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The paper summarizes previously derived constitutive parameters for temperatures of 575, 590, 600, 620 and 640 8C. Values of the multi-axial stress rupture parameter, #, are reviewed and recorded. This constitutive parameter set is used to determine the thickness of the Type IV material zone (0.7 mm). Values of Type IV multi-axial stress rupture parameter # are determined for a wide range of butt-welded pipe and cross-welded uni-axial specimens, and an interpolation equation is derived in terms of temperatures and stress level. Finally, continuum damage mechanics (CDM) analyses are performed for pipes and cross-welded testpieces, which include a coarse-grained heataffected zone (CG-HAZ). It is shown that the constitutive parameter set, which corresponds to a minimum creep rate ratio of 1/2.5, with respect to the parent material, gives accurate predictions of lifetimes and damage distributions.

The paper reports the use of three-dimensional creep continuum damage mechanics techniques to study the creep failure of a medium-bore low-alloy ferritic-steel cylinder-cylinder branched pressure vessel welded connection, tested at a constant pressure of 4 MPa, at a uniform temperature of 590 • C. The development of computational techniques is reported to analyse this problem with a four-material model of the welded connection which includes: parent, type IV, heat-affected zone (HAZ) and weld materials. The results of analyses are presented for two sets of creep damage constitutive equations. For both equation sets, lifetimes are conservatively, yet accurately predicted; however, the results of metallographic examinations of a tested vessel are not accurately predicted. To overcome this deficiency further analyses of the vessel are recommended which include: a coarse-grained HAZ (CGHAZ), adjacent to the weld material; and, more-refined finite element modelling.

The paper reports three-dimensional creep continuum damage mechanics (CDM) analyses of creep failure in a medium bore Cr–Mo–V low alloy ferritic steel welded branched-pressure vessel that has been tested under a constant pressure of 4 MPa, at a uniform temperature of 590 °C. The use of the CDM computer software Damage XXX to analyse the initiation and growth of creep damage and subsequent failure in the branch weld is reported for a five-material model that includes: parent, Type IV, refined heat affected zone (R-HAZ), coarse grained heat affected zone (CG-HAZ) and weld materials. The results of the analyses are presented for two cases: the first without the CG-HAZ; and, the second with the CG-HAZ included. For both cases, lifetimes are conservatively, yet accurately predicted. It is shown that it is necessary to use a Type IV thickness of 0.7 mm to accurately predict the failure location and mode. The results of metallographic examinations of a tested vessel and the predicted damage fields are in close accord. Failure is predicted to take place, by steam leakage, from the interior of the vessel, through the Type IV zone adjacent to the main pipe, connecting through the R-HAZ to the CG-HAZ, where leakage takes place at the weld toe in the crotch plane.

For axi-symmetrically notched tension bars [Dyson, B.F., Loveday, M.S., 1981, Creep Fracture in Nimonic 80A under Tri-axial Tensile Stressing, In: Ponter A.R.S., Hayhurst, D.R. (Eds.), Creep in Structures, Berlin, show two types of damage propagation are shown: for low stress, failure propagates from the outside notch surface to the centre-line; and for high stress, failure propagates from the centre-line to the outside notch surface. The objectives of the paper are to: identify the physics of the processes controlling global failure modes; and, describe the global behaviour using physics-based constitutive equations.Two sets of constitutive equations are used to model the softening which takes place in tertiary creep of Nimonic 80A at 750°C. Softening by multiplication of mobile dislocations is firstly combined, for low stress, with softening due to nucleation controlled creep constrained cavity growth; and secondly combined, for high stress, with softening due to continuum void growth. The Continuum Damage Mechanics, CDM, Finite Element Solver DAMAGE XX has been used to study notch creep fracture. Low stress notch behaviour is accurately predicted provided that the constitutive equations take account of the effect of stress level on creep ductility. High stress notch behaviour is accurately predicted from a normalized inverse cavity spacing d/2' = 6, and an initial normalized cavity radius r hi /' = 3.16 · 10 À3 , where 2' is the cavity spacing, and d is the grain size; however, the constants in the strain rate equation required recalibration against high stress notch data. A void nucleation mechanism is postulated for high stress behaviour which involves decohesion where slip bands intersect second phase grain boundary particles. Both equation sets accurately predict experimentally observed global failure modes. Nomenclature Stress r 1Uni-axial stress r ij Stress tensor r e (=3r ij r ij /2) 1/2 Effective stress r 0Normalizing stress S ij Stress deviation tensor J 1 Normalized first stress invariant R ij ð¼ r ij =r 0 Þ Normalized stress tensor S ij ð¼S ij =r 0 Þ Normalized stress deviation tensor R e (=r e /r 0 ) Normalized effective stress Strain _ e 1 Uni-axial creep strain rate _ e ij Creep strain rate tensor _ e o ð¼ A ¼ P Þ Steady-state uni-axial strain rate e o (= r 0 /E) Normalizing strain k ij (= e ij /e o ) Normalized strain tensor _ e e ð¼ 2_ e ij _ e ij =3Þ 1=2 Effective creep strain rate tensor