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In order to evaluate creep deformation mechanism of heat resistant steels, stress change tests were conducted during creep tests. In this study, it was confirmed that the dislocation behavior during the creep tests was in viscous manner, because no instantaneous plastic strain was observed at stress increments. Transient behavior was observed after stress changes for all kinds of steel in this work. Mobility of dislocation was evaluated by the observed backward creep behavior after stress reduction. Internal stress was evaluated by the change of creep rate in stress increment, and mobile dislocation density was evaluated with the estimated mobility of dislocation and the change of creep rate in stress increment. It was found that the variation of mobile dislocation density during creep deformation showed the same tendency as the variation of creep rate. Therefore mobile dislocation density is the dominant factor that influences the creep rate variation in creep deformation of heat resistant steels investigated in this work. The mobility of dislocation showed a good correlation with 1/T and it is related with the amount of solute Mo that is a solution strengthening element. Microstructure of crept specimens was observed by TEM to discuss the validation of these results.

Theoretical and experimental investigations of transmission coefficients of longitudinal waves through metal plates immersed in air have been carried out for noncontact and nondestructive testing (NDT) of metal plates. Transmission coefficients through metal plates in water and through polymer plates in air have also been shown for comparisons. Theoretical relations between the transmission coefficients and the Lamb wave dispersions were described. It was confirmed that the transmission coefficients took high values when the Lamb waves were generated without exception. Quantitative evaluations for the transmission coefficients were investigated for the use of the air-coupled ultrasounds. The experimental verifications were carried out using 1-, 3-, and 5-mm-thick aluminum plates. The experimental results of the transmission coefficients were in fairly good agreement with the theoretical predictions.

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