Few comprehensive studies on the effects of stacking sequence and rein forcement form (unidirectional versus woven) have been published to date and much of the available data is contradictory. In the present study, instrumented impact tests were used to characterize such effects for carbon fiber reinforced thermoplastic toughened epoxy laminates. Impact resistance was characterized in terms of load and energy parameters measured during penetration tests. These parameters were related to damage in the lami nates by conducting rebound tests followed by ultrasonic imaging and microscopy. The results clearly demonstrated a relationship between the onset of damage and the first peak in the load versus deflection plots obtained in the penetration tests. No major effects of stacking sequence or reinforcement form were apparent in terms of the energy required for the onset of damage in the laminates. Energy to maximum load was found to be highly de pendent on stacking sequence. Substitution of woven reinforcement for unidirectional tape in a quasi-isotropic layup resulted in a substantial decrease in the energy to maximum load. The results in terms of peak load showed similar trends. No effects of stacking se quence or reinforcement form were observed in terms of energy after peak load. It is ap parent from this work that stacking sequence and reinforcement form can have significant effects on impact resistance particularly at higher impact energies.

Little, if any, information regarding the effects of seawater immersion on the impact resistance of composite materials has been published to date. In the present study, instrumented impact tests were used to characterize such effects for two glass fiber reinforced epoxy composites. One material consisted of continuous nonwoven E-glass fibers in a conventional epoxy resin and the other consisted of woven E-glass fibers in a rubber toughened epoxy resin formulated specifically for marine applications. In the conventional epoxy system, the energy for incipient damage increased significantly following seawater immersion due to plasticization of the matrix by the absorbed moisture. Both glass/epoxy systems experienced substantial reductions in peak load and energy absorbed at peak load as a result of moisture-induced degradation of the fibers and fiber/matrix interface. Energy after peak load decreased in the conventional epoxy system following immersion. Total energy absorbed was reduced significantly for both glass/epoxy systems following seawater immersion. These results indicate that moisture-induced degradation can significantly reduce the impact resistance of glass fiber reinforced epoxy composites.

No abstract

Deformation and damage behavior of a 1070 steel (class U wheel steel) under thermo-mechanical and isothermal loading have been examined. Fatigue lives under thermo-mechanical and isothermal loading were compared for similar conditions of strain and temperature. For most cases, maximum tensile stresses developed in thermo-mechanical tests exceeded those obtained under isothermal conditions (at Tmin) for the same strain amplitude. Cyclic hardening was observed at 200 to 300°C in isothermal tests. Under thermo-mechanical loading, static strain aging resulted in added hardening due to alternate exposure of the material to high and low temperatures. When thermal recovery effects become dominant at the high temperature end, strengthening upon cooling was suppressed. Oxide scales readily formed at high temperatures (≥400°C) and resulted in an increase in damage accumulation rates. Oxygen penetration into crack flanks and the ensuing loss of carbon was identified using Auger spectroscopy. The severity of oxide penetration into the base metal increased with increase in surface crack density, longer oxidation times and higher temperatures.

The fatigue lives of 1070 steel (Class U wheel steel) under (1) isothermal loading, (2) thermo-mechanical constant amplitude loading, and (3) thermo-mechanical block loading have been examined. Fatigue lives for these three cases were compared based on the same mechanical strain range and maximum temperature level, with similar strain rates. Predictions of thermo-mechanical block loading cases were nonconservative based on isothermal data, while the predictions improved to within a factor of 1.5 of experimental lives when thermo-mechanical constant amplitude data were utilized. Oxide scales readily formed at high temperatures (>500°C) and resulted in increased damage due to a localized high strain state and an inherent lack of ductility. Oxides of different composition and morphology form depending on the temperature and type of loading. Stratified and non-stratified oxide layers (predominantly Fe3O4) were identified in the experiments.

No abstract

The present research was undertaken to evaluate the effects of mechanical constraint on the response of polymer matrix composites during thermal cycling. Analytical and experimental techniques were used to characterize the response of carbon-fiber-reinforced cyanate ester and bismaleimide composites. Cross-ply laminates were subjected to thermal cycles from 24 to 177°C in the unconstrained, fully constrained, and overconstrained conditions. Laminate response, damage mechanisms, and residual compressive properties were characterized for each material and degree of constraint. The results of this research indicate that the level of constraint can have a significant effect on the response of polymer matrix composites during thermal cycling. However, longer-term testing is required to determine if the observed changes in response will ultimately affect the final failure mode and fatigue endurance of the materials.

No abstract