System Development for In-Line and In-Process Measurement of Spot Weld Quality with Optical Measurement

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Resistance spot welding is widely used for the construction of thin sheet structures, and the weld quality is typically evaluated by the weld size as well as the strength, which are measured by various destructive methods. In recent automated processes, however, in-line or in-process inspection is demanded for the quality assurance of the spot welds, but there is few techniques applicable to the production line. In this study, therefore, optical measurement were tried to develop the in-line inspection and in-process monitoring for the fast evaluation of weld size and strength of spot welded structures, as follows.1.Nondestructive Evaluation of Spot Welds by Infrared ThermographyIn order to develop a in-line measuring system, infrared measurement were proposed for the nondestructive evaluation of weld quality and weld size. Since this measurement uses the thermal response to the heat input, heating method is important for the fast and large area inspection and the acquisition of the rea … More dily interpretable image of spot welded region. In this study, Joule effect heating due to the electric current flow through the back plate and across the welded region were used for the fast inspection. These heating methods also produced the readily interpretable images with high temperature at the boundary of welded region in the surface. The shape obtained from the maximum temperature position in the image indicated the welded area, and the evaluated size was in the accuracy of 0.3 mm. This method performed the measurement within 0.1 second, while the image processing requires for the measurement of weld size was 2.5 second. It is necessary to shorten the image processing time within 1 second, which is possible by use of high performance CPU.2.Nondestructive Evaluation of Spot Welds by Spectroscopic AnalysisIn order to develop a in-process measuring system, spectroscopic analysis was tried to monitor the weld quality during welding process. Two methods were used for the evaluation of weld size. First method is a measurement at the surface immediately after removal of electrode chip from specimen, and second one is a measurement with the fiber inserted in the electrode. Both measurement at the wave length 800 and 900 nm showed that spectrum intensity had a good relation with weld size, regardless of surrounding condition. However, this relation depended upon the plate thickness, materials and measuring time in the first method and upon the material in the second method. Therefore, second method is recommended for the in-process measurement, although the improvement is required in the electrode design, since the detecting hole in the electrode affects the appearance of spot welds.3.Prediction of Strength of Multi-Spot Welded StructuresIn addition to the weld size, the weld quality was also evaluated by the strength of welded structures. For this purpose, the prediction method was proposed for the strength of multi-spot welds. Since the strength of multi-spot welds depends upon the weld size, material properties, weld pitch, plate width and thickness, the relation between these factors and strength of single spot welds was clarified theoretically and experimentally. As a result, the theoretical stress analysis showed that the strength is given by the summation of fracture load of half sized single spot welded elements in the multi-spot welds. The estimated strength well agreed with the experimental results. This prediction enabled the in-line mad in-process assessments based upon both weld size and strength in the spot welded structures. Less