Thesis (MScIng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: Vesiculated beads consist of aerated microvoids encapsulated in a solid spherical continuous polymeric shell. The difference in refractive index between the voids and polymer granules causes effective scattering of incident light on the particles, presenting it with a white appearance. The size of these beads generally range in the region of 0.5 – 40 μm, making it suitable for use as pigment extender in the surface coatings or paint industry. Currently, titanium dioxide pigment is predominantly used as opacifying agent in paint formulations, but due to the high cost associated in purchasing this pigment, as well as fluctuation in import prices, paint manufacturers are looking for alternative products to replace or at least partially replace this pigment. As an alternative, opaque vesiculated polymer particles can be produced locally at a cheaper price and in existing vessels available in the paint industry. Approximately five years ago a paint company in Mexico and member of the Nova Club, started research in developing vesiculated beads for production in their factories. However, it was found extremely difficult to scale-up the production to industrial size, since the system was very sensitive to process variables. A local paint company and member of the Nova Club acquired this technology and continued further research in developing vesiculated beads on large scale in existing Cowles disperser systems found in the paint industry. The beads consist mainly of an organic phase comprising of unsaturated carboxylated polyester and styrene. A polyamine is also added to assist the formation of vesicles in the organic phase. This phase is slowly added under agitation to an aqueous phase consisting of deionised water, a thickener and colloid stabilisers to form an oil-inwater emulsion. Agitation is continued for a specified period of time, also known as the emulsification period, to allow sufficient time for the organic globules to break-up to smaller particle sizes. These globules are subsequently catalysed with a freeradical initiator and redox activator and left static overnight to allow formation of the solid beads. To determine the most important process parameters during production of vesiculated beads, a fully integrated laboratory scale Cowles reactor system was designed and constructed, geometrically analogous to the vessels found in the paint industry. The system measures and controls production temperature, mixing speed and component addition rates. Production runs were performed where various process parameters were varied to investigate the effect on properties, which include average particle size and particle size distribution, pH, viscosity and opacity. The most important process parameters that were found to play a significant role include production temperature, organic phase addition rate, emulsification time, the Cowles impeller diameter and mixing speed. Production runs were performed in geometrically similar 5l and 20l vessels on the laboratory-scale system to investigate the effect of scale-up. A model presented by Klein et al. (1996) was used as basis for describing the average particle size as a function of mixing speed, impeller diameter, vessel diameter and emulsification time. The applicability of this model was tested on average particle size data obtained from industrial scale runs performed on the plants and proved to be reasonably accurate. / AFRIKAANSE OPSOMMING: Sferiese polimeerpartikels met klein lugholtes vasgevang in ‘n harde omhulsel word al jare in die verf industrie aangewend as pigment. Weens die verskil in brekingsindeks tussen die soliede polimeerpartikel en die vasgevange lugholtes, word invallende lig versprei op so ‘n manier dat die partikels ondeursigtig (of wit) voorkom. Hierdie partikels kan geproduseer word met deursneë wat strek van 0.5 – 40 μm, wat dit geskik maak vir gebruik in verf formulasies. Tans word titaandioksied poeier hoofsaaklik gebruik in verf as pigment, maar weens die hoë koste van die invoer en aankoop van hierdie produk, het verfmaatskappye begin soek na goedkoper alternatiewe. Aangesien hierdie ondeursigtige polimeerpartikels plaaslik goedkoper vervaardig kan word in bestaande mengvate beskikbaar in verf aanlegte, dien dit as moontlike plaasvervanger. Ongeveer vyf jaar gelede het ‘n Mexikaanse verfmaatskappy, wat lid is van die Nova Klub, navorsing begin doen om hierdie polimeerpartikels in hul fabrieke te produseer. Dit was egter vir hulle onmoontlik om die produksie op te skaal na industriële vervaardiging aangesien die proses baie sensitief was vir produksieveranderlikes. Sekere eienskappe soos die gemiddelde partikelgrootte, partikelverspreiding, pH, viskositeit en deursigtigheid van die partikels kon nie van lot tot lot herhaal word nie en verdere navorsing is gestaak. ‘n Plaaslike verfmaatskappy (ook lid van die Nova Klub) het die tegnologie oorgeneem en die proses verder ontwikkel. Die proses is aangepas sodat “Cowles” mengers, wat wydverspreid in die verf industrie beskikbaar