Residual Stress Measurement on Different Length Scales Using Neutron and Synchrotron X-ray Diffraction

Abstract

This thesis presents the analysis of micro and macro residual stress distribution in different materials exploiting the synchrotron X-ray and neutron diffraction technique. This work can be broadly classified into two parts; the first part investigates the feasibility of using synchrotron X-ray diffraction for high resolution crack tip strain and stress mapping around a fatigue crack inside a bulk material under in-situ loading and the second part presents the investigation of macro residual stress distribution on several welded specimens of different alloys using neutron diffraction. Synchrotron X-ray diffraction was used to capture the strain and stress distribution around a fatigue crack under different loading conditions. The investigation was employed in two fine-grained A1-5091 compact tension (CT) specimens, with both the samples having a fatigue crack of 10mm where one of them was subjected to a 100 % overload after the fatigue crack was grown. The results show the presence of significant triaxial tensile stresses in front of the fatigue crack in the as-fatigued sample and the overloaded sample, with maximum stresses of 250 MPa and 550 MPa observed respectively in the longitudinal direction. The crack tip stress value reduces to zero in the as-fatigued sample when unloaded, whilst in the overloaded sample, a compressive stress region is observed in front of the crack tip in the unloaded condition. A constant magnitude of compressive stress is observed along the crack wake just behind the crack tip in both samples under loaded and unloaded conditions; however the extent of this compressive zone was found to be greater in the overloaded sample. The macro-stress distribution in different welded samples has been investigated using the neutron diffraction technique in the second part of work. This study can be sub-divided into three parts. The first part discusses the 3D residual stress distribution in a 316L austenitic stainless steel bead-on-plate welded specimen using neutron diffraction and the results are compared to the results obtained from the contour method. In the second part, an investigation was carried out to study whether the condition of similitude exists in a laboratory size welded component when extracted from the original structure. To study this phenomenon residual stress analysis was carried out in metal inert gas (MIG) welded A1-2024 and A1-7150 alloy compact tension specimens. A substantial amount of stress relaxation is observed in the compact tension specimen when the results are compared with the original welded specimens from where the CT specimens were extracted, which clearly shows that similitude does not apply in the case of welded specimens and care must be taken in using the results from laboratory size fracture and fatigue specimens extracted from real structures. Finally, the third part discusses the residual stress redistribution phenomenon due to the presence of a fatigue crack. To study this redistribution phenomenon an investigation has been carried out in a variable polarity plasma arc-(VPPA)-welded A1-2024 sample with different crack lengths. Stresses in these samples were analysed using neutron diffraction at HMI facility, Berlin. The result shows significant redistribution of residual stress in both samples.