Theoretical And Experimental Studies Of Nonlinear Ultrasonic

Many existing conventional non-destructive technologies (NDTs), which are suitable for testing visible flaws and macro-cracks, are incapable of monitoring the performance degradation of materials or components in the early working-time. Comparatively, nonlinear ultrasonic testing technology is a very good solution in this field.Nonlinear ultrasonic technology mainly based on non-linear dynamics theory and dislocation theory. When a finite-amplitude ultrasonic guided wave propagates through the materials with significant plastic deformation or damage, it will cause the ultrasonic speed variation and waveform distortion. This behavior mainly comes from the nonlinear crystal lattice or dislocation in solid materials. To characterize this type of nonlinear behavior, a nonlinear ultrasonic parameter,β, is defined.βis related with the second-order, third-order and even higher-order elastic constant of materials. These elastic constants are relative to the amplitude of higher order harmonic. Therefore, the nonlinear ultrasonic parameterβquantifies the performance degradation of materials. At present, the nonlinear ultrasonic experimental study mainly concentrates on a few metals or alloys. No systematical experimental results have been received. Moreover, the nonlinear equation does not have the analytic solution in theory research. Only some approximate analytical solutions limited in one-dimensional longitudinal wave or two-dimensional surface wave are received by the perturbation method.This article focuses on theoretical and experimental studies of the nonlinear behaviors of materials. In theory, the one-dimensional nonlinear longitudinal wave equations are deduced and the approximate analytical solutions are received. Based on the relationship between the material elastic constants and the strain energy, the three-dimension SOEC and TOEC tensor expressions are obtained by using tensor's derivation principle. In experiment, the nonlinear ultrasonic testing platform is established. The off-line and on-line tests are carried out by using 45# steel standard specimens at different plastic strains. Some experiment results are received including the typical curves of the responding function in receiving transducer, the material stress-strain curve and the normalized nonlinearity parameter at different plastic strains. In the experiment, some testing measures are adopted to ensure geting reliable results. For example: coupling liquid is used between the transducer and the specimen to ensure efficient transmission. In order to reduce the system nonlinearity, the loading power of the amplifier is enlarged. Considering that the large plastic deformation capability of 45# steel make it similar to a kind of porous material, the band gap is researched by using the finite difference method (FDTD). Based on the numerical results, the nonlinearity parameter affected by the plastic strain is explained.As considering the extensive applications of nonlinear ultrasonic technology in the industry, more experiment and theoretical studies should be carried.