Structural health monitoring using digital speckle photography

Abstract

Infrastructure aging is of current national concern and the need to monitoring its state of health is paramount. The prevailing monitoring techniques are mostly point-wise and qualitative. In this paper we present some results on using the technique of DSP (Digital Speckle Photography) to monitor the deflection and strain distribution of a model bridge. A truss wood bridge with a span of 10 ft was constructed with pine wood beams of various sizes. A load of 400 lbs was distributed across the span using sand bags to simulate uniform load. One surface of the wood bridge was painted with a thin coat of retro-reflective paint. A 300w flood light was used to illuminate the bridge from one side of a 3008 x 2000 pixel CCD camera. Specklegrams before and after the application of load were recorded digitally and analyzed using a special program called CASI to yield deflection of the entire truss bridge as well as regional local strain distributions. The deflection result was compared with the finite element prediction and the strain results were compared with strain gage readings. Reasonable agreements were obtained.