doi: 10.1111/j.1460-2695.2007.01173.x A critical investigation of techniques for stress determination and equivalent static analysis in fatigue life estimation A. P. GROV ´ E, F. VAN TONDER and P. S. HEYNS Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria 0001, South Africa Received in final form 3 July 2007 ABSTRACT This study entails the fatigue analysis of a complex plate-like structure subjected to ran- domloading.Thestressandfatiguelifeassessmentisperformedbymeansofexperimental strain gauge measurements, finite element analysis and a quasi-static fatigue assessment procedure known as the fatigue equivalent static load (FESL) methodology. Firstly, the integrity of shell elements for accurately capturing the stiffness properties and stress dis- tribution in the vicinity of welds is investigated. Furthermore, the stress response of the structuretorandomdynamicloadingisinvestigatedandvalidatedintermsofitssuitability for assessment by the FESL methodology. Nominal stress and hot-spot stress fatigue life predictionsaremade,basedonmeasurementsaswellastheFESLprocedure.Theviability andintegrityoftheFESLmethodologyiscriticallyassessedafterwhichtheactualfatigue life of the structure under particular loading conditions is determined for comparison. Keywords: fatiguestrength;finiteelementmethod;weldmodelling;quasi-staticmeth- ods; fatigue equivalent static load. NOMENCLATURE C d = constant relating to design logS r –logN curve D = fatigue damage parameter D e = damage caused by an equivalent stress range m = inverse slope of logS r –logN curve N = number of cycles to failure N e = number of cycles to failure under equivalent stress range N i = number of cycles to failure for a discrete stress range n i = number of cycles completed at a discrete stress range σ gload = stress due to an inertial load of 1g σ x = stress in x-direction σ z = stress in z-direction σ e = fatigue equivalent stress range σ i = discrete stress range σ measured e = fatigue equivalent measured stress range INTRODUCTION Manyplate-likestructures(especiallyintheearth-moving and heavy-vehicle industries) are subjected to random loading, which causes a fatigue risk. Due to frequent fa- tigueproblemscausedbythenatureoftheloadingcondi- tionsonsuchstructures,properfatigueanalysissupported by accurate and practical stress determination is essential. Thequantificationofinputloadsforstressdetermination Correspondence: F.VanTonder.E-mail:francoisv@bell.co.za and finite element analysis also plays a dominant part in fatigue analysis and design. In the case of finite element analysis of large, complex structures, the use of shell or plate elements is prefer- able to solid elements due to practical and economical considerations. However, the physical modelling of the weld itself by means of shell elements is not yet clearly established and there are still some uncertainties and dis- crepancies surrounding it. The way that the weld should be modelled also depends on the type of fatigue analysis thatwillbeperformed.Thisisbecausedifferentstresspa- rameters are needed for different assessment procedures. 1030 c  2007 The Authors. Journal compilation c  2007 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 30, 1030–1043