Detection Method Of Metal Fatigue Damage By Electromagnetic Ultrasonic Same Side Mixing

Sheet metal is widely used in electrical equipment,mechanical processing,aerospace and many other fields.During its service,it is inevitably affected by cyclic load,high temperature and corrosion,resulting in dislocation and fatigue damage inside the sheet metal.If these small damages can not be detected and measures taken in time,the fatigue damage will continue to expand under the external load and appear macro cracks,which will eventually cause serious accidents of plate fracture.Therefore,the accurate detection of early fatigue damage and fatigue life evaluation of metal components are the key to prevent the failure of large equipment.In this paper,the ultrasonic nonlinear characteristics of fatigue damaged aluminum plate are studied based on the same side mixing method of electromagnetic ultrasonic.It mainly includes the following contents:1)The same side mixing and the different side mixing are compared and analyzed,and the same side mixing method of the new excitation mode is adopted.According to the frequency selection formula,two excitation signals with different frequencies and modes are selected and the mixing effect of these two signals is analyzed.In order to test the different positions of the test piece,the position and spacing of the two fundamental wave coils are analyzed,and more reasonable parameters of the coil position and spacing are selected.Based on the double-sided excitation theory,a mixed electromagnetic ultrasonic transducer is designed,which can excite two kinds of Lamb waves with different frequencies and modes2)Based on the spring interface force model and double-sided excitation theory,a two-dimensional simulation model of mixed electromagnetic ultrasonic is constructed,excitation of single mode Lamb wave is analyzed.The time and frequency domain of the received signal is determined to determine whether the aluminum plate has fatigue damage under the condition of the unbounded force model,and under the condition of unbounded surface force model,the time and frequency domain changes of the received signals are studied to determine whether there is fatigue damage in the aluminum plate.The damage location is determined by detecting the change of nonlinear coefficient at different positions of aluminum plate.The effects of interface force parameters,transducer structure parameters and crack depth on ultrasonic nonlinearity are studied.3)Based on the two-dimensional simulation model,a three-dimensional simulation model of mixed electromagnetic ultrasonic is established,which is more suitable for the actual crack fluctuation.The propagation and crack fluctuation of electromagnetic ultrasonic in the three-dimensional model are analyzed.Based on the three-dimensional simulation model,the aluminum plate with fatigue damage is detected,the damage area is located,and the influence of crack length on ultrasonic nonlinearity is studied.4)Building a nonlinear detection system of mixing electromagnetic ultrasonic and the excitation of mode wave and the frequency selection of mixing excitation signal are studied.The relationship between fatigue period and crack length is analyzed.The mixing test is carried out on the specimen with fatigue damage,and the crack area is located.The experimental results are consistent with the theoretical analysis.The influence of different crack length and fatigue cycle loading period on mixing nonlinearity is studied.On this basis,the relationship between composite nonlinear parameters and fatigue life is fitted,and the feasibility of fitting function curve for fatigue life prediction is verified by test samples.