The concept of achieving low cost (≈ $20/m²) and ultra low weight (5 kg/m²) for heliostats is explored theoretically and experimentally. The objective of this work is to significantly improve the cost and performance of the structure under concern, without sacrificing strength and efficiency. The focus is on an innovative design of stretched-membrane heliostats. A reflective membrane of thin film is supported by a taut fishnet structural membrane consisting of graphite fiber-polymer matrix composite. The reflective and structural membranes are attached to a ring frame made of wood. The nonlinear problem of stress-strain analysis is formulated and solved using the finite-element code NASTRAN. The analysis is done for loads which include the initial stretching of the film and structural membrane and the pressure load due to wind. The scope of the present work is limited to analyzing the structural deformation behavior of flat-plate heliostats and partial extension to parabolic and semi-hemispherical dish reflectors.

Submitted in fulfilment of the degree of Doctor of Dental Material Science in the Department of Dental Services, Faculty of Health Sciences, Durban University of Technology, Durban, South Africa, 2008. / Introduction: Incorporating a feldspar chemical bond between alumina filler particles is expected to increase the wear resistant and flexural strength properties, while reducing flexibility of dental composites. Aims and Objectives: An investigation was carried out to evaluate the influence of the feldspar chemical bonding between alumina filler particles on wear, flexural strength and flexibility of experimental alumina/feldspar dental composites. It was hypothesized that wear resistance and flexural strength would be significantly increased with increased feldspar mass, while flexibility was expected to decrease. Methods: Alumina was chemically sintered and bonded with 30%, 40%, 50% and 60% feldspar mass, silanized and infiltrated with UDMA resin to prepare the dental restorative composite material specimens. Results and conclusions: Significantly higher wear resistant characteristics resulted with increased feldspar mass (p<0.5). Improvements in flexural strength characteristics as the feldspar mass was increased was not statistically different (p>0.5). Flexibility characteristics as the feldspar mass was increased was not statistically different (p>0.5). The alumina/feldspar specimens showed lower flexibility (mm displacement) than SR ADORO (p<0.05). Feldspar chemical bonding between the alumina particles may improve on the wear resistance and Flexibility of alumina/feldspar composites when compared to SR ADORO. This study evaluated the influence of a chemical feldspar bond between alumina filler particles.

published_or_final_version / Dentistry / Master / Master of Dental Surgery

published_or_final_version / Dentistry / Master / Master of Dental Surgery

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

The dynamic response of subsurface cracks in fiber reinforced composites is analytically studied. The response of layered half-space and three-layered plate with two interface cracks excited by a plane SH-wave and line load respectively are studied by formulating the problem as integral equations in the frequency domain. The governing equations along with boundary, regularity and continuity conditions across the interface are reduced to a coupled set of singular integral equations by using Betti's reciprocal theorem along with the Green's functions. In addition, the transient response of an orthotropic half-space with a subsurface crack subjected to inplane line load at an arbitrary angle is analyzed. Two new Green's functions for the uncracked medium are developed and used along with the representation theorem to derive the scattered field. Satisfaction of the traction free condition at the crack surfaces gives rise to a system of singular integral equations. Singular integrals involved in the analysis are computed numerically by removing the poles. Part of the integrals containing the poles are then obtained analytically by using residue theorem. The solution of singular integral equations are obtained by expanding the unknown crack opening displacements (COD) in terms of a complete set of Chebychev polynomials. The problem is first solved in the frequency domain, the time histories are then obtained numerically by inverting the spectra via Fast Fourier Transform (FFT) routine. Numerical results are presented for isotropic and anisotropic materials for several different crack geometries. The results show significant influence of crack geometries and material properties on the COD and surface response of composites.

A thesis submitted in candidacy for the Degree of Doctor of Technology: Electrical Engineering, Technikon Natal, 2001. / This thesis details a study conducted to investigate the dynamic stability of an existing active control model (ACFl) of a composite structure embedded with a piezoelectric sensor and actuator for the purpose of vibration measurement and control. Criteria for stability are established based on the second method of Lyapunov which considers the energy of the system. Results show that ACFl is asymptotically stable although piezoelectric control effects persist when the feedback gain is set to zero. Meanwhile, it is required that there should be no control effects occurring through the piezoelectric actuator when the gain is set to zero. In this study, a new active control model (ACF2) is developed to satisfy the stability criteria, which satisfies the requirement of no piezoelectric control effects when the gain is set to zero. In ACF2 - as well as ACFl - the displacement and potential fields are discretised using the finite element method. In light of the locking phenomena associated with discrete displacements - which is expected to be pronounced in the case of discrete potentials due to their element geometry, ACF2-mixed is developed. ACF2 and ACF2-mixed control methodologies are similar except that in ACF2 both the displacement and potential field are discretised whereas in ACF2-mixed, only the displacement field is discretised and the potential field is continuous. Consequent to ACF2 and ACF2-mixed, stability analysis of the resulting time integration scheme is investigated as well. The results show that the damping forces due to the piezoelectric effect do not add energy to the structure. Hence, asymptotic stability is achieved. The time integration scheme yielded a small error, consistent with the literature. Numerical results revealed that ACFl exhibits a high degree of locking which is relaxed in ACF2 whereas ACF2-mixed exhibits envisaged results when compared with the other two models. Therefore, the ACF2 and ACF2-mixed will provide engineers with an alternative simulation model to solve actively controlled vibration problems hitherto. / D

