The paper discusses various partial solutions used for estimating fatigue life under variable amplitude multiaxial loading in the high-cycle fatigue domain. The concurring effects are treated, and their proposed solutions are commented upon. The major focus is on the categories of the phase shift effect and of cycle counting, and on the scope and quality of data, which support discussed theories. Results of own new experimental data set on specimens from S355 steel are provided. Fatigue life estimates for McDiarmid and Findley multiaxial methods and for two different methods of load path decomposition to cycles are shown to highlight some of the points open for discussion. It is concluded that the available experimental data are not sufficient to substantiate a clear decision to follow a definite algorithm.
The state function based on fatigue accumulation model has a great influence on accuracy of fatigue reliability analysis of components. A nonlinear fatigue accumulation model considering the interaction of loads is proposed in this paper. By introducing the square ratio of front and back loads as the new load interaction factor into the conventional toughness exhaustion model, thus it is improved to reflect the load sequence and interaction effect simultaneously. Moreover, the fatigue state function based on proposed model is constructed, which is analyzed by the probability density evolution method. The time-varying fatigue reliability curve is obtained by analyzing the generalized density evolution equation. The proposed model are verified using four experimental data, and results shows that the predication residual life fraction such obtained is more accurate. And accuracy and efficiency of the proposed time-varying fatigue reliability analysis method is validated using multi-level loading experimental data.
Using thin-walled cone-pipe welded joints of stainless steels, fatigue tests under bending loads were carried out. The test data were statistically analyzed with the Benard’s approximation, Gaussian, 2P-Weibull, and 3P-Weibull distributions. Stress–life curves at different failure probabilities by a constant strength scatter band model were obtained. The metallographic structures were investigated, and the stress concentration states were analyzed to elucidate the causes of the strengths and scatters. In the high-cycle fatigue regime, the 3P-Weibull distribution was mostly in agreement with the Benard’s approximation, and the coefficient of determination was 0.9642. The microstructure of the weld metal with a high weld opening angle was mainly ferrite phase with 20% austenite distribution. The crack initiation point was close to the weld interface, but the propagation direction was at a right angle, and initially penetrated the heat affected zone of the cone, leading to the high fatigue strength. The stress concentration factors depended on the weld opening angles, indicating the main factor which affected strengths and scatters.
Complex civil structures require the cooperation of many building materials. However, it is difficult to accurately monitor and evaluate the inner damage states of various material systems. Based on a convolutional neural network (CNN) and the acoustic emission (AE) time-frequency diagram, we used the transfer learning method for classifying the AE signals of different materials under external loads. The results show the CNN model can accurately classify cracks that come from different materials based on AE signals. The recognition accuracy can reach 90% just by re-training the full connection layer of the pre-trained model, and its accuracy can reach 97% after re-training the top 2 convolutional layers of this model. A realization of cracking source identification mainly depends on the differences in mineral particles in materials. This work highlights the great potential for real-time and quantitative monitoring of the health status of composite civil structures.
This work is an extension of applying a previously developed fracture mechanics cracking damage model to predict the fatigue lifetime of un-notched round specimens made of a ferrite-pearlite 0.4C-70/30 carbon steel in some cases of variable amplitude loading VAL. The model simulates the collective behavior of growing short fatigue cracks originating from the specimen surface roughness. Material grains of different phases, sizes and strengths are randomly distributed over the minimum circumference. Possible activities of surface cracks are predicted against loading cycles. Relevant published experimental data were utilized for comparison. The present predictions are in agreement with the corresponding experimental results.
In this paper a new interpretation and modification of the SWT function in terms of the total damaging energy density is proposed and discussed. The total damaging energy density is the sum of the damaging part of the strain energy density and complementary energy density corresponding to the first quadrant in damaging σD-εD axes. For cyclic loading with positive mean stress (σm≥0) the proposed function reduces to the original SWT formulation. For cyclic loading with negative mean stress (σm<0) the maximum stress is augmented by 1/3 of absolute value of the mean stress. The proposed approach shows a consistent correlation of the mean stress effects for both positive and negative mean stresses.
The present paper investigates the variability in the static and cyclic properties of two nominally identical supplies of the aeronautical Al grade 7075-T6. Samples were extracted from extruded bars of 15 mm and 60 mm diameter and with slightly different chemical composition. Noticeable differences were found in tensile strength, total elongation, low- and high-cycle fatigue strength, despite the nearly identical hardness value. The diverse mechanical behavior has been imputed to different extrusion ratio and therefore work hardening along with a more or less fine distribution of precipitates and dispersoids. The high-cycle fatigue strength was found to be in direct correlation with the monotonic yield strength and the size of the largest intermetallic precipitate. A simple equation based on Murakami sqrt(area) parameter is proposed to predict the fatigue endurance. Tensile tests and microstructural analyses are recommended instead of conventional hardness tests to have a tighter quality control on the mechanical properties of semifinished products.
