Fatigue Strength Values Research Articles

Practical fatigue strength diagrams for compression springs based on the FKM-Guideline “Analytic Strength Assessment for Springs“

Metal springs are used extensively in technical products. The mathematical relationships and Goodman diagrams contained in the DIN EN 13906–1 standard form the essential basis for the design and calculation of cylindrical helical compression springs. They are used not only nationally, but internationally in the spring industry and by spring users. However, the diagrams are more than 50 years old and no longer reflect the current status of modern spring materials and spring manufacturing technologies. This results in great uncertainty for users of the standard, which currently has to be compensated by costly fatigue tests.In order to overcome the problems, the research project IGF 19693 aimed to renew the Goodman diagrams of the DINEN13906–1 standard in accordance with the state of spring technology. Therefore, the FKM guideline “Analytic Strength Assessment for Springs and Spring Elements“ was used to calculate permissible fatigue strength values for standard springs. Additionally, an extensive experimental program was carried out with fatigue tests on cold-formed helical compression springs to validate the calculations. The main results of the project are presented in this manuscript, which strengthens SMEs in designing competitive springs, which they can offer in a shorter time and at a lower cost due to lower development costs.© 2024 The Authors. Published by Elsevier B.V.

Influence of Chemical Composition on Structure and Mechanical Properties of Vacuum-Carburized Low-Alloy Steels.

This study presents research results concerning the vacuum carburizing of four steel grades, specifically conforming to European standards 1.7243, 1.6587, 1.5920, and 1.3532. The experimental specimens exhibited variations primarily in nickel content, ranging from 0 to approximately 3.8 wt. %. As a comparative reference, gas carburizing was also conducted on the 1.3532 grade, which had the highest nickel content. Comprehensive structural analysis was carried out on the resultant carburized layers using a variety of techniques, such as optical and electron scanning, transmission microscopy, and X-ray diffraction. Additionally, mechanical properties such as hardness and fatigue strength were assessed. Fatigue strength evaluation was performed on un-notched samples having a circular cross-section with a diameter of 12 mm. Testing was executed via a three-point bending setup subjected to sinusoidally varying stresses ranging from 0 to maximum stress levels. The carburized layers produced had effective thicknesses from approximately 0.8 to 1.4 mm, surface hardness levels in the range of 600 to 700 HV, and estimated retained austenite contents from 10 to 20 vol%. The observed fatigue strength values for the layers varied within the range from 1000 to 1350 MPa. It was found that changing the processing method from gas carburizing, which induced internal oxidation phenomena, to vacuum carburizing improved the fatigue properties to a greater extent than increasing the nickel content of the steel.

Fatigue strength of cruciform joints with weld imperfections: A comprehensive numerical study

The paper presents an investigation into the fatigue resistance of load-carrying cruciform joints that exhibit weld imperfections. To accomplish this, a finite-element model of the joint is validated by a comparison to experimental investigations. The model includes both a global and a submodel that integrates the effective notch stress concept into its design. This enables the simulation of various types of weld imperfections, such as lack of penetration, lack of root fusion, undercut, excessive convexity, incorrect weld toe, excessive asymmetry of fillet welds and linear misalignment, to assess their impact on the fatigue strength of the cruciform joint. The analysis reveals that these imperfections lead to an increase in stress at the notches of the cruciform joint, which can be characterized as a function of the joint's geometrical properties. Global geometry influences on the fatigue strength of different cruciform join geometries can be regarded independently from the influence of the weld imperfections that is given by stress concentration factors. The normalised stress concentration factor enables a comparison between imperfection influences and shows, that the notch-softer design with full penetration welding at the cruciform joint is in a relative relation more strongly influenced by imperfections than the design with fillet welds. Also, the undercut represents the imperfection with the strongest effect in a relative relation within the sizes considered.Being able to evaluate the fatigue strength values of steel components with weld imperfections eliminates the need for costly repairs and new fabrications, which would otherwise result in a loss of resources and economic efficiency.

