Plain Fatigue Research Articles

Impact of biological environment on bending fatigue lifetime in additive-manufactured polylactic acid fabricated by 3D-printing

Polylactic acid (PLA) has become desirable for biomedical applications, particularly implantable devices. However, the degradation of PLA in biological environments under mechanical stress remains incompletely understood and requires further investigation. This study compared the plain fatigue (PF) and the biodegraded fatigue (BDF) behavior of 3D-printed PLA. For this purpose, two sets of standard fatigue specimens were additively manufactured by the fused filament fabrication (FFF) method. One set was used for plain fatigue testing, and the other was immersed for 330 days in simulated body fluid (SBF). After immersion, the samples were dried and weighed before fatigue testing. The fully reversed rotary bending fatigue tests were conducted on both sets of specimens, and the stress-lifetime (S-N) curves were obtained. Additionally, the fatigue properties of PF and BDF specimens were evaluated. Moreover, the fracture behaviors of the materials were studied using field emission scanning electron microscopy (FESEM). The outcomes implied that the weight of the samples extended during the immersion period, primarily due to water absorption by the PLA. However, after drying, the final weights did not change compared to the weights before immersion. The SBF immersion significantly reduced the fatigue performance of the biodegraded samples comparing the PF result.

Fretting fatigue life of galvanized bridge support cable wire: Experimental study and strain-based fracture mechanics analysis

The galvanized wires used in cable bridges can fail due to fretting fatigue. Experimental results have shown that defects resulting from the galvanization process are a possible source of initiation for fatigue cracks and make the life shorter than that of a wire without a galvanized layer. The investigation presented in this paper uses strain-based fracture mechanics to analyze the influence of these defects on the fatigue life of galvanized wire. The presented analysis considers nonlinear material behaviour. A 3D finite element model is used to compute the required stress field. Stress intensity factors are obtained by a proposed numerical point load weight function, and shape factors are found for the surface and deepest points of the semielliptical crack. Plain fatigue tests of galvanized wires are performed to calibrate key model parameters. The calibrated model is then shown to provide reasonably accurate but systematically conservative estimates of the fatigue life under fretting conditions typical of bridge stay cable saddle supports.

Numerical investigation to the effect of wear on fretting fatigue life using CDM and fracture mechanics

Fretting fatigue is a common type of problem in the engineering fields. Due to its multiaxial characteristics, it leads to a shorter overall fatigue life compared to plain fatigue. Fretting fatigue is caused by the micro slip at the contact surface, which is always accompanied by interface wear. This paper presents a refined elastic numerical simulation method combining interfacial wear with continuum damage mechanics (CDM) and fracture mechanics to predict fatigue life. The crack initiation life is determined by damage evolution law and the initiation position is determined by the total dissipated energy. Crack propagation life and path can be predicted using stress intensity factor. The stress redistribution and geometric change of contact surface caused by wear are considered in the whole fatigue process. Results indicate that the fretting fatigue simulation method incorporating wear has higher accuracy compared with the experimental results in the literature. It is noteworthy that wear retards nucleation damage process and crack propagation. As stress amplitude decreases, fatigue life increases, and delay effect of wear becomes more obvious. Additionally, wear causes the crack initiation position to move out, but crack initiation region is still near the slipping zone and propagation path does not change.

Enhancing plain fatigue strength in aluminum alloys through shot peening: Experimental investigations and a strain energy density interpretation

This study addresses the complex interplay of shot peening-induced effects on the fatigue strength of high-strength aluminum alloys. The inherent poor fatigue properties of aluminum alloys, particularly exacerbated in components manufactured through additive processes, pose significant challenges. Shot peening, a recognized surface treatment, proves effective in enhancing fatigue resistance through mechanisms such as compressive residual stresses, work hardening, and microstructural changes. However, optimizing shot peening and developing accurate fatigue prediction models remain challenging due to the intricate and unclear contributions of the individual mechanisms. In this investigation, five distinct shot peening treatments were employed to emphasize specific effects. Experimental results, analyzed through a strain energy density approach, shed light on the crucial role of compressive residual stresses in maximizing fatigue strength. This comprehensive exploration contributes with valuable insights for designing shot peening treatments to optimize the fatigue performance of high-strength aluminum alloys.

