Strain-life Approach Research Articles

Fatigue Life Modelling of Steel Suspension Coil Springs Based on Wavelet Vibration Features Using Neuro-Fuzzy Methods

This study proposed wavelet-based approaches to characterise random vibration road excitations for durability prediction of coil springs. Conventional strain-life approaches require long computational time, while the accuracy of the vibration fatigue methods is unsatisfactory. It is therefore a necessity to establish an accurate fatigue life prediction model based on vibrational features. Wavelet-based methods were applied to determine the low-frequency energy and multifractality of road excitations. Strain-life models were applied for fatigue life evaluation from strain histories. ANFIS modelling was subsequently adopted to associate the vibration features with the fatigue life of coil springs. Results showed that the proposed wavelet-based methods were effective to determine the signal energy and multifractality of vibration signals. The established vibration-based models showed good fatigue life conservativity with a data survivability of more than 90%. The highest Pearson coefficient of 0.955 associated with the lowest RMSE of 0.660 was obtained by the Morrow-based model. It is suggested that the low-frequency energy and multifractality of the vibration signals can be used as fatigue-related features in life predictions of coil springs under random loading. Finally, the proposed model is an acceptable fatigue life prediction method based on vibration features, and it can reduce the dependency on strain data measurement.

Prediksi Umur Lelah Pegas Ulir Suspensi Depan Minibus yang dikemudikan di Atas Permukaan Jalan Datar, Menanjak dan Menurun Berazaskan Pendekatan Strain-Life

This study aims to evaluate the effect of road conditions on the fatigue life of coil springs using the strain life approach. This approach is accomplished by measuring the strain signal in the coil spring at the front of the car with a strain gauge at the component's critical point. Three different types of roads, namely flat roads, uphill roads, and downhill roads were chosen to test the strain signals obtained. After analysis, it was found that the fatigue life of the coil spring on the downhill road had the lowest value of 1.52e+4 cycles before it broke. This value is 54% lower than the fatigue life on flat roads and 96% lower than on uphill roads. This result is due to the braking factor on the way down which puts a higher tension on the coil springs, reducing their fatigue life. This study can contribute to the automotive industry to consider different road conditions in the design and testing of their products, especially in critical components such as coil springs. The strain life approach has also been shown to be effective in evaluating the fatigue life of automobile components, which can help improve vehicle quality and safety.

Prediksi Umur Lelah Lower Suspension Arm Minibus yang Dikemudikan pada Permukaan Jalan Lurus dan Berbelok Berbasis Pendekatan Strain-Life

In the previous studies, prediction of fatigue life at the lower suspension arm is only done based on strain signals obtained on a straight road. Considering the main function of the lower suspension arm is to stabilize the vehicle when turning, the purpose of this study is to predict the fatigue life of the lower suspension arm when the vehicle goes straight and turns clockwise and counterclockwise. The three roads are passed by vehicles with a speed of 30 km/hour. Measurement of strain signals is done by attaching a strain gauge to the lower suspension arm on the left side of the vehicle. Based on the simulation results based on the strain-life approach the lowest fatigue life is given by turning clockwise direction with 2.56E+6 cycles so that it breaks using the Coffin-Manson model. This value is low than the age of fatigue life when the vehicle goes straight and turns counterclockwise, each with 5.85E+6 cycles so it breaks and 5.08E+7 cycles so it breaks. This value is also comparable to that produced by the Morrow and SWT models. When the vehicle turns right, the lower suspension arm on the left side receives a strain that is greater than when the vehicle turns left, which is 5%. Strain received by the lower suspension arm can shorten the fatigue life of the component. Turning roads shorten the fatigue life of lower suspension arm so that 44% compared to straight roads.

