Stress-life Curves Research Articles

Fatigue crack extension mechanism and mode-type analyses of martensitic steels for proposing fatigue strength evaluation method: Example of 18% Ni BCC martensitic steel

The fatigue properties of martensitic steels tend to differ significantly from those of low- and medium-strength steels. Even if the hardness is the same, the fatigue strength varies substantially depending on the microstructure. Thus, for using martensitic steel rationally and developing materials with high fatigue strength, it is desirable to establish a fatigue strength evaluation method that can consider the microstructural effects on fatigue properties. For this purpose, 18% Ni bcc martensitic steel was used as a model metal to conduct rotating-bending fatigue tests in this study. The fatigue crack behavior on the smooth specimen surface was examined, and the fracture surface morphology on both sides of the broken specimen and the microplastic strain distribution near the fatigue crack tip were analyzed. The results clarified the mechanism underlying fatigue crack extension (FCE). The analytical results revealed three types of FCE mechanisms dependent on the stress amplitude, stress state (i.e., plane stress or plane strain), and martensitic microstructure. Considering these mechanisms, a unique stress–life curve was predicted, and a particular fatigue crack shape and fractography were confirmed. The material indices were speculated to be the representative properties of martensitic steels.

Fatigue properties of titanium-clad bimetallic steel butt-welded joints

Titanium-clad (TC) bimetallic steel is a high-performance constructional material that yields promising application potential in future ocean and coastal civil engineering. However, the understanding on the mechanical behaviors of the corresponding weld joints is still limited. In this paper, a tensile-tensile fatigue experimental study was conducted on two types of butt-weld joints fabricated of hot-rolled TA2 +Q355B TC bimetallic steel. Based on the test results, the fatigue stress-life curves (S-N curve) of the two types of weld joints were obtained and subsequently compared with that of TC parent materials, other representative structural metals, that of butt-weld on other metals and the codified S-N curves provided by six structural/fatigue design standards. Most specimens of the two types of TC weld joints are initially cracked at the clad-to-cap weld. The bonding strength between clad and substrate layers exhibit a promising reliability under fatigue loadings. The fatigue resistance of the two types of TC weld joints is considerably less than the different TC parent materials studied. The fatigue lives of clad material (TA2) and substrate material (Q345/Q355B) are greater than that of TC weld joints. The S-N curves of two TC weld joints are considerably lower than that of high-strength steel series but higher than that of aluminum alloy 6061-T6. The design S-N curves of EC3 (Class 36) and IIW (Class 36) are too conservative for the two TC weld joints.

Effects of heat treatment and hot isostatic pressing on microstructure and fatigue improvements in Ti-6Al-4V alloy fabricated by selective laser melting

SLM plays a critical role in in-space intelligent manufacturing and defense industries, while inherent hurdles due to internal defects in the SLM parts restrict practical reliability. Post-processing of heat treatment and hot isostatic pressing was comparatively applied to reveal the microstructure and fatigue performance. In this work, the resulted microstructure was revealed by XRD, SEM and TEM. Also, the stress-life curve of high cycle fatigue performance was obtained. This work provides a direct comparison for two post-processing techniques, contributing to more reliable application for SLM Ti alloy industrialization.

Notch effect in tension-tension fatigue of short glass fibre reinforced polyphenylene sulfide composites

An experimental investigation of the fatigue behaviour of plain and notched specimens made of 40 % wt. short glass fibre-reinforced PolyPhenylene sulfide is presented. To this end, plain and V-notched specimens (with notch root radius ranging from 0.2 mm to 10 mm) were produced by injection moulding, and the final geometries were obtained by means of two different technological processes (i.e., directly after the injection moulding process and by machining them from injected plates). To investigate the effect of notch root radius and fibre orientation, tension–tension fatigue tests were carried out in load control mode, and the experimental results were reanalysed in terms of net stress amplitude defining the traditional stress-life curves. During the fatigue tests, the damage evolution was monitored using a travelling microscope to define the number of cycles spent for fatigue crack nucleation. Furthermore, the fracture surfaces were observed at the microscopic level to investigate the damage mechanisms and the actual fibre orientation was evaluated by computed tomography analysis. Then, by employing coupled finite element analysis of the injection moulding process and the structural behaviour, four different approaches available in the open literature were used to correlate the geometrical and/or manufacturing process effects in the fatigue strength of the considered material.