Plain and fretting fatigue characteristics of 16MnCr5 steel under cyclic bending loading with the application of engine piston pin

The current study examined the plain fatigue and the fretting fatigue behaviors of the piston pin alloy (16MnCr5 steel) under fully-reversed rotating bending loading. The standard fatigue testing sample was cut out from the piston pin used in internal combustion engines. Furthermore, the failure mechanisms of samples were studied using field emission scanning electron microscopy. The obtained result indicated that the fatigue lifetime of 16MnCr5 steel was significantly affected by the fretting damage. The investigation of the failure mechanisms determined that the fracture behavior of the specimens under the plain fatigue and fretting fatigue conditions was brittle; however, it was considerably influenced by the casting defects in the samples. The test results can merely be used to compare the fatigue performance of the studied material under different conditions. More naturalistic experiments are required for further optimization in designing components made from this material to improve their reliability and service life, especially in contacting surfaces under rotating bending conditions.

Experimental and numerical study on press-fitted railway axles: Competition between fretting and plain fatigue

To investigate the competitive relationship between plain and fretting fatigue in press-fitted railway axles, the plain fatigue limit and fretting fatigue strength were tested, respectively, by changing the depth of the stress relief groove. Next, detailed information on fretting and plain fatigue, as well as on fretting wear, was gathered. Finally, an evaluation methodology integrating the finite element (FE) simulation with the Modified Wöhler Curve Method (MWCM) was established to evaluate the damage in both fretting and plain fatigue scenarios. It was found that as the groove depth increased, the plain fatigue limit decreased while the fretting fatigue strength increased. The optimal groove depth, hopt, that equalized the anti-fatigue capabilities of the groove and wheel seat, was affected by the number of test cycles. For this press-fitted railway axle, hopt was 0.2 mm < hopt < 0.4 mm for 107 test cycles, and hopt > 0.6 mm for 108 cycles. The calculated critical local stress condition, σCri, required for initiating a crack, was 161.8 MPa, which almost agreed with the theoretical value. The calculated σMWCM values can be used to characterise the fretting and plain fatigue damage. The local stress condition for determining an optimal groove depth was: σMWCM in the wheel seat and groove root simultaneously reached σCri.

Effect of shot peening forming and shot peening strengthening post-treatment on the fatigue behavior of bolt-connected 2024HDT alloy

To improve surface integrity and suppress fretting fatigue of bolt-connected 2024HDT alloy plate-hole structure, the effect of shot peening forming (SPF) and shot peening strengthening (SPS) post-treatment on the surface integrity and fatigue behavior of 2024HDT alloy was investigated. The results showed that the SPF treatment increased the fretting fatigue resistance of the 2024HDT alloy plate-hole structure, and the failure mode changed from fretting fatigue to plain fatigue. The SPS post-treatment with three different intensities investigated here all improved the plain fatigue resistance of the SPF sample, whose failure mode also manifested as plain fatigue. This was due to the SPS treatment that flattened the surface of the SPF sample and introduced a compressive residual stress (CRS) field with a larger value and deeper distribution, while increasing the dislocation density and hardness of the SPF sample, which suppressed the initiation and propagation of fatigue cracks. SPS with a medium intensity produced an excellent surface CRS field with the lowest roughness and slightest surface damage, resulting in an improved plain fatigue resistance of 7.92%.

Fretting Fatigue of Riveted Lap Joints in Aircraft Fuselage

Failure of riveted lap joints originating from fretting fatigue has been responsible for a number of structural failures of aircraft in midflight. Using a combination of extended finite element method analysis and experimental data, this paper investigates the formation and propagation of cracks initiated by fretting, and how it is influenced by rivet squeeze force and by the applied fatigue stresses. It is found that fretting and plain fatigue crack growth life increase with the increase of rivet squeeze force. However, the rise of applied fatigue stress reduces fretting and plain fatigue crack growth life. Moreover, enhancing rivet squeeze force or applied fatigue stress decreases the ratio of fretting to total fatigue crack growth life.