Prediction of the Fatigue Strength of Steel Based on Interpretable Machine Learning

Most failures in steel materials are due to fatigue damage, so it is of great significance to analyze the key features of fatigue strength (FS) in order to improve fatigue performance. This study collected data on the fatigue strength of steel materials and established a predictive model for FS based on machine learning (ML). Three feature-construction strategies were proposed based on the dataset, and compared on four typical ML algorithms. The combination of Strategy Ⅲ (composition, heat-treatment, and atomic features) and the GBT algorithm showed the best performance. Subsequently, input features were selected step by step using methods such as the analysis of variance (ANOVA), embedded method, recursive method, and exhaustive method. The key features affecting FS were found to be TT, mE, APID, and Mo. Based on these key features and Bayesian optimization, an ML model was established, which showed a good performance. Finally, Shapley additive explanations (SHAP) and symbolic regression (SR) are introduced to improve the interpretability of the prediction model. It had been discovered through SHAP analysis that TT and Mo had the most significant impact on FS. Specifically, it was observed that 160 < TT < 500 and Mo > 0.15 was beneficial for increasing the value of FS. SR was used to establish a significant mathematical relationship between these key features and FS.

Experimental and Numerical Investigations of the Fatigue Life of AA2024 Aluminium Alloy-Based Nanocomposite Reinforced by TiO2 Nanoparticles Under the Effect of Heat Treatment

Using aluminium metal matrix nanocomposites has recently gained increased attention in the industry due to their high strength and ductility. In this paper, TiO2 nanoparticles in volume percentages of 5 wt. % were added to the AA2024 alloy using the stir casting method. Using a novel powder injection system, TiO2 nanoparticles with an average particle size of 30 ± 5 nm was added to the matrix. The influence of TiO2 content on the fatigue life before and after heat treatment was studied. The results showed the fatigue properties of AA2024 with TiO2 nanoparticles increased after heat treatment. The optimum improvement in fatigue properties was obtained at 5 wt. % TiO2 after heat treatment, with an improving fatigue life in 14.71% compared with sample based. This is due to an increased number of fine precipitates besides its uniformly distributed after heat treatment. The fatigue life of the composite materials with added nanoparticles was investigated using a finite element-based ANSYS workbench. There was a good match between what happened in the experiments and what happened to the numerical fatigue strength. For the composite materials, the difference between the experimental and numerical values of fatigue strength was not greater than 4% for the matrix. The results also, indicated that, after ageing, the precipitate-free zone at the inter-dendritic zone disappeared or became smaller. However, after adding 5 wt. % of titanium and, also, performing heat treatment, it is not possible to precipitate the Al2CuMg precipitates, and, instead of it, the Al3TiCu and Al7TiCu phases precipitates have been formed.

Fatigue strength optimization of gray cast iron processed by different austempering temperatures

It is generally accepted that the mechanical properties of gray cast iron (GCI) can be enhanced by microstructure refinement, while this perhaps needs to be reconsidered to improve the fatigue properties treated by austempering technology. Although the maximum tensile strength can be up to 480 MPa for austempered gray iron (AGI) with finer bainite fabricated by austempering transformations, the value of fatigue strength is less than the coarse counterpart with lower tensile strength. This suggests that tensile strength can be effectively improved by microstructure refinement, while fatigue cannot. It is mainly attributed to the initial defects induced by graphite and the structural toughness increased by austenite in AGI. Therefore, the general relationship between tensile strength and fatigue strength of GCI and AGI was investigated and the roles of fitting parameters were analyzed. The inverted relation provides some new insights into the fatigue strength optimization of GCI.