Balanced bending fatigue life for helical gear drives to enhance the power transmission capacity through novel rack cutters

This article proposes an idea for balancing the bending fatigue life between the gear and pinion of a novel helical gear drive. In this study, the helical gear is developed through analytical equations based on a novel rack cutter tooth thickness factor, and the estimation of principal stress and strain at the critical root fillet of the helical gear teeth is carried out through finite element analysis. Further, the fatigue module in COMSOL- Multiphysics software is employed to determine the bending fatigue life through strain life approach based on Smith-Watson-Topper with Neuber’s rule and fatigue usage factor based on Findley criteria. The effect of major gear parameters such as helical angle, pressure angle, teeth number, gear ratio, addendum height and addendum modification factor (S+, S− and S0) on bending fatigue life are computed based on novel helical gear with different rack cutter tooth thickness factor of gear and pinion. Finally, the above parametric study shows an optimum rack cutter tooth thickness factor between the gear and pinion to achieve a balanced bending fatigue life and fatigue usage factor, which aids in improving the power transmission capacity of the helical gear drive.

Computational model for bending fatigue life and failure location prediction of surface-hardened running gears

In this paper, a numerical model is proposed to predict the location and the number of cycles for bending fatigue failure of surface-hardened gears. The predictions are validated against experimental results. Linear-elastic stresses and strains obtained by a finite element simulation are imported into the fatigue model. Bending fatigue lives at the observed tooth root region are estimated based on the multilayer method, hardness method, and strain-life approach. The critical plane approach is employed due to non-proportional loading. Good agreement between actual and predicted bending fatigue lives was observed, as well as between the estimated and actual subsurface failure location.

Fatigue behaviour of re-entrant auxetic structures made of the aluminium alloy AA7075-T651

An investigation of the fatigue behaviour of the re-entrant auxetic structures made of the aluminium alloy AA 7075-T651 is presented in this study. The analysed auxetic structures represent a new class of cellular structures that show anomalous deformation responses such as negative Poisson’s ratio. In the proposed work, the influence of the unit cell orientation on the crack path and fatigue life was studied using experimental and computational approaches. The Low Cycle Fatigue (LCF) tests were performed at load control with the load ratio 0.1 in tension. In the LCF tests five loading levels were selected, and at least two tests were performed at each loading level. The same loading conditions were then applied in the computational model in the framework of the ANSYS software package, where a nonlinear kinematic material model was applied to obtain the stress–strain relationship. The strain-life approach, with consideration of the Morrow mean stress correction, was then used to obtain the fatigue life of the analysed auxetic structures. The experimental and computational results showed that the unit cell’s orientation has a minor influence on the fatigue life of both analysed auxetic structures, but impact on the direction of the fatigue failure path significantly.

Failure behaviour of strain and acceleration signals using various fatigue life models in time and frequency domains

This study aims to determine the appropriate frequency domain methods for durability predictions of lower suspension arm based on real-life random vibration data. In frequency domain, the power spectral densities (PSD) of the vibration signals were obtained and then analysed using three PSD cycle counting models, including the Lalanne, Dirlik and Narrowband methods. Correlation between fatigue lives predicted by time domain and frequency domain methods were evaluated to determine the appropriate frequency domain methods for durability prediction suspension lower control arm. Results show good correlation between fatigue life predicted with vibration fatigue and strain-life approaches. Therefore, it was deduced that road surfaces affect the fatigue life of the lower control arm, whereby the rougher the surface, the lower the fatigue life of the lower control arm. Lalanne and Dirlik PSD cycle counting algorithm were found to be the most suitable method for vibration fatigue analysis of lower suspension arm.

Fatigue crack initiation and propagation in a PM-gear tooth root

A computational model for the determination of the fatigue life of sintered gears is presented in this study. The total fatigue life of sintered gear, N, is divided into the crack initiation life, Ni, and crack propagation life, Np. The number of cycles for crack initiation in a gear tooth root, Ni, is determined by using the strain-life approach and the low-cycle fatigue parameters of the sintered gear material available in the literature. The number of cycles for crack propagation, Np, to a critical length is obtained by using the simple Paris law and the material parameters taken from the authors’ previous work. The comprehensive computational analyses in the framework of the Ansys-software were performed to obtain the strain-stress field in a gear tooth root, and to determine the stress intensity factor at the crack tip. The numerical results were compared with the experimental data available in the literature.

