Research Experience
Research Experience
Embedded Files
1. Surface Stress of Soft Matter
1. Surface Stress of Soft Matter
Briefly, my research on surface tension of soft matter is to study the effect of surface stiffening on regularizing the surface deformation especially when it behaves large deformation. In a recent experiment, people found that when soft surface were stretched extremely it shows a stiffening phenomenon, which indicates the surface stress is directly proportional to the surface stretch. However, before this study there was no theory that addresses how surface stiffening affects the way of a load's transmission to a homogeneous soft substrate. In this study, we combined analytical analysis and numerical method to crack open the topic of the effect surface stiffening on the surface deformation. Part of our work is published in Soft Matter and more work is undergoing.
Briefly, my research on surface tension of soft matter is to study the effect of surface stiffening on regularizing the surface deformation especially when it behaves large deformation. In a recent experiment, people found that when soft surface were stretched extremely it shows a stiffening phenomenon, which indicates the surface stress is directly proportional to the surface stretch. However, before this study there was no theory that addresses how surface stiffening affects the way of a load's transmission to a homogeneous soft substrate. In this study, we combined analytical analysis and numerical method to crack open the topic of the effect surface stiffening on the surface deformation. Part of our work is published in Soft Matter and more work is undergoing.
H. Wu, Z. Liu, A. Jagota, C.-Y. Hui, Effect of large deformation and surface stiffening on the transmission of a line load on a neo-Hookean half space, Soft Matter. 14 (2018) 1847–1855. doi:10.1039/C7SM02394D.[Link]
2. Adhesion of Structured Surface
2. Adhesion of Structured Surface
H. Wu, N. Moyle, A. Jagota, C.Y. Khripin, F. Bremond, C.-Y. Hui, Crack propagation pattern and trapping mechanism of rolling a rigid cylinder on a periodically structured surface, Extreme Mechanics Letters. 29 (2019) 100475. doi:10.1016/j.eml.2019.100475. [Link]
My research on the topic of adhesion of structured surface was inspired by the recent experimental findings which showed that when a surface was structured by a special architecture (film-terminated ridge-channel structure), the spacing of these surface structure actually controls the resistance to a roller rolling over it. The presence of these surface structures was found to increase the rolling resistance compared with the flat surface, which is called rolling enhancement. This increment in rolling resistance was attributed to the adhesion between the roller and the structured surface. In this work, I combined finite element simulation and the experiments to study the mechanism of this rolling enhancement. The work is published in Extreme Mechanics Letter.
My research on the topic of adhesion of structured surface was inspired by the recent experimental findings which showed that when a surface was structured by a special architecture (film-terminated ridge-channel structure), the spacing of these surface structure actually controls the resistance to a roller rolling over it. The presence of these surface structures was found to increase the rolling resistance compared with the flat surface, which is called rolling enhancement. This increment in rolling resistance was attributed to the adhesion between the roller and the structured surface. In this work, I combined finite element simulation and the experiments to study the mechanism of this rolling enhancement. The work is published in Extreme Mechanics Letter.
3. Steady-state Soft Surface Lubrication
3. Steady-state Soft Surface Lubrication
H. Wu, N. Moyle, A. Jagota, C.-Y. Hui, Lubricated steady sliding of a rigid sphere on a soft elastic substrate: hydrodynamic friction in the Hertz limit, Soft Matter. (2020) 10.1039.C9SM02447F. https://doi.org/10.1039/C9SM02447F. [Link]
My research on the topic of elasto-hydrodynamic lubrication (EHL) is more about revisiting a classical EHL problem: a sphere slides on a lubricated homogeneous substrate. Different from previous research, this research focused on a regime where the normal force is much higher and sliding velocity is smaller. This regime is difficult to handle since the deformation of substrate is significant and the liquid layer is thin. We showed that governing equations could be normalized properly and driven only by a single parameter. I developed numerical scheme to solve the highly nonlinear governing equation in polar coordinate, which significantly accelerates the calculation speed compared with previous studies. The work is published in Soft Matter.
My research on the topic of elasto-hydrodynamic lubrication (EHL) is more about revisiting a classical EHL problem: a sphere slides on a lubricated homogeneous substrate. Different from previous research, this research focused on a regime where the normal force is much higher and sliding velocity is smaller. This regime is difficult to handle since the deformation of substrate is significant and the liquid layer is thin. We showed that governing equations could be normalized properly and driven only by a single parameter. I developed numerical scheme to solve the highly nonlinear governing equation in polar coordinate, which significantly accelerates the calculation speed compared with previous studies. The work is published in Soft Matter.
