Estimation Of Fatigue Strength Research Articles

Effect of natural aging by multifunction cavitation on plane bending fatigue behaviour of heat-treatable Al-Si7Mg aluminum alloys and its fatigue strength estimation

The effect of natural aging by multifunction cavitation (MFC) on the fatigue behaviour of heat-treatable Al-Si7Mg aluminum alloys was examined. Surface observation and plane bending fatigue tests were conducted for the MFC-treated aluminum alloys at a stress ratio, R, of −1. The hardness of aluminum alloy without T6 treatment was significantly increased by MFC due to both the work hardening and natural aging. MFC treatment improved the fatigue lives and the fatigue strength at N = 107 cycles of aluminum alloys due to the generation of compressive residual stress and increasing the surface hardness; however, surface pits were large enough to easily nucleate an initial fatigue crack. In addition, the fatigue strength at N = 107 cycles of MFC-treated aluminum alloys can be estimated considering the residual stress, pit size and surface hardness.

Experimental estimation and numerical comparison of fatigue strength of A356/SiCP composite in T6 condition

Experimental estimation and numerical comparison of fatigue strength of A356/SiCP composite in T6 condition

Microstructure, corrosion behavior, and fatigue resistance of laser powder bed fusion-produced precipitation-hardening martensitic M789 stainless steel

Additive manufacturing has facilitated the fabrication of stainless steel components with intricate designs, providing exceptional strength and corrosion resistance. This advancement, however, poses challenges in accurately predicting fatigue resistance under high-cycle loading, especially when surface features resulting from the printing process and corrosion are overlooked. This study investigates the microstructure of laser powder bed fused Böhler M789 AMPO stainless steel aiming to elucidate its relationship with corrosion resistance and fatigue properties.Through a comprehensive examination of the microstructural features using El-Haddad’s model and extreme value extrapolations, this research reveals the predominant influence of surface roughness, resulting from the printing process, on the fatigue strength of net-shaped specimens. For a stress ratio of 0.1, the estimated fatigue strength ranges between 69 MPa and 162 MPa. Upon mechanically polishing the surface to eliminate the surface roughness effects, the significance of other features is highlighted. Particularly, the effect of corrosion pits, shallow pores and oxides, and internal discontinuities follow in importance.Moreover, this study evaluates the impact of the as-received-oxide layer on corrosion resistance. Upon removal of this layer via surface etching or polishing, an improvement in the stability of the re-formed passive layer was observed. This enhancement is evident in the expanded potential range between the pitting and corrosion potentials, increasing from 0.2 V for the net-shaped surface to 1.05 V for the polished one.This work emphasizes the significance of characterizing microstructural characteristics to predict both fatigue and corrosion resistances. It also highlights potential research areas in additively-manufactured stainless steels.

Fatigue strength estimation of net-shape L-PBF Co–Cr–Mo alloy via non-destructive surface measurements

One of the primary challenges in utilizing additive manufacturing for load-bearing metal components in the aerospace industry lies in the relatively low fatigue strength and significant variability stemming from the typically rough as-built surfaces. The goal of this research is to develop a model able to robustly correlate parameters obtained by non-destructive measurements to the fatigue strength of a generic surface state for a cobalt-chrome alloy manufactured via laser powder bed fusion (L-PBF). The results show that a relatively accurate estimation of fatigue strength of a generic surface quality can be performed via high-quality non-destructive roughness measurements coupled with statistical considerations and fracture mechanics-based assessments. This provides the capability of comparing different surface states and selecting the best option for fatigue strength with limited experimental effort and might prospectively set the basis for qualification of L-PBF components in the presence of rough surfaces.

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.

A new multiaxial fatigue endurance model for high strength steels taking into account the presence of small defects

In the present work, a new multiaxial fatigue model is proposed for metallic material with small defects. This model correlates the uniaxial fatigue limit obtained from the area parameter of a high strength steel containing small defect with values associated with the amplitude of the principal stresses, whose calculation for non-proportional loadings is not trivial. The performance of the model was evaluated with experimental data from AISI 4140 steel, under various loading conditions, in which smooth cylindrical bars were considered, regarding: (i) the effect of non-metallic inclusions in the material, and (ii) superficial micro hole with a straight bottom machined in the material (550 μm in diameter and depth). The proposed model has provided accurate fatigue strength estimates with average error not exceeding 16 %.

