| Metal clad plates are composed of two or more layers of metals with different properties,which have better comprehensive properties and are widely used.Due to the influence of the composite process,the defects of interface delamination,uneven thickness of each layer and large residual stress are very easy to occur in the metal clad plates,which seriously affect the quality of products.To study and realize on-line non-contact nondestructive testing of interface delamination,thickness of each layer,and longitudinal residual stress of metal laminates is the key to improve the composite quality of metal clad plates and to produce them on a large scale and stably.Laser ultrasonic detection method is a non-destructive non-contact detection method,which uses a pulsed laser to excite ultrasonic waves at a long distance on the surface of the object to be measured and uses an optical method to detect ultrasonic waves at a long distance.This method can not only measure the internal defects and stress of materials,but also measure the thickness.Theoretically,it is likely to be used for the on-line detection of composite quality of metal clad plates.In order to realize the on-line automatic detection of composite quality of metal clad plates produced by roll-bonding process,this paper explores the non-contact non-destructive detection of interface delamination defects,thickness of each layer,and longitudinal residual stress of metal clad plates by using the laser ultrasonic detection method.The main research contents and achievements of this paper are as follows.(1)In view of the complex propagation phenomenon of ultrasonic wave excited by pulsed laser in metal clad plates,the finite element models of ultrasonic wave excited by pulsed laser in metal and metal clad plates were established based on thermos-elastic mechanism.On the basis of studying the propagation laws of guided waves,surface waves,and bulk waves excited by pulsed laser in homogeneous metal plates,and revealing the influence of the rising time and spot radius of pulsed laser on laser ultrasound,the propagation laws of Rayleigh-Lamb(R-L)guided waves,Lamb waves and bulk waves excited by pulsed laser in Cu/Al clad plates were further studied.(2)The propagation law of ultrasonic bulk waves induced by pulsed laser in Cu/Al clad plates by thermos-elastic mechanism is studied,and the sound field characteristics of the intact area and the delamination area are compared.The influence of the size and position of delamination defect on the amplitude of different modes of bulk waves is studied.It is concluded that the shear wave reflected by the interface is the best feature to characterize the delamination defects.A set of laser ultrasonic testing system for delamination defects of metal clad plates was established,and a sample of Cu/Al clad plate containing delamination defects was fabricated.The characterization ability of different modes of ultrasonic bulk waves to delamination defects was experimentally studied,and the correctness of the simulation analysis was verified.Finally,the C-scan imaging of the Cu/Al clad plate was carried out with the characteristics of the shear wave reflected by the interface,which verified the effectiveness of the delamination detection method based on bulk waves.(3)The propagation laws of Lamb waves induced by pulsed laser in Cu/Al clad plates and stainless steel/carbon steel clad plates by thermos-elastic mechanism were studied,and the interaction mechanism between Lamb waves and interface delamination defects was analyzed.The influence of the size and location of the delamination defects on the amplitude of Lamb waves in Cu/Al clad plates was studied.The influence of the distance between the excitation point and the receiving point(PRD)on Lamb waves in Cu/Al clad plates was discussed,and a reasonable distance was given.The influence of delamination size on the amplitude and frequency of Lamb waves in stainless steel/carbon steel clad plates was studied.Finally,the C-scan imaging of Cu/Al clad plates and stainless steel/carbon steel clad plates were carried out with the amplitude of the Lamb waves as the characteristic,which verified the effectiveness of the delamination detection method based on guided waves.(4)The influence of tensile stress on the lowest order symmetric mode(S0)and antisymmetric mode(A0)of Lamb waves is studied.A non-contact laser ultrasonic detection device was built to detect the stress distribution of the strips,and an ultrasonic signal processing system for stress detection was developed.Firstly,for a non-oriented silicon steel plate,the relationship between tensile stress and the phase advance and wave packet delay of A0 mode was studied,and the tensile calibration experiment was carried out to obtain its acoustic-elastic equation.Then,the acoustic-elastic effect of Cu/IF steel/Cu clad plate was studied.It was found that the first wave velocity of A0 mode had a high linearity with the tensile stress.The relationship between the first wave velocity of A0 mode and the tensile stress and the corresponding acoustic-elastic equation were obtained.(5)The thicknesses of the copper layer and aluminum layer in Cu/Al clad plates were measured by pulse reflection method and pulse transmission method.The influence of PRD and spot radius of pulsed laser on thickness measurement results was analyzed by finite element analysis method.It was found that a larger spot radius of pulsed laser can reduce the influence of PRD on measurement error.The simulation results show that it is feasible to measure the thickness of an extremely thin copper layer by laser ultrasonic echo method under the condition of waveform aliasing,but the sampling frequency of data acquisition card must be extremely high.The experimental results show that when the detection laser is located on the copper side,the waveform quality of interface echo is the best,because the copper layer is thin,the propagation distance of the interface reflection wave is short and the attenuation is small.According to the experimental data and thickness calculation formulas,the thicknesses of the copper layer and aluminum layer and the total thickness of the sample are calculated,which are basically consistent with the true values. |
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