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A method to measure the mixed-mode fatigue behavior of environmentally degraded adhesive joints was developed. Firstly, the absorption and desorption of water in two different rubber-toughened epoxy adhesives was measured gravimetrically. The water absorption in both adhesives showed anomalous behavior that was fitted to a new “sequential dual Fickian” (SDF) model. The water desorption in both adhesives was modelled accurately using Fick’s law, and there was a significant difference in the amount of retained water after drying in the two adhesives. The effects of long-term aging were studied using open-faced specimens made with two different rubber-toughened epoxy adhesives. The contrasting results illustrated the effects of environmental degradation on the matrix and toughener. Furthermore, the differences in the degradation behavior of both adhesives, combined with gravimetric and dynamic mechanical thermal analysis (DMTA) results, were used to illustrate the role of retained water in degrading the toughening mechanisms. The measured fatigue results invalidated the environmental index (EI) hypothesis for fatigue behavior, at least for the relatively short aging times studied here. Compared with aging under constant humidity, the fatigue performance of joints was found to be superior after aging in a cyclic salt-spray environment due to the lower water concentrations in the adhesive. The effects of test environment humidity and temperature on the fatigue behavior were also studied using closed, un-aged specimens. Both individual and combined effects of temperature and humidity on fatigue behavior were studied. In elevated temperature and humidity environment, joint performance at higher crack growth rates was degraded solely due to the effect of the increased temperature, whereas fatigue performance at low crack growth rates degraded predominantly because of elevated moisture. Finally, to generalise the techniques developed to automotive aluminum sheets, a reinforced specimen was developed that avoids yielding of thin aluminum sheet adherends while loading. Fatigue testing with these reinforced specimens revealed that the fatigue behavior was sensitive to the loading phase angle and the orientation of rolling lines on the sheet. These reinforced specimens were also used to study the effects of long-term aging and the effects of test environment.

A method to measure the mixed-mode fatigue behavior of environmentally degraded adhesive joints was developed. Firstly, the absorption and desorption of water in two different rubber-toughened epoxy adhesives was measured gravimetrically. The water absorption in both adhesives showed anomalous behavior that was fitted to a new “sequential dual Fickian” (SDF) model. The water desorption in both adhesives was modelled accurately using Fick’s law, and there was a significant difference in the amount of retained water after drying in the two adhesives. The effects of long-term aging were studied using open-faced specimens made with two different rubber-toughened epoxy adhesives. The contrasting results illustrated the effects of environmental degradation on the matrix and toughener. Furthermore, the differences in the degradation behavior of both adhesives, combined with gravimetric and dynamic mechanical thermal analysis (DMTA) results, were used to illustrate the role of retained water in degrading the toughening mechanisms. The measured fatigue results invalidated the environmental index (EI) hypothesis for fatigue behavior, at least for the relatively short aging times studied here. Compared with aging under constant humidity, the fatigue performance of joints was found to be superior after aging in a cyclic salt-spray environment due to the lower water concentrations in the adhesive. The effects of test environment humidity and temperature on the fatigue behavior were also studied using closed, un-aged specimens. Both individual and combined effects of temperature and humidity on fatigue behavior were studied. In elevated temperature and humidity environment, joint performance at higher crack growth rates was degraded solely due to the effect of the increased temperature, whereas fatigue performance at low crack growth rates degraded predominantly because of elevated moisture. Finally, to generalise the techniques developed to automotive aluminum sheets, a reinforced specimen was developed that avoids yielding of thin aluminum sheet adherends while loading. Fatigue testing with these reinforced specimens revealed that the fatigue behavior was sensitive to the loading phase angle and the orientation of rolling lines on the sheet. These reinforced specimens were also used to study the effects of long-term aging and the effects of test environment.