Strain-Controlled Fatigue Behavior of a Nodular Cast Iron in Real Off-Highway Axles: Effects of Casting Skin and Strain Ratio

Nodular cast irons are widely adopted in off-highway vehicles, since they allow to obtain components with complex shapes as well as good mechanical properties. However, the fatigue strength of such components is a major issue, which is typically addressed by adopting the strain-life approach, since it considers the local strains at stress concentrators where fatigue cracks are prone to initiate. In the fatigue design of off-highway components, the detrimental effect of casting skin, due to both surface and subsurface features, must be accounted for in all cases where machining is not needed. Moreover, the local strain ratio at stress raisers can be different from the nominal one due to forward plasticity. In this paper, static tensile as well as strain-controlled fatigue tests have been executed on specimens taken from real EN-GJS-450-10 off-highway axles. Static tensile tests have been performed on specimens with machined surfaces, while fatigue tests at a strain ratio equal to −1 have been carried out on specimens with surfaces in both machined and as-cast conditions. In addition, machined specimens have been tested at strain ratios equal to 0.1 and 0.5 to investigate the material sensitivity to the mean strain. Finally, the high-cycle downgrading effect of the casting skin has been evaluated, and experimental data of machined specimens tested at different strain ratios have been summarized by using the Smith–Watson–Topper expression.

A study on design optimization for compressor blisks

Abstract Bladed disks are important components of gas turbine engine. Rotor disk spool drum assemblies of gas turbine engine constitute 20–25% of total engine weight. Increasing thrust-to-weight ratio and engine life is paramount for designers. Blisk reduces significantly weight of rotor, compared against conventional disks for aero engines. This paper brings out specific challenges faced while re-designing bladed disk into blisks including structural integrity aspects under various operating loads. This paper presents a case study on re-design of typical compressor bladed disk into a blisk, without changing the flow path or airfoil configuration, within space constraints. Weight reduction of rotor disk is carried out using shape optimization technique. Blisk configuration is derived from existing bladed disk general arrangement. This paper describes methodology of weight optimization of blisk using ‘HyperStudy’ tool considering static and dynamic 3D models with ANSYS solver. APDL fatigue life macro is developed for fatigue life prediction, using strain-life approach. In this paper 3D bladed disk, baseline and optimized 3D blisk modal analyses results are used to ensure minimum interferences for engine operating conditions. The developed methodology for optimization can be appreciated by significant weight reduction (30%), while meeting design criteria and increased fatigue life.

Distribution characterisation of spring durability for road excitations using maximum likelihood estimation

This paper presents the determination of a suitable probability function for fatigue-related reliability assessment of automotive coil springs under various road excitations using maximum likelihood estimation (MLE). The fatigue-related reliability assessment is a critical element for determining the component warranty period. The fatigue lives of the coil spring under various road excitations were obtained from experimental analysis using strain life approaches. The parameters for various distribution approach were estimated through maximum likelihood estimation (MLE). The Akaike information criterion (AIC) was used to determine the most suitable approach for fitting. As the outcome, the Nakagami distribution has shown the lowest AIC value at 330 which indicating as the most suitable fitting among other distributions. For reliability assessment, the mean time to failure (MTTF), reliability function and hazard rate was analysed. Based on these results, the suitable distribution for fatigue-related reliability characterisation were determined for improving spring life cycle assessment.

