Kiet Tieu

Asymmetric cryorolling for fabrication of nanostructural aluminum sheets.

Nanostructural Al 1050 sheets were produced using a novel method of asymmetric cryorolling under ratios of upper and down rolling velocities (RUDV) of 1.1, 1.2, 1.3, and 1.4. Sheets were rolled to about 0.17 mm from 1.5 mm. Both the strength and ductility of Al 1050 sheets increase with RUDVs. Tensile strength of Al sheets with the RUDV 1.4 is larger 22.3% of that for RUDV 1.1, which is 196 MPa. The TEM observations show the grain size is 360 nm when the RUDV is 1.1, and 211 nm for RUDV 1.4.

A combined experimental-numerical approach for determining mechanical properties of aluminum subjects to nanoindentation

A crystal plasticity finite element method (CPFEM) model has been developed to investigate the mechanical properties and micro-texture evolution of single-crystal aluminum induced by a sharp Berkovich indenter. The load-displacement curves, pile-up patterns and lattice rotation angles from simulation are consistent with the experimental results. The pile-up phenomenon and lattice rotation have been discussed based on the theory of crystal plasticity. In addition, a polycrystal tensile CPFEM model has been established to explore the relationship between indentation hardness and yield stress. The elastic constraint factor C is slightly larger than conventional value 3 due to the strain hardening.

An application of nonlinear feature extraction - A case study for low speed slewing bearing condition monitoring and prognosis

This paper presents the application of four nonlinear methods of feature extraction in slewing bearing condition monitoring and prognosis: these are largest Lyapunov exponent, fractal dimension, correlation dimension, and approximate entropy methods. Although correlation dimension and approximate entropy methods have been used previously, the largest Lyapunov exponent and fractal dimension methods have not been used in vibration condition monitoring to date. The vibration data of the laboratory slewing bearing test-rig run at 1 rpm was acquired daily from February to August 2007 (138 days). As time progressed, a more accurate observation of the alteration of bearing condition from normal to faulty was obtained using nonlinear features extraction. These findings suggest that these methods provide superior descriptive information about bearing condition than time-domain features extraction, such as root mean square (RMS), variance, skewness and kurtosis.

A study of microstructural evolution around crack tip using crystal plasticity finite-element method

The microstructure plays a significant role in crack initiation and crack propagation in metals. In this paper, a crystal plasticity finite-element method has been developed to simulate the tensile tests on rectangle-notched, triangle-notched, and circle-notched aluminium samples. Polycrystalline aggregates are approximated by a Voronoi structure. Two sets of initial orientations are randomly assigned to the aggregate. Notch geometries after deformation, the maximum shear stresses, and crystal orientation rotations are presented and discussed. Simulation results reveal that deformation of polycrystalline aggregate is greatly influenced by both notch geometry and initial crystal orientation. The crystal orientation profoundly rotates around the thickness direction of the samples for all the notch geometries and the initial orientations.

Coupled grain boundary motion in aluminium: The effect of structural multiplicity

The shear-induced coupled grain boundary motion plays an important role in the deformation of nanocrystalline (NC) materials. It has been known that the atomic structure of the grain boundary (GB) is not necessarily unique for a given set of misorientation and inclination of the boundary plane. However, the effect of the structural multiplicity of the GB on its coupled motion has not been reported. In the present study we investigated the structural multiplicity of the symmetric tilt Σ5(310) boundary in aluminium and its influence on the GB behaviour at a temperature range of 300 K–600 K using molecular dynamic simulations. Two starting atomic configurations were adopted in the simulations which resulted in three different GB structures at different temperatures. Under the applied shear deformation each GB structure exhibited its unique GB behaviour. A dual GB behaviour, namely the transformation of one GB behaviour to another during deformation, was observed for the second starting configuration at a temperature of 500 K. The atomistic mechanisms responsible for these behaviour were analysed in detail. The result of this study implicates a strong relationship between GB structures and their behaviour, and provides a further information of the grain boundary mediated plasticity in nanocrystalline materials.