Epoxy resins are widely used as coatings, encapsulations, structural composites, castings, and adhesives in a number of electrical applications. Recently, novel uncatalyzed co-anhydride cured epoxy formulations that exhibit a high performance property profile, have been introduced. The objective of this thesis was to perform a comprehensive material characterization of these new resin/hardener combinations, which are potentially used as electrical insulation material in medium and high voltage engineering. The thermal cure behaviour of commercial tetraglycidyl-diamino-diphenylmethane (TGDDM) and a co-anhydride mixture consisting of maleic anhydride (MA) and hexahydrophthalic anhydride (HHPA) was extensively studied. Different analytical real-time methods, such as FT-Raman spectroscopy, differential scanning calorimetry, and chemo-rheological methods were applied to investigate the principal polymerization mechanism and the related curing kinetics. It was demonstrated that kinetic parameters that were based on isothermal measurements provided consistent and reliable results. On the other hand the general limitations of different dynamic methods for kinetic parameter calculations were outlined and discussed. Temperature modulated differential scanning calorimetry provided a powerful technique to confirm a TGDDM/MA and TGDDM/HHPA sub-network structure of the co-anhydride cured epoxy. A generalized time-temperature-transformation-diagram was developed in order to predict the complex material transformations (e.g. gelation and vitrification) occurring during the entire isothermal curing process. In the selected system, the mechanical deformation and fracture behaviour as a function of temperature, strain rate, inorganic filler fraction, particle size, and filler/matrix-adhesion were thoroughly studied by using compression-, tension- and double torsion fracture-tests. The potential of hyperbranched polymers (HBPs) as low viscosity toughening modifiers for highly crosslinked anhydride-cured epoxy networks was experimentally evaluated. The effects of the HBP molecular structure, in particular the specific shell chemistry, on thermo-mechanical properties, final morphology, and blend concentration, were assessed. For the neat investigated epoxy-system the most efficient toughening modifier was obtained for a molecular HBP-design that provided a pseudo-homogeneous blend morphology. Thus, by using suitable HBPs in a concentration of 20% w/w, the fracture toughness, expressed by the critical stress intensity factor (Klc) of 0.58 MPam(to the power of)0.5, was increased by over 50% to 0.88 MPam(to the power of)0.5. The corresponding Young's modulus and glass transition temperature were only affected to a limited extent by the addition of the HBP-additive. The toughest epoxy blend (critical stress intensity factor of about 1.6 MPam(to the power of)0.5) was achieved by the incorporation of 60% w/w inorganic silica particles. The application of hybrid concepts by utilizing synergistic toughening mechanisms (HBP and silica), revealed only moderate benefits within the investigated highly crosslinked materials. As the examined epoxy/anhydride formulations are generally considered for high temperature applications, it was essential to determine their long-term thermooxidative ageing performance. The long-term thermo-oxidative ageing behaviour has been investigated by means of thermo-gravimetric analysis (TGA), dynamic mechanical analysis (DMA) and vibrational infrared spectroscopy (FT-IR) methods with special emphasis on fundamental understanding of the ageing mechanism. Effects of the thermal ageing on the characteristic viscoelastic and flexural behaviour, weight-loss and oxidation susceptibility of the examined epoxy networks were assessed and discussed, thus providing an understanding of the principal material endurance properties. The thermo-mechanical behaviour and the related structural changes with thermal ageing were examined by Cole-Cole plots in combination with a molecular theory previously developed by Perez. It was demonstrated that this new methodology provides a connection between conversion, glass transition temperature, and mechanical relaxation data and allows a fundamental molecular interpretation with respect to physical and chemical ageing phenomena. A variety of thermo-gravimetric experiments were carried out in order to determine and model the specific weight-loss profile as a function of anhydride nature and ageing temperature. The influence of different inorganic fillers on the thermooxidative response was systematically studied. Different models were applied to extract meaningful kinetic parameters in order to describe the thermo-oxidative degradation and to facilitate an extrapolation of weight-loss data outside the experimental time- and temperature-scale. FT-IR micro ATR-spectroscopy was used to identify, localize and quantify the complex oxidation behaviour. A thermo-oxidative degradation mechanism, that involved predominantly radical oxidation processes and C-N as well as C-O chain scissions, was proposed to account for the experimental observations. Specific oxidation-front profiles were constructed to describe the heterogeneous oxidation processes. Finally, the comprehensive material characterization of these novel co-anhydridecured amino-glycidyl resins in terms of curing mechanism and kinetic, deformation and fracture behaviour, and thermo-oxidative ageing performance, allows the assessment of the potential of these materials for demanding applications in electrical and electronic industries.

