The aim of the research was to develop predictive models for the interfacial degradation of adhesively bonded joints when exposed to aggressive environmental conditions. Four different joint configurations using the same adhesive system were exposed to a variety of conditions including immersion at 50°C, 96%RH at 50°C and 80%RH at 70°C. In addition data from joints for other adhesive systems were also incorporated into the investigation. Moisture has a degrading effect on the strength of adhesively bonded joints. Therefore the diffusion into the bulk material was determined by gravimetric experiments. However, the mobility of the water molecules at the interface between the adhesive and the substrate may be higher than in the bulk material. In order to assess this, the spatial moisture distribution in bonded epoxy laminates was detennined by a nuclear reaction analysis (NRA) technique. The moisture profile found experimentally and the modelling undertaken of the interfacial diffusion indicated that the ingress in the interfacial region was a few times faster than in the bulk material for the adhesive system investigated. Both hygroscopic (swelling) and thermal residual strains may affect joint durability. The thermal expansion was determined by means of a bi-material beam and the hygroscopic expansion was determined by measuring the expansion of bulk samples at various moisture levels. Creep properties for the adhesives studied were determined to investigate the relaxation of residual stresses during the aging process. The coefficients of thermal expansion and hygroscopic expansion were of the same order of magnitude for the adhesives investigated. Creep was seen to be enhanced in the presence of moisture. The AVl19 adhesive was seen to creep much more than FM73 and also absorbed more moisture. As a consequence, the residual stresses in the joints bonded with A Vl19 were seen to relax nearly totally whilst the residual stresses in the joints bonded with FM73 relaxed to about half of their original magnitude. Different interfacial fracture tests were carried out in order to assess which was most appropriate. Notched coating adhesion tests (NCA) were carried out initially. However, it was very difficult to produce a repeatable notch and the adhesive often cracked before the coating debonded. Good results were obtained then these samples were immersed in water. Another test investigated was a split beam specimen. However this test was of limited use as the secondary bond was weaker than the aged interface of interest. Finally, a mixed mode flexure specimen (MMF) was selected to determine the fracture energy of the adhesive systems in the 80%RH and 96%RH environments. The fracture energy degraded rapidly initially with moisture content and then at a slower rate as more moisture reached the interface. The fracture energy was found to be a function of the amount of moisture at the interface. No further degradation was found when the joints were held at equilibrium. The degradation and the progressive damage were simulated with a cohesive zone model (CZM). The model was extended from 2D to 3D. This was ~eful when predicting where 2 the crack initiated in the width direction and how the initiation site changed after aging for a L-joint configuration. When using a CZM the interfacial strength was defmed by a traction-separation law. The parameters governing the traction-separation law were determined using the interfacial fracture tests (NCA and MMF). The parameters were the tripping traction and the fracture energy. It was shown to be essential to incorporate elasto-plastic adhesive continuum behaviour in order to simulate the complete joint response correctly. The tripping traction was determined by correlating the deviation of the load-displacement curve with the simulated result. The fracture energy was then determined by correlating the experimental load-crack length response with the simulation. This gave a unique set of moisture dependent CZM parameters for various moisture concentrations. These parameters were then used to predict the response of other joint configurations. For most of the joints, the residual strength was predicted closely using the moisture dependent CZM parameters. However, in some cases other degradation mechanisms were active. These included stress enhanced degradation and cathodic delamination. When these mechanisms were included in the modelling, the prediction of the durability of all joint configurations was good.

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This thesis covers both experimental and computer investigations into the dynamic behaviour of mechanical seals. The literature survey shows no investigations on the effect of vibration on mechanical seals of the type common in the various process industries. Typical seal designs are discussed. A form of Reynolds' equation has been developed that permits the calculation of stiffnesses and damping coefficients for the fluid film. The dynamics of the mechanical seal floating ring have been investigated using approximate formulae, and it has been shown that the floating ring will behave as a rigid body. Some elements, such as the radial damping due to the fluid film, are small and may be neglected. The equations of motion of the floating ring have been develofed utilising the significant elements, and a solution technique described. The stiffness and damping coefficients of nitrile rubber o-rings have been obtained. These show a wide variation, with a constant stiffness up to 60 Hz. The importance of the effect of temperature on the properties is discussed. An unsuccessful test rig is described in the appendices. The dynamic behaviour of a mechanical seal has been investigated experimentally, including the effect of changes of speed, sealed pressure and seal geometry. The results, as expected, shorf that high vibration levels result in both high leakage and seal temperatures. Computer programs have been developed to solve Reynolds' Equation and the equations of motion. Two solution techniques for the latter program were developed. The unsuccessful technique is described in the appendices. Some stability problems were encountered, but despite these the solution shows good agreement with some of the experimental conditions. Possible reasons for the discrepancies are discussed. Various suggestions for future work in this field are given. These include the combining of the programs and more extensive experimental and computer modelling.

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The diffu~ion and permeation of aviation fuel (Avtur) and water in commercial and 'model' polysulphide sealants have been studied using simple gravimetric techniques. For Avtur, the mass uptake behaviour is explicable in terms of normal, Fickian diffusion, and the diffusion coefficient determined has been found to be independent of the experimental method and concentration of the liquid. Water, however, exhibits anomalous diffusion behaviour. Mass uptake by the polysulphides is high and equilibrium is not reached after several months. There is no correlation between the results of permeation, absorption and desorption experiments except at low water concentration ... ,,~ . These anomalies have been explained in terms of the formation of water droplets within the rubber. This study shows that the amount of water absorbed is dependent upon the amount and nature of the curing agent residues. The results have been analysed in terms of the osmotic and water vapour pressure and two equations have been derived which satisfactorily predict the amount of water absorbed at equilibrium. It has been shown that the effect of the water droplets on permeation is negligible, and hence the diffusion coefficient found is a measure of the true rate of diffusion. In contrast, the effect of the droplets on mass uptake experiments is large and the apparent overall diffusion rate is greatly reduced. The apparent diffusion coefficient found is dependent upon the water concentration and the concentration of the water-soluble impurities. An equation has been derived showing the relationship between these variables. The results of this work show that the elastic strength of the rubber has negligible effect in determining the water uptake at equilibrium and the apparent diffusion coefficient. However, deterioration in adhesive and cohesive strength occurs with water uptake. At high uptake some of this loss is irreversible. The deterioration in properties is mainly due to breakage of weak physical bonds and plasticisation effects. -