Hong Chul Hyun

A Conical Indentation Technique Based on FEA Solutions for Property Evaluation

The sharp indenters such as Berkovich and conical indenters have a geometrical self-similarity in theory, but different materials have the same load-depth curve in case of single indentation. In this study, we analyze the load-depth curves of conical indenter with angles of indenter via finite element method. From FE analyses of dual-conical indentation test, we investigate the relationships between indentation parameters and load-deflection curves. With numerical regressions of obtained data, we finally propose indentation formulae for material properties evaluation. The proposed approach provides stress-strain curve and the values of elastic modulus, yield strength and strain-hardening exponent with an average error of less than 2%. It is also discussed that the method is valid for any elastically deforming indenters made of tungsten carbide and diamond for instance. The proposed indentation approach provides a substantial enhancement in accuracy compared with the prior methods.

Analysis of Cracking Characteristics with Indenter Geometry Using Cohesive Zone Model

In this study, we investigated the effect of the indenter geometry on the crack characteristics by indentation cracking test and FEA. We conducted various cohesive finite element simulations based on the findings of Lee et al. (2012), who examined the effect of cohesive model parameters on crack size and formulated conditions for crack initiation and propagation. First, we verified the FE model through comparisons with experimental results that were obtained from Berkovich and Vickers indentations. We observed whether nonsymmetrical cracks formed beneath the surface during Berkovich indentation via FEA. Finally, we examined the relation between the crack size and the number of cracks. Based on this relation and the effect of the indenter angle on the crack size, we can predict from the crack size obtained with an indenter of one shape (such as Berkovich or Vickers) the crack size for an indenter of different shape.

Evaluation of Indentation Fracture Toughens in Brittle Materials Based on FEA Solutions

The purpose of the present invention is to provide a method for evaluating the indentation fracture toughness of brittle materials based on finite element analysis (FEA) solutions, which is capable of evaluating the indentation fracture toughness of brittle materials based on indentation crack characteristics observed with a crack analysis applied with a cohesive zone model (CZM). To achieve this, the method for evaluating the indentation fracture toughness of brittle materials based on FEA solutions comprises the steps of: presenting standards defining the ″well-developed″ crack of a material; selecting strain e_o, a Poisson′s ratio ν, and a Young′s modulus E absolute value as indentation variables affecting the size of a crack in the material, and presenting an indentation fracture toughness formula using the regression relationship among the e_o, the v and, E_R (=E/E_1000) and K_o (=P_max/c^3/2); excluding the influence of the Young′s modulus using an indentation contact length a of the material, and presenting a fracture toughness formula using the relationship among the e_o, the v and k(=P_max/c^ia^1.5-i); and presenting a regression function using E/H, a ratio of the Young′s modulus E to hardness H, and the Poisson′s ratio ν in order to obtain the e_o.

Forming Limit Diagrams of Zircaloy‐4 and Zirlo Sheets for Stamping Process of Spacer Grids of Nuclear Fuel Rod

In this work, we investigated the theoretical forming limit models for Zircaloy-4 and Zirlo used for spacer grid of nuclear fuel rods. Tensile and anisotropy tests were performed to obtain stress-strain curves and anisotropic coefficients. The experimental forming limit diagrams (FLD) for two materials were obtained by dome stretching tests following NUMISHEET 96. Theoretical FLD depends on FL models and yield criteria. To obtain the right hand side (RHS) of FLD, we applied the FL models (Swifts diffuse necking, M-K theory, S-R vertex theory) to Zircaloy-4 and Zirlo sheets. Hills local necking theory was adopted for the left hand side (LHS) of FLD. To consider the anisotropy of sheets, the yield criteria of Hill and Hosford were applied. Comparing the predicted curves with the experimental data, we found that the RHS of FLD for Zircaloy-4 can be described by the Swift model (with the Hills criterion), while the LHS of the FLD can be explained by Hill model. The FLD for Zirlo can be explained by the S-R model and the Hosfords criterion (a = 8).