This thesis systematically investigates the properties of the aluminium adherend which influence the fracture toughness of aluminium-epoxy adhesive joints in humid environments. The fracture energy of the adhesive joint exposed to a humid environment in comparison with the fracture energy in a dry environment provides a measure of the joint durability. A 500C and 95% relative humidity environment is used to simulate aging of an adhesive joint over several years under normal service conditions. Initially, surface roughness is found to have a significant influence on the fracture toughness of the adhesive joint in humid conditions. A direct correlation between the bond durability and the angle of deliberately machined micro-roughness in the aluminium surface is determined. Consequently a model is developed which initially has the capacity to describe the bond durability performance. The preparation of aluminium surfaces involves the use of a novel ultramilling tool to produce well defined and controlled surface topography. This work represents the first time surface angles of features in the 1????m to 10????m range have been systematically varied and a direct relationship with bond durability has been determined. The use of surface analytical tools aids in elucidating mechanisms involved in the failure of the adhesive joint and contributes to the development of the stress based diffusion model. Examination of the aluminium oxide hydration level reveals this property has a negligible effect on the fracture toughness of the aluminium-epoxy joints exposed to humid environments. This information confirms the dominant role of the physical properties of the aluminium surface in determining the adhesive joint durability. This is the first occasion that planer oxide films grown in an RF plasma have had their hydration state adjusted in a controlled manner and their properties subsequently assessed in terms of bond durability properties. Further alteration of the aluminium surface chemistry is achieved through the application of an organo-silane coupling agent and a series of novel organo-phosphonate compounds. This work further develops the stress based diffusion model developed in conjunction with the micro-machining studies. The components of surface roughness and the ability of interfacial bonds to co-operatively share load are essential for the maintenance of fracture toughness of adhesive joints exposed to humid conditions. The ability of the silane coupling agent to share load through a chemically cross-linked film is a significant property which provides the superior fracture toughness in comparison with the phosphonate treated joints. Although the organo-phosphonate treated aluminium provides hydrolytically more stable bonds than the silane coupling agent, the film is not cross-linked via primary chemical bonds and the reduced load sharing capacity of interfacial bonds increases the bond degradation rate. The stress based diffusion model evolving from the initial work in the thesis can be used to predict the performance of more complex systems based on a thorough characterisation of the aluminium surface chemistry and topography. The stress based diffusion model essentially describes the concept of the production of micro-cavities at the epoxy-aluminium interface under mode 1 load, as a result of the distribution of strong and weak adhesive bonds. Alternatively, micro-cavities may result from an inhomogeneous stress distribution. In areas where the adhesive bonds are weak, or the local stresses are high, the interfacial load produces larger micro-cavities which provide a path of low resistance for water to diffuse along the bond-line. The water then degrades the adhesive bond either through the displacement of interfacial epoxy bonds or the hydration of the oxide to form a weak barrier layer through which fracture can occur. Alternatively, the water can hydrolyse the adhesive in the interfacial region, leading to cohesive failure of the epoxy resin. The bond durability performance of a series of complex hydrated oxide films used to pre-treat the aluminium adherend provides support for the stress based diffusion model. Whilst surface area is an important property of the aluminium adherend in producing durable bonding, the best durability achievable, between an epoxy adhesive and aluminium substrate, requires a component of surface roughness which enhances the load sharing capability in the interfacial bonding region. This component of durability performance is predicted by the model. In more specific terms, a boiling water treatment of the aluminium adherend indicates a direct correlation between bond durability, surface area and topography. The characterisation of film properties indicates that the film chemistry does not change as a function of treatment conditions, however, the film topography and surface area does. The overall bond durability performance is linked to both of these properties. The detailed examination of the hydrated oxide film, produced by the boiling water treatment of aluminium, is the first time the bond durability performance has been related to the film topography. It is also the first occasion that the mechanism of film growth has been examined over such a large treatment time. The combination of surface analysis and bond durability measurements is invaluable in confirming the properties, predicted by the stress based diffusion model, which are responsible in forming fracture resistant adhesive bonds in humid conditions. The bond durability of high surface area and low surface area hydrated oxide films indicates that surface area is an important property. However, this study confirms that the absence of the preferred surface topography limits the ultimate bond durability performance attainable. The fracture toughness measurements performed on aluminium adherends pre-treated with a low surface area film also supports the mechanism of load sharing of interfacial adhesive bonds and its contribution to the overall bond durability. The role performed by the individual molecules and particles in an oxide film is similar to the load sharing performed by the silane coupling agent molecules. Further support for the stress based diffusion model is provided by films produced on aluminium immersed in nickel salt solutions. The topography of these film alters as a function of treatment time and this is directly related to fracture toughness in humid environments. This work provides the first instance where such films have been characterised in detail and their properties related to bond durability performance. The study is also the first time that the growth mechanism of the film produced on the aluminium substrate has been examined in detail. The film growth mechanism supports the film growth model proposed for the hydrated oxide film produced by the boiling water treatment. The major findings presented in this thesis are summarised as the direct correlation between surface profile angle, the importance of co-operative load sharing of interfacial adhesive bonds and the relative insignificance of surface oxide hydration in the formation of durable aluminium-epoxy adhesion. This information is used to develop a stress based diffusion model which has the capacity to describe the fracture toughness of a range of aluminium-epoxy adhesive joint systems in humid environments. The stress based diffusion model is also capable of predicting the relative performance of the bond systems examined in the final chapters of the thesis, where complex interfacial oxide films are involved in the formation of adhesive bonds.

Universiẗat, Diss., 2006--Frankfurt (Main).

Thesis (Ph. D.)--University of New South Wales, 1998. / Also available online.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references (leaves 83-87). Also available in electronic version.