An experimental investigation into the influence of incorrect root gaps in welded-in transverse stiffeners on fatigue performance

The paper presents a comprehensive set of special experimental fatigue tests designed to determine the fatigue strength sensitivity of steel components, under circumstances where fabrication quality levels deviate from regulatory standards. Precise monitoring of the manufacturing process of test specimens containing internal imperfections facilitates accurate documentation of the phenomena that contribute to the eventual shape of the imperfection. Welded-in stiffeners with incorrect root gaps of varying sizes were subjected to fatigue testing, as such studies have never been carried out in experimental form before, as the background data to EN 1993-1-9 disclosed. The experimental investigation confirms numerical estimations, which predicted that even large root gaps do not result in lower fatigue strength values, when compared to stiffeners that were properly fitted. Moreover, details in welded beams that feature an intersection of three welds at the stiffener exhibit no discernible decrease in fatigue performance compared to rolled sections.

Numerical Investigation of Production-Related Characteristics Regarding their Influence on the Fatigue Strength of Additively Manufactured Components

Abstract In order to further enhance the application of additive manufacturing (AM) processes, such as the laser powder bed fusion (L-PBF) process, reliable material data are required. However, the resulting specimen properties are significantly influenced by the process parameters and may also vary depending on the material used. Therefore, the prediction of the final properties is difficult. In the following, the effect of residual stresses on the fatigue strength of 316L steel, a commonly used steel in AM, is investigated using a Weibull distribution. The underlying residual stress distributions as a result of the building process are approximated for two building directions using finite element (FE) models. These imply significantly different distributions of tensile and compressive residual stresses within the component. Apart from the residual stresses, the impact of the mean stress sensitivity is discussed as this also influences the predicted fatigue strength values.

Mathematical Model of Properties and Experimental Fatigue Investigation at Elevated Temperatures of Functionally Gradient Materials

Fatigue that occurs at elevated temperatures is called thermal fatigue. High temperatures and cycling loads cause thermal fatigue that can cause component failures. In this paper, induced the mathematical models for functionally gradient materials and three models of functionally gradient materials (FGMs) manufactured by permanent casting were tested to predict their thermal fatigue life. FGMs have been tested to determine the effect of fatigue and temperature interactions. FGMs models were of FGM1, FGM2, and FGM3 respectively, with the following volume fractions and gradations: [100%Al-50%Al50%Zn-50%Zn50%Al-100%Zn], [100%Al-30%Zn70%Al-70%Zn30%Al-100%Zn] and [100%Al-70%Zn30%Al-30%Zn70%Al-100%Zn]. The experimental procedure presented the mechanical properties as modulus of elasticity at two levels of temperatures (80 oC, and 160 oC). Through the results of the tensile test at high temperatures, it was noted that the reduction percentage was high in the second type. This was especially for the yield strength value, where the percentage reached 43% at 160 °C. In the second type, the ultimate strength was affected more at 160 °C: the decrease percentage reached 33%, and the elastic modulus at the same temperature fell by 32%. The third type is the least affected at high temperatures, as the percentage of properties decrease at 160 °C reached 16, 17, and 16% for modulus of elasticity, ultimate strength, and yield strength, respectively. Fatigue strength is the most significant among the mechanical properties, where the highest value of the fatigue limit for the third type was experimentally 149.68 MPa. While the lowest value of fatigue strength was the second type, and at the same time, on the contrary, it was less affected at high temperatures, with a rate of 8%, which reached the decrease. The simulation results from the ANSYS program regarding fatigue at high temperatures were acceptable and gave reasonable variation ratios and were very close to the experimental results.

Energy-Based Fatigue Assessment of Defect-Afflicted Cast Steel Components by Means of a Linear-Elastic Approach

Cast steel components are affected by manufacturing process-based imperfections, which severely limit their fatigue strength. In this work, the linear-elastic strain energy density concept is applied to assess the fatigue behaviour of bulk defect-afflicted components made of high-strength cast steel alloy G12MnMo7-4+QT. Based on analytical calculations, an energy-based design limit curve is derived which merges experimental results of notched and unnotched small–scale specimens into a statistically proven scatter band. The stress ratio dependency is also investigated. Moreover, a numerical methodology is introduced, which facilitates the energy-based fatigue assessment of complex spatial imperfections on the basis of radiographs. Validation of the established framework utilizing experimental results of defect-afflicted large–scale specimens leads to sound accordance of numerically and experimentally derived fatigue strength values, showing an average deviation of about only eight percent.

