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Rickhey, F.,Lee, J.H.,Lee, H. Elsevier Ltd 2016 Materials & Design Vol.107 No.-
<P>In sharp indentation of brittle materials, cracks form below the impression or at its corners and propagate further during unloading. The dimensions of radial-median cracks can be exploited to derive the fracture toughness. Knoop indentation has the merit that only one large crack is produced. This and the shallow plastic zone make it the preferred method for crack growth experiments, but which require information on the initial crack shape. For investigation of the complex indentation stress field, which is moreover perturbed by cracking, the extended finite element method (XFEM) is an ideal tool. Results show that the point load assumption holds for sufficient loading, which means that Knoop indentation can be used for fracture toughness evaluation. During loading the plastic zone evolves linearly with depth h, while crack depth c(z) is found to evolve according to h proportional to c(z)(3/4). For well-developed cracks, the crack aspect-ratio rho equivalent to c(z)/c (c is the length of the crack on the surface) is load independent. The XFE model is validated through comparison with experimental results,from the literature. Based on parameter studies, we establish functions that allow determination of fracture toughness and crack aspect-ratio. It is demonstrated that the mapping functions work well. (C) 2016 Elsevier Ltd. All rights reserved.</P>
Evaluation of the fracture toughness of brittle hardening materials by Vickers indentation
Rickhey, F.,Marimuthu, K.P.,Lee, J.H.,Lee, H.,Hahn, J.H. Pergamon Press 2015 Engineering fracture mechanics Vol.148 No.-
We propose a Vickers indentation cracking-based method for evaluating the fracture toughness of brittle materials exhibiting strain hardening. The approach is an extension of the recent method by Hyun et al. (2015) for non-hardening materials to hardening materials. The hardening material is simplified by an equivalent non-hardening material with an elevated modified yield strain so that the fracture toughness of a hardening material can be evaluated with the formula proposed by Hyun et al. The proposed extension is verified by comparison with experimental results from nanoindentation tests on Ge(100) and Si(100).
Evaluation of combined hardening coefficients of zircaloy-4 sheets by simple shear test
Rickhey, F.,Kim, M.,Lee, H.,Kim, N. BUTTERWORTH - HEINEMANN 2015 MATERIALS AND DESIGN Vol.65 No.-
Springback prediction in the simulation of multi-step stamping of zircaloy-4 metal sheets requires accurate understanding of the material behavior and in particular of the Bauschinger effect. In this study, issues concerning the one-sided simple shear test (specimen size, tightening torque and measurement of simple shear displacement) are discussed and suggestions regarding the test set-up are made. Forward/reverse simple shear tests are then performed with zircaloy-4 sheets to obtain shear load-displacement (P-δ) curves. In addition, applying the combined isotropic/kinematic hardening model the test is simulated in Abaqus/Explicit. Finally, the hardening coefficients for the combined isotropic/kinematic hardening model for zircaloy-4 sheets are determined by fitting the P-δ curve from finite element analysis (FEA) to the experimental curve. Applying the results to springback analysis will serve to improve the accuracy of multi-step stamping of zircaloy-4 sheets.
An efficient way of extracting creep properties from short-time spherical indentation tests
Rickhey, Felix,Lee, Jin Haeng,Lee, Hyungyil Cambridge University Press (Materials Research Soc 2015 Journal of materials research Vol.30 No.22
<▼1><B>Abstract</B><P/></▼1><▼2><P>Indentation as a means to extract creep properties has the advantage that it can be applied directly to micro/nano-structures. Many studies on indentation creep reported at least partially poor agreement with creep parameters derived from uniaxial test. One important reason for the incompatibility is the neglect of transient creep. Another one is the choice of equivalent stress and strain measures to relate the different material responses. Applying a material model that accounts for transient creep effects we propose an efficient method for deriving creep properties from short-time spherical indentation tests. We first determine a subsurface point where the material response is very close to that observed in uniaxial tests. We then map the load-displacement data to the material response, expressed in terms of two dimensionless variables, at this point. Converting the dimensionless variables data to stress, strain, and strain rate data, we finally determine the material's creep coefficient and exponent.</P></▼2>
Rickhey, F.,Lee, J.H.,Lee, H. Scientific and Technical Press ; Elsevier Science 2015 Materials & Design Vol.84 No.-
The presence of residual stresses (RS) in a material causes a shift of the indentation load-displacement curve. The resulting change in the Kick's law coefficient C can hence be exploited to estimate these RS. By contrast with axisymmetric indenters, when employing the Knoop indenter with its large aspect-ratio, C is further sensitive to the indenter's orientation with respect to the principal RS directions. For a wide range of material properties, maximum and minimum C values are obtained by finite element analysis. It is observed that the RS ratio can be estimated directly from the C ratio, independent of magnitude and sign of RS. Further, the finding that C values for the non-equibiaxial RS case can be converted to equivalent C values for two equibiaxial RS cases, made for conical indentation (J.H. Lee et al., 2010, J Mater Res 25: 2212-2223), is shown to apply to Knoop indentation, too. The magnitude of RS can thus be determined from the equibiaxial RS case. The equibiaxial RS case is investigated in detail and mapping functions are established between C and the corresponding RS value. Finally the method is validated experimentally by comparison with Knoop indentation of bended cross-shaped steel specimens.
Indentation cracking of monocrystalline silicon considering fracture anisotropy
Rickhey, Felix,Marimuthu, Karuppasamy Pandian,Lee, Kwangmin,Lee, Hyungyil Elsevier 2019 Theoretical and applied fracture mechanics Vol.100 No.-
<P><B>Abstract</B></P> <P>Despite its inherent anisotropy, in indentation cracking monocrystalline silicon has often been treated as an isotropic material. An attempt at accounting for its anisotropy is made here. We propose a (simplified) material model that allows the simulation of anisotropic cracking in Vickers indentation of monocrystalline silicon. Anisotropy is confined to the elastic domain; the complex plastic deformation is considered through an effective yield strength, which irreversible deformations are merged into. Numerically obtained hardness values agree well with literature values. The well-known Lawn-Evans-Marshall equation is modified to account for the anisotropy of cubic materials. The variation of the crack size for diverse orientations on Si(0 0 1), Si(1 1 0) and Si(1 1 1) is analyzed. Numerical results are found to be in a generally good agreement with the experimental results by Ebrahimi and Kalwani (Mater Sci Eng A 1999;268:116–26), which shows that the simplifications regarding the material model are appropriate. The model can be applied also to other cubic materials such as germanium.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A simplified material model that allows investigation of fracture anisotropy is proposed. </LI> <LI> Lawn-Evans-Marshall equation is modified to account for the anisotropy of cubic materials. </LI> <LI> Material anisotropy must be considered when determining the residual field intensity. </LI> <LI> Numerically obtained crack lengths agree well with experimental data. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
XFEM simulation of radial-median crack formation in brittle medium underneath sharp indenters
Felix Rickhey,Minsoo Kim(김민수),Hyungyil Lee(이형일) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
The evolution of radial-median cracks occurring in sharp indentation of brittle materials is simulated using the extended finite element method (XFEM). Issues associated with XFEM are discussed for the particular problem. For the evaluation of fracture toughness, hardness is considered as dependent of indenter angle and Poisson’s ratio. Parameter studies are then carried out to assess the influence of indentation test, indenter and material parameters on fracture toughness. The results can be used to improve fracture toughness evaluation by indentation.