No description available.

A ramjet inlet duct structure has been designed using nonlinear structural analysis to allow local plastic deformations which result in structural weight reduction. To establish the feasibility of reduced shell thickness, the Lower-Bound Theorem of limit analysis has been applied using an existing finite element analysis computer code intended for elastic analysis. The results of the analysis were verified by structural testing. Predicted stresses and deflections showed good correlation with measured values up to the point of failure. This work was supported by the U.S. Air Force Aero Propulsion Laboratory, Wright-Patterson Air Force Base, Ohio, under Contract No. F33615-72-C-1366.

No description available.

Over the past several decades, the use of composite materials has grown considerably. Typically, fiber-reinforced polymer-matrix composites are modeled as being linear elastic. However, it is well-known that polymers are viscoelastic in nature. Furthermore, the analysis of complex structures requires a numerical approach such as the finite element method. In the present work, a triangular flat shell element for linear elastic composites is extended to model linear viscoelastic composites. Although polymers are usually modeled as being incompressible, here they are modeled as compressible. Furthermore, the macroscopic constitutive properties for fiber-reinforced composites are assumed to be known and are not determined using the matrix and fiber properties along with the fiber volume fraction. Hygrothermo-rheologically simple materials are considered for which a change in the hygrothermal environment results in a horizontal shifting of the relaxation moduli curves on a log time scale, in addition to the usual hygrothermal loads. Both the temperature and moisture are taken to be prescribed. Hence, the heat energy generated by the viscoelastic deformations is not considered. When the deformations and rotations are small under an applied load history, the usual engineering stress and strain measures can be used and the time history of a viscoelastic deformation process is determined using the original geometry of the structure. If, however, sufficiently large loads are applied, the deflections and rotations will be large leading to changes in the structural stiffness characteristics and possibly the internal loads carried throughout the structure. Hence, in such a case, nonlinear effects must be taken into account and the appropriate stress and strain measures must be used. Although a geometrically-nonlinear finite element code could always be used to compute geometrically-linear deformation processes, it is inefficient to use such a code for small deformations, due to the continual generation of the assembled internal load vector, tangent stiffness matrix, and deformation-dependent external load vectors. Rather, for small deformations, the appropriate deformation-independent stiffness matrices and load vectors to be used for all times can be determined once at the start of the analysis. Of course, the time-dependent viscoelastic effects need to be correctly taken into account in both types of analyses. The present work details both geometrically-linear and nonlinear triangular flat shell formulations for linear viscoelastic composites. The accuracy and capability of the formulations are shown through a range of numerical examples involving beams, rings, plates, and shells. / Ph. D.

The plane strain solution is obtained for the natural vibrations and impulse response of a thin circular cylinder containing an added line mass. The solution for a uniform cylinder is derived by taking the added mass to be zero. Numerical calculations of the frequencies and mode shapes for several of the lower modes are presented in graphical form for various values of the added mass. The general impulse response solution for arbitrary initial conditions is obtained by normal mode theory. For both the natural vibrations and impulse response, the theory is found to be in reasonable agreement with available experimental results. In a particular mode, four distinct solution states are found to exist: a symmetrical and anti-symmetrical branch for each class of vibration, flexural and extensional. Noteworthy features revealed by this investigation are the difference in frequency and mode shape of each solution state and the presence of coupling between the flexural and extensional classes, particularly noticeable in the extensional class mode shapes. In comparing impulse response solutions for velocity with and without the added mass, the major influence of the added mass is found to be an increased participation of the flexural class modes, including the rigid body translation, and decreased participation of the extensional class oscillatory modes. / Ph. D.

This study developed a finite element program suitable for analyzing one quarter of a rectangular tank. A rectangular plate element capable of both extension and flexure was used with appropriate coordinate transformations to enable interaction of the floor and wall plates. Moment values throughout the tank were determined but not collected into tables because of their dependence on the width-to-length ratios and the height of the tank. A moment distribution type of method was developed so that critical vertical moment values could be rapidly determined without the direct use of a complex computer program. / Master of Science

