Creep and creep–fatigue crack growth

Abstract

Creep and creep-fatigue considerations are important in predicting the remaining life and safe inspection intervals as part of maintenance programs for components operating in harsh, high temperature environments. Time-dependent deformation associated with creep alters the crack tip stress fields established as part of initial loading which must be addressed in any viable theory to account for creep in the vicinity of crack tips. This paper presents a critical assessment of the current state-of-the-art of time-dependent fracture mechanics (TDFM) concepts, test techniques, and applications and describes these important developments that have occurred over the past three decades. It is concluded that while big advances have been made in TDFM, the capabilities to address some significant problems still remain unresolved. These include (a) elevated temperature crack growth in creep-brittle materials used in gas turbines but now also finding increasing use in advanced power-plant components (b) in predicting crack growth in weldments that inherently have cracks or crack-like defects in regions with microstructural gradients (c) in development of a better fundamental understanding of creep-fatigue-environment interactions, and (d) in prognostics of high temperature A. Saxena (B) College of Engineering, University of Arkansas, 130 A John A. White Hall, 790 W. Dickson Street, Fayetteville, AR 72701, USA e-mail: asaxena@uark.edu component reliability. It is also argued that while these problems were considered intractable a few years ago, the advances in technology do make it possible to systematically address them now and advance TDFM to its next level in addressing the more difficult but real engineering problems.

Atomistic aspects of fracture

“…Systematic, materialspecific studies of crack nucleation are therefore fundamental for the development of physics-based models of material failure of such structures. Mechanism-based models for crack nucleation are also very important for fatigue (Saxena 2015) in the very high cycle (VHCF) domain, where the fatigue life is almost completely governed by the crack nucleation and initial stages of growth (Mughrabi 2006) and crack nucleation at featureless sites in the interior of the material is often reported (Bach et al 2014).…”

Frequency and hold time influence on crack growth behavior of a 9–12% Cr ferritic martensitic steel at temperatures from 300 °C to 600 °C in air

“…The shape of the "hook" tends to decrease with decreasing temperature and increasing hold time. Saxena [20] found this "hook" in case of a Cr-Mo-V creep ductile steel at 538 °C with a hold time of 60 s at maximum load, Narasimhachary et. al.…”

“…The crack growth behavior of engineering alloys under creep-fatigue conditions has also been investigated in several studies [11][12][13][14][15][16][17][18][19]. The influence of maximum load holding times in the temperature range from 600 °C to 625 °C on fatigue crack propagation in 9 -12 % Cr steels was investigated in [15,16,18,20]. Contrary to that, only limited or no crack propagation data are available for X20CrMoV12-1 steel in the temperature range from 300 °C to 600 °C, which is most relevant for flexibly operated power plants.…”

Procedures for handling computationally heavy cyclic load cases with application to a disc alloy material

“…the repeated starts and stops for a stationary gas turbine balancing the power grid or the many take-offs and landings for an aircraft engine operating midrange distances. Under these circumstances a component such as a turbine disc will experience thermomechanical fatigue [19][20][21], low-cycle fatigue [22][23][24], highcycle fatigue [25], creep [26], mean stress relaxation [27], creep-fatigue crack growth [28], dwell crack growth [29] etc., and thus, an appropriate constitutive model needs to be utilised to account for the behaviour of the material. Focusing attention on disc alloys, as a non-linear hardening behaviour is generally adopted, see e.g.…”