Time dependent crack growth analyses can be carried out with Zencrack and these require data in the form of da/dt against K (rather than da/dn against deltaK for fatigue). As with fatigue growth data, a variety of options provides flexibility of input:

• COMET equation (for K data)
• Paris coefficients (for K data)
• Paris coefficients (for Ct data)
• tabular option (da/dt vs K curves)
• user subroutine

At high temperatures under both sustained and cyclic loading conditions, microstructural changes interact synergistically with time dependent mechanisms such as creep, oxidation and corrosion and affect the crack growth rate. 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. The time dependent crack growth law, COMET (Creep Oxidation Microstructure Environment Temperature), considers the effect of these combined processes using a temperature dependent parameter based on an Arrhenius equation. Further information on the COMET crack growth law, including a description of calculation of the parameters A and B, can be found in the following reference:

A time dependent crack growth law for high temperature conditions
C. Timbrell, R. Chandwani (Zentech International Limited, London), D.W. MacLachlan, S.J. Williams (Rolls-Royce plc, Derby)
NAFEMS European Conference: Multiphysics Simulation, 16-17 October 2012, Frankfurt, Germany

The COMET law has an implied temperature based threshold condition, but for other options a threshold definition can be defined. As with crack growth data, user subroutine options allow for completely general definition of a threshold condition.