A pre-damage method was developed to determine whether rubber particle cavitation is responsible for craze initiation, or vice versa. Tensile tests were carried out on pre-strained high impact polystyrene (HIPS) specimens which had been annealed above 100 °C to heal any crazes formed in the PS matrix during pre-straining. Moderate prestraining followed by annealing was found to reduce the yield stress of the HIPS, but not the post-yield flow stress. These observations are related to cavitation for the rubber particles, which results in a fibrillar structure within the rubber membranes of a typical "Salami' particle. The reduction in yield stress provides evidence for primary chain scission in the rubber phase during yielding: on subsequent loading of the annealed tensile bar, the rubber particle exhibits a reduced resistance to cavitation because less energy is required to form a void. It was found in the follow-up pre-straining tests on HIPS blends and at different testing temperatures that the critical pre-strain is a function of rubber content and temperature. Comparable effects are seen in creep tests. Small levels of pre-straining have little effect if the specimen is not annealed before reloading. The TEM work also confirms rubber particle cavitation as a rate-determining step in the deformation of HIPS and supports the view that cavitation precedes crazing. A modified cavitation model is proposed to account for the deformation of a cavitated particle under tension. The predictions of this model agree with the experiments that the resistance of the particle is weakened by cavitation and crazes could initiate at the lower tension. The technique of pre-straining and annealing specimens, before submitting them to conventional tensile tests, provides valuable insight into the mechanisms and kinetics of toughening.

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.

The main objective of this work was to develop a framework to predict the fracture toughness degradation of highly toughened adhesive joints using fracture test data obtained by accelerated open-faced degradation method. First, the mixed-mode fracture resistance (R-curve) behavior of two rubber-toughened epoxy-aluminum adhesive systems was measured and could be fit in a bilinear R-curve model. Then, open-faced DCB (ODCB) specimens of the same adhesive systems were aged over a relatively wide range of temperature, relative humidity (RH) and time, dried and tested to characterize the irreversible evolution of the mixed-mode fracture R-curves. The R-curve bilinear model parameters of adhesive system 1 varied significantly with degradation while that of adhesive system 2 remained unchanged. The absorption and desorption of water in the adhesives cast wafers was measured gravimetrically. The absorption data were fitted to a new sequential dual Fickian (SDF) model while water desorption was modeled accurately using Fick’s law. A significant difference was observed between the amounts of retained water in the two adhesives after drying. An exposure index (EI) was defined as the integral of water concentration over time and calculated at all points in the ODCB and closed DCB joints. The fracture toughness of the closed joints was then predicted from these calculated EIs by making reference to fracture toughness data from the ODCB specimens degraded to various EI levels. To verify the predictions, fracture experiments and analyses were carried out for closed DCB joints. Good agreement was found between the predicted and experimentally measured fracture toughness values for the degraded closed DCB joints. Furthermore, the crack path and fracture surface characteristics were evaluated as a function of the degree of aging using optical profilometery. The unexpected crack path in the mixed-mode fracture of unaged open-faced DCB specimens was addressed. The results showed a strong relationship between fracture surface parameters and the critical strain energy release rate, Gcs, irrespective of the type of adhesive and exposure condition.

The main objective of this work was to develop a framework to predict the fracture toughness degradation of highly toughened adhesive joints using fracture test data obtained by accelerated open-faced degradation method. First, the mixed-mode fracture resistance (R-curve) behavior of two rubber-toughened epoxy-aluminum adhesive systems was measured and could be fit in a bilinear R-curve model. Then, open-faced DCB (ODCB) specimens of the same adhesive systems were aged over a relatively wide range of temperature, relative humidity (RH) and time, dried and tested to characterize the irreversible evolution of the mixed-mode fracture R-curves. The R-curve bilinear model parameters of adhesive system 1 varied significantly with degradation while that of adhesive system 2 remained unchanged. The absorption and desorption of water in the adhesives cast wafers was measured gravimetrically. The absorption data were fitted to a new sequential dual Fickian (SDF) model while water desorption was modeled accurately using Fick’s law. A significant difference was observed between the amounts of retained water in the two adhesives after drying. An exposure index (EI) was defined as the integral of water concentration over time and calculated at all points in the ODCB and closed DCB joints. The fracture toughness of the closed joints was then predicted from these calculated EIs by making reference to fracture toughness data from the ODCB specimens degraded to various EI levels. To verify the predictions, fracture experiments and analyses were carried out for closed DCB joints. Good agreement was found between the predicted and experimentally measured fracture toughness values for the degraded closed DCB joints. Furthermore, the crack path and fracture surface characteristics were evaluated as a function of the degree of aging using optical profilometery. The unexpected crack path in the mixed-mode fracture of unaged open-faced DCB specimens was addressed. The results showed a strong relationship between fracture surface parameters and the critical strain energy release rate, Gcs, irrespective of the type of adhesive and exposure condition.

