N. Chollacoop - Academia.edu (original) (raw)

Papers by N. Chollacoop

Scripta Materialia, 2005

Experimental assessment of the reverse algorithms that enable the extraction of plastic propertie... more Experimental assessment of the reverse algorithms that enable the extraction of plastic properties from the load-depth of penetration curves was conducted. Results show that they predict the stresses at 3.3% and 5.7% representative strains for Berkovich and 60°cone-equivalent three-sided pyramidal indenters, respectively, with good accuracy. It was shown that the uniaxial stress-strain curves could be reconstructed from the indentation data.

Materials Science and Engineering: A, 2006

Algorithms for extracting mechanical properties from instrumented sharp indentation results (P-h ... more Algorithms for extracting mechanical properties from instrumented sharp indentation results (P-h curves) were recently developed on the basis of extensive finite element simulations coupled with the identification of a representative strain underneath the indenter and construction of characteristic dimensionless functions. The representative strain varies with the cone angle and a critical experimental verification of this concept was performed recently. In this paper, we ascertain the robustness of the algorithms, by subjecting the load versus depth of penetration curves (generated using finite element simulations on solids that were subjected to different levels of prior-plastic strains) to the reverse algorithms and comparing the extracted flow stress and the work-hardening exponent to the uniaxial data generated experimentally. A good match between the experiments and simulations validates the key assumptions made during the algorithm development.

Applied Energy, 2013

h i g h l i g h t s " Energy demand modeling in Thai road transportation sector was developed. " ... more h i g h l i g h t s " Energy demand modeling in Thai road transportation sector was developed. " Such model was used to assess environment impact by ethanol bus technology (ED95). " Ethanol bus technology (ED95) shows beneficial impacts to Thailand. " Increase in ethanol demand and decrease in GHG emission in Thailand by ethanol bus. " Ethanol bus (ED95) has been successfully demonstrated in Thailand.

The 2007 NSTDA Annual Conference (NAC2007), Mar 1, 2007

Acta Materialia, 2004

The compression behavior of a two-phase (Mo solid solution + T2) Mo-6.1Si-7.9B (at.%) alloy is ex... more The compression behavior of a two-phase (Mo solid solution + T2) Mo-6.1Si-7.9B (at.%) alloy is examined as a function of strain rate in the 1000-1400°C range and compared to that of commercially available powder-metallurgy-processed TZM (often referred to as MT104). A limited number of tests was also conducted on a three phase alloy with a composition of Mo-8.6Si-8.7B (at.%) that lies in the three phase field, Mo solid solution + T2 + Mo 3 Si. These compression studies confirmed that deformation in the temperature-strain rate space evaluated is matrix-controlled, yielding an activation energy of $415-445 kJ/mol. As a consequence, the response of the three-phase material overlaps that of the two-phase material. In all instances evaluated, the Mo-Si-B alloys exhibit superior flow stress relative to their TZM counterpart. Examination of the deformed microstructure illustrates that recovery and in some instances, recrystallization occurs in the Mo solid solution matrix whereas the T2 phase either cracks or deforms plastically depending on the temperature and strain rate used. Finite element analysis assuming an elastic-plastic matrix and an elastic second-phase illustrates strain localization in the matrix, the extent being more severe when the work-hardening rate in the matrix decreases (i.e., increasing temperature and decreasing strain rate), while the T2 particles are highly stressed. However, if plastic deformation is permitted in the T2 particles, strain distribution is homogenized substantially and the level of stress build up in the T2 particles diminishes by an order of magnitude. The interplay between matrix and T2 properties and their dependence on temperature and strain rate are used to explain the observed deformed microstructure.

Acta Materialia, 2003

A methodology for interpreting instrumented sharp indentation with dual sharp indenters with diff... more A methodology for interpreting instrumented sharp indentation with dual sharp indenters with different tip apex angles is presented by recourse to computational modeling within the context of finite element analysis. The forward problem predicts an indentation response from a given set of elasto-plastic properties, whereas the reverse analysis seeks to extract elasto-plastic properties from depth-sensing indentation response by developing algorithms derived from computational simulations. The present study also focuses on the uniqueness of the reverse algorithm and its sensitivity to variations in the measured indentation data in comparison with the single indentation analysis on Vickers/Berkovich tip (Dao et al. Acta Mater 49 (2001) 3899). Finite element computations were carried out for 76 different combinations of elasto-plastic properties representing common engineering metals for each tip geometry. Young's modulus, E, was varied from 10 to 210 GPa; yield strength, s y , from 30 to 3000 MPa; and strain hardening exponent, n, from 0 to 0.5; while the Poisson's ratio, n, was fixed at 0.3. Using dimensional analysis, additional closedform dimensionless functions were constructed to relate indentation response to elasto-plastic properties for different indenter tip geometries (i.e., 50°, 60°and 80°cones). The representative plastic strain e r , as defined in Dao et al. (Acta Mater 49 , was constructed as a function of tip geometry in the range of 50°and 80°. Incorporating the results from 60°tip to the single indenter algorithms, the improved forward and reverse algorithms for dual indentation can be established. This dual indenter reverse algorithm provides a unique solution of the reduced Young's modulus E * , the hardness p ave and two representative stresses (measured at two corresponding representative strains), which establish the basis for constructing power-law plastic material response. Comprehensive sensitivity analyses showed much improvement of the dual indenter algorithms over the single indenter results. Experimental verifications of these dual indenter algorithms were carried out using a 60°half-angle cone tip (or a 60°cone equivalent 3-sided pyramid tip) and a standard Berkovich indenter tip for two materials: 6061-T6511 and 7075-T651 aluminum alloys. Possible extensions of the present results to studies involving multiple indenters are also suggested.