Fatigue strength evaluation of scale railway axle with surface defect considering mean stress effect

In order to evaluate the notch fatigue strength of transition groove in railway axle, a series of fatigue tests were conducted on small specimens and press-fitted axles containing surface defects. Based on the experimental and finite element (FE) analyses, a novel method considering mean stress effect was proposed to evaluate the fatigue strength of scale railway axle with surface defect. It is found that press-fitted axles exhibit significantly lower notch fatigue strength than small specimens. Furthermore, the theory of critical distance (TCD) provides a good correlation between the notch fatigue strength of small specimens and experimental results. FE analyses revealed that the press-fitting reduces the fatigue strength of the transition groove through increasing stress concentration and introducing a mean tensile stress that decreases with an increase in the applied loading. To account for the mean stress effect, the axial stress at the groove is corrected as an equivalent stress amplitude at R = -1 based on Goodman relation. Through the combination of TCD and Goodman relation, an estimated fatigue strength with an error of less than 11 % can be achieved for the scale railway axle with surface defect.

Analytical prediction of fatigue resistance of additively manufactured aluminium alloy based on Murakami method

This research is devoted to analyze the stress state and fatigue strength of two specimens made by a newly developed high-strength aluminium alloy produced by the wire arc additive manufacturing (WAAM) process. The relationship between fatigue strength and flaw size was calculated based on the root squared area – a parameter by conventional Murakami’s equation which is a widely used analytical approach for predicting fatigue resistance in metallic materials. The research involves the metallographic preparation process on two aluminum alloy labeled HS-Al-A and HS-Al-B followed by Vickers hardness measurement. Further, the image of the observed pores was processed and dimensioned using an open-sourced software ImageJ by considering pixels and actual distance as well as by defining image threshold value for measuring pore sizes. The analytical approach is conducted in order to describe the maximum stress intensity factor Kmax at the crack front and to assess the fatigue strength σFS. As final results, specimen A has an average pore area of ≈65 µm with Kmax of 333.75 MPa∙√m and σFS of 137 MPa, while specimen B has an average pore area of ≈42 µm with Kmax of 325.13 MPa∙√m and σFS of 153 MPa. Overall, this research allows the formulation of a method for estimating fatigue strength of large defects leading to a conclusion that flaws can influence the fatigue resistance of the material so that the bigger the flaw size is, the lower σFS and the higher the Kmax.

Probabilistic evaluation of the Step-Stress fatigue testing method considering cumulative damage

A general testing and analysis framework for the Step-Stress fatigue testing method is identified, utilizing interval-censored data and maximum likelihood estimation in an effort to improve estimation of fatigue strength distribution parameters has been performed. The Step-Stress method’s limitations are characterized, using a simple material model that considers cumulative damage to evaluate load history effects. In this way, the performance including cumulative damage was evaluated and quantified using a probabilistic approach with Monte-Carlo simulations, benchmarked against the Staircase method throughout the work. It was found that the Step-Stress method, even when cumulative damage occurs to a wide extent, outperforms the Staircase method, especially for small sample sizes. Furthermore, positive results reaches further than the increase performance in estimating fatigue strength distribution parameters, where improvements in secondary information, i.e. S-N data gained from failure specimens, are shown to be distributed more closely to the fatigue life region of interest.

Fatigue strength estimation of a CF/PEKK composite through self-heating temperature analysis using cyclic bending tests at 20 kHz

Very high cycle fatigue (VHCF) testing at conventional frequencies is highly time-consuming and unrealistic for large sample sizes. Though testing at ultrasonic frequencies can accelerate the testing, any undesirable thermal effect arising from testing at 20 kHz can cause significant heat generation in the specimens, increasing the likelihood of thermally induced failure. When mitigated, the temperature behavior during ultrasonic fatigue testing may conversely provide knowledge of the fatigue behavior of the composite. We study this hypothesis in this work by estimating the fatigue strength of a CF/PEKK composite through its temperature evolution during 3-point bending tests at 20 kHz with different cyclic load amplitudes. The fatigue strength of the composite was estimated using the temperature characteristics from an increasing cyclic amplitude test. This estimation was validated with four constant cyclic amplitude tests. Further, the estimation model was extended to different pulse–pause sequences by two constant cyclic amplitude tests. The results indicate a reliable and rapid method for fatigue life estimation as a material property. Finally, the criticality of the self-heating phenomenon during the non-stationary self-heating condition was evaluated.

