Assessment of notched tubular ferritic oxide-dispersion-strengthened components subjected to multiaxial creep loading at 1100°C

Abstract

Notched tubular components of a ferritic oxide-dispersion-strengthened material, MA 956, were subjected to multiaxial creep loading at 1100°C by applying a constant internal pressure. The components proved to be extremely insensitive to circumferential notches of up to 80 percent of the wall thickness due to the high axial creep rupture strength of this material. However, the components were sensitive to both externally and internally axially aligned notches, and displayed similar stress rupture behaviour but consistently longer rupture lives than plane components at the same ligament stress level. Failure was found to be due to strain controlled cavitation in the ligament rather than as a consequence of creep crack growth from the notch. A direct current/potential drop method was shown to provide a reasonable indication of the development of cavitation in these tests. It is shown that the low ductility failure of notched MA 956 components is best described by a creep fracture mechanism rather than by fracture mechanics.