Abstract
High-temperature components in power plants may fail due to creep and fatigue. Creep damage is usually accompanied by the nucleation, growth, and coalescence of grain boundary cavities, while fatigue damage is caused by excessive accumulated plastic deformation due to the local stress concentration. This paper proposes a multiscale numerical framework combining the crystal plastic frame with the meso-damage mechanisms. Not only can it better describe the deformation mechanism dominated by creep from a microscopic viewpoint, but also reflects the local damage of materials caused by irreversible microstructure changes in the process of creep-fatigue deformation to some extent. In this paper, the creep-fatigue crack initiation analysis of a modified 12%Cr steel (X12CrMoWvNBN10-1-1) is carried out for a given notch specimen. It is found that creep cracks usually initiate at the triple grain boundary junctions or at the grain boundaries approximately perpendicular to the loading direction, while fatigue cracks always initiate from the notch surface where stress is concentrated. In addition to this, the crack initiation life can be quantitatively described, which is affected by the average grain size, initial notch size, stress range and holding time.
Highlights
The increasing industrial demand of efficiency puts forward higher requirements for the service conditions of process equipment [1]
To investigate the influence of the initial notch size on creep and fatigue crack initiations, the microstructures of three different initial notch sizes are modeled in the section
The creep crack initiation is predicted by the cavity coalescence damage model at grain boundary, while the fatigue crack initiation is predicted by accumulated equivalent plastic strain both at grain and grain boundary
Summary
The increasing industrial demand of efficiency puts forward higher requirements for the service conditions of process equipment [1]. McDowell and Dunne [23] explored the sensitivity of fatigue crack initiation to microstructure using a crystal plasticity finite element model. Tang et al [25] developed a crystal plasticity model to simulate geometrically necessary dislocations (GNDs) in a titanium alloy under high-cycle fatigue loading. Few studies have implemented both creep and fatigue meso-damage mechanisms into crystal plasticity modeling, nor have they distinguished contributions of the two physical mechanisms to crack initiation at a microstructural level. Based on the crystal plasticity theory, this paper introduces a creep meso-damage mechanism with nucleation, growth and coalescence of grain boundary cavities under the framework of the rate-dependent cyclic constitutive model and combines the equivalent cumulative plastic strain to predict fatigue crack initiation. The finite element implementation of the model in ABAQUS is realized by developing a UMAT subroutine that calculates damages in grains and at grain boundaries
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