Fatigue Strength Exponent Research Articles

Mechanical fatigue in microtubules

Mechanical failure of biological nanostructures due to sustained force application has been studied in great detail. In contrast, fatigue failure arising from repeated application of subcritical stresses has received little attention despite its prominent role in engineering and potentially biology. Here, paclitaxel-stabilized microtubules are up to 256 times bent into sinusoidal shapes of varying wavelength and the frequency of breaking events are observed. These experiments allow the calculation of fatigue life parameters for microtubules. Repeated buckling due to 12.5% compression–equal to the compression level experienced by microtubules in contracting cardiomyocytes – results in failure after in average 5 million cycles, whereas at 20.0% compression failure occurs after in average one thousand cycles. The fatigue strength (Basquin) exponent B is estimated as − 0.054±0.009.

Effect of in-situ activated core-shell particles on fatigue behavior of carbon fiber reinforced thermoplastic composites

In this unique study, the effect of adding core-shell particles (CSPs) on fatigue performance of carbon-fiber reinforced PA6 (CF-PA6) laminates is investigated. The thermoplastic laminates were prepared using compression molding and were reinforced at ply interfaces with 2 wt% and 4 wt% CSPs of the polymer mass. A manual method was used to disperse CSPs using a sieve and carefully selected process parameters. The cyclic tests were conducted and assessed, considering S–N curve, stiffness degradation, and energy dissipation. Consequently, the fatigue life of modified composites improved respectively by eight and four times when 2 wt% and 4 wt% CSPs were used. The results showed that an optimal improvement was achieved with a 2 wt% CSPs. The fatigue strength coefficient and fatigue strength exponent of CF-PA6 composites improved by 22.13 % and 9.85 %, respectively. The findings have the potential to establish a new frontier in thermoplastic research and would help designers to enhance the fatigue properties of thermoplastic laminates in specific elastic tailoring structures.

Improved Equivalent Strain Method for Fatigue Life of Automobile Aluminum Alloy

Improved Equivalent Strain Method for Fatigue Life of Automobile Aluminum Alloy

Optimization towards Walker’s equation using intrinsic damage dissipation energy

In this work, a fatigue stress equation associated with equivalent life is proposed based on the intrinsic damage dissipation energy. This method can be treated as an optimization towards Walker’s equation owing to the similarity between them. Compared to Walker’s equation, a modification of SWT function, the proposed equation is attached to a material parameter directly correlating with the fatigue strength exponent, which eliminates the need to conduct additional HCF experiments. The analysis of experimental results from the existing literature allowed for the comparative validation of the suggested equation with two existing equations that have wide acceptance and simplicity, namely the Goodman relationship and SWT function, which revealed a better performance of the suggested model. The proposal provides great potential for its simplicity in the evaluation of HCF lifetime under tensional fatigue loading with mean stress, further optimizing towards Walker’s equation.

Grain size effects on high cycle fatigue behaviors of pure aluminum

Friction stir processing (FSP) was used to prepare coarse-grained (CG), fine-grained (FG), and ultrafine-grained (UFG) materials to investigate the grain size effects on high cycle fatigue properties. A linear relationship similar to the Hall-Petch relationship was acquired between the fatigue strength and grain size. Excellent grain boundaries stability in FSP UFG material inhibited the formation of large-scale shear bands and grain growth, which contributed to higher fatigue strength exponent and fatigue damage resistance. Compared with other UFG materials, increasing the fraction of high angle grain boundaries can significantly improve the fatigue strength.

