• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1453-1463
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• 2013

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.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.1
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• pp.17-28
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• 2012

In this study, we suggest a method for material property evaluation by dual-triangular pyramidal indenters using the reverse analysis. First, we demonstrated that load-displacement curves of conical and triangular pyramidal indenters are different for the same material. For this reason, an independent research on the triangular pyramidal indenter is needed. From FE indentation analyses on various materials, we then investigated the relationships among material properties, indentation parameters and load-displacement curves. From this, we established property evaluation formula using dual-triangular pyramidal indenters having two different half-included-angles. The approach provides the values of elastic modulus, yield strength and strain-hardening exponent within an average error of 3% for various materials.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.2
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• pp.129-137
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• 2013

A force calibration of a nanoindenter and a 3D morphology observation of indenters were carried out in this study. A microbalance calibrated with standard weights was used for measuring the loads generated by a nanoindenter. The indentation load could be calibrated from the ratio of measured and generated loads and the first contact load also could be detected from the microbalance data. By analyzing atomic force microscopy images of two indenters, curvature radii of apexes were determined by $19.71{\pm}3.03$ and $1043.94{\pm}50.91$ nm, respectively, for the nearly new indenter A and the severly worn indenter B. Corresponding bluntness depths were estimated by 1.22 and 64.56 nm for the both indenters by overlapping their profiles on the perfect pyramidal shape. In addition, nanoindentation curves obtained from a fused silica reference material with the both indenters showed a depth difference corresponding to the bluntness depth difference along the indentation depth axis. By shifting amounts of the bluntness depths along the horizontal axis, whole nanoindentation curves overlapped on themselves and resulted in nanohardness values consistent within 1.11 % without considering the complex indenter area function of each indenter.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.9
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• pp.859-869
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• 2009

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.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.9
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• pp.943-951
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• 2014

The tip geometries of the pyramidal and conical indenters used for micro/nano-indentation tests are not sharp. They are inevitably rounded because of their manufacturability and wear. In many indentation studies, the tip geometries of the pyramidal indenters are simply assumed to be spherical, and the theoretical solution for spherical indentation is simply applied to the geometry at a shallow indentation depth. This assumption, however, has two problems. First, the accuracy of the theoretical solution depends on the material properties and indenter shape. Second, the actual shapes of pyramidal indenter tips are not perfectly spherical. Hence, we consider the effects of these two problems on indentation tests via finite element analysis. We first show the relationship between the Poisson's ratio and load-displacement curve for spherical indentation, and suggest improved solutions. Then, using a possible geometry for a Berkovich indenter tip, we analyze the characteristics of the load-displacement curve with respect to the indentation depth.

• Journal of the Microelectronics and Packaging Society
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• v.26 no.4
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• pp.95-100
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• 2019

Nanoindentation has been widely used for evaluating mechanical properties of nano-devices, from MEMS to packaging modules. Residual stress is also estimated from indentation tests, especially the Knoop indenter which is used for the determination of residual stress directionality. According to previous researches, the ratio of the two stress conversion factors of Knoop indentation is a constant at approximately 0.34. However, the ratio is supported by insufficient quantitative analyses, and only a few experimental results with indentation depth variation. Hence, a barrier for in-field application exists. In this research, the ratio of two conversion factors with variation in indentation depth using finite elements method has been attempted at. The magnitudes of each conversion factors were computed at uniaxial stress state from the modelled theoretical Knoop indenter and specimen. A model to estimate two stress conversion factor of the long and short axis of Knoop indenter at various indentation depths is proposed and analyzed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1503-1512
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• 2013

In this study, we proposed an indentation evaluation method for fracture toughness using cohesive finite element simulations. First, we examined the effect of material properties (yield strain, Poisson's ratio, and elastic modulus) on crack size during Vickers indentation and then generated a regression formula that explains the relations among fracture toughness, indentation load, and crack size. We also proposed another indentation formula for fracture toughness evaluation using the contact size a and E/H (H: hardness). Finally, we examined the relation between the crack size and the indenter shapes. Based on this, we can generate from the formula obtained using the Vickers indenter a formula for an indenter of different shapes. Using the proposed method, fracture toughness is directly estimated from indentation data.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.1
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• pp.81-90
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• 2012

The indentation test, which is one of the testing methods for evaluating the mechanical properties of materials, can be applied to the evaluation of creep properties. Many studies related to the indentation creep test, however, have just focused on the characteristics of the steady-state creep, so there are wide discrepancies between the uniaxial test and the indentation test. To obtain accurate creep properties, it is therefore important to consider the effects of transient creep. In the present work, the Ogbonna et al.'s work on the spherical indentation test including the transient creep was expanded and applied to the conical indentation creep test. The characteristics of the transient creep were analyzed via finite element simulations and compared with those obtained through spherical indentation. Other effects, such as elastic strain, indenter shape, contact area, and representative strain, which have not been considered properly in prior studies on the creep test, are also discussed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.1
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• pp.35-42
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• 2008

Instrumented sharp indentation test is a well-directed method to measure hardness and elastic modulus. The sharp indenter such as Berkovich and conical indenters have a geometrical self-similarity in theory, but the self-similarity ceases to work in practice due to inevitable indenter tip-blunting. In this study we analyzed the load-depth curves of conical indenter with finite tip-radius via finite element method. Using the numerical regression data obtained from Kick's law, we first confirmed that loading curvature is significantly affected by tip radius as well as material properties. We then established a new method to evaluate Young's modulus, which successfully provides the value of elastic modulus with an average error of less than 2%, regardless of tip-radius and material properties of both indenter and specimen.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.6
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• pp.464-469
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• 2017

This study evaluated the displacements of a Cu bar in the Y-direction and the relationship between swaging pressures and total contact forces to increase the productivity of the rotor core swaging process. Elastic-plastic numerical analyses of four different Cu bar shapes were performed with a constant swaging pressure to evaluate the displacements of the Cu bar in the Y-direction and the contact force distributions at the contact surfaces during the swaging process. Based on the numerical analysis results, the following conclusions were obtained. First, a simplified 2-dimensional cyclic symmetric analysis model was developed for the numerical analysis of the rotor core swaging process. Second, the final displacements of the Cu bar in the Y-direction were nearly the same as the change of the Cu bar size at a constant swaging pressure. Third, a linear relationship between the swaging pressures and the total contact forces, the so called resistance forces, was suggested.