Detection of brazing defects in stainless steel core plate using the first peak value of pulsed eddy current testing signals

As a novel building structure element, stainless steel core plate (SSCP) has seen growing application in modular construction. It is composed of two stainless steel skin panels and a number of stainless steel core tubes, which are brazed by elevated temperature inert gas. The underlying defects in brazing joints severely damage the SSCP's structural integrity and weaken its mechanical properties. In this study, a nondestructive testing (NDT) method based on pulsed eddy current testing (PECT) was employed to contactlessly detect the missing solder and non-bonding brazing defects from the plate surface. The defect-free and defective brazing joints are simplified to stratified structures and analytical modeling for a PECT probe over the brazing joint is thus performed. The time -domain features of the calculated PECT signal due to the structure parameter variation are investigated. The first peak value is selected to characterize the brazing quality due to its distinct variation w.r.t. well-bonded brazing, missing solder, non-bonding and skin panel. Experiments are also carried out to validate the analytical modeling. With the aid of an industrial robotic arm, one-dimensional (1D) line-scan and two-dimensional (2D) planar scan of the PECT probe over the SSCP specimen are implemented and the detection results are represented in images. In contrast to the well-bonded joint, the brazing joints with missing solder defects are imaged as notched rings, on which the notch orientations and sizes agree with the actual openings of the brazing foils, while the non-bonding brazing joint is imaged as a full ring with faded color. The proposed method shows a promising aspect in practical application for automated NDT of the brazing defects in SSCPs.