Bi-iterative moving enhanced model for probability-based transient LCF life prediction of turbine blisk

The low cycle fatigue (LCF) life of aeroengine turbine blisk endures uncertain and transient loads from multi-physical fields, so that it is necessary to evaluate the LCF life for improving the safety of turbine blisk and aeroengine from a probabilistic perspective. In this paper, bi-iterative moving enhanced modeling (BIMEM) approach is developed to conduct the probability-based transient LCF prediction of turbine blisk with fluid-thermal-structural interaction, by integrating extremum thought, Kriging model, moving least squares (MLS) technique and modified particle swarm optimization (MPSO) algorithm. The extremum thought is used to simplify the processes of transient responses of turbine blisk into extremum values in time domain. Both MLS technique and Kriging model are employed to establish the relationship functions between output response and input parameters, for deriving transient reliability sensitivity analysis of turbine blisk LCF life. The MPSO algorithm is applied to find the optimal values of hyperparameters in Kriging model with the effective samples; from the probability-based transient LCF prediction of turbine blisk, it is illustrated that turbine blisk reliability degree is 0.9986, when the allowable value of LCF life is 2968 cycles; and the primary factor affecting turbine blisk LCF life is strength coefficient, followed by gas temperature, fatigue strength exponent, and so forth; the developed BIMEM approach is better in modeling precision and simulation efficiency in the probabilistic-based LCF life prediction, relative to other methods. The efforts of this paper offer a promising way for the health evaluation and prediction of complex structures besides turbine blisk.

Low‐cycle fatigue parameters and fatigue life estimation of high‐strength steels with artificial neural networks

AbstractFatigue life estimation is essential for life safety and cost reasons. Fatigue parameters at low cycles must be estimated with high accuracy to correctly estimate fatigue life. Conventional equations for parameters at low cycles are inadequate and unable to calculate the values correctly. Artificial neural networks (ANNs) are used to estimate and optimize the parameters and are more accurate than traditional equations. This study aims to estimate the fatigue parameters at low cycles and fatigue life by ANN with basic tensile properties of high‐strength steels (HSSs), which can be easily obtained from the literature. In particular, the fatigue strength exponent and fatigue ductility exponent primarily characterize the strain–life curve, and the estimation of these parameters is extremely important. To improve the accuracy of the estimation, activation functions, epoch numbers, training functions, elapsed times of training functions, and the number of hidden neurons is compared and determined.

High-Cycle Fatigue Behavior and Fatigue Strength Prediction of Differently Heat-Treated 35CrMo Steels

In order to obtain the optimum fatigue performance, 35CrMo steel was processed by different heat treatment procedures. The microstructure, tensile properties, fatigue properties, and fatigue cracking mechanisms were compared and analyzed. The results show that fatigue strength and yield strength slowly increase at first and then rapidly decrease with the increase of tempering temperature, and both reach the maximum values at a tempering temperature of 200 °C. The yield strength affects the ratio of crack initiation site, fatigue strength coefficient, and fatigue strength exponent to a certain extent. Based on Basquin equation and fatigue crack initiation mechanism, a fatigue strength prediction method for 35CrMo steel was established.

Original vs compatible stock abutment- implant connection: An in vitro analysis of the internal accuracy and mechanical fatigue behaviour.

To assess the internal accuracy and mechanical behaviour under cyclic loading after artificial aging of implant-supported crowns restored with original stock abutments and two compatible non-original stock abutments. Forty-eight original internal hexagon connection implants were connected to different stock abutments. The samples were divided into three groups depending on the manufacturer of the abutment components (one original, two non-originals).Firstly, samples were cross-sectioned and observed by using Scanning Electron Microscope (SEM) to evaluate the internal accuracy in three different implant-abutment interface locations (platform, internal and screw).Further, cyclic fatigue loading was carried out according to the ISO Norm 14801 using dynamic testing machine under sinusoidal loads for 2,000,000 cycles at test frequencies of 2 Hz in air after thermocycling with 10,000 cycles at 5 °C and 55 °C in artificial saliva for aging simulation. Original abutment components presented the highest percentage of surface with tight contact with the implant in the three implant-abutment interfaces studied. Additionally, original configuration showed highest fatigue limit value and fatigue strength exponent (280 N and -0.054) than non-original 1 (225 N and -0.109) and non-original 2 (200 N and -0.101). Original abutment components provide better fit and mechanical results under cyclic loading than non-original configurations. The results obtained in this study seem to suggest that the use of the original stock abutments to implants leads to a more homogeneous load distribution between the components that can influence the long-term success of the restorations.

