“…The underlying principle is that, since cavities and microcracks tend to be preferentially oriented along the direction of main stress, the difference between velocities in two orthogonal directions will change with creep life. It has been shown that birefringence variations are indeed monotonic with porosity and creep test time [48] and, albeit small (typically less than 1% even in the late stage of creep), they are significantly higher than the typical scatter on an intact specimen caused by material inhomogeneities and measurement accuracy [46,51,65]. It has been recently observed that changes in the orientation of grains due to plastic deformation cause variations of opposite sign to those due to creep voids and cracks: while this means that in theory the two effects could cancel each other, in practice industrial components are usually subjected to stress levels below the yield limit, therefore the contribution of aligned cavities and cracks should prevail [51].…”