Precracking of NiAl single crystals by compression-compression fatigue and its application to fracture toughness testing

Abstract

NiAl is a candidate material for high-temperature applications. However, it suffers from insufficient ductility at lower temperatures. In order to evaluate its suitability for use in hostile environments, careful examinations of its damage tolerance towards defects are necessary. A sufficiently high fracture toughness is desirable. Values of fracture toughness should be known as accurately as possible, and are usually obtained by tests with bending or CT specimens. For this purpose, specimens with sharp precracks offer two major advantages over those with a more or less rounded notch: the stress field is more accurately known, and because of the high stress concentration at the sharp notch, the influence of undesired stress concentrators like particles is lowered. Previous investigations of fracture toughness of NiAl suffered from the impossibility of producing sharp starter cracks. In this work, precracking by compression-compression fatigue, originally developed for ductile metals and later also extensively studied on ceramics and other brittle materials by Suresh and co-workers is modified for semi-brittle intermetallics like NiAl.