Fatigue Curves Research Articles

Statistical Characterization of Strain-Controlled Low-Cycle Fatigue Behavior of Structural Steels and Aluminium Material.

Probabilistic evaluation of the resistance to low-cycle deformation and failure of the critical components in the equipment used in the energy, engineering, metallurgy, chemical, shipbuilding, and other industries is of primary importance with the view towards their secure operation, in particular, given the high level of cyclic loading acting on the equipment during its operation. Until recently, systematic probabilistic evaluation has been generally applied to the results of statistical and fatigue investigations. Very few investigations applying this approach to the low-cycle domain. The present study aims to substantiate the use of probabilistic calculation in the low-cycle domain by systematic probabilistic evaluation of the diagrams of cyclic elastoplastic deformation and durability of the materials representing the major types of cyclic properties (hardening, softening, stabilization) and investigation of the correlation relationships between mechanical properties and cyclic deformation and failure parameters. The experimental methodology that includes the calculated design of the probabilistic fatigue curves is also developed and the curves are compared to the results of the experiment. Probabilistic values of mechanical characteristics were determined and calculated low-cycle fatigue curves corresponding to different failure probabilities, to assess them from the probabilistic perspective. A comparison of low-cycle fatigue curves has shown that the durability curves generated for some materials using analytical expressions are not accurate. According to the analysis of the relative values of experimental probabilities of low-cycle fatigue curves, the use of analytical expressions to build the curves can lead to a significant error. The results obtained allow for the revision of the load bearing capacity and life of the structural elements subjected to cyclic elastoplastic loading in view of the potential scattering of mechanical properties and resistance parameters to low-cycle deformation and failure. In addition, the results enable determination of the scatter tolerances, depending on the criticality of the part or structure.

Fatigue assessment of U-rib full penetration welded joints based on local methods

Experimental and numerical methods were used to carry out fatigue assessment of double-sided full penetration U-rib welded joints with different deck thicknesses in Orthotropic Bridge Decks (OBD). The local stress field analysis was conducted by the finite element method. S-N curves for different thicknesses can be obtained based on fatigue test results. The Nominal Stress (NS) method, the Effective Notch Stress (ENS) method, and the Theory of Critical Distances (TCD) were applied to estimate the fatigue life. The feasibility of these three approaches was validated by fatigue test results. The results show that two types of fatigue crack initiation sites (inner and outer weld toes) and typical smooth and rough regions are found. The maximum principal stress decreases by about 30% when the deck thickness increasing from 14 mm to 18 mm, while the fatigue life increases by 6–11 times for different stress ranges. The stress concentration factors (SCFs) for U ribs with 14 mm, 16 mm, and 18 mm deck thicknesses are 2.71, 3.16, and 3.15, respectively. Double-sided U ribs welded joints with thicker decks have better fatigue performance. All predicted results using the NS method and the ENS method are greater than the FAT71 and FAT225 fatigue curves, respectively. The Point Method (PM) and Line Method (LM) in the form of TCD are found to be successful in predicting the fatigue life and recommended to fatigue assessment for double-sided welded U-ribs.

Time-temperature equivalence and unified characterization of asphalt mixture fatigue properties

This work presents a new method for evaluating the time-temperature equivalence and unified characterization of the fatigue performance for asphalt mixtures. For this purpose, direct tensile strength tests and direct tensile fatigue tests were carried out on asphalt mixtures at different temperatures and loading rates. A time-temperature-dependent stress ratio was established. By exploring the correlation between stress ratio and fatigue life, a new fatigue model considering the effect of temperature and loading time was built. The time-temperature equivalent of fatigue performance was confirmed by analysis of the fatigue dates under several temperatures and loading frequencies. The results showed that the direct tensile strength of the asphalt mixture changed non-linearly with the variation of the loading rate or temperature. The stress ratio was also related to temperature and loading rate. The time-temperature stress ratio was linearly related to fatigue life in a double logarithmic coordinate system, and a new SN fatigue curve was formed. The fatigue life under several temperatures and various loading frequencies can be described in one fatigue model and the time-temperature equivalent can be explored. The master curve of fatigue life or stress ratio was built. The time-temperature superposition principle was applicable for fatigue life estimation and stress ratio calculation. The fatigue life under different loading frequencies and various temperatures of the pavement in service can be completely estimated. The relative error between the test results and the calculated values is less than 30%.

