New samples with artificial voids for ultrasonic investigation of material damage due to creep

Prediction of the Mechanical Properties of P91 Steel by Means of Magneto-acoustic Emission and Acoustic Birefringence

“…Variations of nonlinear acoustic parameters were investigated in the course of creep [15]. In late creep stages, when numerous voids have been created in the bulk steel specimens, an ultrasonic technique based on acoustic birefringence was used to detect material damage [16].…”

A review of non-destructive techniques for the detection of creep damage in power plant steels

“…In general, longitudinal wave velocity decreases monotonically with increasing porosity or damage grade [44,45,[47][48][49][50][51]. In the initial stages the change is small (order of magnitude 0.1%) and therefore it may be difficult to detect in the field [52], also because of the large variations due to material inhomogeneities observed even within a single specimen [53]; as soon as microcracks start forming, the velocity starts decreasing noticeably (2-5%).…”

“…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].…”

“…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]. It was also shown in creep tests on copper specimens that the velocity of longitudinal waves in the direction of the applied load decreases more rapidly than that of longitudinal waves in perpendicular directions, because of the preferential orientation of voids [50].…”

“…The values plotted represent the maximum (B max ) and average (B av ) birefringence measured in the gauge section of a sample tested at 500 1C and 250 MPa, the average birefringence in the reference (non-deformed) section of the same sample (B non ), and the maximum birefringence measured on a sample of the same material but tested at room temperature (B pl )[51].…”