Analysing the heterogeneity of traveller mode choice preference using a random parameter logit model from the perspective of principal-agent theory

When travellers entrust their desire for a transport mode that is customer-focused (i.e., reliable) to the Transport for NSW (TfNSW), this creates a metaphorical contract between travellers and the TfNSW, known as an agency contract. This contract is often characterised by agency uncertainty because both are most likely to act in their own self-interest. It can be assumed that where there is a high use of public transport, the TfNSW is performing the entrusted tasks as per travellers’ demand, which indicates an improvement in agency uncertainty. On the other hand, where there is a high use of private transport, it is likely that the TfNSW is acting largely in its own self-interest, and the agency problem remains unresolved. From the results, it is shown that the probability of car use is significantly higher than public transport, which indicates that an agency problem exists in the traveller-TfNSW relationship. It is recommended that integrating traveller preferences in transport projects would help to resolve this problem.

Simulation of polycrystalline aluminum tensile test with crystal plasticity finite element method

The crystal plasticity was implemented in the finite element method(FEM) software ABAQUS through the user subroutine UMAT. By means of discretizing the space at the grain level with the Voronoi diagram method, a polycrystal model was built and used in the FEM analysis. The initial orientation of each grain was generated based on the orientation distribution function(ODF). The developed model was successfully applied in simulation of polycrystalline aluminium samples deformed by the tensile tests. The theoretical strain—stress relation was in good agreement with the experimental result. The simulation results show that the grain size has significant effect on the deformation behavior. The initial plastic deformation usually occurs at grain boundaries, and multiple slip often results in an enhanced local hardening at grain boundaries.

Degradation Trend Estimation and Prognosis of Large Low Speed Slewing Bearing Lifetime

In many applications, degradation of bearing conditions is usually monitored by changes in time-domain features. However, in low speed (< 10 rpm) slewing bearing, these changes are not easily detected because of the low energy and low frequency of the vibration. To overcome this problem, a combined low pass filter (LPF) and adaptive line enhancer (ALE) signal pre-conditioning method is used. Time-domain features such as root mean square (RMS), skewness and kurtosis are extracted from the output signal of the combined LPF and ALE method. The extracted features show accurate information about the incipient of fault as compared to extracted features from the original vibration signal. This information then triggers the prognostic algorithm to predict the remaining lifetime of the bearing. The algorithm used to determine the trend of the non-stationary data is auto-regressive integrated moving average (ARIMA).

Mechanics of Thin Strip Steering in Hot Rolling

The hot rolling of thin strip can result in several problems in hot rolling, for instance, the control of strip steering, strip shape and flatness and surface roughness etc. Therefore, the hot rolling of thin strip brings out a requirement of innovative technologies such as the extended control of shape and flatness, steering control and reduction of load by roll gap lubrication. In this paper, the authors focus on the analysis of thin strip snaking movement, as well as solve the related problems such as the shape and flatness due to a larger reduction applied when the strip is thinner. A finite element method was used to simulate this nonsymmetricity rolling considering the non‐uniform reduction along the strip width. The calculated spread is compared with the measured values obtained from the rolling mill in laboratory and the friction effect is also discussed.

Molecular dynamics simulation on generalized stacking fault energies of FCC metals under preloading stress

Molecular dynamics (MD) simulations are performed to investigate the effects of stress on generalized stacking fault (GSF) energy of three fcc metals (Cu, Al, and Ni). The simulation model is deformed by uniaxial tension or compression in each of [111], [11-2], and [1-10] directions, respectively, before shifting the lattice to calculate the GSF curve. Simulation results show that the values of unstable stacking fault energy (γusf), stable stacking fault energy (γsf), and unstable twin fault energy (γutf) of the three elements can change with the preloaded tensile or compressive stress in different directions. The ratio of γsf/γusf, which is related to the energy barrier for full dislocation nucleation, and the ratio of γutf/γusf, which is related to the energy barrier for twinning formation are plotted each as a function of the preloading stress. The results of this study reveal that the stress state can change the energy barrier of defect nucleation in the crystal lattice, and thereby can play an important role in the deformation mechanism of nanocrystalline material.