Experimental Study on Vibration Fatigue Behavior of Aircraft Aluminum Alloy 7050.

It has been previously noted that the development of aerospace material technology and breakthroughs are inseparable when obtaining great achievements in the aerospace industry. Materials are the basis and precursor of modern high technology and industry. As one of the most powerful aluminium alloys, 7050 is widely used in the aerospace field. In this manuscript, the vibration fatigue behaviour of aircraft aluminium alloy 7050 is studied based on experiments. A vibration fatigue experiment and the traditional fatigue testing of aluminium alloy 7050 were performed. We found that there was an extreme difference between the vibration fatigue and the traditional fatigue curves. In addition, the experimental end criteria for the vibration fatigue experiment of aluminium alloy 7050 was obtained from the acceleration reduction and the frequency reduction value. For the acceleration experimental end criterion, 2% was the acceleration reduction value for the vibration fatigue experimental end criteria of aluminium alloy 7050. For the frequency experimental end criterion, 2% was the frequency reduction value for the vibration fatigue experimental end criteria of aluminium alloy 7050.

Application of the fuzzy data theory in durability estimation

Aim. Problems associated with the study of material fatigue, while being relevant in terms of engineering practice, have a significant degree of uncertainty. The fatigue curve is censored (which indicates the presence of items that have passed the planned load cycle and destroyed by the end of the tests), while the load block made for calculating durability can be designed in a number of ways with a sufficient share of subjective decisions. The load block is intended for calculating durability and defining test plans. It is to fully reflect the entire expected operational history. Both factors are considered in the paper as elements of fuzzy logic. The author examines the creation of a scientifically substantiated load block that would take into account the possible operating modes in a right proportion and taking into account the variability. That is due to the fact that fatigue damage accumulates over the entire life of a machine and is to be scientifically evaluated for an adequate probabilistic assessment. Methods. As the modes of operation of a certain part are not precisely defined (and cannot be defined by virtue of the logic of random use of machines), projections of random fuzzy distributions are considered. A finite set of operating modes in a reasonable proportion was successfully scientifically substantiated. Using the example of load analysis of a critical part of rolling stock, distributions were constructed and the possible distribution of a part’s life was estimated. The output of the developed method will allow assessing the operational risks and predict the required number of spare parts. By taking into account the censored sample elements in the process of fatigue curve construction, the estimation of the fatigue curve parameters can be made more consistent. Conclusions. The use of fuzzy sets may prove to be very useful when examining fatigue curves and estimating durability variation. Examples are given of applying the proposed method.

Effect of severe thermo-oxidative aging on the mechanical behavior and fatigue durability of short glass fiber reinforced PA6/6.6

The work deals with the fatigue lifetime estimation of Short Fiber Reinforced Thermoplastics (SFRP), with a focus on conjugated effects of thermal aging. Two materials containing 35% (V35) and 50% (V50) weight ratio of short glass fibers were aged for 500 h at 200°C in air and compared to the same materials in a Dry-As-Molded (DAM) state. Monotonic and fatigue tests were performed in samples machined out of injected plates and cut along three different orientations to the injection one (0°, 45°, 90°) in order to capture the anisotropy of the skin-core microstructure, classical in injected SFRP.Monotonic tensile tests evidenced the stiffening and embrittlement of the Polyamide matrix reported in the literature, nevertheless with an acuity depending on the matrix ratio and fiber orientation. Stress-controlled fatigue tests were performed at constant amplitude, frequency (10 Hz), temperature (200°C) and stress ratio R = 0.1. The fatigue curves of V35 are more affected by aging than those of the V50 material. The combined results from the mean strain evolutions and SEM observations confirmed that the initiation approach for fatigue lifetime estimation was still valid in aged composites. Several fatigue criteria, among the most recently reported in the literature, were evaluated from this database. The effect of aging on the cyclic evolution of the different Fatigue Indicator Parameters (FIP) involved in the criteria was analyzed preliminarily. For the present fatigue conditions, the best criteria for unaged materials (i.e cyclic creep energy-based ones) were shown to still be the best for aged materials. Finally, the ability to predict fatigue lifetime for aged composites from the identification of the fatigue criterion in the unaged state was evaluated.

