Fatigue Life Of Leaf Spring Research Articles

Structural optimization of ultra-thin leaf spring for weakening elastic hysteresis effect and enhancing fatigue life

Small-size ultra-thin leaf spring as the key elastic sensitive member is widely used in the precision electro-mechanical products. However, owing to the unavoidable elastic hysteresis effect of leaf spring, it always suffers from a significant graceful degradation of designed deflection and subsequently fatigue fracture under the condition of cyclic loading. In order to weaken the elastic hysteresis effect, structural optimization according to sequential quadratic programming method for reducing stress concentration of leaf spring was first carried out by simulations, and then the identification of elastic hysteresis effect for optimal leaf spring was carried out by experiments. Compared with the original design, the maximum deflection deviation for single cycle is decreased from 0.026 mm to 0.002 mm. For the sake of predicting the fatigue life of optimal structure of leaf spring being on the service, a loading block with multistage variable stress amplitudes was experimentally recorded and fitted by normal probability density function, and then a statistical probability method by mathematical analysis was proposed. Compared with the original design, the predicted fatigue life of leaf spring is 736000 loading blocks, which is ten times than that before structural optimization. A structural optimization for reducing stress concentration can significantly weaken the elastic hysteresis effect and enhance the fatigue life, which is benefit for the elastic stability of leaf spring during engineering application.

Fatigue life-based reliability assessment of a heavy vehicle leaf spring

Purpose This study aims to determine the reliability assessment based on the predicted fatigue life of leaf spring under random strain loading. Design/methodology/approach Random loading data were extracted from three various road conditions at 200 Hz using a strain gauge for a duration of 100 s. The fatigue life was predicted using strain-life approaches of Coffin–Manson, Morrow and Smith–Watson–Topper (SWT) models. Findings The leaf spring had the highest fatigue life of 1,544 cycle/block under highway data compared uphill (1,299 cycle/block) and downhill (1,008 cycle/block) data. Besides that, the statistical properties of kurtosis showed that uphill data were the highest at 3.81 resulted in the presence of high amplitude in the strain loading data. For fatigue life-based reliability assessment, the SWT model provided a narrower shape compared to the Coffin–Manson and Morrow models using the Gumbel distribution. The SWT model had the lowest mean cycle to failure of 1,250 cycle/block followed by Morrow model (1,317 cycle/block) and the Coffin–Manson model (1,429 cycle/block). The SWT model considers the mean stress effects by interpreting the strain energy density that will influence the reliability assessment. Research limitations/implications The reliability assessment based on fatigue life prediction is conducted using the Gumbel distribution to investigate the behaviour of fatigue random loading, where most previous studies had concentrated on a Weibull distribution on random data. Originality/value Thus, this study proposes that the Gumbel distribution is suitable for analysing the reliability of random loading data in assessing with the fatigue life prediction of a heavy vehicle leaf spring.

Program Load Spectrum Compilation for Accelerated Life Test of Parabolic Leaf Spring

A method of program load spectrum compilation suitable for ALT (accelerated life test) of parabolic leaf spring was presented. In order to shorten the design and test period, road test is conducted on proving ground roads. The displacement spectrum of shock absorber connecting the point of the parabolic leaf spring seat on a commercial vehicle exposed to roads was obtained. The FEA (finite element analysis) method was used to obtain stress spectrum of hot spots on leaf spring by the displacement spectrum, and the fatigue life of the leaf spring is predicted using Palmgren-Miner’s rule. Statistical characteristics of the amplitudes and means of the displacement spectrum is investigated. The amplitudes and means of displacement spectrum found in experiment obey Weibull distribution and normal distribution respectively, and the distributions of amplitudes and means are independent. Based on the Conover principle, the two-dimensional spectrum with eight levels is established by applying the cycle extrapolation method, and then it is converted into one-dimensional spectrum with the variable mean method. Follow the theory of equal damage, the program load spectrum is compiled suitable for bench test of fatigue life of leaf spring. The method of program load spectrum compilation can provide reference for fatigue life test and life prediction of parabolic leaf spring assembly.