Fatigue strength estimates for composite wing panels of prospective supersonic transport aircraft

Fatigue strength estimates for composite wing panels of prospective supersonic transport aircraft

Multi-domain optimization of cast iron components in wind turbines

The continuously rising demand for renewable energies leads to increased installations of wind turbines with higher power. While the current power-to-weight ratio of up to 20 metric tons of cast iron per megawatt is stagnating, cast iron components of modern wind turbines are facing new challenges in terms of weight, manufacturability, and castability. These challenges can be addressed by systematically using multi-domain optimization approaches to reduce component weight and increase local component utilization.In order to meet the requirements for modern cast iron components, this multi-domain approach must employ methods from casting simulation, micromechanical analysis, topology optimization, and strength assessment. Here, casting simulation is used to determine local microstructure descriptors, which are subsequently used in micromechanical shakedown analysis to estimate the local microstructure-dependent fatigue strength. In parallel to the fatigue strength estimation, topology optimization is performed iteratively in combination with a castability analysis. The component strength is evaluated using a strength assessment approach based on the previously determined local material properties in combination with the topology optimized component.In this study, the overall concept of the proposed multi-domain approach is presented and requirements for the application of such an approach are formulated. The use case of this study is a planet carrier of a wind turbine gearbox manufactured from austempered ductile cast iron ADI-GJS-1050‑6. For this use case, a weight reduction of 17% was achieved while maintaining the required stiffness, such that the microstructure variance along the component was significantly reduced. Furthermore, the potentials and limitations of the presented approach are outlined and discussed in the context of the design of heavy-section castings.

Effect of galvanostatic anodic polarization on hydrogen evolution rate and residual fatigue strength of Mg alloys for implant use

For surgery, biodegradable magnesium alloys are considered promising candidates. The low corrosion resistance is advantageous since the implant is degraded in the presence of aqueous body fluids, supporting the human bone for a defined time and be fully degraded after this functional phase, making a second surgery for implant removal unnecessary. To design this phase and prevent early implant failure, precise knowledge of the degradation behavior over months and the correlating mechanical stability is essential. In vitro tests of the required duration are associated with enormous time and cost efforts. Therefore, a short-time method is developed to accelerate the degradation progress by anodic polarization without affecting the corrosion mechanism. The method is based on the corrosion behavior of Mg alloys under polarization, accompanied by increasing hydrogen evolution for increasing current densities, allowing conclusions to be drawn about the corrosion rate. Three-week immersion tests of the alloy WE43 with and without plasma electrolytic oxidation are the starting point. The time-dependent corrosion rates are to be reproduced by applying anodic polarization within three days. An experimentally determined relationship between current density and corrosion rate is used to design the polarization. The corrosion morphology of both testing strategies is analyzed by µCT. To evaluate the influence of the morphology on the mechanical stability, ex situ multiple amplitude tests are performed, allowing an estimation of the residual fatigue strength. The results show that a qualitative simulation of the hydrogen evolution rate is possible by applying polarization, achieving an enormous time saving. Due to a changed corrosion morphology with increased pitting tendency and the associated reduced residual fatigue strength, the method has to be considered as a worst-case evaluation allowing to exclude unsuitable Mg-based biomaterials right from the beginning, in particular before further preclinical studies.

Fatigue Strength Estimation of Ductile Cast Irons Containing Solidification Defects

The goal of the present paper is to discuss the accuracy and reliability of a procedure for the fatigue strength estimation of defective metals by considering some experimental data available in the literature. In particular, the fatigue behaviour of three ductile cast irons (DCIs) containing solidification defects (i.e., micro-shrinkage porosity) is simulated through the above a procedure, based on the joined application of the area-parameter model and the Carpinteri et al. multiaxial fatigue criterion. The fatigue strength of such DCIs subjected to both uniaxial (rotating bending or torsion) and biaxial (combined tension and torsion) cyclic loading is evaluated and compared with the experimental results.

A comparative analysis of step loading and staircase testing for fatigue strength estimation of an engine component

AbstractStaircase testing is a standard method for evaluating the fatigue strength of components. However, staircase testing assumes a normal distribution, while components can display bimodal behavior due to flaws in material, or issues during the manufacturing process. Three unique step loading data sets on different production crankshafts provide evidence that step loading reliably identifies material or manufacturing issues, which lower a component's fatigue strength. Staircase testing has an 87% or greater chance of overestimating the component's fatigue strength, which in turn overestimates the component's expected reliability. For example, a component with a 99.9% reliability based on staircase testing would only have a 74% reliability based on step loading. If a component contains an undetectable manufacturing defect, staircase testing has a 99% chance of overestimating the component's fatigue strength. Step loading reliably improves the estimation of a component's fatigue strength distribution while providing insights into a component's defect tolerance.

