12 Strain mapping

Neutron strain scanning techniques have been developing since the early 1980s U-3J and synchrotron X • ray techniques rapidly since the 1990s [4, 5]. The first neutron measurements used single detector instruments at medium flux reactor sources, 'gauge volumes' that were relatively large and coarsely defined, manual positioning of samples and computer-aided but labour-intensive Interactive Gaussian peak fitting routines. Statistical data quality was often high but spatial resolution, data collection rates and data processing speeds were all relatively low. Even in comparatively simple experiments, for example, scans through thin sections of ferritic steel test samples with moderate residual strain gradients, only 11 handful of measurements could be made and processed in a day. Consequently most neutron strain investigations were-restricted to linear scans at a dozen 0 so points, usually in three orthogonal directions so that stresses could be derived. Area mapping, which typically might require orthogonal measurements at a bundred or more points was not then generally a practical proposition due to time and resource cost COIl si derations. With the introduction of high flux neutron sources, multldetectors, automated sample positioning, optimisation of data acquisition, fast and cheap computers, fully computerised. peak fitting and commercial surface fitting software. the situation has changed dramatically. The data for an adequately defined neutron diffraction peak might now be collected in a few minutes, or even less, and be processed near on-line, so that multipoint neutron area strain scanning is DOW practicable [6,7], Synchrotrons produce near-parallel X-ray beams with extremely high photon fluxes, typically billions of times the equivalent neutron flux of even the most powerful nuclear reactors. On the other hand X-ray attenuation lengths, at similar wavelengths to those used with neutron strain scannmg (1 ~ 1.5 A), tend to be several orders of magnitude smaller than the corresponding neutron srtenuation lengths, Consequently, at these wavelengths the attcnuarion of X-ray beams is generally much higher than for neutron beams, especially through thick samples. However, for harder more penetrating Xsrays (A ~ 0.3 A), attenuation lengths can be similar (~mm), for light clement materiels, to those for neutrons. Scattering par am ters and attenuation lengths for neutrons and for X-rays of typically used wavelengths a 'e shown for elements that 'Comprise (he majority component of common engineering alloys in Table 12.1. In favourable circumstances. such as when measuring through relatively thin samples made of lower attenuating light element materials, synchrotron X-ray COWll rates. even …