Low cycle thermo‐mechanical fatigue: damage operator approach

Abstract

A B S T R A C TThe strain-life approach is standardized and widely accepted for determining fatigue damage under strain-controlled low cycle fatigue (LCF) loading. It was first extended to non-isothermal cases by introducing an equivalent temperature approach (ETA). The paper presents its extension that is the damage operator approach (DOA) enabling online continuous damage calculation for isothermal and non-isothermal loading with mean stress correction. The cycle closure point, cycle equivalent temperature, threshold temperature and separate rainflow counting obligatory for the ETA are not necessary for the DOA any more. Both approaches are equivalent for the second and subsequent runs of block loading if temperature is constant. However, for non-isothermal cases, the DOA is within the worst and the best case scenarios of the ETA. The approaches are compared to the simple stress histories and several thermo-mechanical fatigue (TMF) cycle types. = cyclic stress-strain curve i = data point index j = fictive yield stress index, fictive yield damage parameter index k = temperature index K (T) = cyclic hardening coefficient K P = limit load ratio n p = index of the top fictive yield damage parameter n r = index of the top fictive yield stress n T = index of the top temperature n (T) = cyclic hardening exponent N f = number of cycles to failure p j = fictive yield damage parameter P , P (t i ) = damage parameter P a (t i ) = damage parameter amplitude P o (t i ) = damage parameter origin r j = fictive yield stress S * = nominal stress Correspondence: M. Nagode.

A New Ductility Exhaustion Model for High Temperature Low Cycle Fatigue Life Prediction of Turbine Disk Alloys

“…The methods reported in [15][16][17] are limited to evaluation of HTLCF life under strain controlled conditions. In addition, many reported methods require specific types of tests to estimate the corresponding parameters or have too many "to be determined parameters or constants", thus restricting their usefulness, such as LDS, FS, SRP, SEP, DRA, GSEDF [19], models reported in [16] and DOA [20]. In summary, further improvement of HTLCF life prediction models is needed.…”

“…Combining (20) with the cyclic stress-strain equation by substituting (12) into (20) results in the following equation:…”

“…Over the past several decades, the issue of accurately predicting HTLCF life of these hot section components has been an area of interest. Many life prediction methods have been reported [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. However because of the complex damaging mechanisms involved, a unified model that can provide accurate life prediction for HTLCF does not exist [1].…”

“…To improve the accuracy of HTLCF life prediction, researchers have presented several models [14][15][16][17][18][19][20]. For example, considering the dynamic viscosity as the damage parameter to describe the flow behavior, Goswami [14] proposed the Ductility Exhaustion (DE) approach for HTLCF life prediction which is based on extensive research of CrMo steels.…”

“…This model has wider applications and is more precise to predict the life of fatigue-creep interaction. Nagode et al [20] proposed a Damage Operator Approach (DOA) based on temperature-dependent cyclic damage calculation under low cycle thermo-mechanical fatigue. This approach focuses on fatigue damage while ignoring the effects of creep, viscoplasticity, or the interaction between them.…”