Papers by Author: Michael R. Hill

Abstract: Welding processes create a complex transient state of temperature that results in post-weld residual stresses. The current work presents a finite element (FE) analysis of the residual stress distribution in an eight-pass slot weld, conducted using a 316L austenitic stainless steel plate with 308L stainless steel filler metal. A thermal FE model is used to calibrate the transient thermal profile applied during the welding process. Time-resolved body heat flux data from this model is then used in a mechanical FE analysis to predict the resultant post-weld residual stress field. The mechanical analysis made use of the Lemaitre-Chaboche mixed isotropic-kinematic work-hardening model to accurately capture the constitutive response of the 316L weldment during the simulated multi-pass weld process, which results in an applied cyclic thermo-mechanical loading. The analysis is validated by contour method measurements performed on a representative weld specimen. Reasonable agreement between the predicted longitudinal residual stress field and contour measurement is observed, giving confidence in the results of measurements and FE weld model presented.

Abstract: The influence of different parameters of laser shock processing applied to a precipitation-hardened aluminium alloy 6082-T651, on residual stress, surface tophraphy and microhardness was investigated. Processing was performed with an innovative Nd:YLF laser with the power densities of 2 and 4 GW/cm², with a uniform pulse duration of 18 ns. Laser shock processing experiments were performed with the closed ablation method to ensure a higher shock-wave pressure. In the first phase, the study was focused on an evaluation of surface topography, with the record of the surface roughness profile and with the surface evaluation at a scanning electron microscope JEOL JXA-8600M. Then followed measurement of microhardness HV_0.2 in the cross section region. In the second phase comparison of residual stresses which were measured using the X-ray diffraction, was performed. Laser shock processing turned out to be a very efficient technique to improve surface properties. On the basis of the micro plastic deformation induced by shock waves, an increased dislocation density in the specimen surface was obtained. The gradient of dislocation piling through the specimen depth improved the variation of microhardness and residual stresses, which, in turn, improves fatigue strength of the material under dynamic loading.

Abstract: The present paper treats results of laser shock processing applied to a precipitationhardened ENAW 6082-T651 aluminium alloy. Processing was performed with a Nd:YLF-yttrium lithium fluoride crystal laser with power densities of 2 and 4 GW/cm2, producing a pulse of 18 ns. Laser shock processing experiments were performed with the closed ablation method, the application of an ablative coating and a transparent tamping medium to obtain a higher shock-wave pressure. In the first phase, the surface study focused on the record of surface profile with a roughness gauge and on an evaluation of surface topography at a scanning electron microscope. In the second phase, residual stresses were measured using the relaxation hole-drilling method at a processed specimen surface. Then followed measurement of microhardness in the cross section. The hardening results obtained were evaluated on the basis of variations of residual stresses and of microhardness, and of macro and microstructural changes of the surface, i.e. the surface layer. The purpose of processing was to improve fatigue strenght and, consequently, extend the life of a machine component in operation.

Abstract: This paper describes a method for extending the capability of the contour method to allow for the measurement of spatially varying multi-axial residual stresses in prismatic, continuously-processed bodies. Currently, the contour method is used to determine a 2D map of the residual stress normal to a plane. This work uses an approach similar to the contour method to quantify multiple components of eigenstrain in continuously-processed bodies, which are used to calculate residual stress. The result of the measurement is an estimate of the full residual stress tensor at every point in the body. The methodology is presented and the accuracy is assessed for a representative test case using a numerical experiment. Finally, a measurement is performed on a thick laser peened plate of 316L stainless steel to show that the approach is valid under real experimental conditions.