Jin Haeng Lee

Evaluation of Material Characteristics by Micro/Nano Indentation Tests

The present work reviews the methods to evaluate elastic-plastic material characteristics by indentation tests. Especially the representative stress and strain values used in some papers are critically analyzed. The values should not only represent the load-depth curve, but also represent the whole of deformed material around the impression. We briefly introduce other indentation techniques to evaluate residual stresses, creep properties, and fracture toughness. We also review some technical problems that are related to the accuracy issues in indentation tests.

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.

An Indentation Theory Based on FEA Solutions for Material Property Evaluation

A new numerical approach of indentation theory is proposed by examining the finite element solutions based on the incremental plasticity theory with large geometry change. First the optimal data acquisition location is selected, where the strain gradient is the least and the effect of friction is negligible. This data acquisition point increases the strain range by a factor of five. Numerical regressions of obtained data exhibit that strain hardening exponent and yield strain are the two main parameters which govern the subindenter deformation characteristics. The new indentation theory successfully provides the stress-strain curve with an average error less than 3%.Copyright

Spherical indentation cracking in brittle materials: An XFEM study

This work aims at characterizing the formation of cone-cracks in brittle materials upon spherical indentation. The cone-cracking is simulated by the extended finite element method (XFEM) in Abaqus / Standard. The element size-dependency is reduced by scaling the damage initiation strength based on mean stress criterion and calibration techniques. The formation of a kinked-cone-crack is observed when the indenter comes into (second) contact with the surface part outside the ring crack. After analyzing the effects of friction, Poissons ratio on cone-crack evolution, a database for enhanced Roeslers constant, which considers the effect of cone-crack-kinking, is provided by performing systematic XFE analyses. This database can be used for the fracture toughness evaluation in brittle materials.

Property Evaluation Method Using Spherical Indentation for High-Yield Strength Materials

In this paper, we propose a method to evaluate the material properties of high-yield strength materials exceeding 10GPa from spherical indentation. Using a regression equation considering four indentation variables, we map the load displacement relation into a stress-strain relation. To calculate the properties of high-strength materials, we then write a program that produces material properties using the loading / unloading data from the indentation test. The errors in material properties computed by the program are within 0.3, 0.8, and 6.4 for the elastic modulus, yield strength, and hardening coefficient, respectively. † Corresponding author, hylee@sogang.ac.kr C 2015 The Korean Society of Mechanical Engineers 최영식 · Karuppasamy Pandian Marimuthu · 이진행 · 이형일 1080 변수들을 재료물성의 함수로 표현했다. 이를 통해 금속재료에 대한 구형압입 하중-해중곡선으로부터 물성치를 얻는 프로그램을 개발했다. 많은 연구중 물성치의 정확성에 있어서 뛰어난 것으로 평가되고 있다. 역해석의 개념은 다음과 같다. 주어진 재료 물성 범위 내에서 물성변수(영률, 항복강도, 변형경화 지수 등)를 적절히 변화시키면서 압입시험을 모사 하는 유한요소해석을 수백회 실시한다. 이로부터 하중-변위 곡선의 데이터와 이에 상응하는 응력변형률을 일대일 대응시키는 함수들을 생성한다. 이 함수들에 기초한 반복계산 과정을 포함하는 프로그램을 만들면, 압입시험 데이터로부터 응력변형률 관계 및 물성치를 구할 수 있다 . 본 연구에서는 Lee 등의 연구를 확장해 일반 금속 재료뿐만 아니라 항복강도가 1GPa 이상되는 고강도 재료 및 세라믹과 같은 취성재료의 물성을 측정할 수 있는 구형압입 시험 물성평가기법을 개발 한다. 구형압입 시험법은 취성재료나 고강도 금속 재료처럼 시편가공 또는 균열 성장을 제어하기 어려운 경우, 인장시험을 대신해 재료의 영률, 항복강도, 변형 경화지수 등을 얻기 위한 물성평가에 용이하게 활용될 수 있음에도, 아직 이들 재료에 대한 연구는 미비한 실정이다. 따라서 이러한 고강도 재료의 기계적 특성들을 정확히 측정하기 위한 고정밀 비파괴 압입시험법을 개발할 필요가 있다. Lee 등이 제안한 압입 물성평가법에서 압입변수들은 압입자 직경으로 무차원화 되어 있어 마이크로 단위의 직경을 갖는 미소 압입자를 이용한 나노 압입시험으로부터 균열 없이 고강도, 취성소재의 물성을 평가할 수 있다. Lee 등 (1) 은 일반적인 금속재료에 대한 구형압입 시에는 압입자의 변형이 미소해 압입접촉직경 계산시 압입자를 강체로 가정했다. 그러나 고강도 재료를 압입하면 동일 압입깊이에서 상대적으로 큰 압입 하중이 필요해 압입자 형상변화가 상대적으로 크게 발생한다. 본 연구에서는 이런 중요한 변화사항을 고려해, 고강도 재료에 대한 개선된 압입변수를 제시 했다. 회귀특성을 분석하여 항복변형률의 구간별로 유효한 3 개의 프로그램을 작성한 후, 이를 하나의 프로그램으로 통합하는 기법을 사용한다. 작성된 프로그램을 압입 시험기에 적용하면 한번의 하중해중으로 광범위한 물성범위에 대한 재료의 물성치를 구할 수 있다.

A Numerical Approach to Spherical Indentation Techniques for Creep Property Evaluation

In this study, the theory of spherical indentation based on incremental plasticity is extended to an indentation method for evaluating creep properties. Through finite element analysis (FEA), the point where the elastic strain effect is negligible and the creep strain gradient constant is taken as the optimum point for obtaining the equivalent strain rate and stress. Based on FE results for spherical indentation with various values of creep exponent and creep coefficient, we derive by regression an equation to calculate creep properties using two normalized variables. Finally a program is generated to calculate creep exponent and creep coefficient. With this method, we obtain from the load-depth curve creep exponents with an average error of less than 1.5 % and creep coefficients with an average error of less than 1.0 %.

Spherical Indentation Techniques for Creep Property Evaluation Considering Transient Creep

Creep through nanoindentations has attracted increasing research attention in recent years. Many studies related to indentation creep tests, however, have simply focused on the characteristics of steady-state creep, and there exist wide discrepancies between the uniaxial test and the indentation test. In this study, we performed a computational simulation of spherical indentations, and we proposed a method for evaluating the creep properties considering transient creep. We investigated the material behavior with variation of creep properties and expressed it using regression equations for normalized variables. We finally developed a program to evaluate the creep properties considering transient creep. By using the proposed method, we successfully obtained creep exponents with an average error less than 1.1 and creep coefficients with an average error less than 2.3 from the load-depth curve.

Failure Assessment Diagrams of Semi-Elliptical Surface Crack with Constraint Effect

For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

An Indentation Method Based on FEA for Equi-Biaxial Residual Stress Evaluation

An indentation method to determine equi-biaxial residual stress is proposed by examining the data from the incremental plasticity theory based FE analyses. We found that hardness is strongly dependent of the magnitude and sign of residual stress and material properties. We then selected some normalized parameters minimally affected by material properties and tip radius. With numerical regressions of the data obtained, we proposed new formulae for residual stress evaluation. The new approach provides a substantial enhancement in accuracy compared with the prior methods.