Improved high cycle fatigue property of ultrafine grained pure aluminum

Ultrafine-grained (UFG) pure Al with uniform and stable microstructure produced by friction stir processing (FSP) significantly increased the high-cycle fatigue (HCF) strength compared with coarse-grained and other UFG materials prepared by severe plastic deformation (SPD). There was no obvious surface damage for FSP-UFG pure Al and the improved fatigue damage resistance can be attributed to the uniform microstructure and high microstructural stability. The ring-island stress distribution of FSP-UFG impeded the formation of large-scale shear bands, and enhanced the coordinated deformation during cyclic deformation. In addition to the enhanced tensile strength, increasing the fatigue strength exponent, which is decided by the microstructural stability, is also an effective method of improving the fatigue strength.

Small-scale high-cycle fatigue testing by dynamic microcantilever bending

The lifetime of cyclically loaded devices is often limited by the fatigue resistance of their individual phases. An advanced method is presented for measuring the high-cycle fatigue behavior of materials at the micrometer scale using a nanoindenter. It is based on the cyclic deflection of focused ion beam-fabricated microcantilevers using the continuous stiffness method (CSM). In line with experimental data on bulk nanocrystalline copper, the specimens exhibit grain coarsening followed by the formation of extrusions and a fatigue strength exponent of −0.10. The method is suitable for characterizing single phases and individual components of further complex systems

Determining the fatigue parameters in total strain life equation of a material based on monotonic tensile mechanical properties

This paper summarized and evaluated a variety of low-cycle fatigue life estimation methods and gives the expression of fatigue strength coefficient, fatigue ductility coefficient, fatigue strength exponent, fatigue ductility exponent, successfully established the relationship among four parameters in total strain life equation and monotonic tensile parameters such as Young’s module, ultimate stress, yield stress, reduction in area, elongation. Therefore a new fatigue life prediction method has been proposed, with which various materials’ fatigue life have been predicted and acquired satisfied life prediction results. The influence of fatigue parameters in the total strain life equation on fatigue life prediction results has been evaluated and in the mid-high (104–106) life stage, fatigue life is very sensitive to the true fracture stress, if the stress is reduced by 5%, the fatigue life will be increased by about 50%. The Material’s Low-Cycle Fatigue Life Prediction System (the MLCFLPS) has been developed based on the Visual Basic which can evaluate parameters in total strain life equation more convenient and fast, which shows strong mechanical significance in life prediction of engineering structures.

Effect of Carbide Density on Fatigue Limit of H21 Die Steel

In the present work, the effect of carbide density on microstructure, hardness, wear rate, and fatigue limit of AISI H21 die steel has been studied. The specimens of H21 die steel have been employed hardening at 1195 °C, double tempering at 540 °C, cryotreatment at − 185 °C for 24 h, and soft tempering at 100 °C. The microstructure, hardness, wear rate, and fatigue limit of the conventionally treated and cryotreated specimens have been examined. The pin-on-disk machine has been used for the dry sliding wear test. The fatigue test has been carried out using constant amplitude rotating bending fatigue test machine which indicates enhanced fatigue limit for cryotreated specimens. The improvement in hardness, wear behavior, and fatigue limit of cryotreated specimens as a result of increased carbide density with reduced carbide size and retained austenite has been observed. The carbides Fe6W6C, Cr23C6 and Cr7C3 played an important role in increasing the hardness, wear behavior, and fatigue limit of the material. The Basquin’s equation was used for the analysis of the fatigue test. The minimum value of the fatigue strength exponent of Basquin’s equation shows the enhanced fatigue limit of cryotreated specimens.

