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42 results on '"Kukureka, S. N."'

4. Effect of Materials and Manufacturing on the Bending Stiffness of Vaulting Poles

Author

Davis, C. L. and Kukureka, S. N.

Abstract

The increase in the world record height achieved in pole vaulting can be related to the improved ability of the athletes, in terms of their fitness and technique, and to the change in materials used to construct the pole. For example in 1960 there was a change in vaulting pole construction from bamboo to glass fibre reinforced polymer (GFRP) composites. The lighter GFRP pole enabled the athletes to have a faster run-up, resulting in a greater take-off speed, giving them more kinetic energy to convert into potential energy and hence height. GFRP poles also have a much higher failure stress than bamboo, so the poles were engineered to bend under the load of the athlete, thereby storing elastic strain energy that can be released as the pole straightens, resulting in greater energy efficiency. The bending also allowed athletes to change their vaulting technique from a style that involved the body remaining almost upright during the vault to one where the athlete goes over the bar with their feet upwards. Modern vaulting poles can be made from GFRP and/or carbon fibre reinforced polymer (CFRP) composites. The addition of carbon fibres maintains the mechanical properties of the pole, but allows a reduction in the weight. The number and arrangement of the fibres determines the mechanical properties, in particular the bending stiffness. Vaulting poles are also designed for an individual athlete to take into account each athlete's ability and physical characteristics. The poles are rated by "weight" to allow athletes to select an appropriate pole for their ability. This paper will review the development of vaulting poles and the requirements to maximize performance. The properties (bending stiffness and pre-bend) and microstructure (fibre volume fraction and lay-up) of typical vaulting poles will be discussed. (Contains 5 figures.)

Published
2012
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12. Self-sensing, self-healing, and crack-arrestor composites

Author

Harris, D., primary, Mahendran, R. S., additional, Brooks, D., additional, Al-Khodairi, F.A. A., additional, Machavaram, V. R., additional, Reynolds, P., additional, Wang, L., additional, Pandita, S. D., additional, Paget, M., additional, Wedderburn, J., additional, Malik, S. A., additional, Ojo, S. O., additional, Kukureka, S. N., additional, and Fernando, G. F., additional

Published
2009
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23. Effect of materials and manufacturing on the bending stiffness of vaulting poles.

Author

Davis, C. L. and Kukureka, S. N.

Subjects
VAULTING, SPORTING goods, GLASS fibers, COMPOSITE materials, KINEMATICS in sports, STRAINS & stresses (Mechanics), ELASTICITY, MICROSTRUCTURE
Abstract

The increase in the world record height achieved in pole vaulting can be related to the improved ability of the athletes, in terms of their fitness and technique, and to the change in materials used to construct the pole. For example in 1960 there was a change in vaulting pole construction from bamboo to glass-fibre reinforced polymer (GFRP) composites. The lighter GFRP pole enabled the athletes to have a faster run-up, resulting in a greater take-off speed, giving them more kinetic energy to convert into potential energy and hence height. The GFRP poles also have a much higher failure stress than bamboo, so the poles were engineered to bend under the load of the athlete thereby storing elastic strain energy that can be released as the pole straightens, resulting in greater energy efficiency. The bending also allowed athletes to change their vaulting technique from a style that involved the body remaining almost upright during the vault to one where the athlete goes over the bar with their feet upwards. Modern vaulting poles can be made from GFRP and / or carbon-fibre reinforced polymer composites (CFRP). The addition of carbon fibres maintains the mechanical properties of the pole, but allows a reduction in the weight. The number and arrangement of the fibres determines the mechanical properties, in particular the bending stiffness. The vaulting poles are also designed for an individual athlete to take into account each athlete's ability and physical characteristics. The poles are rated by 'weight' to allow athletes to select an appropriate pole for their ability. This paper will review the development of vaulting poles and the requirements to maximize performance. The properties (bending stiffness and pre-bend) and microstructure (fibre volume fraction and lay-up) of typical vaulting poles will be discussed. [ABSTRACT FROM AUTHOR]

Published
2004

28. Tensile Properties of Polymer Samples Containing Millimetre Scale Silicon Microchips.

Author

Kolonko, A. M., Metje, N., Chapman, D. N., and Kukureka, S. N.

Subjects
TENSILE strength, POLYMERS, POLYETHYLENE, INTEGRATED circuits, SILICON
Abstract

In order to produce monitoring systems for polyethylene pipes, one option is to distribute sensors within the plastic matrix. Currently, no information is available on the effects of small, individual sensors on the structural integrity of polyethylene pipes if these are embedded in the pipe wall. In order to assess the influence of these sensors, small samples were subjected to tensile tests with different sizes and shapes of microchips investigated at different positions and orientations. It was discovered that a square microchip improves the stiffness the most but reduces the ductility of the sample the most, while a microchip with the same area, but of circular shape also acts as reinforcement when its volume is sufficiently large relative to the volume of the polymer matrix, causing the smallest reduction in ductility. The optimal microchip orientation is parallel to the tension direction. [ABSTRACT FROM AUTHOR]

Published
2012
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29. Frequency dependence of viscoelastic properties of medical grade silicones.