Fretting fatigue tests and microstructure analysis of bridge stay cable wires

AbstractSaddle systems are a popular method for supporting the cables at the pylons of cable‐stayed bridges. Fretting fatigue failure of bridge stay cables is a major design consideration for saddle systems. Current design provisions require large‐scale tests of these anchoring systems. However, such tests are costly and time‐consuming. With this in mind, the current paper presents a small‐scale fretting fatigue setup to evaluate the fretting fatigue behavior of bridge stay cable wires at saddle supports. In this paper, fretting tests are conducted on bare and galvanized wires. Hourglass samples are then used to evaluate the material properties and plain fatigue performance of the studied wires. Finally, the microstructure of the wires is evaluated, and the influence of defects on the fretting fatigue life of the wires is discussed.

Effect of friction stir processing on plain fatigue and fretting fatigue behaviour of the AA6063 alloys

The present work investigated the effect of friction stir processing (FSP) on the plain fatigue and fretting fatigue of the AA6063 alloys. The FSP had noticeably smaller grains than the base material. The ultimate tensile strength of the FSP increased by 31.6 %. The fatigue life of the FSP specimen was improved by approximately 10 % in plain fatigue and 28.3 % in fretting fatigue. Increment in life is caused by grain refinement and increased compressive stress. The fretting wear damage and value of the coefficient of friction were observed to be more significant in the case of base material instead of FSP. For base and FSP materials, the life was reduced by 1.6 and 1.18 times, respectively, due to fretting fatigue over plain fatigue.

Experimental study on the fretting fatigue of Inconel 718 superalloy

Fretting fatigue is a surface damage phenomenon appearing in mechanical assemblies subjected to time-varying contact forces. This type of failure is a very frequent problem in many engineering components, with special relevance in the blade-disc connections in aircraft gas turbines. In this study, different plain and fretting fatigue tests were done using different load levels and also under two different temperatures, at room temperature and high temperature (650 °C), in order to obtain the fretting fatigue behaviour in both conditions and compare them with that of plain fatigue test. The contact pair used to carry out these tests is the frequently used cylindrical pad pressed against a dog-bone test specimen with a rectangular cross-section. The material used to manufacture both elements is the often used Nickel alloy Inconel 718, which is widely used in the manufacture of gas turbine parts. Moreover, and because the material was received in annealed condition, this material was heat treated to improve its mechanical properties. Results of tests under different conditions are analysed and compared. Subsequently, the fracture surfaces for different specimens are analysed and fatigue crack growth rate under different testing conditions are presented and compared with data from the literature.

An improved Artificial Neural Network for the direct prediction of fretting fatigue crack initiation lifetime

Fretting fatigue is a common type of problem in the aviation and other engineering fields. Due to its multiaxial characteristics, it leads to a shorter overall fatigue life compared to plain fatigue conditions. Fretting fatigue crack initiation lifetime is a crucial part of the total lifetime, and the currently dominant method for research on fatigue behavior is the combination of the theoretical and numerical models. With the advent of the era of data science, machine learning has been widely used to predict fatigue behavior, but there are no many applications in the field of fretting fatigue. This paper proposed an improved Artificial Neural Network (ANN) using Balancing Composite Motion Optimization (BCMO) for quick prediction of fretting fatigue crack initiation lifetime. Physical-mechanical reasoning parameters, axial stress amplitude, shear stress amplitude, half contact width, and half stick zone width are considered as input parameters, and fretting fatigue crack initiation lifetime is set as the output feature. The main aim of BCMO is to improve the robustness of the ANN based on the influential parameters, namely bias and weight. The provided results using ANN-BCMO are robust compared to ANN and traditional techniques from the literature. The Matlab code of improved ANN using BCMO can be found at https://github.com/Samir-Khatir/BCMO-ANN.git

The influence of thermal oxidation on the microstructure, fatigue properties, tribological and in vitro behaviour of laser powder bed fusion manufactured Ti-34 Nb-13Ta-5Zr-0.2O alloy

