I am Associate Professor at Department of Mechanical Engineering. I got my Ph.D in Materials Science from Royal Institute of Technology (KTH) in Sweden. My specicial research interests are nano-/micromechanics of materials, multiferroics materials and biomimetics study. I am teaching materials science and engineering related subjects at NUS.
For detail information of my research group at NUS, please visit my group website:
http://sites.google.com/site/zengsresearhgroup/ Address: Department of Mechanical Engineering
National University of Singapore
9 Engineering Drive 1
Singapore 117576
Ferroelectricity has been believed to be an important but controversial origin of the excellent p... more Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.
Surface damage of polymers in the nanometer-range is examined and results correlated with materia... more Surface damage of polymers in the nanometer-range is examined and results correlated with material characteristics and surface roughness of epoxies. Under a constant loading and constant scratch rate testing condition, surface roughness plays little or no role in surface damage formed during the course of this study. Material characteristics influence the damage occurred in terms of variations in elastic recovery, damage pattern and damage mechanism. The variations in scratch head geometry, which, in turn, lead to the variations in magnitude of stress and stress field distribution, give rise to various scratch features on the polymer.
Residual stress evaluation in thin films at the sub-micron scale was achieved in the present stud... more Residual stress evaluation in thin films at the sub-micron scale was achieved in the present study using a semi-destructive trench-cutting (ring-core) method. Focused Ion Beam was employed to introduce the strain relief by milling the slots around an “island” and also to record the images for strain change evaluation by digital image correlation analysis of micrographs. Finite element simulation was employed to predict the curves for strain relief as a function of milling depth, and compared with the experimental measurements, showing ...
We report a non-monotonous behavior of the hardness of amorphous Cu–Zr films as a function of com... more We report a non-monotonous behavior of the hardness of amorphous Cu–Zr films as a function of composition, by applying a combinatorial deposition and nanoindentation method with unparalleled compositional resolution. Distinct peaks in hardness were identified at particular compositions to correlate well with the previously reported density peaks. Our results not only facilitate the discovery of new glass-forming alloys, but also raise the possibility of developing bulk metallic glasses with enhanced plasticity and/or ductility for engineering applications.
The shapes of the interfacial delamination crack and stress states during wedge indentation in a ... more The shapes of the interfacial delamination crack and stress states during wedge indentation in a soft-film-on-hard-substrate system were investigated systematically using the three-dimensional (3D) finite element simulation and wedge indentation experiment. In the simulation, a traction–separation law was used to characterize the failure behaviors of the interface. The effects of the wedge indenter tip length and the film thickness on the onset and growth of interfacial delamination were analyzed. It was shown that a two-dimensional (2D) to 3D transition of stress states occurred depending on the ratio of indenter length to film thickness. Furthermore, the interfacial delamination process by wedge indentation was conducted experimentally, and comparisons between the computational and experimental results yielded quantitative good agreement. Finally, a straightforward criterion based on the curvature of the delamination crack front was proposed to indicate the transition of stress states during the interfacial delamination. A guideline was therefore proposed to classify the 2D and 3D stress states for extracting the interface adhesion properties.
This article presents studies on nanoindentation creep for polymeric materials at room temperatur... more This article presents studies on nanoindentation creep for polymeric materials at room temperature. A semi-empirical method has been established from a generalized Kelvin model. This model includes the elastic-viscoelastic-viscous effects on indentation creep. It is shown that this method can fit the experimental creep data well, to give the exact changes of creep behavior during nanoindentations for numbers of amorphous
International Journal of Computational Methods, 2011
This study combines the edge-based smoothed finite element method (ES-FEM) and the extended finit... more This study combines the edge-based smoothed finite element method (ES-FEM) and the extended finite element method (XFEM) to develop a new simulation technique (ES-XFEM) for fracture analysis of 2D elasticity. In the XFEM, the need for the mesh alignment with the crack and remeshing, as the crack evolves, is eliminated because of the use of partition of unity. The ES-FEM uses the generalized smoothing operation over smoothing domain associated with edges of simplex meshes, and produces a softening effect leading to a close-to-exact stiffness, "super-convergence" and "ultra-accurate" solutions for the numerical model. Taking advantage of both ES-FEM and XFEM, the present method introduces the edge-based strain smoothing technique into the context of XFEM. Thanks to strain smoothing, the necessity of sub-division in elements cut by discontinuities is suppressed via transforming interior integration into boundary integration. Hence, it simplifies the numerical integr...
