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 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.