RTM6 has for more than 20 years been the main commercial epoxy system for infusion processing qualified by the aerospace industry. In common with other aerospace-grade epoxy systems RTM6 is mechanically strong but brittle, producing carbon-fibre (CF) composites with relatively low impact resistance and damage tolerance. This thesis reports an approach to toughening epoxy-CF composites without modification of the resin. Thus, a T300 carbon fabric (ES-fabric) coated with 20 weight % of a poly (aryl ether ketone) (PAEK) was used to toughen the composite. The initial stage of the study was the manufacturing process. DSC and oscillatory-shear rheology were used to determine flow times and cure conditions, and to produce laminates with fibre volume fractions ≥55% a hybrid resin infusion/hot-press process was developed. Dynamic mechanical thermal analysis also showed that the PAEK coating produced relatively little plasticization of the epoxy matrix, with values of the matrix glass transition temperature shifting from 186±4.4 to 181± 1.4 ºC when using the ES-fabric. The main body of the study focussed on the toughening effect afforded by the PAEK coating relative to an uncoated fabric system as a reference. Mode I and Mode II interlaminar fracture toughness behaviour were studied using dual cantilever beam (DCB) and four-point end-notch flexure (4ENF) tests, respectively. The measured mode-I fracture energy, GIC, increased three-fold, from 216 ± 7.2 Jm-2 to 751 ± 105 Jm-2, due to the toughening effect of the PAEK coating; whereas the mode-II fracture energy, GIIC, increased almost four-fold from 857 ± 99 Jm-2 to 3316 ± 372 Jm-2. Damage resistance was studied using low-velocity impact testing and damage tolerance using a miniature compression-after-impact (CAI). A comparative study of damage tolerance was performed using open-hole compression (OHC) testing. The impact damage resistance significantly improved with the use of the PAEK-coated ES-fabric as well as the CAI and OHC behaviour. Impact testing showed the PAEK -toughened system exhibited higher energy abortion than the untoughened system, larger damage area was created in the T300/RTM6-2 after impacted with same energy. The CAI results indicated that the normalized CAI strength is major related that damage width rather than other factor. OHC results are predicted by using W-N criteria, for ES/RTM6-2: ASC a0 = 9.35 mm and PSC d0 = 2.72mm; whereas for T300/RTM6-2: ASC a0 = 7.95 mm and PSC d0 =2.43 mm, indicates that the compressive strength of T300/RTM6-2 is more sensitive to the size of the hole, thus ES/RTM6-2 perform better damage tolerance. The results from mechanical testing indicate that the PAEK coating toughened the composite system and significantly improved damage tolerance. Scanning electron microscopy indicated that these improvements in fracture behaviour were due to morphological changes induced by the PAEK coating in the matrix-CF interfacial region, where such changes can provide the maximum benefit. Small particles of RTM (approximately 1 µm in diameter) were observed imbedded within a continuous PAEK phase. Thus, during testing crack propagation was deflected (or bifurcated) by the RTM6 particles or stopped by shearing of the continuous PAEK phase of this multiphase region. This morphology is proposed to have formed in the interfacial region during processing by dissolution of the PAEK coating within the matrix resin system, followed by reaction-induced phase separation and then phase-inversion as the matrix cures.

