Three-dimensional speckle interferometric investigation of the stress intensity factor along a crack front

The variation in mode I stress intensity factor throughout the thickness of an ASTM standard compact tension specimen was determined using Scattered Light Speckle Interferometry. Two very thin sheets of coincident coherent light traveling in opposite directions were passed through a plexiglas specimen normal to the crack faces. A double exposed photograph of the scattered light speckle pattern was taken while the specimen was subjected to a small load increment. From this double exposed photograph the change in the crack opening displacement could be determined. From the information about the crack opening displacement in the region of the crack tip, the stress intensity factor was calculated for various interior planes and on the surface of the specimen. For the compact tension specimen tested, the stress intensity factor did not vary throughout the specimen's thickness. The method of Scattered Light Speckle Interferometry proved to be very powerful in solving this complex three-dimensional problem.