M. S.

A numerical-perturbation approach is used to study modal interactions in the dynamic response of infinitely long circular cylindrical shells to an external harmonic excitation. The excitation frequency is near the linear natural frequency of the breathing mode (i.e., primary resonance of the breathing mode) and the linear natural frequency of the breathing mode is approximately twice that of a flexural mode (i.e., two-to-one internal or autoparametric resonance). The method of multiple-time scales is used to derive a set of autonomous first-order nonlinear differential equations that describe the modulation of the amplitudes and phases of the interacting modes. The same approach is used to study the axisymmetric dynamic response of spherical shells to a radial harmonic excitation having a frequency near one of the linear natural frequencies of a flexural mode (i.e., primary resonance of a flexural mode) and in the presence of a two-to-one internal resonance between the excited mode and a lower flexural mode. The modulation equations derived for infinitely long circular cylindrical shells and for axisymmetric spherical shells are scaled to the same form and their fixed points, periodic solutions, and chaotic solutions are studied as the amplitude or the frequency of excitation varies. As the excitation amplitude varies, the fixed-point solutions of the modulation equations exhibit the jump and saturation phenomena. They also undergo supercritical and subcritical Hopf bifurcations as the frequency or the amplitude of excitation varies. Between the two Hopf-bifurcation frequencies, the fixed-point solutions are unstable and limit cycles exist. Some limit cycles experience symmetry-breaking (pitchfork) bifurcation followed by an infinite cascade of period-doubling bifurcations culminating in chaos. Other limit cycles lose stability through cyclic-fold bifurcations causing a transition to chaos. The same procedure is used to study the nonlinear dynamic response of infinitely long circular cylindrical shells to a subharmonic excitation of order one-haIf of the breathing mode in the presence of a two-to-one internal resonance. The force-response curves exhibit saturation, jumps, and Hopf bifurcations. They also show that the shell does not respond until a certain threshold level of excitation is exceeded. The frequency-response curves exhibit jumps and pitchfork and Hopf bifurcations. For certain parameters and excitation frequencies between the Hopf-bifurcation values, limit-cycle solutions of the modulation equations are found. As the excitation frequency changes, the limit cycles deform and lose their stability through either pitchfork or cyclic-foId (saddle-node) bifurcations. Some of these saddIe-node bifurcations cause a transition to chaos. The pitchfork bifurcations break the symmetry of the limit cycles. Period-three motions are observed over a narrow range of excitation frequencies. Lastly, a computerized symbolic manipulator is used to analyze the dynamic response of an infinitely long circular cylindrical shell to radial harmonic excitations. The excitation frequency is near the linear natural frequency of a flexural mode (i.e., primary resonance of a flexural mode). Due to the complete circular symmetry of the shell, each natural frequency corresponds to two orthogonal mode shapes. The mode with the same spatial variation as the excitation is called the driven mode and the other mode is called the companion mode. Modal interactions between the driven mode and the companion mode are studied. The steady-state response of the shell can involve either the driven mode alone (single-mode response) or both the driven and companion modes (two-mode response). The frequency-response curve exhibits jumps and Hopf bifurcations. Between the Hopf·bifurcation frequencies, the modulation equations exhibit multiple limit-cycle solutions. As the excitation frequency varies, these limit cycles go through either saddle-node collisions or incomplete sequences of period-doubling bifurcations. Some of the saddle-node bifurcations result in the birth of limit cycles and some result in transition to chaos. / Ph. D.

