Nondestructive Testing Method Based On Magnetic Technology

Non-destructive testing (NDT) technique is to evaluate the performance ofa component based on the changes of thermal, acoustic, optical, electric, andmagnetic signals. The changes are caused by the variation/degradation in amaterial. With modern analytical and computational techniques, NDT hasbecome an important testing means of quality control and quality assuranceinspection in modern industrial production.Several NDT techniques such as X-ray diffraction, eddy current,Barkhausen emission, and metal magnetic memory can provide a detection ofnear surface and surface defection. But for the sub-surface defectioncharacterisation, especial for the early stages of cracking, these techniquesare limited. In this paper, the magnetomechanical effect is studied for NDT, adetect system based on LabView environment has been designed andfabricated. The main research are listed as follow.The physical theoretical model of magnetomechanical effect isstudied. When the stress level was in elastic stage, plastic strain appeared atsome weak grain because the strength of the grains is different at the samestress direction. The yield grains formated the stress concentration. Themagnetization curve and magnetic hysteresis loop reflects magneticcharacteristics of a magnet material. The magnetic parameters such ascoercivity, hysteresis loses and permeability were derived from hysteresisloop. These magnetic hysteresis parameters are so sensitive to stress andmicrostructure changes, and the correlations between magnetic parameterswith stress, microstructure, chemical composition have been studied andmodeled. The widely utilized magnetic sensor for NDT consists of a singleU-shaped yoke, a magnetizing coil, and an induction coil. The two coils aredirectly wound on the yoke. The magnetizing coil and induction coil are forsample magnetization and determination of the system flux, respectively. Thesensor is convenient to use and with simple structure. But the U-shaped probeis hard to ensure the measured precision and reproducibility in magnetic stressmeasurement. Many factors can influence magnetic measurement, such asthe contact quality between yoke and sample, stray field and temperature. Inorder to extend the applicability of active magnetic techniques to industries,many measures were taken. Daubing the magnetism cement between theprobe head can reduce the gap magnetic resistance greatly. Establishing anoffset magnetic field along compressed direction can counteract the stray field.Temperature compensation can improve the system measurement accuracy.The best frequency range of induction signal was chosen.Most magnetic apparatuses for NDT employ a sinusoidal or a triangularwave as the excitation. Pulsed excitation has been introduced in hysteresismeasurement in recent work. The voltage of induction coil and the current ofmagnetizing coil are measured. Task of computer in the measurement iscontrolling data acquisition cards, saving the measured data andpost-processing of experimental data.The pulsed field is generated by the discharge of a capacitor over themagnetizing coil. The duration of the generated pulse varies within the limits of10–2to10–3s. In order to obtain a whole hysteresis loop, two capacitors andfour insulated gate bipolar transistors (IGBTs) are used to generate two pulsesin opposite directions. The signals of the excitation current and inducedvoltage in the induction coil are directly fed to a data acquisition card (NationalInstruments, model NI PCI6251). LabView-based software has beendeveloped in order to process the signals and plot the hysteresis loops.The relationship between magnetic parameters and stress wasestablished. The magnetic properties derived from the loops and curves are sensitive to applied stress. Remanence Br did not change with the externalmagnetic field, so it is more accurate and suitable to characterize the stress ina material.The hysteresis loops of a test sample were measured when the samplewas under the tensile stress during the cold-working hardening. In elasticstage, the tensile stress facilitated domain walls movement and result in easymagnetization in a material. Remanence Br increased at the tensile stressdirection. In the plastic strain range, with the stress increasing, there are twofactors affecting the magnetic properties, the tensile stress to ease the domainwalls movements and the dislocations caused by the strain to impede them.Because the hindering effect of dislocation on the domain walls movements ismuch stronger than the effect of tensile stress, the combined influences ofthese two factors result in a rapid decrease of Remanence Br with the appliedtensile stress.The surface magnetic field was measured by a Hall sensor after thepulsed field magnetized a sample. The measurements were performed on thetwo sides of the sample, the samples were made from different thickness. Ifthe surface field strength of the two sides was near consistent, it showed thatthe pulse field penetrating depth reached the sample thickness.An experiment has been set up to investigate the capabilities of thesystem to detect sub-surface defects. The sub-surface defects had influencenot only on permeability and remanence but also on coercive force. The nearsurface defect had great influence on these magnetic parameters. Accordingto the change of coercive force, the existence of the sub-surface defects couldbe judged and the depth of the defect could been calculated.