A B S T R A C T The paper deals with the fatigue and failure analysis of serial shot-peened leaf springs of heavy trucks emphasizing on the influence of thermal treatment and shot peening on fatigue life. Experimental stress-life curves are determined by investigating smooth specimens subjected to fully reversed rotating bending conditions. These test results are compared to corresponding ones determined from cyclic three-point bend tests on shotpeened serial leaf springs in order to reveal the influence of the applied thermal treatment and shot peening process on the fatigue life of the high-strength steel used for leaf spring manufacturing, dependent on the load level. Microstructure, macro-and micro-hardness analyses are performed to support the analyses and explain the effects resulting from the certain shot peening process on the surface properties of the high-strength spring steel under investigation. The assessment of the fatigue results reveals nearly no life improvement due to the manufacturing, emphasizing the necessity for mutual adjustment of shot peening and thermal treatment parameters to take account for life improvement. D = diameter E = Young's modulus f 1 , f 2 = fatigue endurance strength correction factors K = slope of the stress-life curve HB = Brinell hardness M σ = mean stress sensitivity factor N f = number of cycles till specimen rupture R = stress ratio R = radius R m = ultimate tensile strength R z = mean roughness depth σ a = stress amplitude σ E,a = endurance stress amplitude σ m = mean stress I N T R O D U C T I O NHigh-strength steels are used in various technological fields, particularly in the industry of spring manufacturing. Especially in the automotive industry, leaf springs constitute the most effective suspension way of commerCorrespondence: G. Savaidis.

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.

Purpose – The purpose of this paper is to provide sound understanding of the mutual interactions of the major leaf spring design parameters and their effects on both the stress behavior of the designed leaf and the steering behavior of the vehicle. Design/methodology/approach – Finite elements analyses have been performed referring to the design of a high performance monoleaf spring used for the suspension of the front axle of a serial heavy truck. Design parameters like eye type, eye lever, spring rate and arm rate difference have been parametrically examined regarding the stress performance and their influence on the wheel joint kinematics. The effect of each design parameter is exhibited both qualitatively and quantitatively. Findings – Eye lever and eye type affect significantly the wheel joint kinematics and therewith the steering behavior of the vehicle. Spring rate and arm rate difference affect solely the stress performance of the leaf spring. Practical implications – Design engineers may use the outcomes of this research as a guide to achieve optimal leaf spring design ensuring its operational strength in conjunction with accurate steering performance of the vehicle. Originality/value – The international literature contains only few, mostly qualitative data regarding the effect of single design parameters on the leaf spring and the corresponding axle kinematics. The present work contains a comprehensive and systematic study of all major leaf spring design parameters, and reveals their effect on both the stress behavior and the steering behavior of the vehicle qualitatively and quantitatively.