deep learning approach was utilized for fatigue behavior prediction, analysis, and optimization of the coated AISI 1045 mild carbon steel with galvanization, hardened chromium, and nickel materials with different thicknesses of 13 and 19 µm were used for coatings and afterward fatigue behavior of related specimens were achieved via rotating bending fatigue test. Experimental results revealed fatigue life improvement up to 60% after applying galvanization coat on untreated material. Obtained experimental data were used for developing a Deep Neural Network (DNN) modelling and accuracy of more than 99%.was achieved. Predicted results have a fine agreement with experiments. In addition, parametric analysis was carried out for optimization which indicated that coating thickness of 10–15 µm had the highest effects on fatigue life improvement.

Microstructure in subsurface and fatigue properties of shot peened maraging steel

Rotating bending fatigue tests were carried out to investigate the effect of microstructure in subsurface on the fatigue properties of a shot peened maraging steel. The steel had a fine-grained microstructure formed at the surface layer with 50-70 μm in depth. Especially a thin surface layer showed a nano-size structure. A surface fracture occurred at high stress levels, while the combined fracture occurred due to coalescence of a surface crack and an internal one initiated independently with decreasing in stress level until 108 cycles. In all of the macroscopic fracture surfaces in the shot-peened specimen, a ring-shaped flat area was observed at the surface layer irrespective of the fracture mode and the depth of the area nearly corresponded to the one of the fine-grained microstructure. The formation mechanism of the ring-shaped area was explained from the initiation and propagation properties of a surface crack in the surface layer in connection with the microstructure.