Acta Materialia, 2007

Indentation is a remarkably flexible mechanical test due to its relative experimental simplicity.... more Indentation is a remarkably flexible mechanical test due to its relative experimental simplicity. Coupled with advances in instrument development, ease of implementation has made indentation a ubiquitous research tool for a number of different systems across size scales (nano to macro) and scientific/engineering disciplines. However, the exploration of different materials systems and the potential usage of indentation as a precise and quantitative method beyond the research laboratory have prompted intense modeling and interpretation efforts for robust analysis of experimental results. In this review, we describe progress in a number of different aspects of this method, including continuum-based modeling of homogeneous and heterogeneous systems, microstructural size effects and atomic modeling of nanoindentation experiments, in situ transmission electron microscopy observations of nanoscale contact, and novel and emerging uses for indentation. A recurring theme is the consideration of what is meant by ''hardness'' in different physical scenarios.

Acta Materialia, 2003

Systematic experiments have been performed to investigate the rate sensitivity of deformation in ... more Systematic experiments have been performed to investigate the rate sensitivity of deformation in fully dense nanocrystalline Ni using two different experimental techniques: depth-sensing indentation and tensile testing. Results from both types of tests reveal that the strain-rate sensitivity is a strong function of grain size. Specifically microcrystalline and ultra-fine crystalline pure Ni, with grain size range of Ͼ1 µm and 100-1000 nm, respectively, exhibit essentially rateindependent plastic flow over the range 3 × 10 Ϫ4 to 3 × 10 Ϫ1 s Ϫ1 , whereas nanocrystalline pure Ni with a grain size of approximately 40 nm, exhibits marked rate sensitivity over the same range. A simple computational model, predicated on the premise that a rate-sensitive grain-boundary affected zone exists, is shown to explain the observed effect of grain size on the rate-dependent plastic response.

Scripta Materialia, 2005

Experimental assessment of the reverse algorithms that enable the extraction of plastic propertie... more Experimental assessment of the reverse algorithms that enable the extraction of plastic properties from the load-depth of penetration curves was conducted. Results show that they predict the stresses at 3.3% and 5.7% representative strains for Berkovich and 60°cone-equivalent three-sided pyramidal indenters, respectively, with good accuracy. It was shown that the uniaxial stress-strain curves could be reconstructed from the indentation data.

Materials Science and Engineering: A, 2006

Algorithms for extracting mechanical properties from instrumented sharp indentation results (P-h ... more Algorithms for extracting mechanical properties from instrumented sharp indentation results (P-h curves) were recently developed on the basis of extensive finite element simulations coupled with the identification of a representative strain underneath the indenter and construction of characteristic dimensionless functions. The representative strain varies with the cone angle and a critical experimental verification of this concept was performed recently. In this paper, we ascertain the robustness of the algorithms, by subjecting the load versus depth of penetration curves (generated using finite element simulations on solids that were subjected to different levels of prior-plastic strains) to the reverse algorithms and comparing the extracted flow stress and the work-hardening exponent to the uniaxial data generated experimentally. A good match between the experiments and simulations validates the key assumptions made during the algorithm development.

Applied Energy, 2013

h i g h l i g h t s " Energy demand modeling in Thai road transportation sector was developed. " ... more h i g h l i g h t s " Energy demand modeling in Thai road transportation sector was developed. " Such model was used to assess environment impact by ethanol bus technology (ED95). " Ethanol bus technology (ED95) shows beneficial impacts to Thailand. " Increase in ethanol demand and decrease in GHG emission in Thailand by ethanol bus. " Ethanol bus (ED95) has been successfully demonstrated in Thailand.

