Crack detection of ceramics based on the method of laser speckle photometry

Laser speckle photometry (LSP) is a contactless, non-invasive, innovation optical non-destructive and monitoring technique based on the detection and analysis of thermally or mechanically activated characteristic speckle dynamics in the non-stationary optical field. The speckle movement caused by the local thermal expansion can be detected by a fast CMOS camera. A special correlation function is used to build the connection between speckle intensity change and local spatial gradient. The existence of defects on the surface will change the thermal distribution of sample surface, and thermal energy accumulates around the defect which changes the local spatial gradient. This principle provides the possibility of crack detection with LSP technique. In the current work the objective is to examine the suitability of LSP technique for the detection of micro-crack located in the surface of ceramics. In this thesis, Chapter 1 introduces motivation of this work. Chapter 2 presents the state-of-the-art of some NDT techniques for crack detection, which are highly related to LSP technique. Chapter 3 clarifies the objective and scope of current work. Chapter 4, 5, and 6 show the experimental parts with which the results were obtained. For current research stage, the detectable resolution of LSP technique is approximately 800 µm in terms of the length of cracks.