Abstract
In the present work, theoretical approaches, based on grain orientation dependent Young’s modulus and Schmid factor are used to describe the influence of local grain orientation on crack initiation behaviour of the coarse grained nickel base superalloy René80. Especially for strongly anisotropic crystal structures with large grain size, such as the investigated material, the local elastic properties must be taken into account for assessment of fatigue crack initiation. With an extension of Schmid’s law, the resulting shear stress amplitude, which triggers local cyclic plastic deformation, can be calculated depending on local Young`s modulus and Schmid factor. A Monte Carlo simulation with 100,000 samples shows how random grain orientation affects these parameters. Furthermore, the product of Young`s modulus and Schmid factor (called E·m) is used as a parameter to determine how grain orientation influences resulting shear stress amplitude for given total strain amplitude. In addition to the theoretical work using that approach, this model is also validated using isothermal LCF experiments by determining local grain orientation influence on the crack initiation site using SEM-EBSD analyses.
Highlights
Due to the increase in the share of regenerative energy in electricity production and the associated decrease in use of fossil sources, i.e. coal and natural gas, it is necessary to compensate the gap between current electricity demand and available renewables [1]
At a given integral normal stress, e.g. in a uniaxially loaded test piece, the local normal stress to be taken into account in Schmids law depends on local elastic grain stiffness, which again is a function of grain orientation
It could be shown that especially for elastic anisotropic materials such as nickel-base superalloys, that, besides the influence of the Schmid factor investigated in earlier studies [10, 21, 23], local elastic stiffness has an important impact on crack initiation
Summary
Due to the increase in the share of regenerative energy in electricity production and the associated decrease in use of fossil sources, i.e. coal and natural gas, it is necessary to compensate the gap between current electricity demand and available renewables [1]. Recent design approaches use probabilistic models for lifetime calculation with improved material models For this purpose, a significantly higher number of influencing parameters must be taken into account to describe fatigue crack initiation behaviour. Research on HCF of Ni-base superalloys with grain sizes in the μm range has already shown that both, Young’s modulus and Schmid factor including their interrelation via grain orientation must be considered to predict the crack initiating grains In this context, Pollock et al showed that crack initiation in René88DT, manufactured by powder metallurgy, is dependent on the occurrence of twins in addition to Schmid factor and Young’s modulus. For this material cracks often initiated in grains with slip systems oriented parallel to twin boundaries [11,12,13]
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