Strain measurement using neutron diffraction

Abstract

This thesis contains a study of improvement and application of strain measurement technique using neutron diffraction. A dedicated neutron strain scanner - ENGIN, funded by Ee. has been developed at ISIS. It has two radial collimators, allowing for the first time, measurements taken simultaneously at two orthogonal directions; uses Pawley refinement permitting both whole pattern and individual peak profile analysis and its positioner allow's specimens weighing 250 kg to be placed with an accuracy of 100 µm. Strain measurement using neutron diffraction has been investigated under two extreme circumstances: very shallow (within 1 mm near surface) and very deep in materials (hundred of mm). Near-surface measurement requires both the accurate determination of the effective measurement position associated with precise location of specimen, calculation of centroid and correction for an anomalous near-surface effect. The strain measurements on a shot-peened surface in titanium alloy were carried out using the methodology mentioned above. The result is comparable to that obtained from X-ray diffraction. The latter was exploited by studying the effect of wavelength-dependent attenuation in materials. Experiments and theoretical analysis on aluminium and iron show that the effect is small for a strain scanner using radial collimator and time-of-flight technique. However, the reduction of the diffraction peak intensity, as a function of the amount of material in the beam path, reveals that great care should be taken when measuring texture as a function of depth in materials. The neutron diffraction technique was applied to several sets of engineering strain measurements. Firstly, 3-D residual stresses surrounding a cold expanded hole in a high strength aluminium alloy plate were measured. The result agrees well with that from modified Sachs' method after taking the effect of gauge volume averaging into account. Second, measurements of strain distributions in a Q-joint under zero and 30 kN load have been carried out, which provides a confirmation of the excellence of the joint design as the fastener system has completely shielded the fastener hole from deleterious tensile strains under operational loading. Finally, calibration strain measurement for an energy dispersive neutron transmission spectrometer was performed for the first time and it is found that the transmission spectrometer is feasible for strain and phase transformation measurement.