Ph. D. (Department of Chemical Engineering, Faculty of Engineering and Technology), Vaal University of Technology. / Investment casting process is the most suitable technique for producing high quality castings which are dimensionally accurate with excellent surface finish and complex in nature. Recently with the ever-changing manufacturing landscape, the process has been increasingly used to produce components for the medical, aerospace and sports industry. The present study looked at three investigative scenarios in the development of a pattern material for investment casting process: (i) the development of wax/ethyl vinyl acetate (EVA) and wax/linear low-density polyethylene (LLDPE) blends as the carrier vehicle materials for the development of pattern material for investment casting; (ii) the development of wax/EVA/polymethyl methacrylate (PMMA) based investment casting pattern and lastly (iii) the development of wax/LLDPE/PMMA based investment casting pattern material. The first part of the studies elucidates the effects in terms of the thermal, mechanical, surface and rheological properties when paraffin wax in blended with poly EVA and LLDPE. The developments involved the extrusion of seven formulations for EVA and also LLDPE using a twin-screw extrusion compounder. The paraffin wax weight percent investigated ranged from 33% to 87% thus encompassing both low and high wax loading ratios. The thermal properties of the developed binary blends were characterized via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The mechanical properties were characterized using three-point bending test. The thermo-mechanical and rheological properties were determined using thermomechanical analysis (TMA) and a rheometer respectively. A scanning electron microscope (SEM) was used to study the surface texture of the extruded blends. The thermal properties indicated that the thermal stability of paraffin wax is improved when it is blended with both EVA and LLDPE. DSC curves showed two endothermic melting peaks and two exothermic crystallisation peaks. In the case of wax/EVA blends, there was no distinct peak showing the independent melting of neat wax and EVA. The peak at a temperature of 50 – 72 °C corresponds to the melting of the wax/EVA blend. In the case of wax/LLDPE blends, the peak at 50 -66 °C corresponds to the melting of wax whereas the large peak at 112 - 125°C corresponds to the melting of the LLDPE. Wax/EVA and wax/LLDPE had improved mechanical properties as compared to that of neat wax. The rheological properties of both the EVA based and LLDPE based blends indicated that the viscosity of the blends increased as compared to that of neat wax. SEM confirmed that EVA alters the wax crystal habit at higher concentrations. In the case of wax/LLDPE blends, at 20-30 % wax content, a heterogeneous surface was observed, indicating the immiscibility of the paraffin wax within the LLDPE matrix. At a high wax content, there was agglomeration of wax. LLDPE allows amorphous structure of wax to disperse easily between the chains. The second part of the studies focussed on the wax/EVA filled with poly (methyl methacrylate) (PMMA) microbeads. TGA behaviour on the pyrolysis of wax/EVA/PMMA showed that the compounds volatilise readily with virtually no residue remaining above 500 °C. The DSC curves indicated that, the incorporation of PMMA reduced the crystallinity of wax/EVA blend. A distinct endothermic peak and another small peak was observed in all the formulations. The mechanical properties of wax/EVA/PMMA improved significantly. The methylene group present in both wax and EVA combined to form a blend with enhanced mechanical properties whereas the PMMA microbeads improved the needle penetration hardness. The melt viscosity of wax/EVA/PMMA increased as the EVA and/or the PMMA content is increased. The rheological experimental data fitted with the data predicted using the modified Krieger and Dougherty expression. The maximum attainable volume fraction of suspended PMMA particles was at max = 0.81. The SEM micrograph of wax/EVA/PMMA revealed a near perfect spherical nature for the filler particles in the wax/EVA polymer matrix. It further shows that the PMMA microbeads were weakly bonded and well distributed in the wax/EVA matrix. The third part of the studies focussed on the wax/LLDPE filled with Poly (methyl methacrylate) (PMMA) microbeads. The incorporation of LLDPE and PMMA into paraffin wax had a strong influence on the thermal properties, tensile properties, flow properties and its morphology. The TGA analysis showed that there was a slight observable decrease in the melting onset temperatures when the wax content was increased. From the DSC curves, the corresponding values of onset temperatures observed are between melting and crystallization temperature of neat paraffin wax and neat LLDPE. The short chains of the paraffin wax and the fragments formed by scission of wax chain have sufficient energy to escape from the matrix at lower temperatures. The slight decrease in peak temperatures associated with melting and crystallization could be attributed to the decrease in the average lamellar thickness of the blends. The tensile properties by three-point bending tests indicated an increase in the stress with an increase in the LLDPE content. This can be attributed to the formation of paraffin wax crystals in the amorphous phase of the blend which may influence the chain mobility. Since the paraffin wax used for this study had a low viscosity as compared to LLDPE, both LLDPE or PMMA had an influence on the viscosities of the blends. The data obtained from the experiments fitted with the data predicted obtained from the modified Krieger and Dougherty expression. The maximum attainable volume fraction of suspended PMMA particles was at max = 0.74. Similar observation with that of wax/EVA/PMMA was made in terms of the morphology of the wax/LLDPE/PMMA blends. The excellent thermal stabilities, the superior mechanical strength of wax/EVA/PMMA and wax/LLDPE/PMMA and the flow properties with relatively high EVA and also with high PMMA loadings, open new opportunities for EVA and LLDPE based pattern material for in investment casting process. It is worth pursuing further comprehensive studies since it offers a strong potential for realizing further technological improvement in the field of investment casting and rapid prototyping technologies.

