Fatigue assessments of components in construction equipment

Abstract: The thesis refers to "Fatigue Assessment of Components inConstruction Equipment". The subject was chosen because ofincreasing interest in higher payloads, lower weight, highervelocities and shorter load cycles in construction equipmentvehicles.The main research goal in this thesis is to investigatetechniques to extend the fatigue life of an induction hardeneddrive shaft, with splines. Areas related to this research areresidual stress, fatigue life assessments, manufacturingprocess parameters and hardening process parameters. Themethods used to achieve the goal are process simulations,fatigue tests, finite element calculations, measurements ofresidual stress and fatigue life assessments. The aim of thepresent research is to cover several aspects of applied fatigueassessment. Most of the theoretical work has been verified withmeasurements and fatigue tests.The residual stresses have been calculated by simulating thehardening process with SYSWELD, a Finite Element program. Thesimulated residual stress beneath the surface was compared toX-ray and neutron diffraction measurements. The conformancewith simulations and X-ray measurements was relatively good,but the conformance with neutron diffraction measurements wasnot so good. The detrimental axial tensile residual stress atthe core was found to be in the order of 800-900 MPa, for theshafts.About 100 shafts have been fatigue tested in torsion, bothin constant amplitude and in variable amplitude and some of theresults have been reported in this thesis. For many of theshafts, crack initiation beneath the hardening layer wasdetected, which depends on the high tensile residual stress inthe core. Fatigue life assessments were made on the shafts,using a multiaxial strain based critical plane model, with themeasured residual stresses as input data. The generalconclusion is that the induction hardening process parametersinfluence the residual stresses to a high extent and thusinfluence fatigue life. Simulations of how different hardeningprocess parameters influence the residual stress profile havebeen done. Low hardening power and low frequency seem to reducethe detrimental tensile residual stress at the core.The load distribution along the axis of the spline teeth hasbeen investigated. The shear stress concentration in a splinehas been calculated by the finite element method, using anon-linear model, and was compared with results found in theliterature. An equation describing how the tooth thicknessshould vary to obtain smooth contact in the axial direction hasbeen derived. Finally, fatigue tests have been made oninduction hardened shafts in torsion with crack initiation atthe spline surface.The influence of pitch errors on fatigue life for splineshas also been estimated, by using a weakest link failureprobability model and combining it with a 2-parameter Weibullfailure distribution model. The conclusion is that pitch errorsin the investigated splines appear to reduce fatigue life byabout 50-70 %, compared with ideal pitch.