Author

Mahomed, A., Chidi, N. M., Hukins, D. W. L., Kukureka, S. N., and Shepherd, D. E. T.

Subjects
DYNAMIC mechanical analysis, SILICONES, SALINE solutions, ARTHROPLASTY
Abstract

Cylinders of medical grade silicone elastomers, (29 mm in diameter and 13 mm thick), immersed in physiological saline solution at 37°C, were investigated by dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 ± 5 N was applied over a frequency range, f, of 0.02–100 Hz. Values of the storage, E′, and loss, E″, moduli for the cylinders were found to depend on f; the dependence of E′ or E″ on the logarithm (base 10) of f was represented by a third‐order polynomial. Above about 0.3 Hz, the cylindrical specimens appeared to be undergoing the onset of a transition from the rubbery to the glassy state. There was no significant difference between results obtained at 37 and 23°C; pretreatment of specimens in physiological saline at 37°C for 24 h and 29 days had no appreciable effect on the results. © 2008 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2009 [ABSTRACT FROM AUTHOR]

Published
2009
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30. In-Situ Copolymerization of Cyclic Poly(butylene terephthalate) Oligomers and ε-Caprolactone

Author

Tripathy, A. R., MacKnight, W. J., and Kukureka, S. N.

Abstract

Copolyesters were synthesized by using ring-opening polymerization of cyclic poly(butylene terephthalate) oligomers (c-PBT) and ε-caprolactone monomer (CL) in the presence of stannoxane catalyst at an elevated temperature. The in-situ copolymerization was conducted with various c-PBT/CL feed ratios. The copolyesters were characterized by GPC, NMR, and FT-IR techniques. Phase separation and the relaxation behavior of the resultant copolyesters were investigated by DSC, DMTA, and dielectric spectroscopy measurements. Mechanical properties (stress−strain behavior) are also reported and can be adjusted by varying the ε-caprolactone content of the copolymer.

Published
2004

37. Twisting fatigue in multilayer films of Ag-alloy with indium tin oxide on polyethylene terephthalate for flexible electronics devices

Author

Mohammed, D.W., Ameen, R. B., Sierros, K. A., Bowen, J., Kukureka, S. N., Mohammed, D.W., Ameen, R. B., Sierros, K. A., Bowen, J., and Kukureka, S. N.

Abstract

Twisting monotonic and fatigue experiments were conducted on multi-layered films of Ag-alloy based indium tin oxide (ITO) deposited on polyethylene terephthalate (PET). In the twisting tests, crack development and electrical resistance were monitored in situ. Cracks initiated at an angle of 39° ± 1.7° and propagated towards the direction of the sample length. Two sets of experiments were performed; the first set of experiments was conducted to study the effect of twisting angle and temperature on the film's electromechanical performance. The other set of experiments was conducted to study the effect of temperature in the absence of cyclic twisting deformation. The change in electrical resistance increased with number of twisting cycles and twisting angle. In addition, the highest change in electrical resistance was observed for samples subjected to cyclic fatigue at 100 °C, which is attributed to crack growth and oxidation of the Ag-alloy layer. The cracks were observed to initiate not only from coating defects but also from edge defects. Development of cracks is accelerated due to the combined effects of the external repeated stress and temperature. Therefore, it is suggested that controlling temperature when using ITO/Ag-alloy/ITO thin film under mechanical stress is important for electrical device performance; temperatures in both fabrication and use should not exceed 50 °C.

38. Mechanical properties of amorphous indium-gallium-zinc oxide thin films on compliant substrates for flexible optoelectronic devices

Author

Mohammed, D. W., Waddingham, R., Flewitt, A. J., Sierros, K. A., Bowen, J., Kukureka, S. N., Mohammed, D. W., Waddingham, R., Flewitt, A. J., Sierros, K. A., Bowen, J., and Kukureka, S. N.

Abstract

Amorphous indium–gallium–zinc-oxide (a-IGZO) thin films were deposited using RF magnetron sputtering on polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) flexible substrates and their mechanical flexibility investigated using uniaxial tensile and buckling tests coupled with in situ optical microscopy. The uniaxial fragmentation test demonstrated that the crack onset strain of the IGZO/PENwas ~2.9%,which is slightly higher than that of IGZO/PET. Also, uniaxial tensile crack density analysis suggests that the saturated crack spacing of the film is strongly dependent on the mechanical properties of the underlying polymer substrate. Buckling test results suggest that the crack onset strain (equal to ~1.2%, of the IGZO/polymer samples flexed in compression to ~5.7 mm concave radius of curvature) is higher than that of the samples flexed with the film being in tension (convex bending) regardless whether the substrate is PEN or PET. The saturated crack density of a-IGZO film under the compression buckling mode is smaller than that of the film under the tensile buckling mode. This could be attributed to the fact that the tensile stress encouraged this crack formation originating from surface defects in the coating. It could also be due to the buckling delamination of the thin coating from the substrate at a lower strain than that at which a crack initiates during flexing in compression. These results provide useful information on the mechanical reliability of a-IGZO films for the development of flexible electronics.