Microstructural, surface and fatigue analysis of stress peened leaf springs

Purpose– The purpose of this paper is to investigate the fatigue and failure of commercial vehicle serial stress-peened leaf springs, emphasizing the technological impact of the material, the thermal treatment and the stress-peening process on the microstructure, the mechanical properties and the fatigue life. Theoretical fatigue analysis determines the influence of each individual technological parameter. Design engineers can assess the effectiveness of each manufacturing process step qualitatively and quantitatively, and derive conclusions regarding its improvement in terms of mechanical properties and fatigue life.Design/methodology/approach– Two different batches of 51CrV4 were examined to account for potential batch influences. Both specimen batches were subjected to the same heat treatment and stress-peening process. Investigations of their microstructure, hardness and residual stress state on the surface’ areas show the effect of the manufacturing process on the mechanical properties. Wöhler curves have been experimentally determined for the design of high-performance leaf springs. Theoretical fatigue analyses reveal the influence of every above mentioned technological factor on the fatigue life of the specimens. Therewith, the effectiveness and potential for further improvement of the manufacturing process steps are assessed.Findings– Microstructural analysis and hardness measurements quantify the decarburization and the degradation of the specimens’ surface properties. The stress-peening process causes significant compressive residual stresses which improve the fatigue life. On the other hand, it also leads to pronounced surface roughness, which reduces the fatigue life. The theoretical fatigue life analysis assesses the mutual effect of these two parameters. Both parameters cancel each other out in regards to the final effect on fatigue life. The sensitivity of the material and the potential for further improvement of both heat treatment and stress peening is appointed.Research limitations/implications– All quantitative values given here are strictly valid for the present leaf spring batches and should not be widely applied. The results of the present study indicate the sensitivity of high-strength spring steel used here to the various technological factors resulting from the heat treatment and the stress-peening process. In addition, it can be concluded that further research is necessary to improve the two processes (heat treatment process and the stress peening) under serial production conditions.Practical implications– The microstructure investigations in conjunction with the hardness measurements reveal the significant decrease of the mechanical properties of the highly stressed (failure-critical) tensile surface. Therewith, the potential for improvement of the heat treatment process, e.g. in more neutral and controlled atmosphere, can be derived. In addition, significant potential for improvement of the serially applied stress-peening process is revealed.Originality/value– The paper shows a systematic procedure to assess every individual manufacturing factor affecting the microstructure, the surface properties and finally, the fatigue life of leaf springs. An essential result is the quantification of the surface decarburization and its influence on the mechanical properties. The methodology proposed and applied within the theoretical fatigue life analysis to quantify the effect of technological factors on the fatigue life of leaf springs can be extended to any engineering component made of high-strength steel.

Fatigue life prediction: A comparative study for a three layer EN45A parabolic leaf spring

Article history: Received 6 January, 2015 Accepted 10 May 2015 Available online 12 May 2015 There are literally several studies accomplished to predict the fatigue life of leaf springs but estimation of fatigue life of a parabolic leaf spring by using CAE tools has not yet been executed in the past. Parabolic spring is an important component in a vehicle suspension system. It needs to have excellent fatigue life and in today’s scenario manufacturers rely on constant loading fatigue analysis. The objective of this work is to perform the fatigue analysis of parabolic leaf spring by three different methods where CAE analysis is performed to observe the distribution of stress fatigue life and damage using Goodman approach. In this work, fatigue life of the parabolic leaf spring is determined as per SAE spring design manual and experimentally by testing on full scale fatigue testing machine. ANSYS is used for CAE solution for the prediction of leaf springs fatigue life considering stress theory. The fatigue life estimated by all three modes is then compared for the purpose of validation. The methodology used in this paper brings a practical approach to the professionals in the industries who are engaged for design of mechanical components. © 2015 Growing Science Ltd. All rights reserved.

