Assessment of thermal degradation by cumulative variation of ultrasonic nonlinear parameter

The ultrasonic nonlinear parameter β has been studied for the evaluation of material degradation. However, there is a limit to the assessment of degradation induced by thermal aging in aluminum alloy because the nonlinear parameter may fluctuate with the aging time and thus it has not one-to-one correspondence with the thermal degradation. In order to overcome such limitation, we propose a cumulative nonlinear parameter βc defined by the accumulation of variation in the nonlinear parameter over the aging time. For verifying the usefulness of the proposed parameter, two kinds of aluminum alloy specimens heat-treated with different aging temperatures and aging times were tested. After the ultrasonic test has been completed, the tensile test was conducted for all specimens to measure the yield strength as a quantitative index of material degradation. Results showed that the nonlinear parameter β fluctuated in directions of both increase and decrease, while the strength decreased one-sidedly with the increase of aging time. On the other hand, the proposed parameter βc was one-sidedly increased, which showed strong exponential negative correlation with the yield strength. These tendencies were similar in different material and different aging temperature.