Automobiles are required to have both lightweight bodies to improve fuel economy and high rigidity to improve safety. Recently, weld bonding which is a method of joining steel materials is expected to be effective. Weld bonding combines spot welding with adhesive to compensate for the reduced rigidity of the steel plate due to its thinner wall thickness. Since the stiffness-enhancing effect obtained by this technique assumes that the adhesive fills the required area, the bonding area is an important inspection item. Conventional sampling inspection has the problems that only the cut surface can be evaluated and the cut part cannot be used. Therefore, we conducted this study to establish an inspection method that enables nondestructive inspection of all parts by infrared temperature measurement.

In this study, an active infrared thermography method that applies a forced thermal load to the surface to be inspected was used. A test specimen bonded by weld bonding was pulse-heated by a flash lamp, and the temperature fluctuations on the surface of the specimen were captured by an infrared camera. The difference in heat conduction between adhesive-applied and non-applied areas appeared as a temperature difference at the measurement surface, and the bonded area could be detected. Singular value decomposition was performed on the obtained time-series temperature variation data. This method enabled us to extract the temperature fluctuation through the adhesive and to identify the adhesive area more precisely.