The 2007 NSTDA Annual Conference (NAC2007), Mar 1, 2007

Acta Materialia, 2004

The compression behavior of a two-phase (Mo solid solution + T2) Mo-6.1Si-7.9B (at.%) alloy is ex... more The compression behavior of a two-phase (Mo solid solution + T2) Mo-6.1Si-7.9B (at.%) alloy is examined as a function of strain rate in the 1000-1400°C range and compared to that of commercially available powder-metallurgy-processed TZM (often referred to as MT104). A limited number of tests was also conducted on a three phase alloy with a composition of Mo-8.6Si-8.7B (at.%) that lies in the three phase field, Mo solid solution + T2 + Mo 3 Si. These compression studies confirmed that deformation in the temperature-strain rate space evaluated is matrix-controlled, yielding an activation energy of $415-445 kJ/mol. As a consequence, the response of the three-phase material overlaps that of the two-phase material. In all instances evaluated, the Mo-Si-B alloys exhibit superior flow stress relative to their TZM counterpart. Examination of the deformed microstructure illustrates that recovery and in some instances, recrystallization occurs in the Mo solid solution matrix whereas the T2 phase either cracks or deforms plastically depending on the temperature and strain rate used. Finite element analysis assuming an elastic-plastic matrix and an elastic second-phase illustrates strain localization in the matrix, the extent being more severe when the work-hardening rate in the matrix decreases (i.e., increasing temperature and decreasing strain rate), while the T2 particles are highly stressed. However, if plastic deformation is permitted in the T2 particles, strain distribution is homogenized substantially and the level of stress build up in the T2 particles diminishes by an order of magnitude. The interplay between matrix and T2 properties and their dependence on temperature and strain rate are used to explain the observed deformed microstructure.

Acta Materialia, 2003

A methodology for interpreting instrumented sharp indentation with dual sharp indenters with diff... more A methodology for interpreting instrumented sharp indentation with dual sharp indenters with different tip apex angles is presented by recourse to computational modeling within the context of finite element analysis. The forward problem predicts an indentation response from a given set of elasto-plastic properties, whereas the reverse analysis seeks to extract elasto-plastic properties from depth-sensing indentation response by developing algorithms derived from computational simulations. The present study also focuses on the uniqueness of the reverse algorithm and its sensitivity to variations in the measured indentation data in comparison with the single indentation analysis on Vickers/Berkovich tip (Dao et al. Acta Mater 49 (2001) 3899). Finite element computations were carried out for 76 different combinations of elasto-plastic properties representing common engineering metals for each tip geometry. Young's modulus, E, was varied from 10 to 210 GPa; yield strength, s y , from 30 to 3000 MPa; and strain hardening exponent, n, from 0 to 0.5; while the Poisson's ratio, n, was fixed at 0.3. Using dimensional analysis, additional closedform dimensionless functions were constructed to relate indentation response to elasto-plastic properties for different indenter tip geometries (i.e., 50°, 60°and 80°cones). The representative plastic strain e r , as defined in Dao et al. (Acta Mater 49 , was constructed as a function of tip geometry in the range of 50°and 80°. Incorporating the results from 60°tip to the single indenter algorithms, the improved forward and reverse algorithms for dual indentation can be established. This dual indenter reverse algorithm provides a unique solution of the reduced Young's modulus E * , the hardness p ave and two representative stresses (measured at two corresponding representative strains), which establish the basis for constructing power-law plastic material response. Comprehensive sensitivity analyses showed much improvement of the dual indenter algorithms over the single indenter results. Experimental verifications of these dual indenter algorithms were carried out using a 60°half-angle cone tip (or a 60°cone equivalent 3-sided pyramid tip) and a standard Berkovich indenter tip for two materials: 6061-T6511 and 7075-T651 aluminum alloys. Possible extensions of the present results to studies involving multiple indenters are also suggested.

Acta Materialia, 2007

Indentation is a remarkably flexible mechanical test due to its relative experimental simplicity.... more Indentation is a remarkably flexible mechanical test due to its relative experimental simplicity. Coupled with advances in instrument development, ease of implementation has made indentation a ubiquitous research tool for a number of different systems across size scales (nano to macro) and scientific/engineering disciplines. However, the exploration of different materials systems and the potential usage of indentation as a precise and quantitative method beyond the research laboratory have prompted intense modeling and interpretation efforts for robust analysis of experimental results. In this review, we describe progress in a number of different aspects of this method, including continuum-based modeling of homogeneous and heterogeneous systems, microstructural size effects and atomic modeling of nanoindentation experiments, in situ transmission electron microscopy observations of nanoscale contact, and novel and emerging uses for indentation. A recurring theme is the consideration of what is meant by ''hardness'' in different physical scenarios.

Acta Materialia, 2003

Systematic experiments have been performed to investigate the rate sensitivity of deformation in ... more Systematic experiments have been performed to investigate the rate sensitivity of deformation in fully dense nanocrystalline Ni using two different experimental techniques: depth-sensing indentation and tensile testing. Results from both types of tests reveal that the strain-rate sensitivity is a strong function of grain size. Specifically microcrystalline and ultra-fine crystalline pure Ni, with grain size range of Ͼ1 µm and 100-1000 nm, respectively, exhibit essentially rateindependent plastic flow over the range 3 × 10 Ϫ4 to 3 × 10 Ϫ1 s Ϫ1 , whereas nanocrystalline pure Ni with a grain size of approximately 40 nm, exhibits marked rate sensitivity over the same range. A simple computational model, predicated on the premise that a rate-sensitive grain-boundary affected zone exists, is shown to explain the observed effect of grain size on the rate-dependent plastic response.