| Plate-like metallic structures are widely used in various engineering equipments.Health monitoring for those structures is an important part of structural integrity monitoring.Fatigue cracking is one of the main damage types in plate-like metallic structures.Early detection and evaluation of fatigue cracks by means of a certain method are the preconditions for the catastrophic accidents such as structural breaks.Fatigue cracks in such structures have the characteristics of diversity and widely distribution.Moreover,initial fatigue cracks in plate-like metallic structures are usually close or open,i.e.the interfaces contact each other,and their positions are usually hidden deep in structure.These characteristics are much challenging for current structural health monitoring techniques.Nondestructive testing methods based on nonlinear ultrasonic techniques have become a hot area of research,because of its convenience and high sensitivity of micro-cracks.Nonlinear ultrasonic techniques,which detect damages based on various nonlinear effects derived from the ultrasound field scattered by the defects,have been proved to be able to identify and even image microscopic material degradation and highly sensitive to micro-scale and closed cracks.As a new nonlinear ultrasonic methods,vibro-acoustic modulation(VAM)technology is widely applied in engineering because of its low sound wave attenuation and less nonlinear sensitivity to the measurement system itself.However,the current understanding of the mechanism of VAM phenomenon is not enough,and using the method to carry out structural health monitoring is still in the initial stage of exploration.Therefore,under the support of the National Natural Science Foundation of China,this paper studies the mechanism of VAM technology and the key technologies of online application.The main research works are as follows:1.The theory of classical and non-classical nonlinear ultrasonic wave in the solid is studied.Based on the nonlinear constitutive relation,the nonlinear response and nonlinear coefficients of the single wave field are derived by using the perturbation approximation method.The mechanism of rough contact interface as well as the constitutive relation is analyzed.Based on the analysis results,the nonlinear scattering response of single wave field induced by an interface and the characterization method of the contact nonlinearity are also studied.Theoretical models of VAM under free and loading state are developed.Moreover,the inner unity of opposites between single wave scattering and VAM is analyzed.The mathematical model of VAM shows that when the external load is changed,the nonlinear coefficient of VAM will no longer be proportional to the damage size.Therefore,when the nonlinear coefficient is used to track the damage propagation in material,the change of external load will lead to the result that the tracking result can not be explained.In addition,the relation between single wave scattering and VAM is one of unity of opposites,and the unity is dominant.2.The mechanism of nonlinearity change induced by the varied macroscopic shape of a crack,when the plasticity induced crack closure(PICC)is taken into account,is studied.Based on the mechanism of fatigue crack propagation,the reasons for the change of the crack face shape under the influence of PICC and external load are analyzed qualitatively.On this basis,through the analysis of PICC closure stress distribution results obtained by different research methods,combined with the actual needs of this study,a linear distribution model was derived.An analysis model which reveals the linear dependence between the crack size and the changing rate of contact area is approached based on linear distribution model.A load-insensitive damage index(LIDI),which has the ability to track the actual length of the crack in the whole load range,is then developed based on this analysis model.3.The application of VAM technology in the early fatigue crack detection and evaluation of the plate metal structure is studied and the data analysis is carried out.First of all,one practical method for measuring the nonlinear coefficient of materials is proposed,and a testing system based on surface-bonded low-profile piezo-ceramic transducers is designed and built,and the reasonable parameters of the measurement are also selected.According to the measured data obtained from the experiment,the tracking performances of damage propagation of conventional damage index(CDI)and the proposed LIDI are compared.In addition,some theoretical conclusions are also verified by experimental results.The experimental results show that the LIDI has the ability to track the actual length of the crack in the whole load range.Moreover,it is more sensitive to the damage propagation of micro cracks,and the overall tracking linearity and sensitivity were both two times higher than those of CDI.Therefore,the LIDI is superior in early detection as well as the overall tracking performance as compared with CDI. |
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