High-speed holographic microscopy for fast-propagating cracks in transparent materials

A method of high-speed holographic microscopy is developed to take three successive microscopic photographs of a crack tip propagating in a transparent poly(methyl methacrylate) specimen at a speed of several hundred meters per second. When a crack is propagating in a specimen, three Q-switched ruby lasers emit three laser pulses successively. The time interval between each laser pulse and the next is 1 mu s or longer. An optical system of angle-multiplexing holography records the crack as three successive holograms on one photographic plate. Crack images are reconstructed and photographed through a conventional microscope. The spatial resolution of the reconstructed images is approximately 114 lines/mm. From the photographs, one can measure crack speed, crack opening displacement, and the dynamic stress intensity factor. The high-speed holographic microscopy makes it possible to study rapid crack propagation in microseconds. (C) 1997 Optical Society of America.