Detection Of Distributed Micro-cracks In Elastic Solids By Nonlinear Rayleigh Wave And Bulk Wave Methods

With the rapid development of manufacturing industry,the safety and durability of the key engineering structures have been paid extensive attention to.In order to ensure the reliability of structures,it is very important to identify and inspect the defects of engineering structures by non-destructive testing technology.As a conventional NDT method,ultrasonic technology has attracted extensive attention.Relevant studies have shown that micro-defects such as internal dislocations,slips and micro-cracks can cause ultrasonic nonlinearity.Therefore,micro-cracks on or near the surface and inside of materials can be characterized by acoustic nonlinearity parameter.In this paper,the propagation mechanisms of Rayleigh surface wave and one-way collinear mixing wave in elastic solids with randomly distributed micro-cracks are studied by numerical simulations.Firstly,a numerical model containing randomly distributed surface micro-cracks is constructed to investigate the propagation phenomenon of Rayleigh surface waves by statistical analysis from numerous results from random micro-crack models.The results reveal a significant ultrasonic nonlinear effect caused by surface micro-cracks,which is mainly represented by a second harmonic with even more distinct third/quadruple harmonics.Based on statistical analysis from the numerous results of random micro-crack models,it is clearly found that the acoustic nonlinear parameter increases linearly with micro-crack density,the proportion of surface cracks,the size of micro-crack zone,and the excitation frequency.This study theoretically reveals that nonlinear Rayleigh surface waves are feasible for use in quantitatively identifying the physical characteristics of surface micro-cracks in structures.Next,the feasibility of locating micro-cracks is further studied in this paper.Numerical simulations on the propagation of bulk waves in elastic solids with randomly distributed micro-cracks by the one-way collinear mixing method are performed to investigate the behavior of the resonant wave.The results show that the resonant wave can be generated due to the effect of micro-cracks,and the position and the size of micro-crack zone can be detected by the time signal of the resonant wave.Based on statistical analysis from numerous results of random micro-crack models,we find that the acoustic nonlinear parameter increases linearly with the micro-crack density and the size of microcrack zone,which is also related to the frequency of resonant wave and friction coefficient of micro-crack surfaces.This study numerically reveals that the one way mixing method can be employed to quantitatively identify and characterize micro-cracks in elastic solids.