Investigation of the relationship between the effective stiffness of three-dimensional material samples and thin layers
Materials in Sintering Processes and Generative Manufacturing Processes
Mechanical Properties of Metallic Materials and their Microstructural Origins
Final Report Abstract
This research project discussed the reduction in stiffness that occurs when materials with microstructures are subjected to tensile testing as either whole body or sliced samples of the body are subjected to a tensile test, and have given two practical examples. The resulting lower modulus of elasticity of the sliced specimen is the result of a lack of scale separation along the slice normal and a restriction to the plane stress state. The direction perpendicular to the slice is not available for the load distribution. Analyzing the phenomenon with synthetic data, we were able to determine the contributions to the reduction in stiffness. The first contribution is the free lateral strain in tensile tests in disks, which is more significant the greater the difference between the Poisson’s ratios of the phases. For the second effect, we used a relaxation scheme that mimics free contraction by equalizing the Poisson’s ratios. The latter approach can be applied to any 3D estimate that can be converted in terms of the Poisson ratios of the phases. The second effect, namely the topological changes when cutting the thin slices, is more difficult to take into account. A 3D microstructure with interpenetrating phases shows properties of a matrix inclusion structure when slices are considered. This leads to a significant change in the load path from 3D to 2D case. We believe that we have only scratched the surface of this interesting and apparently not very well understood phenomenon and that it deserves more attention. For example, grain structures have not been considered here. Possible benefits of further investigations would be: more accurate material characterizations when structural gradients are present; representative surface elements with less numerical effort, and the technological use of the potentially drastic reduction in stiffness at the transition from full-body to thin structures.
