Experimental Study On Detection Of Weld And Crack By Magnetic Adaptive Testing

The magnetic adaptive testing technique is also known as the differential permeability detection method.It's a new method for nondestructive testing by using the most sensitive differential permeability parameters for stress concentration and fatigue damage,and can also be used to detect the microstructure changes of martensite austenite phase transition,dislocation defect density,mechanical toughness and mechanical brittleness of ferromagnetic materials,as well as macroscopic defects.This paper discusses the design of the sensor,the mechanical properties of the weld and the detection and evaluation methods of the crack of the ferromagnetic component from the theoretical derivation and the experimental analysis.A differential sensor with the shape of "M" is designed,and the material of the magnetic core is manganese-zinc ferrite.The influence of sensor core type and size,coil turns,winding wire diameter and excitation voltage on the optimal excitation frequency and detection sensitivity were studied.The results show that the optimal excitation frequency of the magnetic adaptive detection system is a fixed value,which does not change with the winding number,winding wire diameter and excitation voltage.However,with the increase of the value of winding wire diameter,winding number and excitation voltage,the detection signal also increases,and the detection sensitivity of the probe increases gradually.The lifting effect has great influence on the detection signal value,and the detection sensitivity is the best when the probe lift is within 1mm.The tensile stress and residual stress of Q235 steel plate are tested by using the magnetic adaptive testing platform.The results show that the residual stress of the weld specimen is more sensitive to the change of permeability than tensile stress.The weld quality is related to the current size and welding method.When the welding quality is poor,the specimen is brittle fracture in the weld zone.When the welding quality is good and without defect,the specimen has plastic fracture in the mother material area.The 45# steel plates with different sizes of artificial defects was tested by using the detection probe of "M".The influence of defect size,scanning offset,scanning angle and residual magnetism on detection signal and detection sensitivity were analyzed.The results show that the width of the defect peak is very close to the actualdefect size.In response to meet the magnetic detection system under the premise of the minimum resolution,as long as the width of defect detection in sensor end covered area,the system can measure the position of the defect,and beyond the limits of the defect detection rate to zero;As the excursion distance of the probe increases gradually,the peak of the detection signal gradually decreases,which will eventually be annihilated by the noise signal.When the size of the defect is certain,the peak of the detection signal will gradually decrease with the increase of the scanning angle,which will eventually reach the same state as the noise signal on the surface of the steel plate.It is concluded that the residual magnetism of the specimen has little influence on the test results.The results show that the detection system can also be used to detect the circumferential defects and longitudinal defects on the surface of seamless steel tubes.Therefore,as a new detection method,magnetic adaptive detection technology can alternative to traditional nondestructive testing to detect the defects of ferromagnetic materials.