Es wird ein einfacher Berechnungsansatz für ein Baukastensystem aus Kastenprofilen verschiedener Querschnittsabmessungen erarbeitet. Diese Profile bestehen aus WVC (Wood Veneer Composites, Holzfurnierlagenverbundwerkstoffen). Der Ansatz bildet den statischen Lastfall und das Ermüdungsverhalten unter schwellende Dreipunktbiegung ab. Am Beispiel eines ausgewählten Strukturbauteils aus handelsüblichen Birkensperrholz wird der Berechnungsansatz konkretisiert und durch Versuche evaluiert. Aufbauend auf dem Kraft-Verformungsverhalten der analysierten Einzelbauteile und der kapillarporösen Struktur des Holzes wird bei dem Berechnungsansatz auf eine Analyse der Spannungen verzichtet. Stattdessen wird als Berechnungskriterium die kritische Normaldehnung in der Randfaser der Strukturbauteile genutzt. Weiterhin wird eine Methode vorgestellt um mittels niederzyklischen Ermüdungsversuchs (LCF, ca. 1e+03 Lastwechsel) den „Knickpunkt“ der Zeitfestigkeitslinie eines einstufigen Ermüdungsversuchs bei hohen Lastspielzahlen (HCF, 1e+06 bis 1e+07 Lastwechsel) für diese Strukturbauteile zu bestimmen. / It was developed a simple approach for the calculation of a modular construction system for box sections (profile structures) of different cross-sectional dimensions. These profile structures consists of WVC (Wood Veneer Composites). The approach maps the static load case and the fatigue behavior under pulsating three-point bending. By using a structural component made from commercial birch plywood, the calculation approach is specified and verified. Based on the force-deformation behavior of the analyzed single components in connection with the capillary-porous structure of the wood, the calculation approach dispense on an analysis of the tensions. Instead, a criterion, which calculates the critical normal strain in the outer fibers of the structural components, is used. Furthermore, a method of a low-cycle fatigue test (LCF, abbr. 1e+03 cycles) is presented. This method detects the “knee point" of the fatigue limit line for the profiles. That point is usually determined by the use of a high-cycle fatigue tests (HCF, 1e+06 until 1e+07 cycles).

The diploma thesis is focused on the evaluation of mechanical fracture parameters of concrete after exposure to high temperatures. In the introductory theoretical part general principles of fracture mechanics with the concentration on a linear elastic fracture mechanics and non-linear fracture models for the concrete are summarized. The meaning of the three-point bending fracture test used for determination of fracture parameters is also explained. Further the influence of high temperatures on the partial components of concrete and general modeling of temperature loading is described. The practical part is concerned with the evaluation of fire experiments on the concrete panels including numerical simulations using GiD and ATENA software. The evaluation of data obtained from the three-point bending test carried out on specimens with initial stress concentrator taken from concrete panels is a main part of the diploma thesis. The values of modulus of elasticity, effective fracture toughness, work of fracture and fracture energy are determined from the measured F–d and F–CMOD diagrams after their proper corrections in the GTDiPS application. The evaluation of the selected mechanical fracture parameters was performed by StiCrack software using effective crack model and work of fracture method and DKFM_BUT software using the double-K fracture model. Finally, the attention is paid to the analysis of the obtained data.