The interface fracture toughness of SnSb11Cu6/20steel was measured by calculating the critical energy release rate and stress phase angle of the interface crack. A three-point bending test was used to introduce cracks into the bonding interface, and the cohesion model of the bonding interface was established through experimental data. Through finite element analysis of load-deflection curves with and without interface crack propagation, the crack initiation point is found. Then the energy calculation model of crack propagation is established, and the critical energy release rate is obtained using the virtual crack growth criterion. The calculation results of the stress phase angle show that the crack propagation is greatly affected by the normal stress after the babbitt alloy layer fractures. If the strength of the substrate material is weaker, the crack will continue to expand in the tangent perpendicular to the crack tip.
This paper describes a microstructure-based multiaxial non-proportional fatigue life prediction model with maximum shear strain and non-proportionality as damage parameters applied to A319 alloy. The materials made with different casting cooling rates and Sr modification are characterized and quantified in terms of secondary dendrite arm spacing (SDAS), size and aspect ratio of eutectic Si particles. Multiaxial non-proportional fatigue tests have been performed on six groups of A319 alloys to systematically analyze the effect of microstructure and loading path on the fatigue properties of Al-Si cast alloy. The first part of the paper is focused on microstructure quantitative characterization to determine the influence of different casting conditions, followed by stress response behavior and fatigue fracture analysis. Finally, quantitative relationship between six fatigue life parameters and microstructure characteristics is established and a new fatigue life prediction model is proposed to predict fatigue life of Al-Si alloy under multiaxial non-proportional loading condition.
A new perspective of localized shear strain accumulation was proposed to elucidate the formation mechanism of fine granular area (FGA) generated in a high strength steel under very-high-cycle fatigue (VHCF). On the one hand, experiments of VHCF under the negative stress ratio of -1 was carried out, and the microstructure of FGA was found and characterized by using Scanning Electron Microscope, Transmission Electron Microscopy, and Transmission Kikuchi Diffraction. The results show that the FGA consists of high-density dislocations, sub-grains, and fine grains with high angle grain boundaries. On the other hand, the evolution of shear strain and fatigue damage at the vicinity of an inclusion was modelled by using crystal plasticity finite element method at both positive and negative stress ratios. The results show that although the overall strain in VHCF is negligible, significant shear strain is accumulated at the vicinity of the inclusion. Such a large local strain is the driving force for the formation of FGA. The results also suggest that with the accumulation of shear strain and damage, the positive stress ratio is gradually evolved into negative. This may explain why FGA has also been reported at positive stress ratios in some literatures.
Unloading compliance (UC) method and normalization method (NM) are two of the most commonly used methods for determining the fracture toughness of materials. However, considerable differences often exist in the fracture toughness determined by these two methods, which solicits a new method to determine the fracture toughness accurately. In this paper, the compliance of crack length differences as measured by the crack length difference ratio Si is discovered, analysed and verified by experiments. Based on this compliance, a new accurate method, known as AJR, is developed and verified by test results. Factors that exhibit the advantages of the developed new AJR method are also investigated. It is found that the J-R curves determined by the new AJR method are more accurate than those determined by UC and NM. The new AJR method should be the first choice for steels with a small strain hardening ratio and low effective yield strength, and thicker CT specimens with shallower initial crack length. This is because the disagreement between UC and NM is unacceptably large. The developed new AJR method and the results presented in this paper can assist engineers and researchers to determine J-R curves and fracture toughness of steels more accurately and can contribute to the body of knowledge of fracture mechanics.
Gear contact fatigue is becoming a primary limitation for the growing demand of power density and service life in gear-driven equipment. The unchecked surface fatigue crack could further cause premature failure and put a serious risk to the safety and reliability of mechanical systems. In this work, an attempt is made to investigate the effects of rolling-sliding and mild wear on contact fatigue behavior. A comprehensive contact model is developed to capture the variation instantaneous pressure and stress field is calculated with the transient mixed EHL approach. Rolling-sliding contact is simulated with the time-varying roughness topography updated by Archard wear equation. The stress cycles are extracted and the relative contact fatigue life is obtained by using Zaretsky criterion. Results suggest that in rolling-sliding contact the contact fatigue life is obviously lower compared with pure rolling. The increases in the number and amplitude of stress micro-cycles is found to be the main contributors to the reduction of fatigue life. Mild wear tends to smooth the surface, subsequently mitigates the stress concentration and reduces stress cycles, then decrease the risk of surface contact fatigue.
