Study On The Detection Mechanism Of Micro-cracks In Glass By The Nonlinear Photoacoustic Method Based On Thermal Modulation

In the past decades,structural health has been the focus of attention,and it is particularly urgent to develop efficient and accurate defect detection methods.Nonlinear photoacoustic method inherits the advantages of nonlinear acoustics and laser ultrasound,has a wide application prospect in the field of micro-crack(micron and submicron scale)detection.However,the physical mechanism behind many nonlinear phenomena is still unclear,this also hinders the development of the qualitative and quantitative evaluation of defects based on nonlinear photoacoustic method.This paper will give a reasonable explanation for these nonlinear phenomena through theoretical and experimental research.In theoretical research,the characterization of crack faces is always a difficult problem,the penalty method is used to deal with boundary contact from the point of contact mechanics in this paper.Two intensity-modulated laser beams in the experiment are simplified as two variable temperature field in the model.The effect of the relative position of the high-frequency and the low-frequency temperature field on the amplitude of side-lobe is discussed,and the conclusion is consistent with the experiment.In the subsequent studies,the perspective shifted from macro to micro,and two typical microscopic contact mechanisms are observed.The relationship between microscopic contact mechanism and the received photoacoustic signal is discussed.The nonlinear acoustic signal is affected by the crack contact state when the nonlinear photoacoustic method is used for micro-crack detection.Due to the complex of the micro-crack itself,the contact state of different positions of the same crack(such as the crack opening position and the crack tip)will be very different.In addition,the contact state of the crack can still be affected by many factors such as pump and probe parameters,which makes the nonlinear phenomenon complex and changeable.In this paper,the stress-strain relationship of cracks in different contact states is discussed.This explains the reason why the nonlinear phenomenon is the best when the crack is in the transition state,and provides theoretical guidance for the selection of excitation parameters in practical operation.Considering the complexity of the crack faces,a spring with constant stiffness is introduced to characterize the weak connection between crack faces.Based on these,a nonlinear bi-modular stiffness model with weak connections is developed in this paper.There will be two types of nonlinearities in the model,on the one hand,the nonlinearity comes from contact mechanics;on the other hand,the nonlinearity comes from the crack "breathing",that the contact stiffness will have a bi-modular characteristic.When the amplitude of the pump source changed in the simulation,the phenomenon can be observed that several side-lobe amplitudes will have clear minima,and the numerical simulation results are in good agreement with the experimental phenomena.Beyond that,some differences between theoretical results and experimental results are analyzed,and the effect of foundation displacement in the experiment is discussed.In the traditional frequency-mixing experiments,a laser intensity-modulated at low frequency is used as the pump source to modulate the crack state,and the laser intensity-modulated at high frequency is used to generate acoustic waves.Traditional experimental scheme is improved in this paper,and the third laser beam is added to serve as a heating source to provide the foundation displacement for the crack,the contribution of foundation displacement to the nonlinear frequency-mixing phenomenon and the effect of foundation displacement on defect location are studied separately in the experiment.