Very high cycle fatigue of laser powder bed fused Al-Cu-Mg-Ag-TiB2 (A20X) Alloy: Stress relief and aging treatments

This study presents a comprehensive exploration of the fatigue response in the very high cycle fatigue (VHCF) regime for an additively manufactured (i.e., laser powder bed fused) A20X aluminum alloy. Although the need for high-performance materials with exceptional fatigue qualities has increased dramatically, the VHCF behavior of Al-Cu-Mg-Ag-TiB2 (A20X) structures remains largely unknown. A series of ultrasonic fatigue tests were performed to assess the prolonged fatigue life of the A20X alloy (in the VHCF domain where the number of cycles to failure is beyond 10 million cycles). The VHCF response, assessed through ultrasonic fatigue testing, was investigated by examining the stress-life (S-N) curves in a statistical framework, the fatigue crack initiation and propagation behavior, and the fracture surfaces. An asymptotic trend was experimentally found at 109 cycles, with a stress amplitude of 110 MPa for stress-relieved (SR) and 125 MPa for artificially aged (T7) materials, indicating the presence of an endurance limit. Furthermore, fracture surfaces showed the typical fisheye morphology, with a fine granular area (FGA) containing an internal crack-initiating site. The findings of this paper can assist in optimizing fatigue and durability design allowable for applications for extended fatigue life in the VHCF domains.

Tensile fatigue behavior of short and continuous carbon fiber reinforced additively manufactured thermoplastic multiscale composite

Fiber reinforced multiscale composites represent a cutting-edge class of advanced materials that have garnered significant attention in the realm of materials science and engineering. These advanced materials intertwine various fiber reinforcements, meticulously tailored at micro, to macro levels, imparting them with remarkable mechanical, thermal, and functional properties. In this research, an experimental investigation was performed to observe the tensile fatigue behavior of short and continuous carbon fiber reinforced additively manufactured multiscale thermoplastic composites. The multiscale composite was fabricated by combining layers of continuous woven carbon fiber (CCF) with short carbon fiber (SCF) reinforced acrylonitrile butadiene styrene (ABS) laminates which were additively manufactured using the fused filament fabrication technique. It was observed that the multiscale composite exhibits a maximum 127% higher strength and 154% higher Young’s modulus than neat ABS polymer. Based on non-normalized stress-life curves, multiscale composites with four layers of CCF have a higher fatigue strength than multiscale composites with one layer of CCF. However, when the applied stress is normalized by the quasi-static strength, the multiscale composite with one layer of CCF had a longer fatigue life than the four layers of CCF reinforced multiscale composites at high normalized cyclic stress, while the multiscale composite with four layers of CCF exhibited a longer fatigue life at low normalized cyclic stress. The four CCF layers reinforced multiscale composites experienced a different extent of fiber pull-out, matrix cracking, and delamination at different stress levels. Conversely, fatigue failure of the one layer of CCF reinforced multiscale composites primarily occurred due to localized fiber breakage with minimal matrix-related damage.

Thermomechanical Rotor Fatigue of an Interior Permanent Magnet Synchronous Motor

Interior permanent magnet synchronous motors (IPMSMs) are extensively used as traction motors today because of their exceptional torque, power density, and wide, constant power operating range. Under real-world usage, an IPMSM rotor undergoes varying electromagnetic, thermal, and mechanical loads. Under such conditions, fatigue life-based design criteria should be used over stress-based design criteria to ensure the structural integrity of the rotor. Moreover, the driving dynamics can change the rotor temperature continuously, which affects the electromagnetic, mechanical, and fatigue properties of the rotor material. This paper proposes a robust thermomechanical rotor fatigue simulation workflow considering significant loads acting on an IPMSM rotor and the temperature variation throughout a drive cycle. It discusses an accelerated fatigue life estimation approach based on the peak valley extraction method to reduce the simulation time significantly for the stress and fatigue analysis. Then, it presents a method for a stress-life curve generation for variable loading. It also presents a sensitivity study with a median S-N curve, and a 90% reliability and 95% confidence (R90C95) S-N curve.

Fatigue behaviour of 10% wt. short glass fibre reinforced recycled Polypropylene with mineral filler in presence of notches

An experimental investigation of the fatigue behaviour of plain and notched specimens made of 10% wt. short glass fibre reinforced partially recycled Polypropylene filled with mineral filler is presented. To this end, plain and V-notched specimens (with notch radius ranging from 0.07 mm to 10 mm) were produced by injection moulding and tested under tension-tension fatigue. During the fatigue test, the damage evolution was monitored using a traveling microscope to define the number of cycles spent for the fatigue crack initiation. The fracture surfaces were analysed at microscopic level in order to investigate the fatigue damage mechanisms of plain and notched specimens. The fatigue tests were first analysed in terms of net stress amplitude to define the stress-life curves. Then, an energy-based approach was proposed to reduce the scatter of the fatigue data of plain and notched specimens. Starting from the damage evolution observed during the fatigue tests, the model relies on the strain energy density numerically evaluated in the matrix and averaged on a structural volume embracing the notch tip.

