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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.

A B S T R A C T The present paper reviews and contains simple modifications to extend the applicability of global and local fatigue concepts to thin-welded specimens with pronounced weld terminations (starts/ends). Experimental fatigue data reported by Kaffenberger and Vormwald have been used for the validation of the proposed procedures. Special attention is paid to the notch strain concept. The material state is explicitly considered by means of corresponding strain-life curves in conjunction with P SWT , P Morrow or the experimental mean stress sensitivity factors proposed by Olivier. The accuracy of Neuber's sharp and mild notch formulas, Seeger-Beste's formulation and Glinka's strain energy density rule within the context of the notch strain approach has been investigated. Simple rules accounting for stress relaxation effects have been additionally proposed and implemented into the calculations. Comparison of experimental with theoretical fatigue lives validates the accuracy and robustness of the proposed procedures to calculate fatigue design S-N curves.Keywords fatigue; effective notch stress; notch strain approach; thin-welded joints; weld terminations. N O M E N C L A T U R EA [%] = elongation at fracture a * [mm] = micro-structural support length A nom [mm 2 ] = nominal sheet area A w [mm 2 ] = nominal weld area b = fatigue exponent c = ductility exponent c f = fatigue notch factor (expressed versus nominal loads) c t = elastic notch factor (expressed versus nominal loads) e* = nominal strain ENSC = effective notch stress concept FAT = fatigue assessment curves FEA = finite element analysis FE = finite element FKM = Forschungskuratorium Maschinenbau HAZ = heat affected zone HCF = high cycle fatigue HRA = Rockwell-A hardness IIW = International Institute of Welding K ' [N/mm 2 ] = cyclic hardening coefficient K f = fatigue notch factor (expressed versus nominal stresses) K p = fully plastic limit factor K t = elastic notch factor (expressed versus nominal stresses) L [N] = nominal load LCF = Low Cycle Fatigue L w [mm] = weld length m = slope Correspondence: Michail Malikoutsakis.

Welding processes often lead to pronounced weld termination points (start/end), which are prone to fatigue events such as crack initiation. The fatigue of weld ends under shear loading, especially in thin sheet structures, is not sufficiently explored yet. In the present investigation, the real geometry of welds was obtained by scanning the three‐dimensional surface, especially of weld ends. Cryogenic breaking of the weld opened the view to the weld root geometry and especially to the critical location, where the weld root migrates to a weld toe at the weld end. In the experimental part of this research, fatigue testing of thin sheet plane and cylindrical specimens as well as large components containing weld ends was performed. Based on the data, weld end life curves have been obtained. The notch stress concept and a fracture mechanics based approach were applied for describing the results of fatigue tests providing sufficient accuracy.

PurposeThe purpose of this paper is to develop a FE based modeling procedure for describing the mechanical behavior of high‐performance leaf springs made of high‐strength steels under damaging driving manoeuvres.Design/methodology/approachThe type and number of finite elements over the thickness of leaves, as well as the definition of contact, friction and clamping conditions, have been investigated to describe the mechanical behavior in an accurate and time‐effective manner. The proposed modeling procedure is applied on a multi‐leaf spring providing complex geometry and kinematics during operation. The calculation accuracy is verified based on experimental stress results.FindingsA FE based modeling procedure is developed to describe the kinematics and mechanical behavior of high‐performance leaf springs subjected till up to extreme driving loads. Comparison of numerically determined stress distributions with corresponding experimental results for a serial front axle multi‐leaf spring providing complex geometry and subjected to vertical and braking loads confirms high calculation accuracy.Research limitations/implicationsThe proposed FE based model is restricted to linear elastic material behavior, which is, however, reasonable for the high‐strength steels used for leaf spring applications.Practical implicationsThe proposed FE procedure can be applied for the design and optimization of automotive leaf springs, especially for trucks.Originality/valueThe proposed procedure is simple and can be applied in a very early design stage. It is able to describe accurately the leaf behavior, especially the stiffness and stress response under the most significant driving events. It goes far beyond today's practice for leaf spring design, which is based on analytical methods not covering complex axle and steering kinematics, large deformations and non‐linearities.