Lightweight design of hub under three working conditions based on Topological optimization

SpaceClaim was used to parameterize the main dimensions of 6.50R16 trailer hub model. The sensitivity diagram of hub parameters was obtained based on Central Composite Design (CCD) algorithm, and the stress area of hub was screened out. Using transient mechanical fatigue analysis was carried out on the wheel under the condition of impact load on the S-N fatigue curve obtained hub fatigue failure area, through Admass STEP function wheel movement geometry curve under impact load is determined, according to the result of wheel hub parameter sensitive figure, fatigue analysis and topology wheel movement curve area under impact load, The shape of the hub model was optimized based on topology optimization, and the new hub model was constructed by SpaceClaim geometric post-processing. After topology optimization, the hub quality is reduced to 16%

Evaluation and Comparison of Traditional Plaster and Fiberglass Casts with 3D-Printed PLA and PLA-CaCO3 Composite Splints for Bone-Fracture Management.

Bone fractures pose a serious challenge for the healthcare system worldwide. A total of 17.5% of these fractures occur in the distal radius. Traditional cast materials commonly used for treatment have certain disadvantages, including a lack of mechanical and water resistance, poor hygiene, and odors. Three-dimensional printing is a dynamically developing technology which can potentially replace the traditional casts. The aim of the study was to examine and compare the traditional materials (plaster cast and fiberglass cast) with Polylactic Acid (PLA) and PLA–CaCO3 composite materials printed using Fused Filament Fabrication (FFF) technology and to produce a usable cast of each material. The materials were characterized by tensile, flexural, Charpy impact, Shore D hardness, flexural fatigue, and variable load cyclic tests, as well as an absorbed water test. In addition, cost-effectiveness was evaluated and compared. The measured values for tensile strength and flexural strength decreased with the increase in CaCO3 concentration. In the fatigue tests, the plaster cast and the fiberglass cast did not show normal fatigue curves; only the 3D-printed materials did so. Variable load cyclic tests showed that traditional casts cannot hold the same load at the same deflection after a higher load has been used. During these tests, the plaster cast had the biggest relative change (−79.7%), compared with −4.8 % for the 3D-printed materials. The results clearly showed that 3D-printed materials perform better in both static and dynamic mechanical tests; therefore, 3D printing could be a good alternative to customized splints and casts in the near future.

The features of the evaluation of the results of experimental data in interlaboratory comparison tests for low-cycle fatigue

Improving the efficiency of assessing the results of low-cycle fatigue (LCF) test in the course of interlaboratory comparison tests (ICT) requires a reduction of the uncertainty associated with a number of random factors independent on the quality of testing and qualification of testing laboratories. Those factors are attributed to a random nature of the initiation and development of the fatigue fracture due to the inhomogeneity of the macro- and microstructure of the material, the state of the sample surface, stress concentrator, etc. A method for evaluating the results of LCF tests at ICT is presented. The method is implemented on the basis of linear regression analysis of experimental values of the mean and standard deviation of the logarithm of the durability at a given stress level. The features of evaluating the results of experimental data of interlaboratory comparison tests for low-cycle fatigue are presented. We propose to use the median estimate of the logarithm of fatigue life (corresponding to a given level (amplitude) of stresses obtained from the median fatigue curve constructed from the results of statistical analysis including regression analysis of the variances of experimental data) as an assigned value for assessing the characteristics of functioning of the laboratories participating in the ICT. The assigned value and estimates of the performance of laboratories is obtained using the median fatigue curve, which is plotted depending on the number of participants in the ICT. With a small number of participants, the median low-cycle fatigue curve is plotted according to the test results obtained by the expert laboratory, whereas for a sufficiently large number of participants according to the test results combined for all laboratories participating in the ICT.