Fatigue behaviour of 25% wt. short glass fibre reinforced recycled Polypropylene filled with mineral filler in presence of notches

An experimental investigation is presented regarding the fatigue behaviour of notched and plain specimens made of 25% wt. short glass fibre reinforced recycled Polypropylene filled with mineral filler. Plain and double-edge notched specimens (with notch radius ranging from 0.2 mm to 10 mm) were produced by injection moulding and tested under tension-compression fatigue to highlight the effect of the notch root radius and notch geometry. During the fatigue tests, the damage evolution was monitored using a traveling microscope to define the number of cycles spent for the fatigue crack nucleation. First, the fatigue tests were reanalysed in terms of net-stress amplitude and the traditional stress-life curves for each specimen's geometry were found. Then, an energy-based approach was proposed for the assessment of fatigue life spent for the crack nucleation. The model is conceptually based on the actual damage evolution observed during the fatigue tests, which was according to the technical literature: the initiation of a macro crack results from the accumulation of damage in the matrix. Thus, the strain energy density evaluated in the matrix and averaged on a structural volume embracing the notch tip was adopted as fatigue damage index to correlate in a single scatter band the fatigue data of plain and notched specimens.

Fatigue-Life Estimation of Vertical-Axis Wind Turbine Composite Blades Using Modal Analysis

Abstract Vertical-axis wind turbines (VAWTs) have gained significant impact due to their belief in sustainable wind energy solutions. The primary challenge with VAWTs is complicated structural dynamics and incorporating the optimal composite blade configuration. This article presents an integrated study of the dynamic performance and fatigue-life characteristics of modern VAWT composite blades. Considering the VAWT’s design parameters, the impact of variables such as stress level, loading conditions, fiber type, laminate stacking sequences, and lamina ply angles on the dynamic behavior and fatigue life of VAWTs is investigated. Stress-life curves are generated for glass/carbon-polyester VAWT composite blade configurations. Multi-response optimization using the Taguchi technique combined with the response surface methodology is employed to model, propose, and elect the optimal VAWT composite blade configurations. Finite element analyses (FEAs) are conducted within the solidworks simulation software to simulate and predict the dynamic performance and fatigue-life characteristics of VAWT composite blades. The results show that the composite blades significantly enhance the dynamic and fatigue-life characteristics of VAWTs. Furthermore, a small damage percentage of 2.5% characterized by the FEA for the optimal VAWT composite blade configuration validated the sustainability of the VAWT composite blades in dynamic loading conditions.

Application of local fatigue approaches on corroded welded joints with consideration of weld geometry and residual stresses

Corrosion can significantly affect the fatigue strength of steel structures. On the one hand, it can cause stress concentrations by changing the surface geometry, and on the other hand, it can lead to embrittlement of the material. In welded steel components, sharp notches are already present due to the weld geometry, which may interact with the notches from corrosion. In addition, residual stresses exist in the fatigue-relevant area, which can also be partially modified by surface preparation methods such as clean blasting. In the current standards, all the mentioned effects are considered by reducing the design stress-life (SN) curves, in which all aforementioned effects are superimposed. One possibility to consider the effects separately is the application of local concepts, with which the influence of the geometry, here obtained from 3D-scans, the influence of measured residual stresses, and the influence of material can be considered individually. For butt and fillet welds investigated in this study, it could be shown that by considering the real geometry via a reverse engineered 3D-solid model, the locations of crack initiation in the fatigue tests, measured by digital image correlation (DIC), matched with the numerically predicted locations in most cases. Moreover, it was shown that by taking into account the real geometry and residual stresses, the use of local approaches can result in a substantially reduction of the scatter in SN-curves from Tσ = 2.17 to Tσ = 1.26, demonstrating the great reliability of the applied method.

