Title: Microstructurally Short Cracks in Polycrystals Described by Crystal Plasticity
Publisher: Nova Science
Publication Year: 2010
JRC N°: JRC61785
ISBN: 978-1-61668-811-0
URI: www.novapublishers.com
Type: Books
Abstract: Microstructurally short cracks with lengths up to about ten grains are known to be strongly influenced by the microstructural features in the neighborhood of the crack tip. These include randomly shaped and oriented crystal grains and strongly orientation dependent deformation behavior of the grains. The goal of our work is to propose computational models aiming to quantify the effects of random grain orientations on the variability of crack tip opening and sliding displacements (CTOD, CTSD). A Voronoi tessellation based computational model has been developed to simulate the random grain structure. The constitutive behavior of individual grains includes randomly oriented anisotropic elasticity and crystal plasticity (where Schmid resolved stress is taken into account). The equilibrium equations are solved with macroscopic boundary conditions at the scale of the component using commercially available finite element solver ABAQUS. The stationary crack configurations studied include transgranular crack extending through about half of a crystal grain, a series of cracks of different sizes simulating the short crack approaching and crossing the first grain boundary and a series of cracks with different lengths extending from one to over a few grains. The FCC material with properties representing industry grade austenitic stainless steel is assumed with macroscopic uniaxial loading approaching macroscopic yield strength of the material. Sufficiently many simulations with different random grain orientations have been performed to arrive at approximate cumulative probability distributions of the CTOD and CTSD. Possible limits of the CTOD/CTSD variability, as for example those derived from a large monocrystal with variable lattice orientations with respect to the crack and from the linear elastic fracture mechanics, are given and discussed. Discussion includes identification of at least two main sources of the CTOD/CTSD variability: grain structure and strain localizations extending over the entire computational domain. Also, the attempt is made to quantify the decreasing influence of the grain structure with increasing crack length. The current computational model is limited to an essentially planar model (plane strain) with lattice rotations around the out of plane axis only. This basically allows for limiting most of the slip to two active (inplane) slip systems. The reason for this is the computational intensity of the simulations. A limited amount of simulations with spatial lattice 3D material orientations has also been performed to quantify the consequences of planar approximation. An outlook towards modeling of as-measured spatial microstructures and intergranular cracks is given.
JRC Directorate:Energy, Transport and Climate

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