A Time Dependent Crack Growth Law For High Temperature Conditions

Chris Timbrell, Ramesh Chandwani, Zentech International Ltd.
Duncan MacLachlan, Steve Williams, Rolls-Royce plc, Derby

NAFEMS European Conference: Multiphysics Simulation, Frankfurt, Germany, Oct 16-17 2012

Alloys, especially nickel based ones used in the aerospace industry, are continuously being improved to provide greater strength against component failure and also to increase resistance against crack propagation. This involves altering their composition and, under controlled conditions, modification of precipitate and grain sizes. At high temperatures under both sustained and cyclic loading conditions, these microstructural changes interact synergistically with time dependent mechanisms such as creep, oxidation and corrosion and affect the crack growth rate (CGR). The individual effects of environmental conditions such as oxidation and corrosion and microstructural evolution of grain size at high temperatures, are generally difficult to evaluate. In addition, thermo-mechanical testing of large numbers of specimens under a variety of conditions can be prohibitively costly. Attempts have been made over the last few decades by a number of investigators to conduct standardised tests under controlled environmental conditions and compare them with the results obtained in neutral environments such as vacuum or inert gas [1-4]. It has been found that these environmental effects interact and their combined effect is generally greater than if they were considered separately. In this paper a time dependent crack growth law, COMET (Creep Oxidation Microstructure Environment Temperature), is described which considers the effect of these combined processes using a temperature dependent parameter based on an Arrhenius equation. Using this time dependent law in conjunction with a fatigue crack growth law, a finite element based implementation has been developed to carry out detailed 3D crack propagation analysis and simulation of a cracked component under the effect of thermo-mechanical loading at high temperatures.