is, gebruik kan word om dit te vervaardig. Die partikels bestaan hoofsaaklik uit ‘n organiese fase wat ‘n onversadigde gekarboksileerde poliëster en stireen insluit. ‘n Poli-amien word ook bygevoeg en is verantwoordelik vir die vorming van die lugholtes in die partikels. Hierdie fase word stadig onder menging by ‘n tweede water fase, bestaande uit gedeïoniseerde water, ‘n verdikker en kolloïdale stabiliseerders gevoeg om ‘n olie-in-water emulsie te vorm. Menging word voortgesit vir ‘n bepaalde emulsifiseringsperiode om die oliedruppels verder op te breek. Gevolglik word hierdie druppels gekataliseer met ‘n vry-radikaal inisieerder en redoksaktiveerder en oornag staties gelos om vorming van die soliede partikels toe te laat. Aangesien eienskappe van die polimeerpartikels so sensitief is vir prosesveranderlikes, is besluit om aanvanklik ‘n ten volle geïntegreerde laboratorium skaal “Cowles” reaktorsisteem te ontwerp en bou. Hierdie sisteem is geometries gelykvormig aan die mengvate wat in verffabrieke gevind word. Die produksietemperatuur, stuwergrootte, mengspoed en materiaal toevoertempo kan effektief gemeet, verstel en beheer word. Eksperimentele lopies is gedoen en die effek van verskeie produksieveranderlikes op eienskappe is ondersoek. Die belangrikste veranderlikes wat die proses beïnvloed, is die emulsifiseringstemperatuur, die toevoertempo van die organiese fase, emulsifiseringsperiode, stuwerdeursnit en mengspoed. Eksperimentele lopies is gedoen op twee geometriese gelykvormige mengvate (5l en 20l kapasiteit) om die effek van opskaling op eienskappe te ondersoek. ‘n Model wat deur Klein et al. (1996) voorgestel is, is as basis gebruik om die gemiddelde partikelgrootte te bepaal as ‘n funksie van mengspoed, stuwerdeursnit, mengvat deursnit en emulsifiseringstyd. Hierdie model is getoets op partikelgrootte data wat verkry is van groot industriële skaal lopies uitgevoer in die fabrieke onder bekende produksie kondisies en daar is gevind dat hierdie model bevredigend gebruik kan word om die gemiddelde partikelgrootte te voorspel.

La rupture ductile des alliages métalliques survient suite à la nucléation, la croissance et la coalescence de microcavités. La première partie de cette thèse est consacrée à l'étude des effets de forme et d'anisotropie plastique sur la phase de croissance des cavités. Dans un premier temps, nous implémentons numériquement le modèle de croissance de Madou et Leblond pour des cavités ellipsoïdales générales plongées dans un matériau isotrope dans un code de calcul par éléments finis, afin d'appliquer le modèle à des cas de rupture où les effets de forme sont importants. On montre que la prise en compte des effets de forme des cavités est nécessaire afin de reproduire la rupture ductile en cisaillement. Ce modèle est ensuite étendu au cas de l'anisotropie plastique, en s'inspirant des travaux de Monchiet et Benzerga. On dérive notamment un critère de plasticité macroscopique pour les matériaux anisotropes contenant des cavités ellipsoïdales générales, que nous validons par analyse limite numérique. La seconde partie de la thèse est dédiée à l'étude des effets de taille sur la rupture ductile des matériaux nanoporeux contenant des cavités sphériques ou sphéroïdales. Enfin, la troisième partie de la thèse est consacrée à l'étude des effets de forme et d'anisotropie plastique sur la phase de coalescence des cavités. Nous dérivons deux nouveaux critères de coalescence en couche que nous validons par analyse limite numérique. Cette étude nous permet de développer un nouveau critère permettant d'unifier les phases de croissance et coalescence. Enfin nous dérivons un critère de coalescence pour les matériaux anisotropes. / Ductile fracture of metallic alloys occurs by the nucleation, growth and coalescence of microvoids. In a first step, we study the influence of void shape effects and plastic anisotropy on the growth phase. we implement numerically in a finite element code the void growth model of madou and leblond for ellipsoidal voids embedded in an isotropic material, in order to apply the model to ductile fracture problems involving important void shape effects. We show that the consideration of void shape effects is necessary in order to reproduce shear-dominant ductile fracture. This model is then extended to plastic anisotropy, in the spirit of the models of monchiet and benzerga. In particular, we derive a macroscopic criterion for anisotropic materials containing general ellipsoidal voids, which is assessed by finite element limite analyses. In a second step, we study the effects of void size on the ductile fracture of nanoporous materials contenant spherical or spheroidal voids. The last part of the thesis is dedicated to the study of void shape effects and plastic anisotropy on the coalescence phase. We derive two new criteria of coalescence by internal necking, which are assessed numerically. Then, we derive a new criterion that permits to unify the growth and coalescence phases. Finally we study the influence of plasticy anisotropy on coalescence by internal necking.

