Evaluation of creep damage accumulation models: Considerations of stepped testing and highly stressed volume

Abstract

A B S T R A C T Many components experience combined temperature and stress loading and are designed to withstand creep. In this study, experimental creep testing was performed under both static and stepped loading conditions with constant temperature for two specimen geometries (tensile and three-point bend). The objective of this study was to evaluate whether existing damage accumulation models accurately predict creep performance when considering step loading and stress gradients. Model predictions, based on static tensile creep data and using a highly stressed volume correction for the three-point bend specimens and the experimental average damage sum, agreed well with experimental data; differences were on average within 38% (static) and 2.2 h (stepped). Comparisons showed more accurate predictions using an exponential Larson-Miller parameter curve and the Pavlou damage accumulation model. Findings of the current study have applicability to component design, where complex geometries often contain stress gradients and it is desirable to predict creep performance from static tensile creep data. N O M E N C L A T U R E a, B = Material constants in stress versus highly stressed volume relation C = Empirical constant used in Larson-Miller parameter D = Damage D * = Damage level which indicates failure E = Young's modulus K c = Creep stress state factor K t = Elastic stress concentration factor LMP = Larson-Miller parameter n = number of tests p = Fitting parameter used in Pavlou analysis R 2 = Coefficient of determination S y = Yield strength (0.2%) S u = Ultimate (tensile) strength t i = Time at stress-temperature combination i t f = Time to creep failure, rupture time T = Temperature T m = Melting temperature V = Highly stressed volume (≥ 95% of max. stress) λ = Exponent in linear damage summation lambda-model

An Explicit Creep-Fatigue Model for Engineering Design Purposes

“…Existing methods based on summation of fatigue damage and creep damage ignore the interaction between fatigue and creep, and thus result in less reliable findings. Although the improved representations of creep and fatigue components have been proposed in the literature, such as the non-linear accumulated damage models for creep [7,8] and fatigue [9,10], the issue caused by ignoring the interacted effect of fatigue and creep is still not fundamentally solved.…”

Evaluation of fatigue damage parameters for Ni‐based super alloys Inconel 825 steel notched specimen using stochastic approach

“…In the past few decades, numerous experimental and theoretical studies on creep damage under constant load have been carried out, and many models have been proposed. [30][31][32][33] Hu et al 34 have outlined few of such damage accumulation rules and presented a new creep damage model for metallic material under variable load condition at elevated temperature based on creep damage tolerance parameters. The life of Ni-based super alloy components is affected by the stress and strain fields resulting from thermomechanical loading, the crystal orientation, and the evolution of their microstructure.…”

A new creep damage assessment method for metallic material under variable load conditions at elevated temperature

“…Since the Larson-Miller parameter can be well correlated with creep rupture data at different temperatures and stresses, the prediction results of the Pavlou method are in good agreement with the experimental results to some extent. 24,25 However, the dispersion of material life under the same load conditions is not considered in the above method. [23][24][25] More importantly, the creep strain, which is the most direct reflection of creep damage, is not considered within the method.…”

“…24,25 However, the dispersion of material life under the same load conditions is not considered in the above method. [23][24][25] More importantly, the creep strain, which is the most direct reflection of creep damage, is not considered within the method. Therefore, in 2009, Batsoulas 26 proposed a creep damage assessment method that not only considered the damage caused by the loading sequence but also used creep strain as the most important parameter for evaluating creep damage.…”