A re-assessment of Ti-685 as a dwell sensitive titanium alloy and a definition for engineering relevant dwell behaviour

It is believed that the near-α titanium alloy Ti-685 was the first to demonstrate dwell sensitive fatigue behaviour whilst under engineering service as a gas turbine fan disc material. New fatigue data, micro-textural characterisation and fractography from a recent study which utilised remnant Ti-685 specimens originally manufactured for previous laboratory campaigns based at Swansea dating back to the late 1980s are now reported. In isolation, the present results failed to demonstrate a significant dwell effect in either aligned or basketweave microstructural variants. On this occasion, the limited sensitivity to dwell loading was explained by the combination of heat treatment artifacts and selection of a relatively small test specimen geometry. However, the present data were also reviewed in the context of the wider fatigue database generated by multiple studies over the past fifty years. In combination, stress-life (SN) curves defined the magnitude in applied peak stress necessary to demonstrate a dwell effect in this alloy. Ultimately, this facilitated a fundamental definition of dwell sensitivity that is relevant to the engineering community.

Wire arc additive manufactured AWS ER100S-G steel: Very high cycle fatigue characterization

This paper presents a comprehensive study on the very high cycle fatigue (VHCF) characterization of a wire arc additive manufactured (WAAM) AWS ER100S-G steel. The demand for high-performance materials with superior fatigue properties has grown exponentially. However, the VHCF behavior of large-format WAAM’d structures remains relatively unexplored. In this study, a series of VHCF tests were conducted under fully reversed cyclic loading conditions to investigate the extended fatigue life (performance) of the WAAM ER100S-G steel. The VHCF properties, relative to conventional fatigue employing a servohydraulic testing system, of the WAAM ER100S-G steel were evaluated by analyzing the stress-life (S-N) curves, fatigue crack initiation, and propagation behavior, in a statistical framework, and the fracture surfaces. The results revealed the controlling mechanisms of VHCF failure of the WAAM ER100S-G steel and the fatigue response of the material beyond the conventional fatigue limit of 107 cycles. The findings provide valuable insights into the influence of WAAM-induced defects/microstructure on the extended fatigue performance of WAAM ER100S-G steel, which can aid in optimizing fatigue and durability design guidelines for additive manufacturing applications in high-cycle and very high-cycle fatigue domains.

A probabilistic stress-life model supported by weakest link principle and highly-stressed volume/surface area concepts

This article introduces a probabilistic fatigue model that combines the principles of the weakest link theory with the highly-stressed volume (HSV) or highly-stressed surface area (HSSA) concepts to estimate stress-life (P-S-N) curves for both unnotched (smooth) and notched large-scale specimens, for any prescribed failure probability. By incorporating probabilistic models, P-S-N curves corresponding to lower probabilities of failure can be obtained. The integration of HSV or HSSA with the weakest link principle enhances the estimation of fatigue strength for real elements or components, which often require testing with reduced-size specimens. The proposed model is verified by experimental fatigue data from the literature, specifically involving unnotched and notched cylindrical specimens subjected to different constant amplitude loadings. The results demonstrate the robustness and effectiveness of the model in estimating characteristic P-S-N curves at lower probabilities of failure.

High-Cycle Fatigue Life Behaviour of Fabricated Glass Fibre-Reinforced Polymer

This study focuses on the fatigue behaviour analysis of glass fibre-reinforced polymer (GFRP) composite specimens under high-cycle fatigue loading conditions. Therefore, property validation is recommended in the material development process upon further investigation of the fabricated GRFP. This study aims to evaluate the behaviour of the fabricated GFRP fatigue specimen when subjected to high-cycle fatigue loads and compare it to existing studies. A GFRP fatigue test sample was fabricated using the hand layup process into a flat rectangular panel, which was then cut into a small dimension of 28×2×0.2 cm fatigue specimen. Fatigue tests were performed on five flat specimens at different constant amplitude loads or stress levels between 40% and 80% of ultimate tensile strength to obtain the stress–life curve for the fabricated GFRP. Results showed that the high-stress levels of 80% contributed to the most reduced fatigue life cycle of GFRP. This result is consistent with previous studies and lies within the published life cycle range, validating the fabricated GRFP. A new parameter called the failure modulus, or Mf, may be used to quantify a particular set of fatigue tests.

Fatigue life prediction approach for through-hole of needled ceramic matrix composite based on volume fraction

An applied approach for predicting the fatigue life of the through-hole for needled ceramic matrix composite was proposed by using the 0° layers volume fraction and the fatigue notch factor, which can be applicable for the needled composite with different lamination types. In the proposed approach, the fatigue limit and stress-life (S–N) curve of the needled ceramic matrix composite with different 0° layer volume fractions were estimated by interpolation method first. On this basis, combined with the fatigue notch factor determined by the smooth and through-hole specimens with arbitrary 0° layer volume fraction, the fatigue life of the through-hole can be predicted. By comparing the predicted lives with the experimental lives, the proposed fatigue life prediction approach has high prediction accuracy.