Thesis (MEng)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: Peach and apricot shells are agricultural waste products. These waste products accumulate around canneries and food-processing plants in South Africa. No effort is being made to utilise these waste products. This study is the first part of the product development from these products i.e. peach shell activated carbon and apricot shell activated carbon. By producing activated carbon from peach and apricot shells the solid waste problem is addressed, but most of all a profit can be made. But why activated carbon? Activated carbons are unique and versatile adsorbent with a vast amount of adsorption applications. It can be produced via a simple oxidation reaction with steam and the nature of peach and apricot shells are such that it is expected that activated carbons with good adsorption properties can be produced from it. The single largest consumer of activated carbon in South Africa is the gold mining industry that uses imported coconut shell activated carbon for gold adsorption in the gold recovery process. Activated carbon is also used as water purification adsorbents. During this study activated carbons were produced in a fluidized bed reactor at various activation conditions: 700 - 900°C, 0.0425 - 0.0629 g steamlg char.min and 30 - 60 min. This was done in order to find the optimum activation conditions within the activation parameter range. The optimal activated carbons were defined as peach and apricot shell activated carbons that showed good microporous as well as mesoporous character. The optimal activated carbons produced are: peach shell activated at 875°C, 0.0533 g stearnlg char. min, 60 min and apricot shell activated carbon at 850°C, 0.0533 g steamlg char.min, 60min. The possible use of these optimal activated carbons and two other activated carbons produced (Peach shell activated carbon 900°C, 0.0425 g steamlg char. min, 60 min and apricot shell activated carbon 900°C, 0.0425 g steamlg char.min, 60min) were tested in gold recovery and water purification. The gold adsorption properties of peach and apricot shell activated carbons were found to be better than two commercial coconut shell activated carbons (Chemquest 650 and GRC 22). No definite conclusions could, however, be drawn about the replacement of coconut shell activated carbon with peach or apricot shell activated carbon, because abrasion test work and thermal regeneration of the experimental carbons still have to be performed. The experimental activated carbons displayed good phenol adsorption characteristic, although further test work is required. / AFRIKAANSE OPSOMMING: Perske- en appelkoospitte is landbouafvalprodukte. Hierdie afvalprodukte versamel rondom inmaakfabrieke en voedselververkingsaanlegte. Tans word daar geen poging in Suid-Afrika aangewend om hierdie afvalprodukte te benut nie. Hierdie studie is die eerste deel van die ontwikkeling van die produkte: Perskepitdop-geaktiveerde koolstof en appelkoospitdop-geaktiveerde koolstof. Deur geaktiveerde koolstof van die perske- en appelkoospitdoppe te maak, word nie net 'n antwoord op die vastestofafvalsprobleem gevind nie, maar daar kan ook geld gemaak word. Hoekom geaktiveerde koolstof? Aktiveerde koolstowwe is veelsydige en unieke adsorbente met 'n groot verskeidenheid adsorpsie toepassings. Dit kan vervaardig word via 'n eenvoudige oksidasie reaksie met stoom en die aard van die perske- en appelkoospitdoppe is sodanig, dat verwag kan word om geaktiveerde koolstowwe met goeie adsorpsie eienskappe daarvan te kry. Die grootste enkelverbruiker van geaktiveerde koolstof in Suid-Afrika is die goudmynbedryf, wat kokosneutdop geaktiveerde koolstof invoer om goud te herwin. Geaktiveerde koolstof word ook gebruik vir watersuiwering. Tydens hierdie studie IS geaktiveerde koolstowwe by verskillende aktiveeringskondisies in 'n gevloeïdiseerde bed vervaardig: 700 - 900oe, 0.0425 - 0.0629g stoornlg gepiroliseerde pitdoppe.min en 30 - 60 mm. Die aktiveringskondisies is gevarieer om sodoende die optimale aktiveringskondisies binne die aktiveringsparameterreeks te kry. 'n Geaktiveerde koolstof is as optimaal geklassifiseer as dit 'n goeie mikro- sowel as mesostruktuur getoon het. Die optimaal geaktiveerde koolstowwe is: geaktiveerde koolstof vervaardig van perskepitdoppe by 875°e, 0.0533 g stoornlg gepiroliseerde pitdoppe.min, 60 mm en geaktiveerde koolstof vervaardig van appelkoospitdoppe by 850oe, 0.0533 g stoornlg gepiroliseerde pitdoppe.min, 60min. Die gebruik van die twee optimale geaktiveerde koolstowwe sowel as twee ander geaktiveerde koolstowwe (perskepitdop-geaktiveerde koolstof, 900oe, 0.0425 g stoornlg gepiroliseerde pitdoppe.min, 60 min en appelkoospitdop-geaktiveerde koolstof, 850°C, 0.0533 g stoom/g gepiroliseerde pitdoppe.min, 60min) is VIr goudadsorpsie en watersuiwering ondersoek. Die goudadsorpsie eienskappe van die perske-en appelkoospitdop-geaktiveerde koolstowwe was beter as die van twee kommersiële kokosneutdop-geaktiveerde koolstowwe (Chemquest 650 and GRC 22). Daar kan egter geen definitiewe gevolgtrekkings gemaak word oor die vervanging van kokosneutdop geaktiveerde koolstowwe met dié van perske of appelkoospitdoppe nie, aangesien daar nog toetsresultate oor die slytweerstand en reaktiverings eienskappe van die eksperimentele geaktiveerde koolstowwe uitstaande is. Die eksperimentele geaktiveerde koolstowwe toon goeie adsorpie ten opsigte van fenol, maar verdere toetswerk is egter nodig.

Cashew nut and coconut shells are two potential renewable and environmentally friendly energy sources that are commonly found as agro-industrial wastes in tropical countries. Despite this fact, they are not yet widely studied as such. Given this lack of specific technical and reliable data, technologies for their conversion into energy cannot be designed with confidence as it happens with other commonly studied biomass feedstock. Thus, the need to generate these data guided this research in order to provide technical information for the designing of appropriate thermochemical conversion technologies for energy generation, particularly, in remote areas, where electricity grid is neither a feasible nor an affordable solution.Among thermochemical processes, pyrolysis plays a key role as it is found in both combustion and gasification at their earlier stages. In both technologies, pyrolysis products are generated and later submitted to further transformations according to the process in use.Hence, pyrolysis was selected for thermal characterisation of cashew nut and coconut shells. The main characteristics envisaged are i) pyrolysis profiles; ii) global, semi-global and individual kinetics; iii) pyrolysis global and individual yields; iv) modelled pyrolysis yields at high heating rates; and, v)char combustion kinetics and reactivity. The main technique used for experimental data generation is thermogravimetry and FTIR spectroscopy. Data experimentally generated from TG and TG-FTIR experiments were processed through different methods and codes, such as the Coats and Redfern model-fitting method, the modelfree methods of Ozawa-Flynn-Wall, Friedman and ASTM E698, for semi-global and global kinetics; DAEM and FG-Biomass were used for pyrolysis individual kinetics and yields determination. Proximate and ultimate analyses were performed as well.The study revealed peculiar characteristics compared to the commonly known lignocellulosic biomass. The volatiles content was above 66%w/w; hemicelluloses DTG peak did not overlap with the cellulose peak; the global pyrolysis activation energies were around 200 and 120 kJ/mol for coconut and cashew nut shells, respectively. Hemicelluloses and cellulose showed varying activation energies as 130-216 and 155-208 kJ/mol, respectively. Char combustion showed two steps with activation energies of 135 and 121 kJ/mol (cashew nut shells); 105 and 190kJ/mol (coconut shells). Individual yields and kinetics were determined for 17 compounds, including tars. These data are of key importance for modelling and the consequent data generation for the designing of appropriate thermochemical energy for these biomasses. / QC 20100914