Research On Vibro-Acoustic Modulation Behavior Of Contact Interface And Defect Detection Method

Vibro-acoustic modulation(VAM) technique is a nonlinear acoustic detection method. Due to its high sensitivity to the contact-type defects and no limitation for the geometries of the testing samples, VAM is conformed to has great potential in the quality evaluation of the engineering structures. In order to improve the reliability and the efficiency of VAM technique in the practical applications, further investigations such as modulation mechanism, parameter optimization and quantitative evaluation still need to be carried out. In this dissertation a VAM testing system is constructed, by taking aluminum rod sampes as the research object, the selection principle of the main exciting parameter is investigated, the interaction mechanism of the vibration/ultrasound on the crack interface is analyzed, a quantitative detection method based on the modified nonlinear modulation model is proposed, and finally, its applications in the subsurface defects detection of the complex structures are studied.To meet the demand of exciting and receiving the vibration/ultrasound signal, the hardware devices are selected, the experimental platform is constructed and the method for exciting, receiving and processing signals are determined. In order to improve the detection efficiency, a Lab VIEW-based testing program is developed and the online testing is implemented.The interaction mechanism of the low frequency(LF) vibration and the high frequency(HF) ultrasound signals on the defect interface are analyzed. Utilizing the numerical simulation method, the low and high frequency sound field in the work piece are analyzed, respectively. And the influence of LF vibration on the HF ultrasound field is studied by the variation of the high frequency response function(HFRF) curve during the test. The results show that the relative motion amplitude of the defect interface will reach the extremum when the LF exciting frequency is the resonance frequency of the work piece. In the meantime, if the HF response of the defect zone is large, the interaction between LF and HF signals will be induced, which is presented as the great variation of HFRF curve with LF vibration. For the aluminum rod samples, there is no interaction between the LF and HF signals when the sample is intact; when the crack size is small, the HFRF variation appears near the resonance peaks, which is presented as the offset and peak value change; when the crack size is large, it’s presented as the variation of the curve amplitude, and appears in the whole HF range. In addition, the sidebands component of the VAM signals is analyzed by comparing them to the simulation results of the signals with different modulation modes. It is indicated that the frequency modulation as well as the amplitude modulation of the vibration/ultrasound signals exists on the defect interface.The optimizing of the main exciting parameter, including the amplitude and frequency selection range or principle of the LF and HF exciting signals, is studied basing on the previous theoretical analysis. The effects of the crack interface motion state on the LF parameter selection are analyzed by the vibration model analysis and the dynamic strain measurement. The results show, with the improving of the LF exciting force and/or the approaching of the LF exciting frequency to the resonance frequency, the opening and closing extents of the crack increases, and the modulation enhances. When the crack interface is fully open, the increasing of the modulation stops. The VAM testing results in different HF exciting parameter are studied, it is demonstrated that the HF amplitude has little influence on the modulation; however, the modulation differences are large in different HF frequencies. In order to improve the detection efficiency, the swept frequency technique is used for optimizing the HF exciting frequency, and the corresponding Lab VIEW program is compiled; synchronous demodulation is selected as the modulation information extraction method of the receiving signal in swept frequency excitation by comparing different demodulation method, and a new modulation intensity evaluation index is defined: swept frequency modulation index MISD. The aluminum rod samples with different size of cracks are tested by VAM in swept frequency excitation, and the validity of the theoretical analysis is demonstrated.A quantitative evaluation method for VAM is proposed basing on the previous theoretical analyses and experimental examinations. The modulation information of the VAM testing signals is analyzed with the synchronous demodulation, the results show that the ultrasound wave in the structure can be separated into two parts, one part passing through the crack interface is modulated by the LF vibration, while the other part has little interaction with the LF. Thus, the nonlinear modulation model is modified. According to this modified model, taking the HF ultrasound field distribution in the work piece into considering, the crack size is characterized by the energy ratio of the modulated ultrasound to the ultrasound in the whole work piece. Comparing with the ultrasonic C-scan results, the feasibility of this method in estimating the fatigue crack size in a metallic rod can be confirmed.At last, the application of VAM in the field test is researched. The feasibility of VAM in the contact extent of the defect interface is investigated with aluminum rod with open slit and bolt connection structure as testing samples. The results show that the modulation increases from no contact to full contact of the interface, however, the modulation intensity decreases when the contact extent is further increased. The detection ability of VAM to internal defects of the complex structure is studied, the results indicates that the quality can be preliminarily evaluated when the proper exciting/receiving location and the HF exciting frequency are selected. And the reliability of this method is verified by the destructive testing results.