This study innovatively investigates the feasibility of thermal oxidation (TO) for improving the wear and fatigue properties of TNT5Zr-0.2O alloys manufactured by laser powder bed fusion (LPBF). Static chemical etching (CE) as a pre-treatment for TO successfully removed the LPBF induced surface adhered powders. A mixture of rutile, Nb2O5, Ta2O5, and ZrO phases were formed as an oxide layer after TO. It can be found a better wear resistance was retained in this β-titanium alloy after TO, as demonstrated by the wear scar features with ploughing grooves changing into brittle peeling of oxides. Plain fatigue strength of CE treated alloy (150 MPa) was 1.5 times higher than the value of CE+TO treated alloy (60 MPa), as a result of multiple premature fatigue cracks possibly developing in the compounds region after TO. In vitro biocompatibility results showed no significant differences in metabolic activity of pre-osteoblasts seeded on the treated surfaces. In addition, early and late mineralisation assays revealed similar levels of 14-day ALP activity, and 28-day mineral deposits formed on the two biocompatible TNT5Zr-0.2O alloy surfaces. Overall, though the oxide layer is corrosion-resistant in the aggressive environment (3 M HCl solution), showing a potential application of TO in additively manufactured titanium medical devices. However, TO should be cautiously exploited due to the deterioration of mechanical properties.

Calculation method for the fail-safe design of steel-steel contacts subject to fretting fatigue based on a worst-case assumption

The lower fretting-fatigue strengths of component connections compared to plain fatigue strengths is a notable characteristic of strength assessments. It can be attributed to the additional tribological stress, which is dependent on the contact pressure, the slip amplitude and the selected material pairing. By using a double-actuated flat-pad test bench with slip control, a separate analysis of these influencing variables can be carried out. The materials investigated are limited to the class of steel materials, specifically to C22 + N, C45 + N, C45 + QT, 34CrNiMo6 + QT, and 42CrMo4 + QT, because of their frequent use in shaft–hub connections. Using fretting factors derived from the flat-pad tests, a local approach within the framework of the FKM guideline (Analytical strength assessment of components in mechanical engineering) was proposed for fail-safe design where there is fretting fatigue. The tribological stress parameters of slip amplitude and contact pressure are taken into account by means of a worst-case-concept. The critical stress parameters were found at a contact pressure of 40MPa and a slip amplitude of 10μm. The calculation method was validated on a connecting-rod connection with a maximum error of 14% in the determination of the degree of utilization.

Probabilistic Fatigue Fragility Curves for Overhead Transmission Line Conductor-Clamp Assemblies

The residual life of transmission line overhead conductors under conditions of fretting fatigue is an important asset management issue for electric network operators. The current industry practice for overhead conductor residual life estimation relies heavily on experimentally generated fatigue curves or rule-based expert systems. The current experiment-based methods do not consider specific conductor-clamp configurations and are based on the simple flexion model. This approach results in large uncertainties in service life predictions and are limited to a failure criterion based on the first wire failure in the conductor. Rule-based expert systems also have limited applicability since they lack physical representation of the fretting fatigue process. Given the limitations of the current methods, the objective of this work is to propose a framework that combines physics-based models and probability theory to estimate the residual life of overhead conductors considering either single or multiple wire failure criteria. To illustrate this procedure, a finite element model of a Bersfort conductor-clamp system is used to assess the contact conditions and internal stress states in the wires of the conductor. Results from the numerical model are then used to develop a fretting fatigue criterion that is a function of the contact energy dissipation mechanisms, contact stresses, and the plain fatigue resistance of the wires. Probability of failure of each contact point between wires and between wires and clamp is computed using the fretting fatigue criterion. With this information, the most probable locations of fretting fatigue failure are identified in the conductor. The predictions for the locations of failure are validated with available literature data for the same conductor-clamp configuration. Given the probabilities of failures at each contact point, the probability of failure of the conductor is derived with the Poisson binomial distribution. Fragility curves are presented for the first through the third wire failures in the conductor. The fragility curves are validated through comparisons with available literature data on the same conductor-clamp configuration. Fatigue curves are also generated from the fragility model for the first wire failure and compared against experimentally generated fatigue curves.