This research focused on a comprehensive microstructural and mechanical property characterisation study of the Ni-Fe-Cr alloys 718 and 945X. The aim of the project was to better understand the relationship between performance and microstructure of existing (Alloy 718) and newly developed (Alloy 945X) high strength nickel alloys focusing on downhole applications. The main difference between the two alloys is that alloy 945X has lower Nb content than alloy 718, which may minimise the tendency to form delta when combined with correct processing. Previous studies have related the hydrogen embrittlement in alloy 718 with the collection of hydrogen by delta phase. Microstructural characterisation of the new alloy 945X after long term isothermal exposure up to 120 hours in the temperature range 650◦C to 900◦C was conducted with scanning electron microscopy (SEM), to generate a time-temperature-transformation (TTT) diagram. The TTT diagram was used as a road map for designing two isothermal heat treatments of alloy 945X on tensile specimens. Then, the effect of hydrogen charging on the tensile properties and microstructure of the 'as-received' and these two variant heat treatments was investigated. Fractographic analysis showed that, in the presence of hydrogen, intergranular fracture occurred for all the heat treatments, regardless the presence of delta phase at grain boundaries. There was no simple correlation between the volume fraction of delta-phase and susceptibility to hydrogen assisted embrittlement. Rather, it was demonstrated that the morphology and distribution of delta-phase along grain boundaries plays a key role and the other precipitate phases also have an influence through their effect on the ease of strain localisation. This study also examined the hydrogen embrittlement sensitivity of nickel alloy 718 given four different heat treatments to obtain various microstructural states. Each heat treatment leads to differences in the precipitate morphologies of γ', γ'' and delta phases. Material characterisation and fractography of the examined heat treatments were performed using a high resolution FEG-SEM. Three specimens of each condition were pre- charged with hydrogen and tensile properties were compared with those of non-charged specimens. It was observed that hydrogen embrittlement was associated with intergranular and transgranular microcrack formation, leading to an intergranular brittle fracture. delta phase may assist the intergranular crack propagation, and this was shown to be particularly true when this phase is coarse enough to produce crack initia- tion, but this is not the only factor determining embrittlement. Other microstructural features play a role, as does the strength of the material. Finally, the evolution of delta-(Ni3Nb) phase in alloy 718 from the early stages of precipitation, with a particular focus on identifying the grain boundary characteristics that favour precipitation of grain boundary delta phase was investigated. Results showed that delta phase was firstly formed on Σ3 boundaries after 5 hours at the examined temperature (800◦C). Increasing ageing time at 800◦C was observed to lead to an increase in size and precipitation of phases γ'-γ''-delta, an increase in fraction of the special CSL boundaries and an evolution in the morphology of twins and the growth of grains.