4. Bulk Viscoelasticity on Soft Lubrication
4. Bulk Viscoelasticity on Soft Lubrication
In this project, we study the effect of the bulk's viscoelasticity on lubricated sliding of a cylinder. The substrate is represented by a foundation model. The solution is controlled by three dimensionless parameters. The first of these, a, measures the time for the indenter to move one contact zone relative to the viscoelastic relaxation time; the second is the ratio of the long time to short time modulus of the substrate, c_Inf/c_0; the third parameter, b, is the ratio of average fluid flow rate to the sliding velocity. The friction force can be decomposed into two parts, one due to viscoelastic dissipation and the other from hydrodynamics. Although these two are generally coupled, in the Hertz limit, we found an important result that the viscoelastic portion of the friction force can be well approximated by the solution of the corresponding “dry” sliding problem, in which there is no lubricating layer. This provides a simple way to decouple the hydrodynamic portion of the friction force from the viscoelasticity of the substrate.
In this project, we study the effect of the bulk's viscoelasticity on lubricated sliding of a cylinder. The substrate is represented by a foundation model. The solution is controlled by three dimensionless parameters. The first of these, a, measures the time for the indenter to move one contact zone relative to the viscoelastic relaxation time; the second is the ratio of the long time to short time modulus of the substrate, c_Inf/c_0; the third parameter, b, is the ratio of average fluid flow rate to the sliding velocity. The friction force can be decomposed into two parts, one due to viscoelastic dissipation and the other from hydrodynamics. Although these two are generally coupled, in the Hertz limit, we found an important result that the viscoelastic portion of the friction force can be well approximated by the solution of the corresponding “dry” sliding problem, in which there is no lubricating layer. This provides a simple way to decouple the hydrodynamic portion of the friction force from the viscoelasticity of the substrate.
C.-Y. Hui, H. Wu, A. Jagota, C.Y. Khripin, Friction force during Lubricated Steady Sliding of a Rigid Cylinder on a Viscoelastic Substrate, Tribology Letters. (2021) https://doi.org/10.1007/s11249-020-01396-5. [Link]
5. Transient EHL on Patterned Surfaces
5. Transient EHL on Patterned Surfaces
A common way to increase friction during lubricated sliding is to increase energy dissipation in the slider or the substrate, for example, by bulk viscoelasticity. It has recently been shown that lubricated friction can also be enhanced by surface architecture, specifically by periodic variation of near-surface stiffness. In order to study this phenomenon, here we consider a rigid cylinder undergoing lubricated sliding on a substrate with a periodic variation in mechanical stiffness.
A common way to increase friction during lubricated sliding is to increase energy dissipation in the slider or the substrate, for example, by bulk viscoelasticity. It has recently been shown that lubricated friction can also be enhanced by surface architecture, specifically by periodic variation of near-surface stiffness. In order to study this phenomenon, here we consider a rigid cylinder undergoing lubricated sliding on a substrate with a periodic variation in mechanical stiffness.
We model the process using the Reynolds theory for transient elasto-hydrodynamic lubrication (t-EHL). We have developed a numerical scheme to solve this t-EHL problem and used it to study how surface variation in stiffness affects the motion of the cylinder, the distribution of hydrodynamic pressure, the liquid film thickness, and the friction force. Our results indicate that increasing the variation of the near-surface stiffness can significantly increase the friction force. The numerical method developed in this work can be used to solve other transient EHL problems with slight modification. Our results provide insight into the mechanism of friction enhancement and can be used to guide design of surfaces to control friction during lubricated sliding.
We model the process using the Reynolds theory for transient elasto-hydrodynamic lubrication (t-EHL). We have developed a numerical scheme to solve this t-EHL problem and used it to study how surface variation in stiffness affects the motion of the cylinder, the distribution of hydrodynamic pressure, the liquid film thickness, and the friction force. Our results indicate that increasing the variation of the near-surface stiffness can significantly increase the friction force. The numerical method developed in this work can be used to solve other transient EHL problems with slight modification. Our results provide insight into the mechanism of friction enhancement and can be used to guide design of surfaces to control friction during lubricated sliding.