The influence of maximum principal stress level on true-triaxial unloading behaviors and the failure mechanism of sandstone samples were comprehensively investigated by laboratory tests and discrete element simulations. The results show that the level of σ1 at unloading point significantly affects the deformation and failure characteristics of sandstone samples under true-triaxial unloading conditions. As the level of σ1 at unloading point increases, the ultimate bearing capacity of sandstone sample is increasingly strengthened, while the sample collapses more easily during the unloading process, and the failure mode of sandstone sample changes from mixed tensile-shear failure to shear failure. With the increase in the level of σ1 at unloading point, the accumulative micro-cracks at the unloading point and micro-crack generation rate during the unloading phase exhibit an increasing trend, while the sum of micro-cracks at the unloading phase and the ratio between the amount of tensile micro-cracks and shear micro-cracks roughly show a downward trend. The formation of macro fracture in sandstone sample is closely related to the stress conditions and material inhomogeneity. The tensile fracture in the upper right part of sample when the level of σ1 is relatively low should be attributed to the superiority in tensile contacts between particles in terms of contact number and corresponding tensile force.
The consideration of realistic load assumptions is important for the fatigue design of highly stressed nodular cast iron components for wind energy application. Especially in case of overloads causing elastic-plastic deformation, residual stresses may have a strong impact on fatigue life. In strain-controlled fatigue tests with constant and variable amplitudes, the influence of overloads on the lifetime was investigated. The overload was applied with the objective to create high tensile residual stresses. During fatigue testing the transient material behavior, cyclic hardening, cyclic relaxation of the residual stresses as well as quasi static creep effects, of the EN-GJS-400-18-LT was recorded and evaluated. To quantify the influence of the transient material behavior on the calculated lifetime, fatigue analyses are carried out with the strain-life approach, both with and without consideration of the transient material behavior. The results show that conservative damage sums are derived if the transient material behavior, especially the relaxation of tensile residual stresses, is neglected.
The main aim of the current study is to evaluate the compressive quasi-static and fatigue properties of titanium alloy (Ti6Al4V) cellular materials, with different topologies, manufactured via Laser Powder Bed Fusion (LPBF) process. The topologies herein considered are lattice based regular and irregular configurations of cubic, star and cross shaped unit cell along with trabecular based topology. The results have indicated that the effective stiffness of all configurations are in the range of 0.3 – 20 GPa, which is desirable for implant applications. The morphological irregularities in the structures induce bending dominated behavior affecting more the topologies with vertical struts. The S – N curves normalized with respect to the yield stress indicate that the behavior of star regular structures is between purely stretching dominated cubic and purely bending dominated cross based structures. Trabecular structures have shown desirable quasi-static and fatigue properties despite the random distribution of struts.
Fatigue behaviour of HIPed and stress relieved Ti6Al4V alloy specimen’s produced by SLM was analysed (Rε = -1). The HIP process caused a microstructural transformation decreasing the hardness and monotonic properties that not allowed fatigue strength increase. A bilinear behaviour in the elastic strain-fatigue life curve was observed, because of Young´s modulus decrease during the cyclic elasto-plastic tests consequence of subgrains formation. The Smith-Watson-Topper and total strain energy density models showed a good concordance between predicted and experimental fatigue lives in notched samples.
Concepts for crack propagation as well as fatigue assessment under variable mechanical and thermal loadings are unestablished. For variable mechanical loadings, the damage parameter PJ is well known, for thermal loadings the damage parameter DTMF is established. Both parameters base on the effective cyclic J-Integral, still the definition is different. The damage parameters PJ considers the effective stress and strain ranges from the upper reversal point of each load cycle to the point of crack closure. Depending on the loading sequence the point of crack closure is treated as a history variable. In addition, with a crack length dependant fatigue limit, the most important sequence effects are considered. A new PJ-based concept is developed by considering additional sequence effects. In comparison to experimental results, the developed concept is able to reduce scattering in the range of constant amplitude loading tests.
This article describes how a step-stress accelerated life test (SSALT) can be designed for testing the fatigue life and reliability of structural components with a single failure mode. With simple numerical simulations of the crack’s propagation in the notched area of the structural part for different loading levels, the slope of the S-N curve for a structural component is initially estimated. Then, a very few fatigue-life experiments are carried out in the high-cycle domain to determine the intercept of the structure’s S-N curve. By considering the scatter from the material’s P-S-N curve, different SSALT designs for the structural component can be composed and checked for their expected acceleration factor. The procedure is experimentally validated for the case of a notched specimen and two different SSALT designs. From the results it can be concluded that the predicted durations of the SSALT experiments correlate well with the real experiments.