The integrity of nearly all engineering structures are threatened by the presence of cracks. Structural failure occurs if a crack larger than a critical size exists. Although most well designed structures initially contain no critical cracks, subcritical cracks can grow to failure under fatigue loading, called fatigue crack growth (FCG). Because it is impossible or impractical to prevent subcritical crack growth in most applications, a damage tolerant design philosophy was developed for crack sensitive structures. Design engineers have taken advantage of the FCG threshold concept to design for long fatigue lives. FCG threshold (DKth) is a value of DK (crack-tip loading), below which no significant FCG occurs. Cracks are tolerated if DK is less than DKth. However, FCG threshold is not constant. Many variables influence DKth including microstructure, environment, and load ratio. The current research focuses on load ratio effects on DKth and threshold FCG. Two categories of load ratio effects are studied here: extrinsic and intrinsic. Extrinsic load ratio effects operate in the crack wake and include fatigue crack closure mechanisms. Intrinsic load ratio effects operate in the crack-tip process zone and include microcracking and void production. To gain a better understanding of threshold FCG load ratio effects (1) a fatigue crack closure model is developed to consider the most likely closure mechanisms at threshold, simultaneously, and (2) intrinsic load ratio mechanisms are identified and modeled. An analytical fatigue crack closure model is developed that includes the three closure mechanisms considered most important at threshold (PICC, RICC, and OICC). Crack meandering and a limited amount of mixed-mode loading are also considered. The rough crack geometry, approximated as a two-dimensional sawtooth wave, results in a mixed-mode crack-tip stress state. Dislocation and continuum mechanics concepts are used to determine mixed-mode crack face displacements. Plasticity induced crack closure is included by modifying an existing analytical model, and an oxide layer in the crack mouth is modeled as a uniform layer. Finite element results were used to verify the analytical solutions for crack-tip stress intensity factor and crack face displacements. These results indicate that closure for rough cracks can occur at two locations: (1) at the crack-tip, and (2) at the asperity nearest the crack-tip. Both tip contact and asperity contact must be considered for rough cracks. Tip contact is more likely for high Kmax levels, thick oxide layers, and shallow asperity angles, a. Model results indicate that closure mechanisms combine in a synergistic manner. That is, when multiple closure mechanisms are active, the total closure level is greater than the sum of individual mechanisms acting alone. To better understand fatigue crack closure where multiple closure mechanisms are active (i.e. FCG threshold), these interactions must be considered. Model results are well supported by experimental data over a wide range of DK, including FCG threshold. Closure-free load ratio effects were studied for aluminum alloys 2024, 7050, and 8009. Alloys 7050 and 8009 were selected because load ratio effects at FCG threshold are not entirely explained by fatigue crack closure. It is believed that closure-free load ratio mechanisms occur in these alloys. Aluminum alloy 2024 was selected for study because it is relatively well behaved, meandering most load ratio effects are explained by fatigue crack closure. A series of constant Kmax threshold tests on aluminum alloys were conducted to eliminate fatigue crack closure at threshold. Even in the absence of fatigue crack closure load ratio (Kmax) effects persist, and are correlated with increased crack-tip damage (i.e. voids) seen on the fatigue crack surfaces. Accelerated FCG was observed during constant Kmax threshold testing of 8009 aluminum. A distinct transition is seen the FCG data and is correlated with a dramatic increase in void production seen along the crack faces. Void production in 8009 aluminum is limited to the specimen interior (plane-strain conditions), promoting crack tunneling. At higher values of Kmax (+_ 22.0 MPaà m), where plane-stress conditions dominate, a transition to slant cracking occurs at threshold. The transition to slant cracking produces an apparent increase in FCG rate with decreasing DK. This unstable threshold behavior is related to constraint conditions. Finally, a model is developed to predict the accelerated FCG rates, at higher Kmax levels, in terms of crack-tip damage. The effect of humidity (in laboratory air) on threshold FCG was studied to ensure that environmental effects at threshold were separated from load ratio effects. Although changes in humidity were shown to strongly affect threshold FCG rates, this influence was small for ambient humidity levels (relative humidity between 30% and 70%). Transient FCG behavior, following an abrupt change in humidity level, indicated environmental damage accumulated in the crack-tip monotonic plastic zone. Previous research implies that hydrogen (a component of water vapor) is the likely cause of this environmental damage. Analysis suggests that bulk diffusion is not a likely hydrogen transport mechanism in the crack-tip monotonic plastic zone. Rather, dislocation-assisted diffusion is presented as the likely hydrogen transport mechanism. Finally, the (extrinsic) fatigue crack closure model and the (intrinsic) crack-tip damage model are put in the context of a comprehensive threshold model. The ultimate goal of the comprehensive threshold model is to predict fatigue lives of cyclically loaded engineering components from (small) crack nucleation, through FCG, and including failure. The models developed in this dissertation provide a basis for a more complete evaluation of threshold FCG and fatigue life prediction. The research described in this dissertation was performed at NASA-Langley Research Center in Hampton, Virginia. Funding was provided through the NASA GSRP program (Graduate Student Researcher Program, grant number NGT-1-52174). / Ph. D.