Hyeok Jae Jeong

Three-Dimensional Characterization of SiC Particle-Reinforced Al Composites Using Serial Sectioning Tomography and Thermo-mechanical Finite Element Simulation

In this study, a serial sectioning technique was employed in order to visualize the three-dimensional (3D) structure, and to accurately simulate the mechanical and thermal behaviors of SiC particle-reinforced Al composites. Sequential, two-dimensional (2D) optical images of the microstructure were acquired after polishing, and then reconstructed to develop 3D geometries for microstructural analyses and finite element modeling. Experimental compressive and thermal expansion tests were performed for comparison with the finite element method results. The Young’s modulus and thermal expansion coefficient of the composite, predicted using the 3D microstructure-based finite element analyses, were in good agreement with the experimental results. Furthermore, the 3D microstructure-based finite element model showed anisotropic thermal expansion behavior that was previously disregarded in the other models used in this study. Therefore, it was confirmed that the combined approach of serial sectioning and finite element modeling provides a significant improvement over 2D and 3D unit-cell modeling.

Nanoindentation analysis for local properties of ultrafine grained copper processed by high pressure torsion

Severe plastic deformation (SPD) techniques have recently been developed for producing bulk ultrafine grained metallic materials. High pressure torsion (HPT) produces finer microstructures than those achieved by other SPD processes because of the higher imposed strain and hydrostatic pressure. It is known that HPT-processed metals show a highly heterogeneous microstructure not only along the radius due to the nature of torsional deformation but also through the thickness. Since the sample size for HPT is small, the local properties of HPT-processed specimens have not been investigated yet. In this paper, we propose a method to obtain stress–strain curves from nanoindenting curves by combining the finite element method and the recursion method. The nanoindentation technique was employed to elucidate the local mechanical properties, especially the stress–strain behavior. The method to extract the stress–strain curves from the load–displacement curves obtained by nanoindentation tests was applied to the edge region of the HPT-processed sample. The extracted properties correlated well with experimental results qualitatively.

Thickness inhomogeneity in hardness and microstructure of copper after the compressive stage in high-pressure torsion

Distinction between plastic deformation occuring in compression and compression-torsion stages is important for understanding the properties and microstructures of materials processed by high-pressure torsion (HPT). In the present study, remarkable through-thickness inhomogeneities of hardness and microstructure were found in the samples processed by compression stage of HPT. Three regions on the radial direction plane of compressed disks were defined to display the inhomogeneity: edge zone (high hardness), radial medium zone (uniform hardness) and center zone (low hardness near the surface and high hardness in the thickness central plane). A low hardness region in the center near the surface was detected and found to stretch along the upper and bottom surfaces of the disks compressed by low pressure. This low hardness region was also found to decrease with increasing the pressure. Not only the hardness but also the microstructure through-thickness inhomogeneity is attributed to stress and strain distribution in the disk as well as to a huge friction between the anvil and the disk during processing.

Local and Global Stress–Strain Behaviors of Transformation-Induced Plasticity Steel Using the Combined Nanoindentation and Finite Element Analysis Method

Transformation-induced plasticity (TRIP) steels have excellent strain hardening exponents and resistibility against tensile necking using the strain-induced martensite formation that occurs as a result of the plastic deformation and strain on the retained austenite phase. Detailed studies on the microstructures and local mechanical properties, as well as global mechanical properties, are necessary in order to thoroughly understand the properties of TRIP steels with multiple phases of ferrite, bainite, retained austenite, and martensite. However, methods for investigating the local properties of the various phases of the TRIP steel are limited due to the very complicated and fine microstructures present in TRIP steel. In this study, the experimental and numerical methods, i.e., the experimental nanoindenting results and the theoretical finite element analyses, were combined in order to extract the local stress–strain curves of each phase. The local stress–strain curves were in good agreement with the values presented in the literature. In particular, the global plastic stress–strain behavior of the TRIP steel was predicted using the multiple phase unit cell finite element analysis, and this demonstrated the validity of the obtained properties of each local phase. The method of extracting the local stress–strain curves from the nanoindenting curves and predicting the global stress–strain behavior assists in clarifying the smart design of multi-phase steels.

Obtaining Mechanical Properties of Fe Powder Using a Combined Nanoindentation and the Finite Element Method

Abstract Stress-strain curves are fundamental properties to study characteristics of materials. Flow stress curves ofthe powder materials are obtained by indirect testing methods, such as tensile test with the bulk materials and powdercompaction test, because it is hard to measure the stress-strain curves of the powder materials using conventional uniax-ial tensile test due to the limitation of the size and shape of the specimen. Instrumented nanoindentation can measuremechanical properties of very small region from several nanometers to several micrometers, so nanoindentation tech-nique is suitable to obtain the stress-strain curve of the powder materials. In this study, a novel technique to obtain thestress-strain curves using the combination of instrumented nanoindentation and finite element method was introducedand the flow stress curves of Fe powder were measured. Then obtained stress-strain curves were verified by the com-parison of the experimental results and the FEA results for powder compaction test.Keywords : Fe powder, Finite element method, Nanoindentation