Es wird ein einfacher Berechnungsansatz für ein Baukastensystem aus Kastenprofilen verschiedener Querschnittsabmessungen erarbeitet. Diese Profile bestehen aus WVC (Wood Veneer Composites, Holzfurnierlagenverbundwerkstoffen). Der Ansatz bildet den statischen Lastfall und das Ermüdungsverhalten unter schwellende Dreipunktbiegung ab. Am Beispiel eines ausgewählten Strukturbauteils aus handelsüblichen Birkensperrholz wird der Berechnungsansatz konkretisiert und durch Versuche evaluiert. Aufbauend auf dem Kraft-Verformungsverhalten der analysierten Einzelbauteile und der kapillarporösen Struktur des Holzes wird bei dem Berechnungsansatz auf eine Analyse der Spannungen verzichtet. Stattdessen wird als Berechnungskriterium die kritische Normaldehnung in der Randfaser der Strukturbauteile genutzt. Weiterhin wird eine Methode vorgestellt um mittels niederzyklischen Ermüdungsversuchs (LCF, ca. 1e+03 Lastwechsel) den „Knickpunkt“ der Zeitfestigkeitslinie eines einstufigen Ermüdungsversuchs bei hohen Lastspielzahlen (HCF, 1e+06 bis 1e+07 Lastwechsel) für diese Strukturbauteile zu bestimmen. / It was developed a simple approach for the calculation of a modular construction system for box sections (profile structures) of different cross-sectional dimensions. These profile structures consists of WVC (Wood Veneer Composites). The approach maps the static load case and the fatigue behavior under pulsating three-point bending. By using a structural component made from commercial birch plywood, the calculation approach is specified and verified. Based on the force-deformation behavior of the analyzed single components in connection with the capillary-porous structure of the wood, the calculation approach dispense on an analysis of the tensions. Instead, a criterion, which calculates the critical normal strain in the outer fibers of the structural components, is used. Furthermore, a method of a low-cycle fatigue test (LCF, abbr. 1e+03 cycles) is presented. This method detects the “knee point" of the fatigue limit line for the profiles. That point is usually determined by the use of a high-cycle fatigue tests (HCF, 1e+06 until 1e+07 cycles).

The purpose of dissertation is the analysis of the calculation of fracture parameters using Double-K fracture model for quasi-brittle specimens with the stress concentrator loaded by three-point bending or wedge splitting. To calculation of these parameters was used the developed DKFM_BUT software in Microsoft Excel application with using of Visual Basic programming language. Furthermore, the adequate shape functions and compliance functions were introduced for the selected wedge splitting test configurations. Main part of this dissertation is the series of comprehensively implemented and evaluated fracture experiments on specimens from advanced building materials, while the attention was paid to the analysis of experimental data. Finally, the selected results obtained using mentioned software support were presented and discussed.

Global-local finite element analyses were used to study the damage tolerance of curvilinearly stiffened panels; fabricated using the modern additive manufacturing process, the so-called unitized structures, and that of adhesive joints. A damage tolerance study of the unitized structures requires cracks to be defined in the vicinity of the critical stress zone. With the damage tolerance study of unitized structures as the focus, responses of curvilinearly stiffened panels to the combined shear and compression loadings were studied for different stiffeners' height. It was observed that the magnitude of the minimum principal stress in the panel was larger than the magnitudes of the maximum principal and von Mises stresses. It was also observed that the critical buckling load factor increased significantly with the increase of stiffeners' height. To study the damage tolerance of curvilinearly stiffened panels, in the first step, buckling analysis of panels was performed to determine whether panels satisfied the buckling constraint. In the second step, stress distributions of the panel were analyzed to determine the location of the critical stress under the combined shear and compression loadings. Then, the fracture analysis of the curvilinearly stiffened panel with a crack of size 1.45 mm defined at the location of the critical stress, which was the common location with the maximum magnitude of the principal stresses and von Mises stress, was performed under combined shear and tensile loadings. This crack size was used because of the requirement of a sufficiently small crack, if the crack is in the vicinity of any stress raiser. A mesh sensitivity analysis was performed to validate the choice of the mesh density near the crack tip. All analyses were performed using global-local finite element method using MSC. Marc, and global finite element methods using MSC. Marc and ABAQUS. Negligible difference in results and 94% saving in the CPU time was achieved using the global-local finite element method over the global finite element method by using a mesh density of 8.4 element/mm ahead of the crack tip. To study the influence of different loads on basic modes of fracture, the shear and normal (tensile) loads were varied differently. It was observed that the case with the fixed shear load but variable normal loads and the case with the fixed normal load but variable shear loads were Mode-I. Under the maximum combined loading condition, the