Fatigue crack growth in the re-entrant auxetic structure

The investigation of fatigue behaviour of the re-entrant and rotated re-entrant auxetic specimens made of Al-alloy 7075-T651 is presented in this study. In the proposed work, the influence of the orientation of the unit cell on the crack path and fatigue life were studied by using both experimental and computational approaches. In the experimental and computational approach, two geometric layouts of the base unit cell were analysed (re-entrant and rotated re-entrant structure). The Low Cycle Fatigue (LCF) tests were performed at load control with the load ratio 0.1 in tensile. In the LCF tests, five loading levels were selected and at least two tests were performed at each loading level. The same loading conditions were then applied in the computational model in the framework of the ANSYS software package. For the fatigue life calculation, the strain life approach was used, based on the Coffin-Manson model with a Morrow mean stress correction. The comparison between computational and experimental results regarding Fa − N fatigue-life curves and observed fatigue failure path showed a reasonable agreement.

Advanced fatigue assessment - the future of wind turbine towers

One of the sustainable development goals of the Green Deal established by the European Union is the demand of affordable and clean energy. Among the renewable energies, the wind energy is already crucial in reaching the ambitious goals and will become even more important in the future. Since economic aspects will gain more importance, an efficient use of materials and resources is essential, especially those related to the fatigue design and service life prediction of wind turbine towers made of steel.This paper introduces an advanced fatigue assessment method, using the strain-life approach and the crack propagation method, named Two Stage Model. This model combines the two phenomenological aspects of crack initiation and propagation for a more reliable fatigue service life prognosis compared to commonly applied and simplified methods. By investigating both parts of the Two Stage Model separately, the paper shows the effect of selected input parameters on the service life for constant stress amplitudes. A Monte-Carlo Simulation is applied to validate the Two Stage Model as a vulnerable and reliable fatigue assessment concept.

Fatigue crack initiation and growth in a tooth root of sintered gears

In this study, the computational model for determination the fatigue life of sintered gears is presented. The total fatigue life of sintered gear, N, is divided into the crack initiation (Ni) and crack propagation (Np) period. The number of stress cycles for crack initiation in a gear tooth root, Ni, is determined using the strain-life approach and with consideration of the low-cycle fatigue parameters obtained previously for the same material as already used for the analysed sintered gear. The number of stress cycles for crack propagation from initial to the critical crack length, Np, is obtained using the simple Paris law and with consideration of fatigue crack growth material parameters taken from the author’s previous work. The comprehensive computational analyses in the framework of Ansys-software are performed to obtain the strain-stress field in a gear tooth root and for determination of the stress intensity factor in the crack tip.

Modelling and predicting of the LCF-behaviour of aluminium auxetic structures

This study presents the experimental and computational analysis for determining the fatigue life of the auxetic cellular structures made of aluminium alloys. For the fatigue life calculation, the strain life approach was applied in the framework of the ANSYS software. The obtained experimental results were evaluated using the comprehensive statistical analysis, and presented in the form of different fatigue-life curves.Based on the reasonable agreement between computational and experimental results the proposed computational model was validated and can be used further for the fatigue studies of various auxetic structures made of Al-alloys.

Evaluation of Fatigue Characteristics of Aluminum Alloys and Mechanical Components Using Extreme Value Statistics and C-Specimens

In this study, the fatigue characteristics of aluminum alloys and mechanical components were investigated. To evaluate the effect of forging, fatigue specimens with the same chemical compositions were prepared from billets and forged mechanical components. To evaluate the cleanliness of the aluminum alloys, the cross-sectional area of specimens was observed, and the maximum inclusion sizes were obtained using extreme value statistics. Rotary bending fatigue tests were performed, and the fracture surfaces of the specimens were analyzed. The results show that the forging process not only elevated the fatigue strength but also reduced the scatter of the fatigue life of aluminum alloys. The fatigue characteristics of C-specimens were obtained to develop finite-element method (FEM) models. With the intrinsic fatigue properties and strain–life approach, the FEM analysis results agreed well with the test results.