Experimental, analytical, and numerical investigations on the flexural and fatigue behavior of steel thin-walled X-section beam

Thin-walled hollow shapes are of great interest in many industries with weight constraints, due to their availability, low price, and strength-to-weight ratio. This paper presents the development, calculation, and production of a thin-walled X-section beam design with a unique section geometry. This hollow X-section beam geometry is a beam that has been developed to use shape-connected jaws on the beam and to use less bolts and mounting elements, and to easily change the positions of the parts mounted on the beam. The bending strength of this unique beam section was investigated, and the fatigue damage of the beam was also handled with technological methods. Using both experimental stress measurements and finite element solution, the static and fatigue strength of the beam were calculated with computer-aided engineering software. The FEM solution was performed nonlinearly by defining the linear elastic and plasticity properties of the material. Validation studies were carried out in the laboratory using three-point and four-point bending tests. These traditional experiments were supported by strain gauge technology that measures strain with 0.05% accuracy. The weight and bending strength of the X-section beam were compared with the hollow section square beam. X-section beam (S355J0H) has 13% lower bending strength than 120 × 120 × 8 mm (S355J0H) beam; however, it is 37% lighter. ANSYS-analytical test results of difference are 3.95%, and ANSYS-experimental test results of difference are 0.85%. Experiments showed that a ∼22% increase in strength is found in these corners depending on the production method. The fatigue behavior of the X-section beam was determined with the nCode DesignLife software using validated FEM solutions and fatigue curves of the materials. X-section beam developed for shape–connected assembly systems can be used especially in the formation of chassis with its superior assembly ability and bending strength, increasing functionality, and production speed.

Procedure for defining of the equivalent vehicle mileage and the procedure approbation results

Introduction (problem statement and relevance). Automotive manufacturers and research engineers around the world are actively engaged in improving the methods of checking the safety and durability of the vehicle, vehicle components and systems. Testing and evaluation methods play an important role in solving this problem. This article reveals the issues of using the methods for determining the equivalent mileage of a vehicle in accordance with accumulation of pseudodamage, which includes well-known methods and procedures, and also describes the results of applying the methods in practice.The purpose of the research is development of the program of accelerated durability testing of the М1С category vehicle based on the application of the methods for determining the equivalent mileage.Methodology and research methods. The method is based on the approaches of the material fatigue curve definition, the rule of linear damage accumulation, the theory of counting the number of loading-unloading hysteresis cycles with the rain flow method approach and subsequent comparative analysis according to values of accumulated pseudo damage.Scientific novelty and results. The main results were obtained in the form of a program of accelerated durability testing of the М1С category vehicle, in addition, the features of using the method for determining the equivalent mileage of a vehicle based on the accumulation of pseudo-damages are disclosed.Practical significance. The obtained result is applicable in the field of studies of the durability of the load-bearing components of the vehicle and its subsystems using the capabilities of the test base and mathematical modeling tools, in addition, the results are of practical importance in the development of accelerated durability testing of competing vehicles, similar in weight and size parameters to the studied.

Investigation of the similarity between physical tests and fatigue simulation of flange yoke made of C45 E material used in cardan shafts

AbstractThis study is focused on experimental and numerical estimation of the fatigue life of a flange yoke. The material's stress‐life (fatigue) curve (S−N curve) for axial loading is constructed using specimens extracted from actual flange yokes. The torsional fatigue lives of components are determined by torsional fatigue tests of the complete cardan shafts. Simulation of the cardan shaft model including the flange yoke is carried out in Catia V5 R6 2016 software, which is then transferred to Altair Hyperworks 2017 software for the stress analyses. To predict the number of cycles to failure, the results are transferred into nCode 2018 DesignLife analysis module. An aim is to employ the stress‐life curves as input in numerical calculations and finite element analyses to minimize the costly and time‐consuming experimental work. The results show that physical testing and analysis results are in accordance when S−N curve of actual component material, as well as surface roughness factors measured from real component surface affecting fatigue life are incorporated into the analysis.