Effect of surface decarburisation, scragging stress and individual leaf camber on fatigue life of 65Si7 leaf springs

The production process of high strength leaf springs, comprises of shearing, punching, heat treatment, hot cambering, shot peening, scraging and subsequently testing for load rate and durability. The processing of the raw material plays a vital role in achieving the required fatigue life. The objective of the present work is to determine the effects of material processing and design parameter on the fatigue life of the leaf springs. The material processing parameters considered are surface decarburisation and scragging stress while the design parameter considered is individual leaf camber. A light commercial vehicle (LCV) leaf spring is taken into consideration for this work. The partial and complete surface decarburisation during the rolling is determined and its effect on the fatigue life is predicted. The effect of the scragging stress on the fatigue life is also determined. An ideal range of scragging stress is proposed for fatigue life enhancement. A suitable individual leaf camber is proposed for the individual leaves. The leaf springs with proposed camber are manufactured, assembled and tested on a full scale leaf spring testing machines. The effect of the assembly stresses due to individual leaf camber on the fatigue life is also determined.

Fatigue life prediction of parabolic leaf spring under various road conditions

Parabolic leaf spring experiences repeated cyclic loading during operating condition. Fatigue life assessment of the parabolic leaf spring is a significant aspect during the component design stage. This paper serves to simulate the fatigue life of a parabolic leaf spring design under variable amplitude loading (VAL). VALs carry the road signal that provokes fatigue failure on leaf spring. In order to seek for comprehensive leaf spring fatigue assessment, VALs signal were gathered through measurements from various road conditions such as highway, curve mountain road and rough rural area road. Subsequently, fatigue life of particular leaf spring design was predicted using finite element (FE) stress–strain model together with VALs signal as load input. For more conservative way, Morrow and Smith Watson Topper (SWT) mean stress correction methods were also applied. The results indicate that fatigue life of leaf spring is lowest during rough road mission, followed by curve mountain road and smooth highway road respectively. Additional design modification to prolong the fatigue life of the parabolic leaf spring is compulsory. The road VALs has provided even more realistic fatigue life estimation of parabolic leaf spring design when compared to traditional controlled laboratory method.

Effect of Assembly Stresses on Fatigue Life of Symmetrical 65Si7 Leaf Springs.

The maximum stress induced plays vital role in fatigue life improvement of leaf springs. To reduce this maximum stress, leaves with different unassembled cambers are assembled by pulling against each other and a common curvature is established. This causes stress concentration or sets assembly stress in the assembled leaf springs which is subtractive from load stress in master leaf while it is additive to load stress for short leaves. By suitable combination of assembly stresses and stepping, it is possible to distribute the stress and improve the fatigue life of the leaf spring. The effect of assembly stresses on fatigue life of the leaf spring of a light commercial vehicle (LCV) has been studied. A proper combination of stepping and camber has been proposed by taking the design parameters into consideration, so that the stress in the leaves does not exceed maximum design stress. The theoretical fatigue life of the leaf springs with and without considering the assembly stresses is determined and compared with experimental life. The numbers of specimens are manufactured with proposed parameters and tested for load rate, fatigue life on a full scale leaf springs testing machine. The effect of stress range, maximum stress, and initial stress is also discussed.

Fatigue life prediction of leaf springs in automotive vehicles using CAE tools

Today, the technologies in leaf springs are changing gradually; therefore, new tools are required to keep aligned with worldwide technological requirement. CAE tools are widely used in the automotive industries. The predictive capability of CAE tools has progressed to the point where, physical prototype testing is not required. The objective of the present study is to provide a CAE solution to determine the stresses and deflection under static loading condition. The authors have also reported the CAE solution (using f-fatigue) where the fatigue life of leaf spring has been determined under fluctuating loads. The experiments have been conducted on 65Si7 conventional leaf springs used by TATA Motors. The experimental results have been compared with CAE results. These results are also validated with the SAE, analytical and graphical approach. It was observed that CAE approach provides fatigue life results within 6%–7% and can be alternate of experimental and analytical approach.

