Influence of surface roughness on measurement of the ultrasonic nonlinear parameter

Ultrasonic nonlinear parameters can be determined by measuring higher harmonic amplitudes generated by propagation of ultrasonic waves in a material. However, regardless of the material, the result may be influenced by the measuring system. The contact condition between the transducer and material should be considered, especially when a contact-type transducer is used; therefore, the effect of surface roughness was experimentally investigated in this study. Al6061-T6 specimens were prepared with different surface roughness values of 0.5, 1.5, 2.2, 2.7 and 3.2 μm. The relative and absolute ultrasonic nonlinear parameters were measured under identical experimental conditions and the absolute ultrasonic nonlinear parameter was obtained using the piezo-electric detection method. The experimental results showed that the relative ultrasonic nonlinear parameter increased according to the surface roughness, which confirms that surface roughness affects the measurement of the ultrasonic nonlinear parameter when a contact-type transducer is used. On the other hand, the absolute ultrasonic nonlinear parameter was stable up to the roughness of 2.7 μm. This implies that the measurement of the absolute ultrasonic nonlinear parameter by using piezo-electric detection can compensate for the influence of surface roughness through the calibration process, within a limited range