A New Method for Characterizing the Fatigue Performance of High-Modulus Asphalt Mixtures

Abstract The current fatigue model of high-modulus asphalt mixture (HMAM) is usually built without considering the effect of loading frequency, which leads to the imprecision in the selection of fatigue parameters for the design of high-modulus asphalt pavement structures. The purposes of this study are to solve the problem of uncertainty in the characterization of fatigue performance of HMAM at various loading frequencies and to optimize the structural resistance design of high-modulus asphalt pavements. In this study, the four-point bending beam was first used to carry out the strength test on the HMAM at ten loading rates. The effects of different loading frequencies (1, 5, 10, 20, and 40 Hz) and stress levels on the four-point bending fatigue life of HMAM were then explored. The test results show that the fatigue curve derived from the nominal stress ratio deviates significantly from the theoretical strength failure point (1,1) for Nf = 1 by extending toward both ends; however, derived from the rate-dependent stress ratio (considering the effect of loading rate), it passes through the point (1,1), which reveals the correlation between the strength failure and the fatigue damage. In addition, a new method for calculating structural strength coefficients is proposed by developing a unified fatigue model for HMAM under different loading frequencies. The results of this study could predict the fatigue life of HMAM at different loading frequencies more precisely.

Application of Modified Fatigue Curve for Evaluation of Fatigue Damage of Steels at Variable Stress Amplitudes. Part 1. Calculation Model and Initial Data at Constant Stress Amplitudes

Application of Modified Fatigue Curve for Evaluation of Fatigue Damage of Steels at Variable Stress Amplitudes. Part 1. Calculation Model and Initial Data at Constant Stress Amplitudes

Characterization of fatigue properties of 3D printed polylactic acid

The goal of this research is to determine how tiredness behavior may be measured (PLA). A To A variety of technical data sets were S-N curves were chosen and statistically re-analyzed. as described in the following section, to generate a negative reference value reverse slope also an improved tolerance limit of 106 2 failure cycles. The average effect of stress on fatigue can be indicated via administering the highest level of stress achievable during the cycle, according to experimental data examined after treatment.Furthermore stress/strength study may be effectively performed until the printing orientation seems to possess minimal influence on PLA's general tiredness behavior. carried out via taking the printing orientation into account. A homogeneous, linearly elastic polymer is described. When acceptable experimental findings are not available, the paper explains how to conduct a fatigue evaluation (with a survival probability better than 95%). The study demonstrates how to do so via utilizing standard fatigue curves with a negative-inverse regression of 5.5 and a tolerance limit (2 106 cycles to failure) equivalent to 10% of the material's maximum σ_t.

A Bayesian framework for calibration of multiaxial fatigue curves

A Bayesian framework is proposed to re-formulate a multiaxial fatigue model and produce probabilistic stress-life fatigue curves from experimental data. The proposed framework identifies the experimentally-driven parameters governing the multiaxial fatigue model, in the form of probability distributions. Classical and hierarchical Bayesian inference strategies are presented, accompanied by rigorous analytical expressions for calculating the joint posterior distributions necessary for in-field implementation. An example illustrates the application of the proposed hierarchical Bayesian inference framework and how it compares to a deterministic approach. This probabilistic treatment makes the existing fatigue models suitable for exercising uncertainty propagation for reliability analysis and design purposes.

Fatigue Factor Assessment and Life Prediction of Concrete Based on Bayesian Regularized BP Neural Network.

