Nugget Shape and Size Classification by Shearography Fringe Pattern in Spot Weld Detection

Spot welding is an important and long established welding technique that has been used extensively in the automotive and aerospace industry. This use has caused industry to actively demand means to measure the quality of resistance spot welds. According to recent research, the strength of spot welds is related mainly to the nugget diameter. There are several existing testing methods that can be used to evaluate the quality of spot welds, including tensile-peel testing, ultrasonic monitoring, acoustic wave detecting and X-ray radiographic testing. These methods are used both in factories and laboratories. However, tensile-peel testing is a destructive method, ultrasonic monitoring and acoustic wave detecting are point-to-point scanning methods that are time consuming, and X-ray requires expert operators due to the threat of radiation. In this paper, a whole field, non-contact, nondestructive detecting laser-based shearography method, is introduced and successfully applied on the welding spot to determine the shape and the diameter of the nuggets. The loading method in the shearography testing has been investigated, and thermal loading is considered to be the best choice. Based on the fringe pattern recorded by the shearography system, a series of fringe modes are identified which reflect the shape of the nuggets. For the circle nugget, a set of criteria can be built up to make it possible to classify samples with different nugget sizes. The experimental results are shown and the repeatability and reliability are proved.

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