Laser Powder Bed Fusion Process Optimization of AlSi10Mg Alloy Using Selective Laser Melting: Dynamic Performance of Fatigue Behaviour, Microstructure, Hardness and Density

The quality of a selective laser melting (SLM) component depends on build orientation and layer thickness, which are directly influenced by processing parameters. The present research of layer-by-layer additive simulation before starting the SLM process has several advantages, such as saving time, cost, and material. In this main investigation, the dynamic performance of fatigue strength, density, and hardness of AlSi10Mg alloy was produced by the SLM-AM according to the design of experiments. The L9 orthogonal array of the Taguchi method was created to perform the experimental development process. Finally, the obtained optimal process parameter with the highest values of fatigue strength, density, and hardness was found at a laser power of 225 Watts, a scan speed of 500 mm/s, and a hatching distance of 100 μm. The experimental density result was achieved with a high density value of 99.6% (2.66 g/cm3) and a defect-free component and hardness of 126±5 HV. The future scope of this study will use optimal process parameters to find out mechanical properties for as built and preheated conditions for aerospace applications.

Influence on Fatigue Strength of Post-Process Treatments on Thin-Walled AlSi10Mg Structures Made by Additive Manufacturing

This work aims to study the fatigue behavior of thin-walled structures and the possible influence of post-process treatments. Specimens with novel geometry were manufactured in AlSi10Mg with different inner diameter values using selective laser melting (SLM) technology and then treated. The different processes applied to the specimens were T6 quenching, microshot peening, and controlled roughness machining. The fracture data were analyzed to obtain the fatigue strength values at 2×106 cycles. The results showed that the mechanical treatments and the T6 quenching improved the fatigue strength by over 55% and over 80%, respectively. Relative density and per cent porosity were measured, and microscopic observations by electron microscope, metallographic microscope, and scanning electron microscope (SEM) were performed. It was possible to conclude that thickness did not affect fatigue life in the studied cases.

On the use of milled shell‐only specimens to study the effect of fiber orientation on the fatigue behavior of a short fiber‐reinforced polyamide

AbstractThe interpretation of the fiber orientation effects on the fatigue behavior of short fiber‐reinforced polymers using specimens extracted from injection‐molded plates is hampered by the typical shell‐core‐shell structure, which affects the stress distribution through the sample's thickness. In this work, local fatigue strength values are obtained using specimens machined to leave only the outer shell layers, thus having a more homogeneous fiber orientation. Results of the finite element analysis of integer and shell‐only specimens, accounting for locally measured fiber orientations, are compared. Conclusions are drawn about the role of the core layer and the limitations of milled specimens in the evaluation of the fatigue strength.

Sizing limitations of ultrasonic array images for non-sharp defects and their impact on structural integrity assessments

Existing structural integrity assessment procedures typically assume flaws to be infinitely sharp when they cannot be considered as local thinned areas. This assumption is often over-conservative, resulting in a pessimistic assessment of structural components and a significant underestimation of their margins of safety against fracture. One of the main challenges while adopting non-destructive evaluation techniques is distinguishing between sharp cracks (e.g., fatigue) and non-sharp defects and identifying the more severe ones. Towards this broader challenge, the present work aims to examine the sizing limitation and accuracy of ultrasonic array image-based techniques for non-sharp defects (surface breaking u-notches) and investigate how these measurements would affect the structural integrity assessment of components. Parametric numerical simulations and experimental measurements are performed to generate full-matrix capture datasets, which are then processed using the total focusing method to form an image. The image-based sizing approach is shown to perform efficiently for notch depths higher than the inspection wavelength (λL), i.e. as small as 0.2 mm, and semi-notch widths as small as 0.1 mm. The influence of ultrasonic measurements on structural integrity assessments is highlighted using different case studies in the context of non-sharp defects of fatigue and fracture strength estimates. For the cases under analysis, resolving non-sharp defects led up to 5× and 3× values of effective fracture toughness and fatigue strength, respectively. We have also seen that a 30 % uncertainty in semi-notch width sizing would result in a 30 % and 20 % error in fatigue strength and fracture toughness estimations, respectively.

