Application of Minature Ring-Core and Interferometric Strain/Slope Rosette to Determine Residual Stress Distribution With Depth—Part I: Theories

The principle of an interferometric strain/slope rosette (ISSR) is based on interference of laser beams reflected from three microindentations on a specimen surface. The ISSR can simultaneously measure the in-plane strains and the out-of-plane slopes. Ring-core cutting is a mechanical stress relief method. When used with the ISSR technique for residual stress measurement, the ring core can be made much smaller than used with the resistance strain rosette. Thus, more localized residual stresses can be measured. The theories of the ISSR/ring-core cutting method are described in this paper. Both mechanical and finite element models are developed for the incremental ring-core cutting process with the application of the ISSR technique. The stress-strain coefficients of the ISSR/ring-core method are calculated and nondimensionalized for general applications. A test example is given to demonstrate how residual stress distribution is determined by using the stress-strain coefficients and the ISSR data.

A Study of Mechanical Stress Relief (MSR) Treatment of Residual Stresses for One-Pass Submerged Arc Welding of V-Grooved Mild Steel Plate

The heat-transfer and thermal stress distributions in one-pass submerged arc welding (SAW) were numerically determined using the finite element method (FEM) for a V-grooved rectangular steel plate in which the weld preparation was filled during welding. A two-dimensional non-linear heat-transfer analysis was performed for a transverse section of the plate. This was followed by a thermo-elasto-plastic transient thermal stress analysis, assuming plane strain to be constant for the same model section. The same stress model was used to simulate a mechanical stress relief (MSR) treatment of the plate. This has frequently been used in the fabrication of large pressure vessels instead of post-weld heat treatment (PWHT). In this way its effect in reducing the residual stress in the welded plate was investigated. MSR was simulated by enforcing a constant displacement loading in the welding direction. The solution of the thermal stress analysis showed that it was possible for the residual stress around the weld centre to be accurately estimated by accurate modelling of the dilution of the filler metal in the fused zone of the base metal. The conclusion derived from the MSR simulation was that it could quantitatively predict the effect of reducing the residual stress in the welded plate. Only limited experimental data were available. The mechanism of stress reduction was plastic straining in regions of high residual stress. The amount of stress reduction at the weld centre had a linear relationship to the magnitude of the external stress relieving load. The numerical results for the MSR simulation agreed fairly well with experimental ones obtained from the MSR test performed on a welded plate.

Reply to Dr. Kaarsberg

Paragraph II: Kaarsberg states that mechanical stress relief should not be ruled out as an explanation of low velocity adjacent the borehole. If the volume of the material stressed increases, the elastic moduli decrease. Therefore, velocity immediately adjacent the bore decreases. This is indeed a powerful point and properly should have been discussed more fully.

LATERAL VELOCITY VARIATIONS NEAR BOREHOLES

Difficulties occur in obtaining accurate two‐receiver velocity logs in formations sensitive either to damage by exposure to drilling mud or to mechanical stress relief. Some shales are so altered by the drilling operation that their elastic properties are modified. Vertical velocity measured immediately adjacent the boreface is lower than if it were measured at a greater radial distance from the bore. These damaged shales require relatively deep penetration by the acoustic signal; consequently, the transmitter‐to‐first‐receiver spacing in a two‐receiver velocity logging system should be long enough to refract the sound waves through virgin formation. Experiments in one predominantly shaly section show a difference of almost 10 percent between times measured using transmitter‐to‐first‐receiver spacing of 4.3 ft compared to 8.8 ft. A limited amount of field data suggest that sodium montmorillonite is the clay type most sensitive to hydration and swelling. Studies of areal prevalence of the shale damage problem are incomplete.