On the strain-life fatigue parameters of additive manufactured plastic materials through fused filament fabrication process

In this paper, the effects of part build directions or raster orientations have been studied on the strain-life fatigue parameters of a wide range of 3D printed plastic materials. These materials have been manufactured through Fused Filament Fabrication (FFF), also known under its trademarked name Fused Deposition Modeling (FDM). To do so, precise analyses of fatigue data with the Ramberg-Osgood form of stress-strain curves were utilized through a strain-based approach to fatigue. The effects of different load ratios and types of loading on fatigue parameters have also been studied. Materials considered in this study were Ultem 9085, Polycarbonate (PC), and Polylactic Acid (PLA). Additive manufactured plastic parts that are FDM-processed exhibited large anisotropy of strain-life fatigue parameters. Hence, the upper and lower bounds for fatigue life prediction were introduced based on the strongest and weakest part build directions or raster orientations of 3D printed materials. For all materials studied in the present paper, fill densities, which seem to have significant impact on fatigue strength of 3D printed parts, have been selected based on the maximum fatigue strength of each part. Results showed that, in some build orientations, the transition fatigue life does not exist. In other orientations, in which the plastic strain components are high enough, transition fatigue lives vary roughly between 20–400 cycles. This means that if the part design in very low cycle fatigue regime is of interest, plastic strains and more complicated plasticity analysis are needed. Results show that the load ratio has no major impact on the fatigue parameters of 3D printed PC parts. In addition, changing in the loading type from tensile fatigue to rotating bending fatigue can significantly impact the fatigue strength coefficient of 3D printed PLA specimens however, it does not noticeably alter the fatigue strength exponents.

Deformation and HCF behavior of A356 aluminum alloys in torsion

In this study, monotonic, cyclic and fatigue properties of the A356 aluminum alloy were identified using torsion tests with rounded solid test specimens of the alloy in four different processing conditions. The monotonic and cyclic shear stress-strain behaviors of the alloys are described using the Ramberg-Osgood model and the materials constitutive parameters were estimated minimizing the difference between the experimental torque and the analytical value. Results of torsion fatigue tests with R = −1 were compared with rotating bending results and the load factor was evaluated. Experimental fatigue results indicated that the alloys at T6 condition showed the best behavior and the results of strontium modified alloys shown slightly lower performance. The fatigue life diagram of T6 alloys has steeper negative slope (shear fatigue strength exponent, b ≅ −0.2) than the others as cast alloys (b ≅ −0.06). The comparisons of experimental rotating bending and reverse torsion fatigue showed exponent b is dependent on the load type for the as cast alloys. For the heat-treated alloys, the variation of exponent b is minimal and its value can be considered constant.

Mechanical fatigue behaviour of different lengths screw-retained restorations connected to two designs prosthetic connection level.

The aim of this work is to compare the mechanical fatigue behaviour of different crown heights of single crown implant restorations made over two different internal hexagonal titanium dental implant connection levels. Two different groups of internal hexagonal connection were studied: (a) TL group (N=26) included implants with a transmucosal connection (12.2mm height and 4.8mm width) and (b) BL group (N=26) composed by implants at a bone level connection (10mm height and 3.8mm width). Two subgroups (N=13) were established for each connection level according to crown heights (TL10=10mm, TL15=15mm, BL12=12mm, BL17=17mm). Dynamic load tests were carried out according to ISO Norm 14801. Failed samples and fracture surfaces were analysed with an optical and scanning electron microscope (SEM). The fatigue limit and the fatigue strength degradation (fatigue strength exponent) decreases with increasing crown height (5mm) in both groups of internal hexagonal titanium dental implant connection levels. Consequently, themechanical integrity of internal hexagonal dental implants varies with the height of the crown. An increase of 5 millimetres in the crown height appears to significantly decrease the fatigue life of both types of dental implants.