largest effective stress intensity factor was very smaller than the critical stress intensity factor. Therefore, considering the critical stress intensity factor of the panel with the crack of size 1.45 mm, the design of the stiffened panel was an optimum design satisfying damage tolerance constraints. To acquire the trends in stress intensity factors for different crack lengths under different loadings, fracture analyses of curvilinearly stiffened panels with different crack lengths were performed by using a global-local finite element method under three different load cases: a) a shear load, b) a normal load, and c) a combined shear and normal loads. It was observed that 85% data storage space and the same amount in CPU time requirement could be saved using global-local finite element method compared to the standard global finite element analysis. It was also observed that the fracture mode in panels with different crack lengths was essentially Mode-I under the normal load case; Mode-II under the shear load case; and again Mode-I under the combined load case. Under the combined loading condition, the largest effective stress intensity factor of the panel with a crack of recommended size, if the crack is not in the vicinity of any stress raiser, was very smaller than the critical stress intensity factor. This work also includes the performance evaluation of adhesive joints of two different materials. This research was motivated by our experience of an adhesive joint failure on a test-fixture that we used to experimentally validate the design of stiffened panels under a compression-shear load. In the test-fixture, steel tabs were adhesively bonded to an aluminum panel and this adhesive joint debonded before design loads on the test panel were fully applied. Therefore, the requirement of studying behavior of adhesive joints for assembling dissimilar materials was found to be necessary. To determine the failure load responsible for debonding of adhesive joints of two dissimilar materials, stress distributions in adhesive joints of the nonlinear finite element model of the test-fixture were studied under a gradually increasing compression-shear load. Since the design of the combined load test fixture was for transferring the in-plane shear and compression loads to the panel, in-plane loads might have been responsible for the debonding of the steel tabs, which was similar to the results obtained from the nonlinear finite element analysis of the combined load test fixture. Then, fundamental studies were performed on the three-dimensional finite element models of adhesive lap joints and the Asymmetric Double Cantilever Beam (ADCB) joints for shear and peel deformations subjected to a loading similar to the in-plane loading conditions in the test-fixtures. The analysis was performed using ABAQUS, and the cohesive zone modeling was used to study the debonding growth. It was observed that the stronger adhesive joints could be obtained using the tougher adhesive and thicker adherends. The effect of end constraints on the fracture resistance of the ADCB specimen under compression was also investigated. The numerical observations showed that the delamination for the fixed end ADCB joints was more gradual than for the free end ADCB joints. Finally, both the crack propagation and the characteristics of adhesive joints were studied using a global-local finite element method. Three cases were studied using the proposed global-local finite element method: a) adhesively bonded Double Cantilever Beam (DCB), b) an adhesive lap joint, and c) a three-point bending test specimen. Using global-local methods, in a crack propagation problem of an adhesively bonded DCB, more than 80% data storage space and more than 65% CPU time requirement could be saved. In the adhesive lap joints, around 70% data storage space and 70% CPU time requirement could be saved using the global-local method. For the three-point bending test specimen case, more than 90% for both data storage space and CPU time requirement could be saved using the global-local method. / Ph. D.

The aim of the thesis is the investigation of fracture-mechanics parameters on specimens made of quasi-brittle materials. The principles of two-parameter fracture mechanics are used. Couple of numerical simulations were done and their outputs are used for two main analysed specimen geometries. For simulations the finite element method software ANSYS is used. In the first part, the thesis focuses on bended specimens. The influence of different geometric parameters on fracture mechanics behaviour of cracked specimen is investigated. For model calibration the outputs of other authors are used. In the second part the specimens for modified compact-tension test (CT test) are analysed. Similar to the first part, the influence of geometric parameters of the specimen (in this case, the specimen size) on fracture mechanics parameters were investigated. The modified CT test was derived from CT test which is commonly used for metal materials testing as the suitable geometry for cement-based composite materials testing. The outputs of both parts are calibration polynomials, which are expressions obtained for different specimen geometries, giving the value of fracture mechanics parameter as the function of specimen geometry. As the example, calibration curves are used to obtain fracture toughness of tested material using the outputs from recent experiment.

