Speckle pattern interferometry excited by pulsed laser for crack size detection

Defect detection is an important part of heritage conservation and speckle pattern interferometry is a common technique for inspecting surface and internal defects. A speckle pattern interferometry coupled with pulsed laser system for non-destructive detection and prediction of different size cracks is introduced and tested. According to photoacoustic effect, ultrasonic waves are generated by pumping a pulsed laser beam to the rear surface of the sample. The ultrasonic waves serve as carrier propagating from the rear surface to the front, thus conveying deformation and crack size information. For obtaining information about simulated crack size, the front surface is detected by a speckle pattern interferometry system. And simultaneously, the generated ultrasound waves are detected by air-coupled transducer. In this study, the introduced system and method were validated by detecting medium density fiberboards with simulated cracks of different width and depth. Differentiated speckle pattern and ultrasound signals are compared and combined to indicate crack presence and to figure out different crack size qualitatively.