Determination of the Cleavage Velocity in Tungsten using Ultrasonic Fractography

Abstract

A CLEAVAGE crack propagating through a solid will also cut through the strain field associated with microscopic surface flaws, and this releases energy in the form of a stress pulse. If the material is elastically isotropic, this pulse will travel outwards from each flaw at a constant velocity in all directions, and the interaction of the stress pulse with the moving crack front will cause a slight localized deflexion of the crack. Thus a series of curved “Wallner lines”¹ are produced, and these are easily visible on the fracture surface. Because the velocity of the stress pulse is a known constant for materials such as glass, the presence of these lines provides a simple method of measuring the cleavage velocity^2,3. Kerkhof⁴ was the first to point out that, because the fracture surface can be modulated by a single stress pulse, it should also be possible to produce a series of faint permanent ripples on the fracture surface using ultrasonics. Knowing the frequency of oscillation, the velocity of fracture is obtained directly from the spacing between the ripples. This technique has been used successfully on glass⁵ and a polymer (polymethyl methacrylate (ref. 6 and private communication from K. Saito)), but its application to crystals with well defined cleavage planes is less obvious, for a large energy is then necessary to force the crack to deviate slightly from this plane. The method can be used on ionic crystals (magnesium oxide⁷, potassium chloride (private communication from M. Schinker)), and we report here the successful application of the technique to a metal.