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Nugget expulsion during resistance spot welding and weldbonding has a detrimental effect on weld nugget integrity and adhesive bond line. This study has concentrated on the detection of expulsion from electrical signals because these are easily accessible on-line. A variety of expulsion events has been studied for which a number of detection methods have been developed. A prototype on-line expulsion monitoring system has been developed based on these techniques. The system does not require the measurement of electrode displacement or acceleration. Laboratory experiments have shown that the system can reliably detect expulsions under a wide range of conditions. The techniques developed provide a practicable method for industry application.