Paper online (Int J Mech Sci) on contact mechancis

Zhai, C., Hanaor, Gan, Y. (2017) Contact stiffness of multiscale surfaces by truncation analysis. International Journal of Mechanical Sciences. 131–132: 305–316.
[DOI][PDF:048_IJMS_2017]

Abstract: In this paper, we study the contact stiffness of a fractal rough surface compressed by a rigid flat plane. A numerical model based on the analysis of flat punch indentation is proposed for simulated hierarchical surfaces, which are generated using statistical and fractal descriptors collected by surface profilometry. The contact stiffness of surfaces under increasing normal load is determined on the basis of the total truncated area at varying heights. The results are compared with experimental data from nanoindentation on four types of treated rough surfaces, showing good agreement with experimental observations below a certain truncation depth. Furthermore, the limits of the model’s validity are discussed by focusing on surface geometries and deformation of contacting asperities. With this proposed truncation method, we present a parametric analysis to establish a correlation between contact stiffness and surface roughness descriptors. The contact stiffness shows a unified power-law scaling with respect to the applied load over a wide range for simulated surfaces with distinct sets of roughness descriptors. The exponent of the power-law relationship is found to correlate positively to the fractal dimension while its amplitude is inversely correlated to the surface roughness amplitude. This study provides an easily implemented and computationally efficient method to connect mechanical behaviour with multi-scale surface structure, which can be utilized in design and optimization of engineering applications involving rough contacts.

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Paper online (Heliyon) on crumple-formed materials

Hanaor, D., Flores Johnson, E.A., Wang, S., Quach, S., Dela-Torre, K.N., Gan, Y., Shen, L. (2017) Mechanical properties in crumple-formed paper derived materials subjected to compression. Heliyon. 3 (2017) e00329.
[DOI][PDF:047_Heliyon_2017]

Abstract: The crumpling of precursor materials to form dense three dimensional geometries offers an attractive route towards the utilisation of minor-value waste materials. Crumple-forming results in a mesostructured system in which mechanical properties of the material are governed by complex cross-scale deformation mechanisms. Here we investigate the physical and mechanical properties of dense compacted structures fabricated by the confined uniaxial compression of a cellulose tissue to yield crumpled mesostructuring. A total of 25 specimens of various densities were tested under compression. Crumple formed specimens exhibited densities in the range 0.8–1.3 g cm−3, and showed high strength to weight characteristics, achieving ultimate compressive strength values of up to 200 MPa under both quasi-static and high strain rate loading conditions and deformation energy that compares well to engineering materials of similar density. The materials fabricated in this work and their mechanical attributes demonstrate the potential of crumple-forming approaches in the fabrication of novel energy-absorbing materials from low-cost precursors such as recycled paper. Stiffness and toughness of the materials exhibit density dependence suggesting this forming technique further allows controllable impact energy dissipation rates in dynamic applications.

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Paper online (Fusion Eng. Des.) on ellipsoidal DEM

Moscardinia, M., Gan, Y., Annabattula, R.K., Kamlah M. (2017) A Discrete Element Method to simulate the mechanical behavior of ellipsoidal particles for a fusion breeding blanket. Fusion Engineering and Design. 121: 22-31.
[DOI][PDF:046_FED_2017_M]

Abstract:

