Abstract
Pulsed holographic microscopy is applied to kake instantaneous microscopic photographs of the neighborhoods of crack tips propagating through AISI 4340 steel specimens at a speed of about 300m/ s. We use a holographic optical system with a Q-switched ruby laser which records scattered light from crack edges and from specimen surfaces as object light. A method for brteaking the specimens is devised to synchronize the laser oscillation with the generation and propagation of a crack. Two microscopic photographs are obtained. From each photograph, the crack opening displacement (COD) is measured along the crack within 10 mm of the crack tip. The COD is of the order of ten to one hundred micrometers, and is proposional to the square root of the distance from the crack tip. Dynamic fracture toughness KID is calculated through the formula of COD in the singular stress field of a propagating crack.
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