Application of a Lower Order Strain Gradient Crystal Plasticity Model on Polycrystalline Deformation and Nanoscratching Analyses

Date

2024-7-24

Author

Günay, Enes

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Strain gradient crystal plasticity frameworks are theoretical and computational models used to describe the mechanical behavior of crystalline materials, particularly how they deform at different length scales. These frameworks incorporate the effects of strain gradients, which are spatial variations in strain, into the conventional crystal plasticity theory. This approach is essential for capturing the size-dependent behavior observed in materials at small scales, such as in microstructures and nanostructures. As the strain gradients in the material increase, geometrically necessary dislocations are to maintain the compatibility of the deformation, leading the material to harden further. Strain gradient theories are categorized as lower and higher order, where the former has a simpler mathematical formulation and boundary conditions, and the latter involves higher order stress terms, additional degrees of freedom in the model, and increased computational complexity. In this thesis, a lower order strain gradient crystal plasticity framework is implemented as a user material subroutine in the commercial finite element method solver software ABAQUS. The implementation steps are laid out in detail for multiple element types, outlining the challenges in the implementation. The strain gradient model implemented in this thesis is used to research size effects observed in engineering problems such as highly anisotropic additively manufactured materials, thin specimens with grain sizes comparable to their thicknesses, and nanoscratch tests that measure the wear, friction, and scratch hardness qualities of the material.

Subject Keywords

Crystal plasticity, Finite element method, Size effects, Nanoscratch testing, Polycrystalline materials

URI

https://hdl.handle.net/11511/110559

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Graduate School of Natural and Applied Sciences, Thesis