Study of vibration frequency-fatigue strength action of 6061-T6 aluminum alloy during fillet welding

The research mainly focuses on the fatigue strength characteristics of 6061-T6 aluminum alloy fillet welds under different vibration frequencies. Firstly, by introducing Stress Life Curve (S-N Curve) and Probability Stress Life Curve (P-S-N Curve), the external vibration stress effects of the main load-bearing points in the stress environment of welded joints are analyzed. Subsequently, a vibration test control system is designed to analyze the relationship between vibration frequency and fatigue strength through nominal stress analysis and hot spot stress analysis. The research findings revealed that under the nominal stress analysis method, the P-S-N fitting curve showed a declining trend with an increase in fatigue life for stress variation curves with survival rates of 50 %, 95 %, and 97.7 % at vibration frequencies of 57.5 Hz, 67.5 Hz, and 77.5 Hz. At the same survival rate stress conditions, the fatigue life variation formed by resonance frequency was smaller, and the fatigue life in the resonance state was relatively lower. There is a certain correlation between vibration frequency and fatigue strength, with resonance frequency corresponding to relatively low fatigue life. This research result helps to reveal the fatigue behavior of 6061-T6 aluminum alloy fillet welds under different vibration stresses, providing a reference for the structural safety design of aluminum alloy components.

Using hybrid physics-informed neural networks to predict lifetime under multiaxial fatigue loading

In this article, a machine learning approach is utilized to predict lifetime under multiaxial fatigue loading. A novel hybrid physics-informed neural network is proposed, where a combination of a LSTM/GRU cell and a fully connected layer is used to extract the damage parameter of a loading cycle. A newly proposed logarithmic activation function is then used to introduce the power law relationship between the damage parameter and the predicted fatigue life. In addition, the selected parameters of the suggested network are physically guided. Two data pre-processing methods are used to ascertain the rotational invariance of the axial–torsional loading conditions. The prediction capability of the suggested approach is demonstrated by the experimental datasets that consist of axial–torsional test results obtained for 42CrMo4 steel and for 2024-T3 aluminium alloy. A good correlation between the predicted and experimental data was achieved. Finally, the extrapolation capability of the proposed approach is demonstrated through modelling the stress-life curves for the data-points outside the experimental data range.

Optimization of S-N curve fitting based on neighborhood rough set reduction with improved firefly algorithm

S-N curve fatigue samples of titanium alloy welded joints have such a comparatively significant scatter, that results in the issue that the fatigue life prediction accuracy is not optimal. In this work, the titanium alloy welded joints' fatigue data is used as analysis data, and the neighborhood rough set reduction with improved firefly algorithm efficient method of fitting stress-life curves is set forth. The welded joint's fatigue decision system is built with fatigue data. The continuous iteration of the firefly algorithm is used as the search strategy, the neighborhood rough set is adopted to decrease attributes, and the major deciding elements of welded joints' fatigue life is identified. The fatigue characteristic domains are divided based on the neighborhood rough set reduction with improved firefly algorithm's key factor set, and the S-N curves can then be fitted to each domain individually. According to the goodness-of-fit analysis, the proposed approach can improve fatigue life accuracy and reduce sample scattering from fatigue.

Electromechanical Fatigue Properties of Dielectric Elastomer Capacitive Sensors Based on Plantarflexion of the Human Ankle Joint

Wearable stretch sensors have potential applications across many fields including medicine and sports, but the accuracy of the data produced by the sensors over repeated uses is largely unknown due to a paucity of high-cycle fatigue (HCF) studies on both the materials comprising the sensors and the signal produced by the sensors. To overcome these limitations, using human physiologically-based parameters, stretch sensors were subjected to quasi-static testing and HCF with simultaneous capture of the signal. The strain produced by the sensor was then compared to the strain produced by testing instrument, and the results suggest that the output from the stretch sensors is strongly correlated with output from the testing instrument under quasi-static conditions; however, this correlation deteriorates under fatigue conditions. Such deterioration may be the result of several factors, including a mismatch between the material response to fatiguing and the signal response to fatiguing. From a materials perspective, the shape of the stress-life curve for the polymers comprising the sensors conforms to the Rabinowitz-Beardmore model of polymer fatigue. Based on these results, consideration of the material properties of a stretch sensor are necessary to determine how accurate the output from the sensor will be for a given application.