The breeder materials proposed for the solid tritium breeding blanket concepts are ceramic lithium-based compounds in the form of pebble beds. Different fabrication processes have been developed to produce pebbles with a high sphericity. However, a small deviation from a perfectly spherical shape exists. In this paper the influence of non-sphericity on the mechanical behaviour of a pebble bed is assessed representing the currently produced pebbles by means of ellipsoidal particles. To this end, the in-house Discrete Element Method code KIT-DEM was further extended. The multi-sphere approach was implemented to generate the ellipsoidal particles while the existing random close packing algorithm was modified to create the assemblies. Uniaxial compression of the assemblies, under periodic boundary conditions, was simulated to investigate the bulk stress-strain behaviour of the bed. Sensitivity studies were carried out with different packing factors of the assembly and several aspect ratios of the particles. In agreement with previous studies carried on assemblies of spherical pebbles, the initial packing factor was found to noticeably affect the mechanical response of the investigated assemblies. Moreover, a remarkable influence of the shape on the mechanical behavior of the simulated assemblies was observed. Therefore it is concluded that for production techniques that result in poor sphericity, DEM simulations with non-spherical particles are necessary to reproduce realistic stress-strain behavior of pebble beds.
Keywords: Discrete element method; Pebble bed thermomechanics; Nuclear fusion; Multi-sphere approach; Ellipsoidal particles

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Paper accepted (Powder Tech.) on packing structures

Title: X-ray tomography investigations of mono-sized sphere packing structures in cylindrical containers

Author(s): Joerg Reimann, Jerome Vicente, Emmanuel Brun, Claudio Ferrero, Yixiang Gan, Alexander Rack

Powder Technology, in press. [DOI]

Abstract: The structure of mono-sized sphere packings (diameter d) in cylindrical containers (diameter D and height H) both with and without inner cylinders (diameter Di) has been investigated in detail by means of advanced X-ray computed tomography. The geometrical parameters were varied in a wide range; in all experiments 1d vertical vibration was applied. Five experiments were selected with characteristically differing local packing structures. The influence of container geometry, filling and vibration procedures on the formation of regular packings is discussed and a simple correlation is presented to assess whether structured packings occupy a significant fraction of the total packed volume.

For a packing with moderate densification, the regular structures are restricted to small wall zones and a random packing exists in the largest part of the packing volume. By selecting appropriate vibration parameters, the zones with regular structures can increase considerably and can persist in the total packed volume. The increasing crystallisation causes an increase of the container packing fraction. For cylinders with H/D > > 1 and moderate D/d, regular structures develop preferentially in radial direction from a hexagonal layer at the concave wall. For H/D < 1 and D/d > > 1, hexagonal dense structures grow preferentially above the flat bottom plate and can occupy a great portion of the total volume. The role of granular convection on these crystallisation processes has been addressed. Previous statements that the thickness of wall zones is ≈(4-5)d are not generally valid for mono-sized sphere packings; the development of a comprehensive correlation is the task of a future work.

Structural details of the packings close to concave, plane and convex walls are analysed via void fraction distributions, sphere centre positions, contact angle distributions, coordination numbers, radial distribution function and Voronoi tessellation. The combination of these methods provides a comprehensive understanding of structural details. Only a few characteristic results are presented; special topics will be the subject of forthcoming publications.

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IWMEM2017 website online

The website of International Workshop on Mechanics of Energy Materials (IWMEM2017) is now online via http://iwmem.org/. The information will be updated in the near future.

The workshop will be hosted in Suzhou, 8-11 November, 2017. The venue is the newly established University of Sydney Centre in Suzhou, in Suzhou Industrial Park. The previous workshop was held in Sydney, some more details can be found here.

The brochure can be downloaded via:

IWMEM2017_Brochure
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Dr Gan visited universities and institutions in China

In April 2017, Dr Gan visited a few universities and institutions in China. These institutions include CAS Suzhou Institute of Nano-Tech and Nano-Bionics, Sydney Centre in China, Tongji University, Tsinghua University, CAS Institute of Plasma Physics, Southeast University, Zhejiang University, and Ningbo University. He discussed with other co-organisers for hosting the 2nd Workshop on Mechanics of Energy Materials (IWMEM2017) in Suzhou, earlier November 2017. In addition, he gave invited departmental seminars on the topic of “Mechanics of Compacted Granular Materials: From Interface, Morphology to Complex Networks” at Tsinghua University, Southeast University and Ningbo University.

The following briefly summarises the highlights of these visits:

(1) Discussion with Mrs Cathryn Hlavka (left) and her team at Sydney Centre in China regarding the organisation of coming workshop on mechanics of energy materials, to be hosted here in November, 2017 (more details).

