Detection Of Micro Defects On Metal Surface By Ultrasonic Nonlinear Response

Metals are closely related to people’s lives,and play a critical role in theconstruction industry,transportation,mechanical processing,aerospace and other fields.Metallic structures and components can be degraded or damaged inevitably in the service process,affecting the reliability of structure and its service life.If thesedefects are not detected in time,they may cause accidents.Wear,breakage and corrosion are three major forms of metal failure which begins to occur mostly from the surface of the metal components,thus it is very important to detect defects on the metal surface.As a nondestructive testing technique,nonlinear ultrasonic method is to evaluate the materials degradation and damages based on the measure of ultrasonic nonlinear response,which generated by the ultrasonic waves propagation in the material.The traditional linear ultrasonic testing technology can only be used to detect the macro defects in the material with less sensitive to early damages,while the nonlinear ultrasonic technology can be employed to detect the micro defects at early stage.In this paper,two kinds of defects,fatigue crack and micro corrosion on the metallic surface were prepared in the materials of 6061 aluminum alloy samples with different degrees.Nonlinear ultrasonic appraoch was used to detect these defects.First,the second harmonic generation of surface wave was used to detect these two kinds of defects.The reflection of surface waves was found in the specimen with fatigue crack.When it cames to the corrosional sample,no reflection can be obsorved,while the second harmonic of surface wave increased with the increasing degree of corrosion.Then the experiment of two collinear surface waves mixing at different frequency was carried out in the specimens.Acoustic nonlinear responses of collinear surface waves propagation in the specimens with various damages were obtained.It is shown that the relative nonlinear response increase with the degree of corrosional damges.Secondly,the method of non-collinear mixing of two bulk waves of different frequencies was conducted to locate surface defects in this paper.The corresponding measruement system was proposed.The frequency combination of the excitation signals was determined and the appropriate mixed signal was selected to construct the nonlinear acoustic parameter.The fatigue and corrosional samples were tested under this experimental system.The results showe that the nonlinear response caused by fatigue crack and surface corrosion with different levels can be identified.This method could locate the defect on surface to some extent.In addition,this approach could isolate the instrument induced nonlinearity effectively and simplify signal processing.