Coupling of damage mechanisms in the prediction of creep failure of a welded branched header pipe

The paper introduces the concept of high-temperature creep Continuum Damage Mechanics (CDM); and then presents the associated physics-based creep constitutive equations used to model CDM. The equations involve the coupling of inelastic creep deformation and damage mechanisms. The calibration of the constitutive equations from available creep data is addressed; and materials constants are presented for parent, weld and intermediate phase materials found in welds used in high-temperature pipework in the power generation industry. The paper continues with the introduction of a ferritic steel welded branched header pipe that is subjected to a constant internal pressure of 4 MPa at a temperature of 590°C. The pressurised branch was tested to failure, and the associated test data is used as a benchmark against which lifetime and damage evolution predictions are assessed. The results of two Finite Element solutions are presented that have been obtained using the FRONTAL and the PGC equation solvers. Excellent correlation is shown between laboratory test results on the branch and the Finite element predictions.