(2) On 12 April, group photo with the research group of Professor Jianzhuang Xiao at Tongji University.

(3) On 14 April, dinner with the research group of Professor Changqing Chen at Tsinghua University.

(4)  On 17 April, photo taken in front of EAST (Experimental Advanced Superconducting Tokamak) fusion system at ASIPP with Professor Songlin Liu’s team.

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Mr Guanzhe Cui joined the group

Mr Guanzhe Cui has just started his Master of Philosophy (MPhil) program in March, 2017, and he will work on numerical simulations of multi-component flow in porous media based on Lattice Boltzmann method (LBM). Guanzhe graduated with Master of Architecture and Civil Engineering from School of Civil Engineering and Mechanics at Kunming University of Science and Technology, China in 2016.

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Paper accepted (Fusion Eng. Des.) on effective thermal conductivity

Title: Influence of gas pressure on the effective thermal conductivity of ceramic breeder pebble beds

Author(s): Weijing Dai, Simone Pupeschi, Dorian Hanaor, Yixiang Gan

Fusion Engineering and Design, in press. [DOI][PDF:045_FED_2017]

Figure: Gas pressure related size dependency of the contact units.

Abstract: Lithium ceramics have been considered as tritium breeder materials in many proposed designs of fusion breeding blankets. Heat generated in breeder pebble beds due to nuclear breeding reaction must be removed by means of actively cooled plates while generated tritiums is recovered by purge gas slowly flowing through beds. Therefore, the effective thermal conductivity of pebble beds that is one of the governing parameters determining heat transport phenomenon needs to be addressed with respect to mechanical status of beds and purge gas pressure. In this study, a numerical framework combining finite element simulation and a semi-empirical correlation of gas gap conduction is proposed to predict the effective thermal conductivity. The purge gas pressure is found to vary the effective thermal conductivity, in particular with the presence of various sized gaps in pebble beds. Random packing of pebble beds is taken into account by an approximated correlation considering the packing factor and coordination number of pebble beds. The model prediction is compared with experimental observation from different sources showing a quantitative agreement with the measurement.

Keywords: effective thermal conductivity; ceramic breeding materials; pebble beds; granular materials; gas pressure

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Ms Yongmei Zhang joined the group

Ms Yongmei Zhang has just started her PhD program in March, 2017, and she will work on the mechanics of interfaces. Yongmei graduated with Master of Engineering in Solid Mechanics from the School of Aerospace, Xi’an Jiaotong University, China in 2016.

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PostDoc Opening in Carbon Storage

A postdoctoral research associate in carbon storage has been advertised in the School of Civil Engineering, Faculty of Engineering and IT at the University of Sydney. The official advertisement by The University of Sydney can be found via https://goo.gl/wqsvLl

The Postdoctoral Research Associate will undertake a project funded by the Australian Research Council on CO2 storage, named “Assessing reservoir performance for carbon storage in saline aquifers.” The Chief Investigators include A/Professor Luming Shen, Dr Federico Maggi, Dr Yixiang Gan, Professor Abbas El-Zein, Dr Zhejun Pan.


The project aims at developing and testing a modelling framework to assess the injectivity, storage capacity, and containment of supercritical CO2 injected in deep saline aquifers.The project broadly covers multiphase flow in porous/fractured geological materials, and emphasizes physical, hydraulic, and geomechanical processes at multiple spatial and temporal scales. The project is expected to provide a validated computational framework that can be used by stakeholders to improve analysis and assessment of reservoir performance under various scenarios of CO2 injection operations and boundary environmental conditions. This opening is fundamentally based on the development of advanced computational frameworks but it is part of a greater effort that also involves in-situ data collection and experimental laboratory tests conducted by other participants to this project. The project is led by A/Prof Luming Shen in the School of Civil Engineering at the University of Sydney, and involves the collaboration of the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia.

 

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