La présente étude porte sur le comportement mécanique de deux types de supports de catalyseurs utilisés industriellement en hydrotraitement des résidus. Ces supports extrudés, fabriqués par IFPEN, sont constitués d’alumine de transition γ avec un taux de porosité proche de 70%. La porosité du premier matériau est uniquement constituée de mésopores (< 50 nm). La porosité du second matériau est constituée de mésopores et de macropores (jusqu’à 20 µm). Les niveaux de sollicitation en service étant très peu connus, cette étude s’attache à décrire de manière précise et exhaustive le comportement mécanique de ces supports sous une large gamme de sollicitations, et à identifier les différents mécanismes de ruine possibles. L’objectif final est de mieux comprendre les relations entre les paramètres microstructuraux et les propriétés mécaniques afin d’identifier des leviers d’amélioration de la tenue mécanique des supports. Dans un premier temps, une méthodologie adaptée de caractérisation mécanique est établie. Le comportement des supports est étudié d’une part en traction, à l’aide d’essais de flexion trois points et d’écrasement diamétral, et d’autre part, en compression sous différents taux de triaxialité, à l’aide d’essais de compression uniaxiale et hydrostatique et d’essais de micro-indentation sphérique. Les différents mécanismes responsables de la ruine des supports sont identifiés au moyen de techniques d’imagerie telles que la microscopie électronique à balayage et la micro-tomographie à rayons X. En traction, le comportement est fragile avec l’amorçage de la rupture sur un défaut critique. En compression, une transition fragile / quasi-plastique du comportement est observée avec l’augmentation du taux de confinement. Cette quasi-plasticité s’exprime en particulier à travers un phénomène de densification de la macroporosité. Dans un deuxième temps, un critère de rupture est identifié pour chaque type de matériau en vue de représenter sur une même surface de charge les différents types de comportement et phénomènes physiques observés. Cette identification est réalisée en couplant les essais d’indentation sphérique à une analyse numérique. Des critères faisant intervenir la pression hydrostatique permettent de rendre compte de la forte dissymétrie du comportement des matériaux en traction et en compression. Enfin, dans un souci de se rapprocher des sollicitations subies par les supports de catalyseurs dans un réacteur en service, le comportement d’un empilement de supports est étudié en compression œdométrique. L’analyse de cet essai par tomographie à rayons X permet de déterminer les différents mécanismes de ruine intervenant au sein d’un empilement, en particulier ceux responsables de la génération de fines. Les résultats illustrent la pertinence de la caractérisation en flexion et en indentation des supports de catalyseurs seuls pour prévoir leur comportement au sein d’un empilement en compression. / In this work, we study the mechanical behaviour of two types of catalysts supports produced by IFPEN and industrially used in residues hydrotreating. Those extruded supports are made of transition γ-alumina with about 70% of porous volume. The first material’s porosity is exclusively composed of mesopores (< 50 nm). The porosity of the second material is composed of both mesopores and macropores (up to 20 µm). Because of the limited knowledge of the stress fields in embedded catalysts supports in use in a reactor, this study aims at precisely and exhaustively describing the mechanical behaviour of those supports under a wide range of stresses, and identifying the possible damage mechanisms. The final objective is to better understand the influence of microstructural parameters on the mechanical properties of the supports in order to propose some leads about how to improve their mechanical strength. First, an adequate mechanical characterization methodology is set. On one hand, the tensile mechanical behaviour of the supports is studied with three-point bending and diametrical crushing tests. On the other hand, their compressive behaviour under various triaxiality rates is characterized in uniaxial and hydrostatic compression, and by spherical micro-indentation. The different damaging mechanisms are identified by imaging techniques such as scanning electronic microscopy and X-ray micro-tomography. Under tensile stresses, the supports exhibit a brittle behaviour and fracture initiates at a critical flaw. Under compressive stresses, a brittle/quasi-plastic transition is observed with increasing the triaxiality rate. The quasi-plasticity is mainly due to the densification of the macroporosity. The second part of the study consists in identifying, for each material, a fracture criterion able to represent every types of behaviour and physical phenomena observed on the same yield surface. This identification is achieved by coupling the spherical indentation tests to a numerical analysis. Fracture criteria involving hydrostatic pressure are well suited to describe the highly dissymmetric mechanical behaviour of the materials in tension and in compression. The last part of this work aims at studying the mechanical behaviour of a stack of supports under œdometric compression in order to produce stress fields more representative of those existing within the supports stacked in a reactor. This test is analysed by X-ray tomography, which allows us to determine/acknowledge the different damaging mechanisms involved in fragments and fines generation. The results illustrate the suitability of the bending and indentation tests to characterize the mechanical properties of a single support and relate them to its mechanical behaviour in a stack of supports under compression.