On the stability and sensitivity of the strain-life approach using the example of mild steel

The strain-life approach is a powerful tool for the assessment of steel constructions experiencing low cycle as well as high cycle fatigue. However, among the various modifications and variations of the original concept, the decision to choose a proper one is challenging. Especially since common guidelines do not provide clear recommendations or do not cover the strain-life approach at all. In this contribution, we compared widely accepted versions as well as somewhat newer ideas of the approach regarding the determination of local strains, mean stress correction or strength hypothesis in an extensive numerical study including three geometries with different sharp notches. We essentially emphasized the incorporation of plastic material behavior directly in the finite-element (FE) calculation using a plastic material law. There are partly similar investigations, but they focus exclusively on elastic calculations with subsequent plasticity corrections. Based on the results, recommendations for the application of the approach are given and aspects worth focusing on in further research are suggested. Additionally, a local and global sensitivity analysis of the material parameters was performed for both, that is elastic calculations combined with a plasticity correction as well as plastic FE-computations. The performed analysis identifies the fatigue strength exponent and the fatigue ductility exponent as the parameters most influencing the estimated fatigue life, given their maximum correlation coefficients of 0.56 and 0.91 respectively. The observed results seem to critically assess current approximation methods of strain-life material properties, since these two parameters are often assumed to be constant values for a range of steel grades.

Effect of SWCNTs and graphene on the fatigue behavior of antisymmetric GFRP laminate

In this relatively unique study, the impact of adding nanoparticles (NPs) on the fatigue properties of antisymmetric glass fiber reinforced polymer (GFRP) laminate has been investigated. Antisymmetric GFRP laminates (+45/02/902/02/-45) were prepared and reinforced once with 0.1 wt% of single-walled carbon nanotubes (SWCNTs) and then with 0.1 wt% of Graphene nanoplatelets (GNPs). The NPs reinforced GFRP laminates are termed here GFNRP nanocomposites. Ultrasonication method was used to disperse the NPs using carefully chosen process parameters. Fatigue tests were analyzed based on S-N curves, stiffness degradation and hysteresis loops. The results showed that the use of 0.1 wt% of SWCNTs led to an increase in the fatigue strength coefficient (FSC) and the fatigue strength exponent (FSE) of GFNRP nanocomposite specimens by 51% and 24%, respectively, while the use of similar wt.% of GNPs enhanced the FSC and FSE by 33% and 25%, respectively. Consequently, fatigue life of GFNRP nanocomposites are surprisingly enhanced by about three and twelve times when GNPs and SWCNTs are used, respectively. The findings would give designers much more confidence in using antisymmetric composite laminates in specific elastic tailoring structures.

Characterization of mechanical properties of aluminum cast alloy at elevated temperature

The tensile response, the low cycle fatigue (LCF) resistance, and the creep behavior of an aluminum (Al) cast alloy are studied at ambient and elevated temperatures. A non-contact real-time optical extensometer based on the digital image correlation (DIC) is developed to achieve strain measurements without damage to the specimen. The optical extensometer is validated and used to monitor dynamic strains during the mechanical experiments. Results show that Young’s modulus of the cast alloy decreases with the increasing temperature, and the percentage elongation to fracture at 100 °C is the lowest over the temperature range evaluated from 25 °C to 300 °C. In the LCF test, the fatigue strength coefficient decreases, whereas the fatigue strength exponent increases with the rising temperature. The fatigue ductility coefficient and exponent reach maximum values at 100 °C. As expected, the resistance to creep decreases with the increasing temperature and changes from 200 °C to 300 °C.

Influence of temperature on the high-cycle fatigue properties of compacted graphite iron

High-cycle fatigue tests of compacted graphite iron (CGI) at 25 °C, 400 °C and 500 °C were performed. The corresponding fatigue damage mechanisms were investigated by the observations on fractographies and longitudinal section morphologies of typical samples. The results show that the decreasing rate of fatigue strength from 400 °C to 500 °C is higher than that from 25 °C to 400 °C. The fatigue strength coefficient of S-N curve fitted with the Basquin relationship decreases from 25 °C to 400 °C and increases from 400 °C to 500 °C; while the fatigue strength exponent performs in an inverse tendency. These are resulted from plastic deformation mode transition of the ferrite at the tip of the crack induced by the graphite debonding: slip band at 25 °C and 400 °C to grain boundary sliding at 500 °C. Finally, the influences of carbide on fatigue properties were discussed.