Automotive suspension component behaviors driven on flat and rough road surfaces

The objective of this study is to identify the behavior of the car suspension components subjected to road surface contours. Strain signals were measured by installing a strain gauge at the critical area of the coil spring and lower arm. The car was driven on a flat and rough road surface with speeds of 30–40 km/h and 10–20 km/h, respectively. According to the fatigue life assessments based on the strain-life approach, it was found that when the car was driven on the rough road, the components received higher stresses, contributing to a shorter fatigue life. The fatigue life of the coil spring when being driven on the rough road was 1,248 cycles to failure, which was more than 14 times shorter when being driven on the flat road, with 19,060 cycles to failure. Meanwhile the fatigue life of the lower arm being driven on the rough surface was 3,580 cycles to failure, which was almost 3,328 times shorter when being driven on the flat road, with 11,914,000 cycles to failure. The useful life of the coil spring was more than 625 times lower than the lower arm when driven on the flat road, whereas when driven on the rough road, the useful life of the coil spring was almost 3 times lower than the lower arm. In conclusion, the coil spring will fail more than 2 times faster than the lower arm. This is because the contour of the road surfaces provide a vertical load, directly working the coil spring which reduces the load vertically, while the lower arm functions to hold the load when turning.

Review of and a new approach to elastic modulus evaluation for fatigue design of metallic components

The elastic modulus is a key parameter for the description of material properties when evaluating metallic components under service loads. In particular, when evaluating strain-controlled tests for the application of the strain-life approach, there are numerous different possibilities in standards for determining this material parameter. These can lead to different results and confront the user with a certain challenge in selecting the most suitable method. In this publication, an overview of different evaluation methods is given and a new evaluation method is presented, in which the elastic modulus is determined using the reversal points of a strain-controlled cyclic test under macroscopic linear-elastic deformation. This allows a simple and clear, widely user-independent determination of the elastic modulus. In addition, the differences between the new and the commonly used evaluation methods are shown, using the example of the aluminum alloy EN AW-7075, and their influence on the strain-life curve, the PSWT and PJ damage parameter curves, and the cyclic stress-strain curve by Ramberg-Osgood are investigated.

The Characteristics of the Fatigue Life of the SAE 1541 Carbon Steel Based on the Strain-Life Approach

The aim of this study was to characterize the fatigue life of the SAE 1541 carbon steel. The fatigue life was predicted based on the strain-life approach through the application of constant and variable strain data with negative, zero, and positive mean values. Each strain data was developed at amplitude range of 200 με, 400 με, 600 με, 800 με, 1,000 με, 1,200 με, 1,400 με, 1,600 με, 1,800 με, and 2,000 με. The results showed there was no significant difference in the fatigue life for negative, zero, and positive mean strain amplitudes using the Coffin-Manson model. This is due to the non-consideration of the mean strain effect by the Coffin-Manson model. However, according to the Morrow and SWT models, negative mean strain data gave a longer fatigue life while positive mean strain data gave a shorter life. In addition, variable strain data gave a longer fatigue life since they contain low and high amplitudes, with the low amplitudes contributing to minimum fatigue damage.

Fatigue Analysis of Engineering Structures: State of Development and Achievement

Fatigue failure is a phenomenon that often occurs in mechanical structures, especially in components that receive direct cyclic loading, e.g., marine facilities, automobiles, critical infrastructure, reservoirs, turbines, nuclear reactors, and features that work in extreme conditions. Several factors can affect fatigue resistance, including applied material type, environment temperature, microstructure state, residual stress, corrosion, and crack initiation. Accurately estimating fatigue life is critical. A variety of variables must be taken into the calculation to reduce the risk of dangerous failure. In this paper, a series of development and achievement reviews on the relevant phenomena and advanced research related to fatigue assessment is conducted. Consideration of fatigue assessment methodology, e.g., laboratory experiment and numerical calculation, is discussed to summarize relevant effects to characteristics of stress life, strain life, frequency-based, and fracture mechanic approaches. The review also presents the relationship of previous research and its relevance to the development of the recent study. The purpose of this paper is to provide state-of-the-art investigations as well as demonstrate the challenges of uncharted problems.