Concrete tensile properties usually govern the fatigue cracking of structural components such as bridge decks under repetitive loading. A fatigue life reliability analysis of commonly used ordinary cement concrete is desirable. As fatigue is affected by many interlinked factors whose effect is nonlinear, a unanimous consensus on the quantitative measurement of these factors has not yet been achieved. Benefiting from its unique self-learning ability and strong generalization capability, the Bayesian regularized backpropagation neural network (BR-BPNN) was proposed to predict concrete behavior in tensile fatigue. A total of 432 effective data points were collected from the literature, and an optimal model was determined with various combinations of network parameters. The average relative impact value (ARIV) was constructed to evaluate the correlation between fatigue life and its influencing parameters (maximum stress level Smax, stress ratio R, static strength f, failure probability P). ARIV results were compared with other factor assessment methods (weight equation and multiple linear regression analyses). Using BR-BPNN, S-N curves were obtained for the combinations of R = 0.1, 0.2, 0.5; f = 5, 6, 7 MPa; P = 5%, 50%, 95%. The tensile fatigue results under different testing conditions were finally compared for compatibility. It was concluded that Smax had the most significant negative effect on fatigue life; and the degree of influence of R, P, and f, which positively correlated with fatigue life, decreased successively. ARIV was confirmed as a feasible way to analyze the importance of parameters and could be recommended for future applications. It was found that the predicted logarithmic fatigue life agreed well with the test results and conventional data fitting curves, indicating the reliability of the BR-BPNN model in predicting concrete tensile fatigue behavior. These probabilistic fatigue curves could provide insights into fatigue test program design and fatigue evaluation. Since the overall correlation coefficient between the prediction and experimental results reached 0.99, the experimental results of plain concrete under flexural tension, axial tension, and splitting tension could be combined in future analyses. Besides utilizing the valuable fatigue test data available in the literature, this work provided evidence of the successful application of BR-BPNN on concrete fatigue prediction. Although a more accurate and comprehensive method was derived in the current study, caution should still be exercised when utilizing this method.

The aspect of the corrosion pitting with fretting fatigue on Aluminum Alloy: A nuclear reactor safety or an aerospace structural failure phenomenon

Since wear and other surface decaying processes as Stress Corrosion Cracking (SCC) and Gridto-rod fretting failure (due to fuel rod vibration) can risk the lifespan of nuclear reactors, surface modification and other life degradation reducing attempts are being adopted for Aluminum alloys in such sophisticated applications like nuclear or aerospace components. In this research work, fretting fatigue and artificial pitting effect on heat-treated Al-Mg-Si alloy (Al 6061-T6) have been studied. Experimental observations have been authenticated by developing a FEM (Finite Element Method) model using simulation software ANSYS. Fretting causes a noticeable reduction in fatigue life at high cycle fatigue tests. However, it is difficult to differentiate between fretting and normal fatigue curve lines at low cycle fatigue. The fracture surface schematic shows how fretting affects crack initiation to final rupture. Interestingly, it has been found that pitting reduces the lifetime of Aluminum more abruptly than normal and even fretting fatigue. This study suggests more concentration on pitting by corrosion or others as well as geometry irregularities than the fretting aspect comparatively.

High cycle fatigue S-N curve prediction of steels based on transfer learning guided long short term memory network

The stress-life (S-N) curve is a fundamental aspect in fatigue analysis. However, fatigue testing using S-N curve is very costly and time-consuming. To solve this, a novel method to predict S-N curve is proposed combining the long short-term memory network (LSTM) and transfer learning. A transfer LSTM framework (TR-LSTM) was developed, wherein the reversed torsion S-N curves prediction of low alloy steels was transferred from rotating bending S-N curves. The prediction results for twelve steel grades prove the rationality of the framework. The generality of the framework with respect to different data amount and model parameters was further investigated. Additionally, the model was also successfully extended for the curve prediction of very high cycle fatigue. This proposed prediction framework can significantly reduce the cost of fatigue property evaluation and realize the conversion among fatigue curves with different test costs.

Investigation of low-cycle fatigue characteristics of nickel-based heat-resistant single-crystal alloy

Low-cycle fatigue under a hard loading cycle of a nickel-based heat-resistant single-crystal alloy with 001 crystallographic orientation at operating temperature is studied. The dependences of the change in the stress asymmetry coefficient on the number of cycles to failure are obtained. The fatigue curve and predictive curves obtained using shortterm strength characteristics are compared. Keywords: low-cycle fatigue, high-temperature alloy, single-crystal structure, nickel alloy, Basquin—Manson—Coffin equation, prediction of fatigue resistance characteristics. lab33@viam.ru