Laser ultrasonic loading and optical interference detection of closed cracks in K9 glass

The traditional optical detection method has a limitation in detecting closed cracks in optical elements. In this Letter, a novel, to the best of our knowledge, method has been developed for the detection of closed cracks in transparent samples. Firstly, behaviors of closed cracks under ultrasonic waves loading are demonstrated through the finite-element method (FEM). Simulation results show that the crack can be opened by the laser ultrasonic shear wave (S-wave). Then, a Mach–Zehnder interferometer system is experimentally established to detect closed cracks under ultrasonic waves loading in the sample. It is demonstrated that closed cracks which cannot be detected by interferometry become visible in interferogram images under ultrasonic waves loading. The width of the opened crack can also be quantitatively estimated. Thus the method would be a promising approach to detect initially closed cracks in optical elements.