Studies on laser peening of spring steel for automotive applications

Present experimental laser shock peening study on SAE 9260 spring steel, performed with an in-house developed 2.5 J/7 ns pulsed Nd:YAG laser, aimed to evaluate laser shock peening process as a possible alternative to existing shot peening practice for enhancing fatigue life of leaf springs. In the investigated range of process parameters, laser shock peening yielded largely comparable magnitude of surface compressive stress and shallower compressed surface layer than those achieved with existing shot peening practice. In contrast to considerably rougher shot peened surface with numerous defects, laser shock peening produced largely unaltered surface finish without peening-induced defects. With respect to shot peening, laser shock peening brought about significant increase in fatigue life. Improved fatigue performance of laser shock peened specimens is attributed to their better surface finish without peening-induced surface defects, which were potential fatigue crack nucleation sites in shot peened specimens.

When shot-peening does not enhance fatigue life of leaf springs

Fatigue life of shot-peened components and assemblies is found to be lower than expected owing to various reasons. In this paper, the fatigue strength of EN45A spring steel shot-peened samples has been evaluated experimentally at various shot-peening conditions. A wide difference was noticed in the fatigue testing results of leaf springs and sample data. Various contributing causes were analysed and the extent in improvement of fatigue life of shot-peened leaf springs has been discussed. The paper discusses circumstances when the shot-peening does not improve fatigue and fretting fatigue resistance of mating components.

A stress approach model for predictions of fatigue life by shot peening of EN45A spring steel

A lot of research has been done to improve fatigue strength of materials by creating compressive residual stress field in their surface layers through shot peening. In this paper, fatigue strength of shot peened leaf springs has been calculated from laboratory samples. The axial fatigue strength of EN45A spring steel specimen is evaluated experimentally as a function of shot peening in the conditions used for full-scale leaf springs testing in industries. Optimum shot peening condition for specimen is found and S/ N curves of the specimens are correlated with leaf springs curve. A mathematical model has been developed which predicts the fatigue life of leaf springs for a given stress at varying shot peening conditions. Predictions from this model are compared with experimental data. The estimation of fatigue life and relaxation of compressive residual stress field are discussed.

Fatigue Design Criterion of LCV Leaf Spring Based on Road Load Response Analysis

Suspension system of light commercial vehicle (LCV) has enough endurance to protect passenger and freight. Leaf spring is major part of LCV suspension system. Thus, fatigue strength evaluation of leaf spring based on road load response was carried out. At first, the strain of leaf spring was measured on the city mode driving condition and proving ground driving condition. And , the damage analysis of road load response was carried out. After that, fatigue test of leaf spring was also carried out. Based on ε-N life relation, fatigue life of leaf spring was evaluated at Belgian mode, city mode and drawing test specification called the 3 steps test mode. Next, it is compared the design life of leaf spring and evaluated fatigue life by the 3steps test mode. From the above, new target of Belgian mode and city mode was proposed to gratify design specification of leaf spring. It is expect that the proposed target can be satisfied leaf spring fatigue endurance at specific road condition.

Fretting fatigue behavior of leaf springs under random loading.

Fretting fatigue tests on leaf springs and small axial-loading specimens were carried out under constant loading and random loading simulating field load spectra. The results of leaf springs were compared with the literature data for similar springs. The behavior of fretting fatigue crack initiation and propagation in the leaf springs was found to be almost the same as those in the small laboratory specimen. No significant differences between their fretted surfaces or between their fatigue fractured surfaces were observed. Random loading reduced the fretting fatigue lives by a factor of 1.5. The effects of shot peening on fretting fatigue were also investigated. Shot peening improved the fretting fatigue strength in both specimens by a factor of two. The residual compressive stress induced by the shot peening process was attributed as being the controlling factor to the improvement. It was suggested that the fretting fatigue life of leaf springs can be estimated from that of laboratory specimens, if the clamping pressures and the relative slip amplitudes of both the specimens have the same magnitudes.

Residual Stress by Corner Press and Fatigue Life of Leaf Springs

Residual Stress by Corner Press and Fatigue Life of Leaf Springs

Cnsiderations for Fatigue Life of Leaf Spring

Cnsiderations for Fatigue Life of Leaf Spring