Bilal Ahmad

Minimization and Mitigation of Wire EDM Cutting Errors in the Application of the Contour Method of Residual Stress Measurement

The contour method of residual stress measurement relies on the careful application of wire electro-discharge machining (WEDM) for the cutting stage. Changes in material removal rates during the cut lead to errors in the final calculated values of residual stress. In this study, WEDM cutting parameters have been explored to identify the optimum conditions for contour method residual stress measurements. The influence of machine parameters on the surface roughness and cutting artifacts in the contour cut is discussed. It has been identified that the critical parameter in improving the surface finish is the spark pulse duration. A typical cutting artifact and its impact on measured stress values have been identified and demonstrated for a contour cut in a welded marine steel. A procedure is presented to correct contour displacement data from the influence of WEDM cutting artifacts, and is demonstrated on the correction of a measured weld residual stress. The corrected contour method improved the residual stress magnitude up to 150 MPa. The corrected contour method results were validated by X-ray diffraction, incremental center hole drilling, and neutron diffraction.

Analysis of Residual Stresses in Laser-Shock-Peened and Shot-Peened Marine Steel Welds

Laser peening is now the preferred method of surface treatment in many applications. The magnitude and depth of the compressive residual stress induced by laser peening can be influenced strongly by the number of peen layers (the number of laser hits at each point) and by processing conditions including the use of a protective ablative layer. In this study, residual stresses have been characterized in laser and shot-peened marine butt welds with a particular focus at the fatigue crack initiation location at the weld toe. X-ray diffraction, synchrotron X-ray diffraction, incremental center-hole drilling, and the contour method were used for determination of residual stress. Results showed that the use of ablative tape increased the surface compressive stress, and the depth of compressive stress increased with an increase in number of peening layers. A key result is that variation of residual stress profile across laser peen spots was seen, and the residual stress magnitude varies between the center and edges of the spots.

Effect of Ultrasonic Peening and Accelerated Corrosion Exposure on the Residual Stress Distribution in Welded Marine Steel

Specimens of DH36 marine steel were prepared with welded attachments. Residual stress measurements were made on the samples as-welded, following an ultrasonic peening treatment, and following accelerated corrosion exposure after ultrasonic peening. Neutron diffraction and the contour method were used for determining the residual stress profiles. The welding introduces tensile near-surface residual stress, approaching the material yield strength, and the ultrasonic peening overlays this with a compressive residual stress. Material removal by corrosion decreases the peak surface compressive stress slightly, by removal of a layer of stressed material, but does not cause significant redistribution of the residual stress profile.

The effect of compressive fatigue loads on fatigue strength of non-load carrying specimens subjected to ultrasonic impact treatment

This paper provides description and results from an investigation aimed to determine whether loading histories specific to marine and offshore structures cause shakedown of compressive residual stress produced by high-frequency mechanical impact (HFMI) treatment and therefore affect fatigue resistance of welded joints improved by these methods. Fatigue tests were carried out to investigate the influence of compressive fatigue loads on fatigue strength of non-load carrying specimens with transverse attachments subjected to ultrasonic impact treatment (UIT). It was found that significant relaxation of compressive residual stresses is achieved under application of compressive cycles in fatigue testing. It was also observed that not only the magnitude of the applied compressive stress/mean stress affects relaxation of the compressive residual stress but also the number of applied compressive cycles. This relaxation may reduce the beneficial effect of HFMI treatment under application of load histories specific to marine structures.

Measurement and modelling of residual stress in wire-feed additively manufactured titanium

ABSTRACT Residual stresses were characterised in a wire-feed additively manufactured titanium alloy component. A numerical simulation based on the inherent strain method was used to model residual stresses arising from the manufacturing process. The contour method was used to experimentally determine the residual stress field. High tensile residual stresses were seen at and around the interface of the substrate and the deposited metal. Compressive residual stresses were present in the substrate and at the top of the deposit. The satisfactory correlation was achieved between the results from the numerical simulation and the contour method, except for the location of the root of the deposit. The effect of pre-heating the sample substrate on the residual stress distribution is also discussed.