Papers by Author: Chong Du Cho

Abstract: Numerical simulation is performed to study the deformation and failure modes of Al 1100 plate of 2.4 mm thickness, subjected to low-velocity impact. Blunt and hemispherical nose shaped impactors are used in this study. The quasi-static tensile test is performed at a strain rate of 0.01/s to obtain the Johnson-Cook material parameters which are used in numerical simulation software, ABAQUS/CAE to perform impact analysis. Mesh convergence study is carried out to decide the appropriate number of elements for numerical analysis. The impact behavior of Al 1100 plate for each impactor shapes are studied at 22 J impact energy. Result indicate that increased in the nose radius of impactor will increase the amount of deformation energy for aluminium plate.

Abstract: The present study investigates numerically the compressive residual strength of indented sandwich composite panel. The composite is made of carbon fiber reinforced plastic (CFRP) face sheets and aluminum honeycomb core. The sandwich is loaded under quasi-static condition and along out-of-plane direction. A commercial finite element analysis software ABAQUS is used. The results show that the indented composite retains significant amount of strength after indentation. And, the post-indentation strength of the composite is about 65% its pre-indentation strength under compression.

Abstract: Microcantilever hotplates use the self-heating phenomenon to achieve the required large, uniform temperature field or the cantilever-tip movement. The present study investigates the effect of self-heating on temperature and its distribution in microcantilever hotplates in monolith and u-shape configuration with different substrate materials, operating environments and applied voltages. The cantilevers are made of Si and SiO2 with a p-type Si resistor embedded. The cantilevers are operated in air and water at different applied voltages. The numerical analysis uses finite element analysis software ANSYS Multiphysics. Results show that monolith and u-shape cantilevers have similar temperature distributions, but the maximum temperature values in u-shape cantilever are higher.

Abstract: In this paper, a new non-contact sensor is presented for detecting torque of a rotating stepped shaft which is frequently employed in power transmission system. This sensor doesn’t require cutting or lengthening the rotating shaft. Torque value is obtained by using two magnetic sensors to sense magnetic field intensity of two permanent rubber magnets fixed at the outer surface of the shaft. The phase difference between these two induction signals is used to determine torque of the stepped shaft. A real-time algorithm based on LabVIEW is employed to obtain the measured torque value. The present work has demonstrated that non-contact torque measurement for rotating stepped shaft by monitoring magnetic field is feasible. It seems like that further development will result in low-cost torque sensor. It is hoped that this kind of sensor can lead to a new development direction of torque sensor for rotating shaft.

Abstract: Residual strength degradation of 35CrMo alloy steel is investigated experimentally and theoretically in this paper. The experimental results were obtained by high-frequency fatigue testing machine and electro-hydraulic servo testing machine at room temperature. The experimental results showed the true tensile strength under static load and the residual strength under different cycle ratio, respectively. An exponential degradation model was proposed relating the initial strength, residual strength to the applied fatigue cycles and the constant amplitude stress. The residual strength distribution was described using three-parameter Weibull probability density function. The mathematical software MatLab was used for parameter estimation. Finally, the distribution function and failure probability density function of the residual strength was received, which was significant in fatigue reliability.

Abstract: The objective of this study is to investigate the effect of the low or high strain rate on the impact fatigue properties of the nickel foam material and to understand the lifetime of this material which is subjected to the repeated impacts at different energy levels. Failures of foam materials under single and repeated impacts analogous to fatigue are essential to designers and users in military and aerospace structures. The material failure induced by repeated impact loading becomes a critical issue because of significant loss of stiffness and compressive strength in the foam material. Testing methods to study impact(that is, high strain rate) fatigue are quite numerous; no single standard testing procedure is defined for studying the impact fatigue property of a material. The increasing application of foam material in aerospace structures, owing to high specific stiffness and strength has attracted a great concern about the high sensitivity to impact damage introduced during manufacture or in service, and the effects of such damage on structural degradation. To investigate this issue, this study sets up an experimental procedure to determine the impact fatigue properties of nickel foam material. This study performs both experimental and numerical investigations to catch the impact fatigue behavior of nickel foam with open type. Design life and probability of failure or survival at specified life can be calculated so that the fatigue life of nickel core material subjected to repeated impact loading is predicted.

Abstract: This paper is devoted to the static friction torque of electromagnetic clutch. The torque maximization is also investigated by optimizing the geometrical shape of armature. For the purpose of designing and optimizing electromagnetic clutch, torque prediction is a very important factor. We construct an axi-symmetric FEM model to analyze static friction torque and use a torque tester to evaluate real torque. In this work, analytically predicted torque is compared with the experimental one to discuss the rationality of numerical process. The analytical result agrees well with experimental data, which proves the validity of the mathematical process. Through optimization of the shape of armature, we also improve the static torque of electromagnetic clutch about 30%.

Abstract: Single or multiple of delaminations have been found frequently on the fracture surface of X70 pipeline steel. In this study, the delamination cracks and their influence on the fracture of pipeline are investigated by both experiment and three-dimensional fracture analyses. It is shown that the three-dimensional stress state is prerequisite for delamination crack and the strength distribution of material influences the form and direction of delamination crack. The delamination cracks are produced on the weak interfaces among the material by the tensile stress perpendicular to them before the fracture passes. The direction of delamination crack depends on the three-dimensional stress fields and strength distribution of material near the crack tip or notch root. The delamination cracks of the fracture through thickness of pipe wall make the effective thickness decrease and the delamination cracks of surface crack are perpendicular to the direction of fracture propagation direction. The delamination cracks reduce the stress triaxiality near crack tip and in turn, improve the fracture toughness of X70 pipeline steel.

Abstract: Superelastic shape memory alloys (SMAs) can experience large strains up to 8~10% and restore residual strains just by removing the stress. By employing this unique characteristic of SMAs, a new beam-to-column connection is presented in this paper. The proposed SMA connection consists of an extended end-plate, eight long shank Nitinol SMA bolts, beam flange ribs, beam web stiffeners and continuity steel plates for reinforcing column flanges. In order to predict the behavior of SMA connection, 3-D solid finite element models are set up in ANSYS. The numerical results indicate that the local buckling of beam is avoided and a plastic hinge forms at beam-to-column interface when the moment-carrying capacity of bolt cluster is below the elastic flexural capacity of connecting beam. The SMA connection shows stable moment-rotation hysteresis curves with re-centering capability, which demonstrates the connection’s self-healing deformations function. Far different from energy dissipated by steel yield in traditional connection, the SMA connection shows moderate energy dissipation capacity, and this amount of energy is mainly dissipated by the superelastic behavior of SMA bolts. Moreover, the connection model shows a large inelastic rotation capacity beyond 0.03 rad. The ductility of SMA connection is deeply influenced by the length of SMA bolts, and the 2.2 times length of normal bolt is suggested for SMA bolts.

Abstract: An enhancement to pre & post processing techniques, which is encapsulated in ‘Tunnel Modelling Wizard (TMW)’ for three dimensional tunnel analysis, is developed and described in this paper. The techniques include geometry modelling, mesh generation, virtual transformation, flying view and utilization and interpretation of analysis results. The TMW automatically generates full analysis data including mesh, loads, boundary conditions and construction stages from the user-defined parameters. The TMW also provides its own file I/O service to help users expedite modelling tasks for similar models and build their own tunnel templates. Implementation of the TMW into a FE Program is described, and applications of the TMW to practical tunnel problems will be demonstrated.