Search Results

A three dimensional gradient of residual stresses is inevitable for almost any welding process due to the thermal cycle. During subsequent cyclic loading, these residual stresses were expected to alter the fatigue life with trends similar to those observed for uniaxial experiments with mean stresses. However, plastic deformation during cyclic events can cause the residual stresses to redistribute and relax. Experiments were performed to determine the uniaxial fatigue performance of a low carbon steel transverse double-sided butt weld specimen. An automated laser speckle interferometry hole drilling system was utilized to determine both the initial residual stresses and those after a specified number of cyclic events. Finite element modeling was used to simulate the evolution of the residual stress during the welding thermal cycle and subsequent cyclic loading at room temperature.

The ability to model inelastic deformation is of practical interest in many design applications, especially with the current emphasis on lighter weight structures and components operating at higher temperatures. Thermomechanical deformation poses several challenges in the sense that material properties change with temperature, and at higher temperatures time dependent phenomena such as creep and/or stress relaxation are active deformation mechanisms. Due to recent success modeling many time-independent plastic deformations, recent modifications of the Armstrong-Frederick type plasticity formulations are utilized as a basis for the current model. The choice to implement a non-unified model is based on the notion that distinct or independent mechanisms govern time dependent and independent plastic deformations. The literature also suggests a similar scenario with regard to the damage accumulation.

Induction hardening of mild steel components often results in significant improvements in the static and cyclic load capability, with comparatively small increases in cost. Members subjected primarily to torsional loading are a relevant subset of the broad range of induction hardened components. Due to the variation of material properties and residual stresses, failures are “initiated” at the traditional geometric locations predicted for homogeneous materials and also at subsurface sites. The introduction of shear based fatigue parameters has necessitated the consideration of the residual stress as a three dimensional quantity, especially when analyzing subsurface failures. Not considering the tensoral nature of the residual stress can lead to serious errors when estimating fatigue life, and for larger magnitude loadings, the residual stress field may relax.

One of the problems commonly associated with fatigue life prediction of various spot welded geometries has been the need to test each geometry. Various high strength low alloy and low carbon spot welded specimen geometries have been tested. Even for the same nugget diameter, specimen width and sheet thickness, the fatigue resistance in terms of the either the maximum or range of remote load differed from one specimen type to another. This could be attributed to the fact that the weld nugget region is associated with multiaxial stress fields, which are neglected in the remote load analyses. An initial attempt to incorporate multiaxiality involved converting remote loads into nominal stresses in the vicinity of the spot weld using geometrical factors such as nugget eccentricity. Findley's parameter, a shear based multiaxial critical plane approach, was employed to evaluate the fatigue performance of the various geometries.