Dynamic Evolution Process Of Meso-damaged Acoustic Emission Source Of Q235 Steel

The deformation and fracture of metal materials are common failure modes.Actual metal materials often produce various microscopic defects during forming,processing and using.The macroscopic mechanical properties of the material largely depend on the development of its internal defects.Acoustic emission phenomenon of materials is a concomitant phenomenon that is consistent with its mechanical process and the evolution of internal damage of materials.After years of development and improvement,acoustic emission testing technology has become an effective tool for studying material damage process.The study of the evolution process of the acoustic emission source during the theriod of material damage has theoretical and practical significance for in-depth understanding of material damage behavior,assessment of material performance and that of the safety and reliability of structure.In this paper,Q235 steel is used as the research object,and the acoustic emission technology as well as in-situ stretching method are used to conduct online acoustic emission monitoring and damage image optical observation on the in-situ tensile damage process of the defective specimen.The acoustic emission characteristics and damage image characteristics of the specimen during the tensile damage process are analyzed,and the evolution law of the acoustic emission source during the in-situ tensile process of the metal material are explored.The following research work are carried out:(1)Taking the Gurson-Tvergaard-Needleman meso-damage model as the object,the numerical evolution of the meso-cavity damage parameters of the specimen during the stretching to fracture process are ananlyed,to obtain the meso-damage acoustic emission source mechanism of the material during the stretching process.(2)In-situ monitoring and optical observation experiments of acoustic emission under tensile load are carried out.In-situ mechanical testing instruments are used to perform in-situ monitoring on pre-prepared defect specimens under the action of a single load,as well as the monitoring of acoustic emission signals and the acquisition of damage evolution images,in order to study the source and damage of materials under load further.(3)The acoustic emission parameter analysis method and time-frequency analysis method are used comprehensively to analyze the acoustic emission signal characteristics of the specimen at different stages produced by different acoustic emission sources.The results show that the acoustic emission signal characteristics are different at different stages of tensile damage.In the elastic deformation stage,the internal damage of the specimen is weak,thus the damage mode is single,and the characteristics of acoustic emission are low signal activity and strength.In the yielding and strengthening stages,for the reason that the specimen is seriously damaged,the damage mechanism becomes complex and diverse,and the acoustic emission signal activity and strength are significant increased.Through the analysis of the characteristics of acoustic emission signals in each deformation stage,the evolution mechanism of micropore nucleation,growth and aggregation during the deformation process of metal components,and the meso-damage evolution in the elastic stage,yield stage,strengthening stage and the final failure stage are studied.(4)According to the time of the acoustic emission signal,the corresponding damage image is collected,and the pseudo color,grayscale,brightness,and contrast are adjusted to improve the display effect of the image.The Sobel operator performs edge detection and extraction and is based on the threshold of the grayscale histogram The regional method are used to divide the images.The neighborhood averaging method is to smooth the image and remove the influence of noise.Using mathematical morphology to process the binary image,and the binary image is processed through the open and close operations.The damage image processed by the watershed segmentation method is calculated based on the area method,and the damage characterization parameter at different stages and different times is extracted;(5)Combining the image of the acoustic emission source obtained by image processing and the data of the acoustic emission signal,two typical acoustic emission sources,namely,inclusion fracture and slip dislocation and their evolution rules are obtained.Using the HHT method,in-depth analysis of the acoustic emission signals generated by two typical acoustic emission sources,through the comparison method,to obtain the signal difference of different acoustic emission sources and the similarity of the same acoustic emission source signal,to further understand the meso damage material The evolution rule of the emission source provides a theoretical basis for the application of acoustic emission technology to the quantitative evaluation of early damage of metal materials.