No description available.

À haute température, l'oxygène réagit avec la matrice organique du matériau composite, ce qui change les propriétés du matériau et réduit la résistance à la rupture globale. Mais la compréhension de la dégradation du composite stratifié oxydé, en particulier son comportement à l'échelle du pli, est aujourd'hui limitée et phénoménologique. Les travaux dans cette thèse ont été réalisés pour donner notre contribution sur les aspects méthodologiques de ce sujet. Des essais sont réalisés pour caractériser expérimentalement le comportement d'éprouvette oxydée : en particulier, un nouveau essai, l'essai 'oxy-délaminage', a été développé pour identifier la ténacité du matériau composite vieilli. Une stratégie a été proposée d'étudier le problème à partir des propriétés de la matrice oxydée à l'échelle fibre/matrice et jusqu'à la reproduction et la prédiction du comportement de fissuration transversale des échantillons oxydés à échelle méso via un procédé d'homogénéisation. / At high temperature, the oxygen reacts with the organic matrix of composite material, which changes the material properties and reduces the resistance to global failure. But understanding on the degradation of the oxidized composite laminate, particularly its damage behavior, is now limited and phenomenological. Work in this thesis has been made to give our contribution to the methodological aspects of this topic. Tests are performed to characterize experimentally the behaviors of the oxidized specimens: in particular, a new test test oxy-delamination ', was developed to identify the tenacity of composite material aged. A strategy has been proposed to study the problem from the properties the oxidized matrix at the fiber/matrix scale up to the reproduction and predicting the behavior of transverse cracking of the sample at the scale of a ply via a homogenization process.

Department of Mechanical, Industrial and Aeronautical Engineering MSc (Mechanical Engineering) / Aramid fibre reinforced epoxy composites were hybridised by the addition of electrospun PAN (polyacrylonitrile) and ECNF (electrospun carbon nanofibre) doped PAN nanomats. One of the major concerns in polymer composites is the effect of the interlaminar properties on the overall mechanical properties of the composite. Electrospun carbon nanofibres were used as doping agents within PAN nanofibres, and coated in between aramid epoxy laminates to improve the interlaminar properties. PAN nanomats and ECNF doped PAN nanomats were created by the use electrospinning on the surface of aramid fibre sheets. Multiscale hybrid aramid reinforced composites were then fabricated. Mechanical characterization was carried out to determine the effect of PAN and CNF doped PAN nanofibre mats on aramid fibre reinforced epoxy. It was found that PAN reinforced nanomats had improved the mechanical properties and more specifically, when doped by ECNFs, the volume fraction of ECNFs played a vital role. An addition of 1% vol. CNF doped 0.1% vol. PAN reinforcement within a 30% vol. aramid fibre composite (control composite), improved the tensile strength and elastic modulus by 17.3% and 730% respectively. The 0.5% vol. PAN reinforced AFC (aramid fibre composite) specimens revealed a major increase in the flexural strength by 9.67% and 12.1%, when doped by both 0.5% vol. ECNFs and 1% vol. ECNFs respectively. The 0.5% vol. CNF doped reinforcement increased the impact energy by over 40%, for both the 0.1% vol. and 0.2 % vol. PAN reinforced aramid hybrid specimens. The 0.5% vol. CNF doped 0.5% vol. PAN had increased by 30% when compared to a non-doped sample. Morphological studies indicated interlaminar shearing between plies was affected by CNF agglomerations. This was discovered when determining the impact properties of the multiscale doped hybrid composites. Electrospun nanofibres however, assisted in improving the interlaminar regions within aramid epoxy by mechanical locking within the epoxy, and creating an adhesive bond using Van der Waals forces and electrostatic charges between nanofibre and macro fibre. Hybridising aramid epoxy with the use of nanofibres assisted in improving various mechanical properties. Impact degradation was one disadvantage of hybridising using CNF doped PAN nanofibre reinforcements. / MT2017