H. Wu, N. Moyle, A. Jagota, C.-Y. Hui, Enhancement of Hydrodynamic Friction by Periodic Variation of Contact Stiffness (submitted, 2020)
6. Self-programmed Finite Element Framework for Crack Propagation
6. Self-programmed Finite Element Framework for Crack Propagation
SENT
Established a house-made Finite Element Simulation Framework for general crack propagation problems. Tested the FE framework using three classical fracture problems.
Established a house-made Finite Element Simulation Framework for general crack propagation problems. Tested the FE framework using three classical fracture problems.
ASTM Standard Test
The three fracture problems are: (a) Single edge notch tension test, (b) ASTM standard test; (c) Three points bending test. Compared the energy release rate for all these three fundamental fracture problems to verify the validation of the FE framework. The results show perfect match between the self-made FE and the theoretical formulation.
The three fracture problems are: (a) Single edge notch tension test, (b) ASTM standard test; (c) Three points bending test. Compared the energy release rate for all these three fundamental fracture problems to verify the validation of the FE framework. The results show perfect match between the self-made FE and the theoretical formulation.
Three Point Bending Test
7. Optimization of arrangement of Shield Tunnel Segments to fit small-radius underground railway line
7. Optimization of arrangement of Shield Tunnel Segments to fit small-radius underground railway line
I earned a Master Degree in Civil Engineering at SWJTU of China. During my study there, part of my research is about designing algorithm to optimize the arrangement of segment ring of a shield tunnel (see figure on the right) to fit a small-radius underground railway line. The shield tunnel is constructed rings by rings in underground environment. Each two adjacent rings have to satisfy certain geometrical constrains so that they can be installed perfectly. While there are thousands of arrangement schemes for these segment rings to be installed correctly, only several arrangements will satisfy that the installed segment rings fit approximately the target curved line. This research is about developing algorithm to find these arrangements of segment rings for different target lines. Our work is published at a prestigious journal on Chinese railway construction.
I earned a Master Degree in Civil Engineering at SWJTU of China. During my study there, part of my research is about designing algorithm to optimize the arrangement of segment ring of a shield tunnel (see figure on the right) to fit a small-radius underground railway line. The shield tunnel is constructed rings by rings in underground environment. Each two adjacent rings have to satisfy certain geometrical constrains so that they can be installed perfectly. While there are thousands of arrangement schemes for these segment rings to be installed correctly, only several arrangements will satisfy that the installed segment rings fit approximately the target curved line. This research is about developing algorithm to find these arrangements of segment rings for different target lines. Our work is published at a prestigious journal on Chinese railway construction.
H. Wu, C. He, Q. Yan, K. Feng, T. Cheng, A Study on Curve Fitting Algorithm of Curved Shield Tunnels Assembled by Universal Wedge Segments and Its Application, Journal of the China Railway Society. 10(2016) 1001-8360
Work Experience
Work Experience
1. On-site data collection of geological condition and tunnels structure health
1. On-site data collection of geological condition and tunnels structure health
I led a team of four people working on structure health inspection of 4 tunnels. We used ground penetrating radar (GPR) system (from GSSI and USRadar company) to check if there were any cavities behind the tunnel structure or were there any flaws inside the structure. Besides, we also used tunnel seismic tomography method for geological prediction which helped provide the geological condition ahead the tunnels' surfaces during their construction. We collected data at these tunnels for 7 months and our work helped the construction being carried out safely.
I led a team of four people working on structure health inspection of 4 tunnels. We used ground penetrating radar (GPR) system (from GSSI and USRadar company) to check if there were any cavities behind the tunnel structure or were there any flaws inside the structure. Besides, we also used tunnel seismic tomography method for geological prediction which helped provide the geological condition ahead the tunnels' surfaces during their construction. We collected data at these tunnels for 7 months and our work helped the construction being carried out safely.
2. Data analysis and visualization
2. Data analysis and visualization
My team collected a large scale on-site data about the tunnels' structure health and their surrounding geological conditions, I combined filtering, Fourier transformation, convolution and other technique to clean, analyze and visualize the data. I was in charged of writing more than 20 reports to update the tunnel constructors the tunnels structure quality and the surrounding geological condition. Our work greatly supported the tunnel construction safety and were highly appreciated by the constructors.
My team collected a large scale on-site data about the tunnels' structure health and their surrounding geological conditions, I combined filtering, Fourier transformation, convolution and other technique to clean, analyze and visualize the data. I was in charged of writing more than 20 reports to update the tunnel constructors the tunnels structure quality and the surrounding geological condition. Our work greatly supported the tunnel construction safety and were highly appreciated by the constructors.
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