Inspection and evaluation of laser welds for transit buses

Laser welds in light-weight transit bus panels were studied by IR imaging. The corrugated structural panels were made from 3 mm thick stainless steel. The panels were welded by a high power laser with lines of equally spaced 1″ stitches. After discovering problems by visual and tapping inspections, the panels were sent to our laboratory for further investigation. The IR thermography method was chosen to study the welds because of its non-contact nature and potential for large area, high-speed inspections. We used thermophysical properties of the panels and finite element modeling to predict temperature variations of "good" and "bad" welds. Surface heating and "heat leak" methods were used to inspect each weld. The IR images clearly showed characteristic temperature signatures of "good" and "bad" welds. We also discovered a number of partial welds and questionable welds. In the follow-up destructive inspection, the welds interfaces were imaged and related to the infrared images. No or partial penetration were found on a number of bad and partial welds. Even in the good welds, the weld stitch appeared to be discontinuous. This study helped the bus manufacturer to assess their welding process and make necessary improvement.