Effect of Hydrothermal Degradation on Flexural Fatigue Strength of Various Cubic-Containing Translucent Zirconia.

The aim of this study is to investigate the fatigue and hydrothermal degradation behavior among the cubic-containing translucent yttrium oxide stabilized tetragonal zirconia polycrystal (Y-TZP). Four groups of commercial Y-TZP (T, ST, XT, and P), containing different amount of cubic crystal, were examined. Artificial aging was accomplished by autoclaving at 122°C under 2 bar pressure for 8 hours. Fatigue simulation parameters were set using an axial 50 N load, a total of 240,000 cycles. Flexural fatigue strength was evaluated. Two-way analysis of variance with Tukey's honestly significant difference test was used to determine the difference in fatigue strength values between various type of zirconia materials within control and aging with a significant level of 5%. Weibull analysis was conducted on the fatigue strength data. All groups showed the mean flexural fatigue strength had no significant difference in hydrothermally aged specimens compared with specimens without aging (p = 0.154). Group T showed significantly higher flexural fatigue strength followed by group ST, XT, and P. Group XT and P showed no significant difference in strength value (p > 0.05). Group T demonstrated a lower probability of failure than group ST, XT, and P whether with or without aging. Weibull modulus in group T and ST with aging condition was increased and higher than all the experimental groups. Cubic-containing zirconia materials (group ST, XT, and P) exhibited lower flexural fatigue strength than non-cubic 3Y-TZP zirconia (group T). However, various cubic-containing translucent zirconia was assumed to have high resistance to hydrothermal degradation.

Bending fatigue properties of structural steel fabricated through wire arc additive manufacturing (WAAM)

Wire arc additive manufacturing (WAAM) has become an essential production method with advantages such as ease of application and economic benefits, along with high material savings and its ability to produce large parts close to their final form in a short time. In recent years, many studies have been carried out on WAAM, most of which have investigated the properties of the manufactured parts. Among these properties, the topic of fatigue was less frequently examined. The reasons for this may include difficulties such as the long duration of fatigue tests and the large number of test specimens required. However, since fatigue is responsible for most of the damage in metal components, it is important to determine the fatigue strength of the parts produced using WAAM. In this study, the WAAM process was applied to low alloy ER70S-6 steel, which is widely used in the metal industry and is also one of the most common materials used in WAAM processes. After the fabrication, horizontal and vertical specimens were obtained from the manufactured part and mechanical and metallurgical tests were applied, including, fatigue tests, with the fatigue properties of the produced WAAMed part determined using a specially designed and produced bending stress fatigue testing machine. Results demonstrated that the fatigue limits of the horizontal and vertical fatigue specimens at 107 cycles were very close to each other, as approximately 180 MPa. The fatigue strength values of the vertical and horizontal specimens were quite similar. However, the fatigue strength of the vertical specimens up to 106 cycles was found to be slightly higher.

