Creep behaviour of Waspaloy under non-constant stress and temperature

Abstract

Current creep models are derived using data from constant stress (or load) creep tests and are capable of accurately predicting creep behaviour when applied conditions are constant or near constant. However, analyses of creep curve shapes for the nickel based superalloy Waspaloy, when applied stress and/or temperature vary greatly during testing, have shown that predictive methods based purely on strain, time or lifefraction are insufficient and cannot predict observed creep rates. This is important when considering stress concentration features where stress relaxation due to creep can significantly alter the distribution of stress and thus affect fatigue life. When both stress and temperature are changed during a creep test, dislocation movement must proceed through a dislocation network formed under different conditions, resulting in greater than expected creep rates. It is proposed that this is due to a reduction in effective internal stress due to changes in dislocation structure.

An analysis of small punch creep behaviour in the γ titanium aluminide Ti–45Al–2Mn–2Nb

“…A constitutive creep model based on the θ -projection technique was developed by Evans [21] and this has been incorporated into the commercially available solver Abaqus using a n CREEP subroutine. This method offers the benefit of relating creep rate to the accumulation of internal material state variables representing dislocation hardening (H), recovery (R) and internal creep damage (W) which has been show to predict creep behaviour better than strain or time based hardening methods for cases where stress evolves greatly over time [22]. Using this method, the instantaneous creep rate can be calculated based on the current state of the material characterised by H, R and W and the initial effective creep rate for the virgin material, _ ε 0 .…”

Small Punch Creep

“…The authors used such an approach to relate creep rate to the accumulation of internal material state variables, which has been previously shown in Ref. [47] to predict creep behaviour better than other strain or time based hardening approaches, for situations where a transient stress state is present, such as in a SPC test. In this research, an axisymmetric model was used to predict the material deformation in a SPC test on γTiAl 45-2-2 (XD) with model geometries replicating those that are used in a SPC experiment, with all features modelled as rigid bodies and surfaces.…”

Advanced Methods for Creep in Engineering Design

“…Transient uniaxial creep tests have been used to test the ability of different hardening methods to predict creep at non-constant stress and temperature for the nickel based superalloy, Waspaloy [8]. These creep tests applied temperature and stress which changed between two sets of conditions, one at high stress and low temperature, the other at low stress and high temperature, with both sets of conditions predicted to give similar lives approximately 10 days.…”

“…where ε F is the creep strain at failure and θ 1À3 are obtain using Eq. (8). W F must then be related to applied creep conditions.…”

“…As deformation continues, the creep rate increases to failure during tertiary creep, giving the creep curve the characteristic shape shown in Figure 1. The shape of a creep curve can vary significantly depending on material and applied conditions [8]. This can have a considerable influence when calculating the lives of engineering components, especially in components where the effects of creep deformation and damage interact with other phenomenon such as fatigue during high-temperature fatigue or thermomechanical fatigue (TMF) [9].…”