ABSTRACT This paper aims to formulate a triangular five-node (T5) singular crack-tip element in G... more ABSTRACT This paper aims to formulate a triangular five-node (T5) singular crack-tip element in G space with strain smoothing to simulate an rλ−1 (0<λ<1) stress singularity for bi-material fracture analyses. In the present formulation, a direct point interpolation with a proper fractional order of extra basis functions is specially employed to construct variable power type singular shape functions that are in a G1 space. Within strain smoothing, the singular terms of functions as well as mapping procedures are no longer necessary to compute the stiffness matrix. Furthermore, thanks to the point interpolation, the proposed singular element eliminates the need to shift the position of the side nodes adjacent to the crack-tip, and is thus quite straightforward and easily implemented in existing codes. The effectiveness of the present singular element is demonstrated via numerical examples of a wide range of material combinations and boundary conditions.Highlights► A novel triangular five-node (T5) singular crack-tip element is formulated. ► A variable stress singularity is accurately captured. ► The singular terms of functions and mapping procedures are no longer necessary. ► The need to shift the position of the side nodes is eliminated. ► The convergence rate, accuracy and computational efficiency are found superior.
The oxidation kinetics of the Zr64Cu16Ni10Al10 bulk metallic glass (BMG) roughly follows a two-st... more The oxidation kinetics of the Zr64Cu16Ni10Al10 bulk metallic glass (BMG) roughly follows a two-stage rate law at both 433 and 593K in air. An oxide film of 940nm can be formed by oxidation at 593K, which is ZrO2-enriched but Cu-depleted on the outer surface. The oxide film leads to a superior passivity in 0.5M NaCl and great corrosion resistance improvements
Ferroelectricity has been believed to be an important but controversial origin of the excellent p... more Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.
Surface damage of polymers in the nanometer-range is examined and results correlated with materia... more Surface damage of polymers in the nanometer-range is examined and results correlated with material characteristics and surface roughness of epoxies. Under a constant loading and constant scratch rate testing condition, surface roughness plays little or no role in surface damage formed during the course of this study. Material characteristics influence the damage occurred in terms of variations in elastic recovery, damage pattern and damage mechanism. The variations in scratch head geometry, which, in turn, lead to the variations in magnitude of stress and stress field distribution, give rise to various scratch features on the polymer.
Residual stress evaluation in thin films at the sub-micron scale was achieved in the present stud... more Residual stress evaluation in thin films at the sub-micron scale was achieved in the present study using a semi-destructive trench-cutting (ring-core) method. Focused Ion Beam was employed to introduce the strain relief by milling the slots around an “island” and also to record the images for strain change evaluation by digital image correlation analysis of micrographs. Finite element simulation was employed to predict the curves for strain relief as a function of milling depth, and compared with the experimental measurements, showing ...
We report a non-monotonous behavior of the hardness of amorphous Cu–Zr films as a function of com... more We report a non-monotonous behavior of the hardness of amorphous Cu–Zr films as a function of composition, by applying a combinatorial deposition and nanoindentation method with unparalleled compositional resolution. Distinct peaks in hardness were identified at particular compositions to correlate well with the previously reported density peaks. Our results not only facilitate the discovery of new glass-forming alloys, but also raise the possibility of developing bulk metallic glasses with enhanced plasticity and/or ductility for engineering applications.