Fatigue performance of rib-to-deck joints in orthotropic steel deck with PWHT

Post-weld heat treatment (PWHT) displays potential for enhancing the fatigue strength of rib-to-deck (RD) welded joints in orthotropic steel decks (OSDs) by mitigating welding residual stress. In this study, residual stress measurements, fatigue tests, and fracture analyses were performed to investigate the influence of PWHT on the welding residual stress and fatigue strength of RD joints. Eighteen full-scale specimens were fabricated, i.e. ten PWHT-processed specimens and eight as-welded specimens without additional treatment. Hole drill (HD) and X-ray diffraction (XRD) were used to measure the residual stress in four PWHT specimens and four as-welded specimens. The PWHT specimen showed a reduction in the peak residual stress by 76% and 78% in the HD and XRD results, respectively. Fatigue tests were carried out on the six PWHT specimens and four as-welded specimens. The comparison was made between the two specimen types using the mean value of equivalent fatigue strength derived at 2 million cycles. The results indicated that PWHT improved the mean equivalent fatigue strength of RD joints by 23% and 33% under nominal stress and hot-spot stress, respectively. Parametric investigations were carried out using finite element-based fracture analysis to elucidate the influence of residual stress mitigation on fatigue crack growth in RD joints. The result indicated a significant decrease in the crack growth rate owing to residual stress mitigation. Such a decrease enhances the fatigue strength of RD joints.

An investigation of the fatigue properties of the aluminium-copper alloy AA2014-T6

ABSTRACT An approach to determine the fatigue behaviour in the heat-affected zone (HAZ) of AA 2014 has not been investigated exhaustively since the HAZ has been treated as one region. In this study, fatigue crack growth (FCG), fatigue strength and low cycle fatigue (LCF) properties of the base metal (BM) and two regions of the HAZ were studied. The two regions of the HAZ were thermally simulated at 590°C and 650°C representing regions of the weldment 4 mm and 5 mm from the weld centreline respectively. The hardness tests and microstructure characterisation of the base metal and the two regions of the HAZ were also carried out. The HAZ region thermally simulated at 590°C was found to offer the least resistance to FCG, had shorter LCF life and lowest value of fatigue strength compared to both the BM and the HAZ region thermally simulated at 650°C. The cause of weakness in this region was associated with the dissolution of strengthening phases. The degradation of the fatigue properties of AA2014 occurred severely at 590°C due to the dissolution of CuAl2 precipitates in the aluminium matrix. The HAZ region thermally simulated at 590°C was confirmed to be the weakest link in the HAZ.

Properties of welded joints of casting aluminum VAL8 alloy and wrought 1151T alloy

The results of the formation of the butt joint of 3 mm thickness plates made of VAL8 and 1151T aluminum alloys in different combination are presented. The ultimate strength of VAL8 and 1151T alloy welded joints during melting welding is 87...88 % of ultimate strength of the VAL8 alloy. The strength coefficient of joints of similar combinations of alloys made by friction stir welding is 0.92...0.93 of the strength coefficient of the VAL8 alloy. The fatigue strength for joints of VAL8 and 1151T alloys is 140 MPa. Joints made by friction stir welding at higher values of fatigue strength (160...200 MPa), withstand approximately 1.5...1.8 times more test cycles than joints obtained by automatic argon-arc welding.

Increase the Toughness of Material of Construction Using Heat Treatment

The objective of this research is study on the fatigue strength and fracture behavior of DIN 34CrNiMo6 steel. The specimens were subjected to repeated torsion loading. The type of this steel generally used for turbine rotor, crankshaft, and gear. These components operate in elevation temperature and subjected by fatigue load. The test of fatigue strength were carried out under repeated torsion with variation of angle 10, 20, 30, and 40. The test temperature are also varied. on 5000C and 7000C and than compared to without heating one. The fracture surface were analyzed using scanning electron microscope (SEM) in order to identified the influence of temperature and variation of angles. The results of studies show that either the fatigue strength or fracture surface behavior are significant different. The value of fatigue strength under cyclic torsion which is heated on 7000C is higher than 5000C and original ones. Characteristic of surface fracture using scanning electron microscope showed that the increasing of temperature also change pattern of fracture surface significantly. On 7000C heating the fracture surface is smoother than others. On the specimen without heating plastic deformation area of fracture surface is clearer than others.