The intensity of the back reflection peak, returning along the incident direction when ultrasonic bean is incident upon a liquid/solid interface at or near Rayleigh angle, increased with the decrease of induced frequency and was governed by the grain scattering rule of ultrasonic waves for commercial aluminum specimens. And the peak intensity increased with Schoch displacement and the inverse of beam width for layered substrate specimens.
These experimental results and theoretical considerations show that the radiation from backward propagating leaky - Rayleigh surface wave and the scattering of leaky waves by subsurface microstructures, can be considered as the mechanisms of the back reflection peak phenomenon.
The velocity dispersions of nitrided AISI 4140 tool steel with continuous gradient and Nikel/Brass, Nikel/Aluminum and Copper/Aluminum with discrete gradients, were measured using reflection minimum and back reflection peak. The comparison between peak and minimum data shows that the back reflection peak is useful in the measurement of velocity dispersion of surface wave and the peak represents thinner region than penetration depth from the surface.
The approximate perturbation theory about velocity dispersion of surface wave by equivalent velocity gradient was derived and modified particle displacement profiles were modeled. Our theory showed good agreements with not only continuous but also discrete perturbations.
상업적인 aluminum 시편에서 back reflection peck 세기는 초음파의 grain 크기에 대한 산란규칙을 따랐고 주파수 증가에 따라 감소함을 보였다. Layered substrate 시편에서 peak 세기는 beam 넓이가 작아질수록 그리고 schoch 변위가 커질수록 증가를 보였다. 역방향 표면파로부터 에너지 radiation에 의한 이론적 peak 세기는 실험과 일치하였으며 back reflection peak의 mechanism으로 크게 radiation과 scattering이 고려되었다.
연속적 gradient인 질화된 AISI 4140공구강과 불연속적 gradient인 Nikel/Brass, Nikel/Aluminum, Copper/Aluminum 시편에 대해 속도 dispersion 이 reflection minimum과 back reflection peak에 의해 측정되었다. peak와 minimum의 비교에서 peak에 의한 속도값이 속도 dispersion의 측정에 아주 유용하며 표면파의 침투깊이보다 표면쪽에 가까운 지역을 대변함을 알 수 있었다.
등가 속도 gradient를 가진 시편에서 표면파의 속도 dispersion에 관한 근사섭동이론이 유도되었다. 수정변위 profile을 도입해서 연속뿐아니라 불연속 섭동에서도 근사섭동이론이 적용되었고 실험치와는 좋은 일치를 보였다.
