Study On Nonlinear Ultrasonic Testing Method For Material Properties

In the process of manufacturing and long-term use of major equipment in aerospace and other fields,the key components affected by cyclic impact load,high temperature and high pressure,high stress concentration and corrosion are prone to early damage,such as fatigue and micro-crack etc.Under the action of cyclic impact load,fatigue damage are prone to be produced at key components.Under the influence of stress concentration,micro-cracks evolved from fatigue are easy to expand into macro-cracks and cause structural damage,which seriously threatens the safety of people’s life and production.Therefore,detection of early damage is very necessary.Compared with linear ultrasonic technology,nonlinear ultrasonic technology is more sensitive to early damage,and has become a research hotspot in recent years.Most researches mainly focus on the quantitatively evaluation of fatigue and micro-cracks by using harmonic method or frequency mixing technology.But,the static component（SC）of ultrasonic wave,as a kind of second-order component,is seldom used to evluate the early damage.In addition,highly attenuating materials,such as composite materials and silicone rubbers,are gradually used as major equipment structural parts in aerospace and other fields due to their advantages of light weight,corrosion resistance,aging resistance and impact resistance.And there is an increasing demand.Up to today,there are few researches on the thickness measurement of highly attenuating materials.When the thicknesses of highly attenuating materials are unknown or change with different positions and their thicknesses need to be determined,it is also necessary and urgent to develop a detection method for measuring the thicknesses of highly attenuating materials.The main content of this study include:（1）Based on the SC of incident primary longitudinal wave（PLW）,a nonlinear ultrasonic system based on self–transmitting and self-receiving for thickness measurement of highly attenuating materials is established.Based on the pulse-echo technique,the thicknesses of high attenuation materials can be measured by the time-difference between two adjacent pulse-echo signals of SC.The effectiveness of the method is verified by measuring four different thicknesses of silicone rubbers.At the same time,the property of the SC featuring the low attenuation is also verified,which shows that the propagation distance of SC is longer than that of the incident PLW and the generated second harmonic.（2）The propagation characteristics of SC in thin plate-like structures are analyzed,and a nonlinear ultrasonic method based on SC for evaluating fatigue state quantitatively is proposed.The cumulative properties of SC with propagation distance are numerically analyzed.According to the relationship between SC and fatigue state,the changing regular of SC amplitude with fatigue state is analyzed.When the group velocities of the primary Lamb wave and the generated SC pulse are matching,the quantitative relationship between SC and fatigue state is experimentally verified in a thin aluminum plate.（3）A characterization method of microcrack orientation based on the second harmonic generated by the interaction between a primary transverse wave（PTW）and a microcrack is developed.The bilinear stress-strain constitutive model of the micro-crack is built.The physical mechanism between PTW and the micro-crack is analyzed.The correctness of the physical model is verified by using the micro-crack working as a secondary sound source（SSS）,i.e.,the orientation of the micro-crack is evaluated according to the directivity of the SSS.（4）A SC based method for locating micro-crack in a thin plate is proposed.Based on the theory of the contact acoustic nonlinearity and the bilinear stress-strain model,a two-dimensional finite element（FE）model is built,which can indicate the interaction between the A0 mode Lamb wave and the micro-crack.And a micro-crack localization method is developed based on the SC generated by the interaction between the A0 mode and a micro-crack.The simulation results demonstrate that the mode of the generated SC is S0 mode Lamb wave at zero frequency.The proposed method can effectively locate micro-crack.At the same time,the influence of the stiffness difference change around the micro-crack on the SC amplitude is analyzed.（5）A frequency mixing technique for locating micro-cracks in a thin plate is proposed.Based on the nonlinear theory of contact acoustic nonlinearity and bilinear stress-strain constitutive model,a two-dimensional FE model is built to investigate the frequency-mixing response induced by the interaction between two primary Lamb waves and a micro-crack.Based on the time of flight of the generated A0 mode at difference frequency,an indictor named normalized amplitude index(β_NAI)is defined,according to which the location and number of microcracks can be judged intuitively.FE simulation results show that one or several micro-cracks can be accurately located by using theβ_NAIbased frequency-mixing technique.In addition,the influence of the dispersion property of Lamb wave on the micro-cracks localization is also investigated.This study can be used for reference in the nondestructive field of materials in early damage stage.This study can be used as a reference for the study of SC generation mechanism in a block and thin plate-like structures.It is a basis for evaluation of the early damage degree of materials and micro-crack localization in theory,numerical simulation and experimental fundamental.In addition,this study enriches the nonlinear ultrasonic frequency-mixing technique and provide a certain reference for the localization of micro-cracks in thin plate-like structures.