Fundamental modelling studies of fatigue crack nucleation & microstructurally short crack growth in superalloy

• Main Author: Wan, Victor
• Other Authors: Dunne, Fionn ; Dye, David
• Published: Imperial College London 2016
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.754657

Current lifing techniques for metal superalloys have largely remained empirical, particularly those addressing the nucleation or initiation of fatigue cracks. However, there is growing interest within both the academic literature and industrial users to move away from the approaches which are dependent on empirical models. Instead, there has been greater focus on the advancement of developing fundamental mechanistically-based models to predict alloy behaviour in order to help better understand and predict defect nucleation processes. Early lifing methods in face centred cubic (fcc) and body centred cubic (bcc) polycrystalline alloys are of interest due to the relevance of cubic alloys in industrial applications, where microstructural fatigue behaviour to crack nucleation and short crack growth are studied. A stored energy criterion for fatigue crack nucleation is introduced which is validated with ferritic steel polycrystal specimens to reveal new evidence and address the scattered cycles to crack nucleation. The criterion is extended into a 3D crystal plasticity finite element model (CPFEM) representation of RS5 Nickel superalloy where the method provided a new perspective to quantify microstructurally sensitive cycles to fatigue crack nucleation in RS5 alloys. Comparisons of experimental and CPFEM investigation of microstructural stress distributions is presented across a polycrystalline copper where fatigue hotspots identified provided new insight of common nucleation hotspots, and is typically associated on/near grain boundaries. The work is extended using FEM to address Microstructural Sensitive Short Crack Growth (MSCG) in microstructurally different ferritic notched specimens. Our assessments and methodology introduced based on Extended Finite Element Method (XFEM) revealed information on the role of anisotropy to better capture the MSCG paths. In addition, user-defined materials and grain boundary properties were introduced to address the difficulties in capturing intergranular cracks where grain boundary properties were introduced to promote the grain boundary cracks as witnessed in experiments.