Advanced Ductility Exhaustion Methods for the Calculation of Creep Damage During Creep-Fatigue Cycling

Abstract

The thermal mechanical cycling of high temperature components can result in creep-fatigue cycles in which the creep dwell has a wide variety of positions within the cycle. During in-phase cycling the creep dwell is placed at the maximum strain in the cycle. Out of phase cycling gives compressive dwells and phase cycling between 0° and 180° gives a cycle in which the dwell is placed before the maximum strain is reached, i.e., at an intermediate position within the cycle. Furthermore, components often experience cycles with a variety of different strain amplitudes, which can result in cycle sequencing effects. The effects of these different types of creep dwells have been investigated as part of the development programme for the R5 High Temperature Life Assessment Procedure. The materials included in the R5 programme were two low alloy ferritic steels; a cast 1CrMoV and a cast 1/2 CrMoV and three austenitic stainless steels; a type 316H steel, a cast type 304L, and a type 347 weld metal. The analysis of these tests has resulted in the proposal of a new method to calculate creep damage. This has been shown to give better predictions for the creep damage at failure in laboratory tests compared with both the current R5 ductility exhaustion approach and the time fraction approach, which is used in typical design codes such as ASME III and RCC-MR. In particular, the new method gives significantly improved predictions of creep damage at failure for creep-fatigue cycles with intermediate dwells and for cycles with low strain ranges, which are of particular relevance to the service cycles in real plant. This paper reviews the findings of the work on the new method, the effects of multiaxial states of stress and the effects of compressive dwells.

The energy density exhaustion method for assessing the creep-fatigue lives of specimens and components

“…A recent example [38] has also used hysteresis energy for predicting creepfatigue endurance of 9Cr1Mo steel at 550°C and similar temperatures. Since both stress and strain are implicated, the procedure is in some ways analogous to the stress-modified ductility-exhaustion approach [4]. Four different (strainrate dependent) techniques have been suggested, involving the alternatives of employing stress relaxation data, or if unavailable for the material, forward creep.…”

“…There are arguments based on metallurgical observations Assessing creep-fatigue lives by energy density exhaustion: R.P. Skelton for the less deleterious effects of a peak compressive dwell [4,39]. Similarly, this paper has avoided discussion of the case of intermediate dwells, i.e., where they occur at some midway point in the hysteresis loop.…”

“…All such damage methods make use of (i) easily and quickly determined continuouscycling fatigue endurances and (ii) existing creep data in some form. Time-fraction methods are now considered inaccurate and great improvements are achieved by using the ductilityexhaustion method or better still, the stress-modified ductility exhaustion method [1][2][3][4]. In this regard, the energy method may be considered similar in principle, since both stress and strain are employed [5][6][7][8][9] Modern energy methods as a form of damage summation descend from Miner's [10] original proposal, which took the 'net work absorbed at failure' Sw j /W fj as an alternative parameter to the now more familiar cycle quantity SN j //N fj where the subscript 'f' denotes a failure criterion, defined further in the Nomenclature, and the subscript 'j' here denotes a cycle of type j. Ostergren [11] applied the principle to laboratory creepfatigue studies while Charkaluk [12] employed the method on a cast iron for assessing the life of exhaust manifolds.…”

A New Ductility Criterion for Fatigue-Creep Life Prediction of High Temperature Components

“…Previous studies have shown that these static mechanical properties are useful in fatigue and the intrinsic ductility degrades during cyclic loading, which corresponds to the energy absorbed by the material [15][16][17][18] . Ductility has been applied in modeling steady-state creep behavior.…”

“…It is worth noting that this concept have been extended successfully in a modified form to predict isothermal and thermo-mechanical fatigue lives 18,23,24 . According to Goswami 32 , material toughness was assumed in terms of the product of saturated tension stress range and ductility by 33 .…”