The shapes of the interfacial delamination crack and stress states during wedge indentation in a ... more The shapes of the interfacial delamination crack and stress states during wedge indentation in a soft-film-on-hard-substrate system were investigated systematically using the three-dimensional (3D) finite element simulation and wedge indentation experiment. In the simulation, a traction–separation law was used to characterize the failure behaviors of the interface. The effects of the wedge indenter tip length and the film thickness on the onset and growth of interfacial delamination were analyzed. It was shown that a two-dimensional (2D) to 3D transition of stress states occurred depending on the ratio of indenter length to film thickness. Furthermore, the interfacial delamination process by wedge indentation was conducted experimentally, and comparisons between the computational and experimental results yielded quantitative good agreement. Finally, a straightforward criterion based on the curvature of the delamination crack front was proposed to indicate the transition of stress states during the interfacial delamination. A guideline was therefore proposed to classify the 2D and 3D stress states for extracting the interface adhesion properties.
This article presents studies on nanoindentation creep for polymeric materials at room temperatur... more This article presents studies on nanoindentation creep for polymeric materials at room temperature. A semi-empirical method has been established from a generalized Kelvin model. This model includes the elastic-viscoelastic-viscous effects on indentation creep. It is shown that this method can fit the experimental creep data well, to give the exact changes of creep behavior during nanoindentations for numbers of amorphous
International Journal of Computational Methods, 2011
This study combines the edge-based smoothed finite element method (ES-FEM) and the extended finit... more This study combines the edge-based smoothed finite element method (ES-FEM) and the extended finite element method (XFEM) to develop a new simulation technique (ES-XFEM) for fracture analysis of 2D elasticity. In the XFEM, the need for the mesh alignment with the crack and remeshing, as the crack evolves, is eliminated because of the use of partition of unity. The ES-FEM uses the generalized smoothing operation over smoothing domain associated with edges of simplex meshes, and produces a softening effect leading to a close-to-exact stiffness, "super-convergence" and "ultra-accurate" solutions for the numerical model. Taking advantage of both ES-FEM and XFEM, the present method introduces the edge-based strain smoothing technique into the context of XFEM. Thanks to strain smoothing, the necessity of sub-division in elements cut by discontinuities is suppressed via transforming interior integration into boundary integration. Hence, it simplifies the numerical integr...
ABSTRACT This paper aims to formulate a triangular five-node (T5) singular crack-tip element in G... more ABSTRACT This paper aims to formulate a triangular five-node (T5) singular crack-tip element in G space with strain smoothing to simulate an rλ−1 (0<λ<1) stress singularity for bi-material fracture analyses. In the present formulation, a direct point interpolation with a proper fractional order of extra basis functions is specially employed to construct variable power type singular shape functions that are in a G1 space. Within strain smoothing, the singular terms of functions as well as mapping procedures are no longer necessary to compute the stiffness matrix. Furthermore, thanks to the point interpolation, the proposed singular element eliminates the need to shift the position of the side nodes adjacent to the crack-tip, and is thus quite straightforward and easily implemented in existing codes. The effectiveness of the present singular element is demonstrated via numerical examples of a wide range of material combinations and boundary conditions.Highlights► A novel triangular five-node (T5) singular crack-tip element is formulated. ► A variable stress singularity is accurately captured. ► The singular terms of functions and mapping procedures are no longer necessary. ► The need to shift the position of the side nodes is eliminated. ► The convergence rate, accuracy and computational efficiency are found superior.
The oxidation kinetics of the Zr64Cu16Ni10Al10 bulk metallic glass (BMG) roughly follows a two-st... more The oxidation kinetics of the Zr64Cu16Ni10Al10 bulk metallic glass (BMG) roughly follows a two-stage rate law at both 433 and 593K in air. An oxide film of 940nm can be formed by oxidation at 593K, which is ZrO2-enriched but Cu-depleted on the outer surface. The oxide film leads to a superior passivity in 0.5M NaCl and great corrosion resistance improvements
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