Monitoring System Of The Weld Crack Germination Based On Eddy Current Testing

At present, as the continuous improvement of the locomotive’s carrying capacity andrunning speed, much higher requirements should be put forward to every component of thelocomotive. Among the components, the locomotive bogie, which has always been weldedand had low fatigue strength and continued surging dynamic load by the increase speed, wasalways a weak link for the bearing capacity. As a essential part of the project called ‘TheResearch of the Surface Nanocrystalline Mechanism and Fatigue Performance of the weldingpoint on Train Bogie’, which was supported by the National Natural Science Foundation ofChina, a on-line monitoring system for crack germination in the welding points wasdeveloped based on the theory of eddy current testing, for monitoring the crack germinationmoments in the welding points of a mass of different test specimens in tensile tests,providing useful technical support for the tensile strength testing.There were two main missions in this paper, including the design of eddy current probeand the design of the software and hardware detection system.Starting with the theoretical analysis, the three main effects in eddy current testing andthe signal processing methods were discussed firstly. By the reflex eddy current testing, theeddy probe’ structural parameters were obtained through the theoretical calculation. And thensensor probes were made for the monitoring system.The effects for the probe output from crack parameters,such as the width, depth, etc,were discussed by relative testing. Besides, the overall arrangement of the probes wasdetermined by then.The overall plan of the monitoring system was designed in this paper. Considering thesize of the test specimen, distance between the toes of a weld, germinating position of thefatigued crack, sixteen sensor probes were used to monitorthe specified area on both sides (asthe Fig4-3shows), working in a way og loop detecting. Several problems in the software andhardware designing were analyzed and discussed emphatically, including the producing ofdrive source signal, signal conditioning, data acquistion and processing, extracting and storingof the data features, etc. Virtual instrument was used in the system, mainly including thesoftware of LabVIEW and the hardware of the NI ELVIS, designing a high precision andstable driving source. The NI1121module was selected and used signal conditioning, so thatmost kinds of the interference would be removed, and the signal gain would be improved, too,providing a much better foundation for the signal analysis. The analysis software system wasstructured based on LabVIEW, for processing the detection signal effectively, extracting and storing the characteristic data, and then achieving the signal’s visualization, what’s more,recording the crack germination moment automatically and sending out the stop instructiontimely.For the reason of the limited time for the research, only some basic work was studied andfinished in this paper, so that further study for the final assembly, debugging, and actualdetecting result of the monitoring system was still necessary and much important.