Research On Laser Ultrasonic Nonlinear Mixing Detection Of Real Microcracks

An all-optical method is adopted to detect the nonlinear frequency-mixing process induced by the interaction between the acoustic waves and the crack breathing in the frequency domain,which realized the purpose of detecting the crack.The exciting and heating laser are modulated by the acousto-optic modulator respectively of a high-frequency f_H(tens of kHz exciting laser)and a low-frequency f_L(?Hz heating laser),and the crack breathing is caused periodically under the irradiation of the heating laser.Then the nonlinear frequency-mixing signal is induced by the interaction of the crack and the acoustic waves whose frequency is f_H,Detection of the nonlinear mixed-frequencies can be achieved with vibrometry or deflectometry techniques.During the experiments,the spots on the sample of the exciting,heating,and the probe beam are always overlapping with each other.Moving the sample to scan at the vicinity of the crack,the locational information can be obtained by detecting the mixing-frequency signal.When the mixing-frequency signal occurs,it means the three beams have reached to crack area.With the three beams moving towards the center of the crack,the amplitude of the mixing-frequency signal is becoming higher.After getting the maximum amplitude(the three beams has reached to the center of the crack),the amplitude of the mixing-frequency signal is becoming lower(the three beams are leaving the crack region)and finally approach to a constant value,at the same time,the mixing-frequency signal disappeared.So we can obtain the locational information of the crack by observing the variation of the amplitude of the mixing-frequency signal.During the experiment of receiving the acoustical signal by the deflectometry technique,we detected the mixing-frequency signal after the improvement by expanding the beam,it indicates that,the all-optical method can be adopted to study the nonlinear mixing-frequency signal,further more,for the advantages of the all-optical method such as non-contact,can be focused to very small and can test the complex sample,there's more practical value of adopting the all-optical method.Detecting the real crack by the nonlinear opto-acoustic mixing-frequency method has a remarkable higher sensitivity than the traditional linear laser-ultrasonic testing method,which provides the opto-acoustic NDT of the crack the new solving ideas and technology.