PSI - Issue 14

Ashok Saxena/ Structural Integrity Procedia 00 (2018) 000 – 000

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Ashok Saxena / Procedia Structural Integrity 14 (2019) 774–781

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Considerations involved in predicting crack growth behavior of components that operate in extreme environments can be daunting because of the complexities of the materials and loading conditions involved and a plethora of damage mechanisms that can potentially be present such as creep deformation and damage, environment assisted cracking, and their synergistic play. These considerations are summarized as follows:  Transient and steady-state thermal stresses  Hold times and static and cyclic stresses due to external loading in fracture critical locations  Environmental effects  Creep deformation (primary, secondary, tertiary creep) and rupture  Creep-fatigue and environmental effects  Varying material properties due to temperature gradients  Varying material properties due to anisotropy or microstructural gradients such as in weldments  Complex crack geometries and variable amplitude loading  In-service degradation of material properties In this paper, progress in developing approaches for predicting crack growth in components that operate in these extreme environments is critically reviewed and existing gaps are identified. If creep deformation and damage dominate, such as in creep-ductile materials , it is shown that time-dependent fracture mechanics concepts are available to predict crack growth rates both under sustained and cyclic loading. Similarly, if environment effects dominate over creep, concepts of linear elastic and elastic-plastic fracture mechanics are available for predicting crack growth rates. In creep-brittle materials , both environmental effects and creep occur simultaneously, and several gaps exist in available models to address crack growth under such conditions. C * A path-independent integral used as a crack tip parameter under steady-state creep conditions C t A crack tip parameter used under transient creep conditions derived from the rate of creep zone expansion rate r radial coordinate distance from the origin located at the crack tip a crack size acceleration in the crack growth rates time rate of crack growth Norton’s power -law secondary creep exponent A Norton’s power -law secondary creep pre-exponent constant E Elastic modulus da/dN Crack growth during one cycle da/dt Time rate of crack growth Nomenclature K stress intensity parameter

2. Growing Crack Considerations in Creep-ductile and Creep-brittle Materials

The growing cracks perturb the stationery crack fields and if this perturbation occurs over a sizeable region near the crack tip compared to the region of dominance of the stationery crack tip fields, crack parameters such as K, C * and C t will be unable to uniquely correlate creep crack growth rates. The material characteristics themselves play a big role in determining which of the above parameters, if any, are appropriate. Thus, a classification of the different material types is in order. High temperature structural materials may be classified as creep-ductile or creep-brittle materials. Creep-ductile materials are ones in which the crack growth is accompanied by substantial amounts of time-dependent creep strains and the region of influence of growing crack fields is small in comparison to the creep zone size. Similarly, in the