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.

Very high cycle fatigue assessment at elevated temperature of 100 μm thin structures made of high-strength steel X5CrNiCuNb16-4

Many components and structures are exposed to very high number of cycles and challenging environmental conditions during operation. This study contributes to a better understanding of the very high cycle fatigue (VHCF) properties of high-strength steel X5CrNiCuNb16-4 at room temperature (RT) and 350 °C. For this purpose, conventional specimens and thin-walled structures are extensively examined with novel high-frequency fatigue testing techniques at elevated temperature. Tests with unnotched specimens at 350 °C show a 21.7% reduction in fatigue strength for 107 cycles and a different failure mechanism compared to RT. In contrast, no temperature influence is observed for mildly notched specimens and even a higher local fatigue strength is found for sharply notched specimens at 350 °C. The decrease in fatigue strength for 109 cycles is more pronounced at 350 °C (−10%) than at RT (−5%), and it is proven that notched specimens adequately represent the VHCF behaviour of structures. The transferability of specimen results to components and structures is given great attention. A new proposal for the VHCF strength assessment of structures with high stress gradients is presented, which is based on specimen results, an extended material-mechanical support factor and a VHCF reduction factor. The prediction model gives conservative fatigue strength estimates for 109 cycles with a maximum deviation of 5.8%. This demonstrates that even complex shaped structures can be successfully evaluated with suitable specimens and methods.

Determination of notch factors for welded T-joints based on numerical analysis and metamodeling

The effective notch stress approach to estimate fatigue strength of welded components requires stress concentration factors of an idealized weld geometry using fictitious notch radii. This paper covers the estimation of those stress concentration factors for welded T-joints in different configurations. In comparison with different existing estimations, new methods using metamodeling by (a) the response surface method based on polynomial regression using coupling terms and (b) based on artificial neural networks are presented. The methods were trained by a large database of reference results obtained by finite element analysis. Besides higher quality of prognosis, the new metamodels enhance the range of allowable parameters of the T-joints compared to existing ones. The resulting methods provide means of obtaining stress concentration factors fast and of sufficient quality which could also be embedded in more complex applications as programmed solutions.

Effect of microstructural heterogeneity on fatigue strength predicted by reinforcement machine learning

AbstractThe posterior statistical distributions of fatigue strength are determined using Bayesian inferential statistics and the Metropolis Monte Carlo method. This study explores how structural heterogeneity affects ultrahigh cycle fatigue strength in additive manufacturing. Monte Carlo methods and procedures may assist estimate fatigue strength posteriors and scatter. The acceptable probability in Metropolis Monte Carlo relies on the Markov chain's random microstructure state. In addition to commonly studied variables, the proportion of chemical composition was demonstrated to substantially impact fatigue strength if fatigue lifetime in crack propagation did not prevail due to high threshold internal notches. The study utilizes an algorithm typically used for quantum mechanics to solve the complicated multifactorial fatigue problem. The inputs and outputs are modified by fitting the microstructural heterogeneities into the Metropolis Monte Carlo algorithm. The main advantage here is applying a general‐purpose nonphenomenological model that can be applied to multiple influencing factors without high numerical penalty.

Fatigue strength assessment of TIG-dressed high-strength steel cruciform joints by nominal and local approaches

According to the IIW recommendation, the fatigue strength of welded steel joints is defined as independent of the base material in case of the as-welded condition. However, post-treatment techniques can improve the fatigue performance of welded structures, especially for increased base material strengths. Therefore, this paper investigates the effect of TIG dressing, as common post-weld treatment method, on the fatigue strength of high-strength steel S700 cruciform joints. The statistically evaluated fatigue test results reveal a significant increase of the nominal fatigue strength from FAT 90 for the as-welded up to 182 MPa for the TIG-dressed state. The experiments are further compared to recommended and suggested design curves applying both nominal as well as local stress approaches. Focusing on the TIG-dressed state, the suggested increase in nominal stresses is well validated leading to a conservative assessment. In addition, the proposed slope in the finite life region with a value of m1 = 4 shows a sound fit to the statistically evaluated value of m1 = 4.7 for the test results. The local fatigue strength estimation is performed based on a recent proposal using the theory of critical distances. Therefore, linear elastic numerical analysis of the investigated specimens is performed. Again, the resulting S-N curves agree well to the experiments validating the proposed local design approach.