39. Twisting fatigue in multilayer films of Ag-alloy with indium tin oxide on polyethylene terephthalate for flexible electronics devices

Author

Mohammed, D.W., Ameen, R. B., Sierros, K. A., Bowen, J., Kukureka, S. N., Mohammed, D.W., Ameen, R. B., Sierros, K. A., Bowen, J., and Kukureka, S. N.

Abstract

Twisting monotonic and fatigue experiments were conducted on multi-layered films of Ag-alloy based indium tin oxide (ITO) deposited on polyethylene terephthalate (PET). In the twisting tests, crack development and electrical resistance were monitored in situ. Cracks initiated at an angle of 39° ± 1.7° and propagated towards the direction of the sample length. Two sets of experiments were performed; the first set of experiments was conducted to study the effect of twisting angle and temperature on the film's electromechanical performance. The other set of experiments was conducted to study the effect of temperature in the absence of cyclic twisting deformation. The change in electrical resistance increased with number of twisting cycles and twisting angle. In addition, the highest change in electrical resistance was observed for samples subjected to cyclic fatigue at 100 °C, which is attributed to crack growth and oxidation of the Ag-alloy layer. The cracks were observed to initiate not only from coating defects but also from edge defects. Development of cracks is accelerated due to the combined effects of the external repeated stress and temperature. Therefore, it is suggested that controlling temperature when using ITO/Ag-alloy/ITO thin film under mechanical stress is important for electrical device performance; temperatures in both fabrication and use should not exceed 50 °C.

40. Mechanical properties of amorphous indium-gallium-zinc oxide thin films on compliant substrates for flexible optoelectronic devices

Author

Mohammed, D. W., Waddingham, R., Flewitt, A. J., Sierros, K. A., Bowen, J., Kukureka, S. N., Mohammed, D. W., Waddingham, R., Flewitt, A. J., Sierros, K. A., Bowen, J., and Kukureka, S. N.

Abstract

Amorphous indium–gallium–zinc-oxide (a-IGZO) thin films were deposited using RF magnetron sputtering on polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) flexible substrates and their mechanical flexibility investigated using uniaxial tensile and buckling tests coupled with in situ optical microscopy. The uniaxial fragmentation test demonstrated that the crack onset strain of the IGZO/PENwas ~2.9%,which is slightly higher than that of IGZO/PET. Also, uniaxial tensile crack density analysis suggests that the saturated crack spacing of the film is strongly dependent on the mechanical properties of the underlying polymer substrate. Buckling test results suggest that the crack onset strain (equal to ~1.2%, of the IGZO/polymer samples flexed in compression to ~5.7 mm concave radius of curvature) is higher than that of the samples flexed with the film being in tension (convex bending) regardless whether the substrate is PEN or PET. The saturated crack density of a-IGZO film under the compression buckling mode is smaller than that of the film under the tensile buckling mode. This could be attributed to the fact that the tensile stress encouraged this crack formation originating from surface defects in the coating. It could also be due to the buckling delamination of the thin coating from the substrate at a lower strain than that at which a crack initiates during flexing in compression. These results provide useful information on the mechanical reliability of a-IGZO films for the development of flexible electronics.

42. Accelerated aging for testing polymeric biomaterials and medical devices.

Author

Hukins DW, Mahomed A, and Kukureka SN

Subjects
Computer Simulation, Algorithms, Biocompatible Materials chemistry, Equipment Failure Analysis methods, Equipment and Supplies, Materials Testing methods, Models, Theoretical
Abstract

Elevated temperature is frequently used to accelerate the aging process in polymers that are associated with medical devices and other applications. A common approach is to assume that the rate of aging is increased by a factor of 2(DeltaT/10), where DeltaT is the temperature increase. This result is a mathematical expression of the empirical observation that increasing the temperature by about 10 degrees C roughly doubles the rate of many polymer reactions. It is equivalent to assuming that the aging process is a first order chemical reaction with an activation energy of 10R/log(e)2, where R is the universal gas constant. A better approach would be to determine the activation energy for the process being considered but this is not always practicable. The simple approach does not depend on the temperature increase, provided that it is not so great that it initiates any physical or chemical process that is unlikely to be involved in normal aging. If a temperature increment theta were to increase a given polymer reaction rate n times, then an elevated temperature would increase the rate of aging by a factor of n(DeltaT/theta).

Published
2008
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