Experimental Study Of On-line Acoustic Test Of Microscopic Damage Of Q245R Steel

In actual industrial production,the damage failure of most pressure vessels and other equipment is caused by the early mesoscopic damage evolution of the internal structure of materials caused by long-term service in high pressure and heavy load environments.The appearance of mesoscopic damage can deteriorate the internal structure and mechanical properties of the material,resulting in the decrease of the carrying capacity of the material.In severe cases,it may affect the industrial production efficiency and even cause catastrophic accidents.Therefore,developing effective detection methods to explore the mesoscopic damage evolution mechanism of container steel under load and realize early detection of material damage is of great significance for long term safety operation and accident prevention of equipment.In this paper,taking the steel Q245 R steel for pressure vessel as the research object,the tensile damage process of the bilateral "V" notch specimens was tested on-line by acoustic emission technique and non-linear ultrasonic technique.the acoustic emission characteristics and nonlinear ultrasonic characteristics of the specimen during tensile damage process were analyzed,the evolution of mesoscopic damage of metal material was explored to provide an effective method for the early detection of material damage.The following results are acquired.(1)Combined with the mesoscopic damage evolution mechanism and transient stress wave release theory of metal materials,it is known that dislocation motion is the mechanism of typical acoustic emission sources in the process of mesoscopic damage.The relationship between the damage and the characteristic parameter of acoustic emission N was established based on the dislocation movement.The results showed that with the increase of material loading,the dislocation movement aggravated,the damage became more serious and the value of N increased continuously;(2)According to the mechanism of mesoscopic damage evolution of metal materials and the theoretical basis of nonlinear ultrasonic,it is found that the evolution of dislocation structure is the main mechanism of ultrasonic non-linear response during the meso-damage of material.The ultrasonic nonlinear coefficient?can characterize the strength of nonlinear ultrasonic response.Based on dislocation string theory,the relationship between damage and?value is established.It is found that with the increase of load,dislocation movement is aggravated,material damage is aggravated,and?value is increasing;(3)The acoustic emission technique was used to monitor the tensile damage of the specimens,the acoustic emission characteristics of the specimens during different stages of tensile damage were analyzed using acoustic emission parameter analysis and time-frequency analysis.The results showed that different characteristics of acoustic emission signals in different stages of tensile damage.In the stage of elastic deformation,damage inside the specimens is weak,the damage mode is single,acoustic emission characteristics and small signal activity intensity;the yield and enhancement stage,the damage of the specimen is serious,the damage mechanism is complex and varied,and the activity and intensity of acoustic emission signal increase significantly.(4)The nonlinear ultrasonic technique is used to test the tensile damage process of the specimen and calculate the?values of the specimens at different positions under different loads.The analysis of the change rule of?value shows that with the increase of load,the value of?in each test point shows an increasing trend.In the stage of elastic deformation,the damage of test piece is weak and the value of?increases slowly.During the plastic injury stage,?value increases rapidly.The change of?value at different positions shows that the position of notch is most seriously damaged and the value of?increases most.With the increase of the distance to the notch,the stress decreases,the degree of damage decreases and the increase of?decreases;(5)The acoustic emission cumulative impact count N_h is selected as the acoustic emission parameter,and the damage factor D is defined by the ratio of N_h under different loads to the N_h under the ultimate load.The relationship between N_h value and load was numerically fitted to elastic deformation and plastic damage respectively.The fitting load was calculated by the simultaneous fitting formula,and the range of the value of D in the stage of elastic deformation and plastic damage was obtained,the acoustic emission quantization characterization of material damage was realized by D value;(6)The ultrasonic nonlinear coefficient?is selected as the nonlinear ultrasonic characterization parameter,and the damage factor D'is defined by the ratio of?value under different loads to the ratio of the beta value under the ultimate load.The relationship between the value of?and the load was numerically fitted to elastic deformation and plastic damage respectively.The yield load of the specimen was calculated by the simultaneous fitting formula.the value of D'in the stage of elastic deformation and plastic damage was obtained,the acoustic emission quantization characterization of material damage was realized by D'value;(7)The yield load values obtained by acoustic emission tests,the yield load values obtained by nonlinear ultrasonic tests and the nominal yield strength values of the specimens were compared.Based on the testing mechanism of acoustic emission and nonlinear ultrasonic techniques,the evolution of mesoscopic damage of materials was further studied,the study provides a new test method for the early detection of metal material damage.