Residual stress measurement in engineering materials and structures using neutron diffraction.

Abstract

This thesis presents the determination and analysis of residual stresses, both at the macrolevel and microlevel, in different engineering materials using the neutron diffraction technique. The macrostress studied is that produced by welding in stainless steel pipes used in power plants. The objective was to see the effect of a pan-thickness repair weld on the pre-existing residual stress field generated from the original weld. The strain values were measured in the three principal directions through the pipe thickness both in the original and repair weld areas and the full stress tensors were calculated. The results show the presence of a large tensile hoop stress in the outer half thickness in and around the original weld with a peak value just below the last weld cap pass. The measured through-thickness stresses show an impressive agreement with finite element predictions. A sharp rise in the axial stress in the heat affected zone is produced by the repair treatment, particularly below the repair depth. The presence of a highly tensile axial membrane stress (~175 MPa) in the repaired area compared to a compressive one (~55 MPa) in the original weld region suggests that an overall bending has been caused in the pipe by the repair work. The through-thickness radial stress is always low with values close to zero in both the original and repair welds. The. microstress investigated is that created between the constituent phases of an Al-SiCp metal matrix composite. This study can be divided into two parts. The effect of plasticity on the fatigue crack-tip stresses, particularly the microlevel misfit stresses has been studied in the first part. All the macro and micro stress components were separated from the total stress measured by neutron diffraction in a plastically deformed and fatigue cracked specimen, both unloaded and elastically loaded conditions and they were compared with those in an undeformed and cracked specimen. 1% plastic deformation has been found to reduce the misfit stresses to almost zero in both phases. No effect of fatigue cracking and elastic loading on misfit stresses has been observed in this study. In the second part of the study, changes in microstresses in bent specimens caused by different heat treatments have been investigated. A plastic deformation of about 0.33% has reduced the misfit stress by. -35% in both phases, which is regenerated by heat treatment. The amount of regeneration depends mainly on the treatment temperature and also time. A sub-zero treatment in liquid nitrogen however, has not made any significant change in the stress states in this study