The phase separation process of a rubber modified epoxy system during cure was analyzed by a model developed on the basis of a thermodynamic description of binary mixture and constitutive equations for nucleation and growth rates. As epoxy resins are cured, rubber molecules are precipitated from the epoxy matrix to a non-equilibrium composition due to the decrease in the configurational entropy and the increase in the viscosity with conversion. If phase separation takes place in a metastable region, this model can monitor the changes of rubber compositions in both phases as well as the changes in the number and size of rubber particles upon conversion of polymerization. The particle size distribution at the completion of phase separation was also calculated. The effect of cure temperature on the final morphologies of a rubber modified epoxy system was discussed. The computed particle size distributions for piperidine and diaminodiphenyl sulfone cured systems showed good agreements with experimentally measured values. Depending on the activation energy for viscous flow of the epoxy matrix relative to that for the polymerization, the particle size distribution may show bimodal or unimodal distribution. The size of rubber rich phase increases to a maximum and then decreases with an increase in cure temperature. However, due to limitations of temperature range to probe in an actual experiment, one may observe only either decreasing or increasing particle size as cure temperature increases. The number of rubber particles per unit volume increases for the DGEBA/DDS/ETBN system as cure temperature increases in the temperature range of 30 °C to 220 °C. Fracture toughness of cured DGEBA/DDS/ETBN system was analyzed in terms of morphologies generated by the temperature variation. Since the volume fraction of rubber particles did not change with cure temperature, the critical stress intensity factor did not vary significantly with cure temperature as expected. However, increases in cure temperature produced smaller but more numerous particles. The critical stress intensity factor normalized by the number density of particles exhibited dependence on the radius of particles to the third power. On the other hand, the critical stress intensity factor normalized by the radius of particles showed a linear dependence with respect to the number density of particles. / Ph. D.

This thesis has focused on the synthesis and characterization of amino functional monomers, principally monomers containing aryl phosphine oxide units. Utilization of these monomers was demonstrated in various types of linear and network polymerizations. The diamines monomers included bis(3-aminophenyl) methyl phosphine oxide (DAMPO), bis(3-aminophenyl) phenyl phosphine oxide (DAPPO), bis(3-aminophenoxy phenyl) phenyl phosphine oxide (BAPPO) and bis(3-aminophenoxy phenyl) methyl Phosphine oxide (BAMPO). From these monomers high molecular weight poly(ether imides), polyurea-urethanes, poly(arylene ether ketones) poly(arylene ether sulfones) and poly(arylene ether phosphine oxides) were. Internal and external fire testing methodologies showed that the new polymers containing phosphine oxide units were fire resistant while maintaining the desirable physical characteristics of carefully selected control systems. In addition, suitable curing schedules for epoxy networks were determined by using dielectric monitoring techniques. The curing rates varied with the structure of the monomers and were slowest for the deactivated control (4,4'aminophenyl sulfone). Epoxy networks containing aryl phosphine oxide units had higher char yields in dynamic thermogravimetric analyses than control specimens. This correlated with their superior flame resistance. The brittle epoxy matrices were subsequently modified with reactive or non-reactive thermoplastic polymers in order to improve their fracture toughness. Poly(ether imides) and poly(ether sulfones) showed good phase separation behavior with tetrafunctional epoxy matrices during the curing reactions, as confirmed by scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA). Mechanical tests showed that reactive thermoplastic modification of the epoxy networks improved the fracture toughness of the systems, without noticeable decreases in other characteristics such as flexural modulus. Reactive systems also maintained chemical resistance in contrast to non-reactive thermoplastic controls. / Ph. D.

No description available.

Metal implants are a type of hard tissue replacement currently used. Metals used for implants include: stainless steel, titanium, chrome, and cobalt alloys. Such implants often fail at the interface with bone. Metal implants fail when the surface of the implant is coated with an osteoconductive material. An osteoconductive material provides scaffolding for cellular migration, cellular attachment, and cellular distribution. A reason for metal implant failure could be the vastly different material properties than bone. Motivation for the research was to find a suitable bone substitute other than metal. Materials considered were: zirconia toughened alumina, carbon fiber reinforced epoxy, and glass fiber reinforced epoxy. Those materials have been used in previous biological applications and can be cast into complex configurations. Objectives of the study were to compare material properties of the composites to bone. A method to create porosity was then tested in the material that was similar to bone in critical material property. Some of the materials were statistically similar to bone in yield strength. Method to create interconnected porosity in those materials resulted in 49% void space.