Fretting Fatigue in Mechanical Joints: A Literature Review

Fretting fatigue is a fatigue damage process that occurs when two surfaces in contact with each other are subjected to relative micro-slip, causing a reduced fatigue life with respect to the plain fatigue case. Fretting has now been studied deeply for over 50 years, but still no univocal design approach has been universally accepted. This review presents a literature study that involves the three main types of mechanical joint affected by fretting fatigue (press-fitted shaft hub joints, dovetail joints, and bolted joints) aiming at giving insight into the progress made in terms of design guidelines for engineers.

Optimal shot peening residual stress profile for fatigue

Shot peening (SP) is frequently used as a palliative measure against metal fatigue in many engineering components. The performance of this surface treatment depends on different factors including the material, shape and loading conditions of the component, as well as process parameters. Fatigue improvement due to SP depends to a great extent upon the in-depth compressive residual stress profile produced in the specimen.In this work, we study the optimum shape for a residual stress profile in terms of fatigue behaviour. For this task, we assume a residual stress profile that is qualitatively similar to that produced by SP in many metals. Based on this generic profile, we analyse the optimum shape for two simple, but noteworthy, fatigue cases: plain fatigue and notch fatigue. The analysis is conducted in the “fatigue damage tolerance design” framework, in which a certain initial defect is assumed to be present in the component under study.

Effect of mean stress in very high cycle fretting fatigue of a bearing steel

Very high cycle fretting fatigue tests of high carbon chromium GCr15 steel were carried out using an ultrasonic fatigue system to investigate the fatigue strength and the mechanism of its wear and fracture in the damaged area. Fretting reduced the fatigue strength in contrast to the case of plain fatigue, which could escalate even further with the existence of mean stress. Some models were proposed based on the morphologies characterized by a scanning electron microscope: (i) four specific regions on the fracture surface were defined according to the evolution of cracks; (ii) formation of fretting fatigue cracks was analyzed; and (iii) how fretting and mean stress affected the crack propagation after the formation of cracks was analyzed.

Plain fatigue and fretting fatigue behaviors of 17-4PH steel subjected to ultrasonic surface rolling process: A comparative study

The effect of ultrasonic surface rolling process (USRP) on surface integrity, plain fatigue (PF) and fretting fatigue (FF) behaviors of 17-4PH steel was contrastively studied, and their internal mechanisms were discussed. The results indicated that USRP processing times had a significant influence on the PF and FF properties of 17-4PH steel. Improved PF life resulting from USRP was much higher than that of FF life due to the differences of PF and FF mechanisms. When the processing times were three cycles (USRP-3), its PF life or/and PF limit was the highest; while six subsequent USRP processing times (USRP-6) led to the maximum FF life. Gradient microstructure, large residual compressive stress and high microhardness were introduced by USRP technology, which contributed to improve PF and FF lives of 17-4PH steel. Additionally, the relatively smooth surface (Ra 0.116 μm) and nanocrystalline structure layer of USRP-3 sample contributed to inhibit crack initiation and propagation, while surface “fish-like scale” or folded micro-topography (Ra 0.618 μm) helped to relieve fretting damage for USRP-6 sample.

A parametric investigation on the fretting fatigue behaviour of heat treated Al 6061-T6 under rotating bending phenomena

ABSTRACT This research investigated the effect of variable fretting parameters on bending fatigue behaviour of heat-treated Al-Mg-Si alloy (Al 6061-T6). Bulk stress, contact pressure, pad geometry, and locations were chosen as variable parameters to measure its consequence on fatigue life. A developed FEM model justified the experimental observation. It observed that, within the lower bulk stress range, fretting action decreases the fatigue life of Aluminium alloy significantly than the plain one. However, this difference almost vanishes at higher-order loading as well as stress in comparison to plain fatigue. Contact pressure and stress distribution along the contact path and non-filleted-edge suggest about the importance of safety design over the fretting zones. The flat on cylinder contacts was found as detrimental at higher-order contact pressure while cylinder on cylinder one reduces notably at low contact pressure. Fuzzy logic, a soft computing technique was implemented to predict the fretting fatigue as well as the effect of pad location from surface graphs. Nevertheless, catastrophic rupture followed the aspect of fretting loading parameter that showed fretting not only affects in quantity but also the quality of the fatigue behaviour of Al 6061-T6.