Thermal, rheological, and mechanical properties of a polymer composite cured at staged cure cycles
Thermal, rheological, and mechanical properties of a polymer composite cured at different one-stage and two-stage cure cycles were studied in this dissertation. A commercial carbon-fiber prepreg, Cycom 977-2 UD, was used. This curing-toughened epoxy resin prepreg is formulated for autoclave or press molding. An encapsulated sample rheometer (ESR) was used to obtain its viscoelastic properties, including complex viscosity, gel time, and minimum viscosity time, as well as glass transition temperature (Tg) and pressure window time for onestage and two-stage cure cycles. A differential scanning calorimeter (DSC) was used to obtain the degree of cure (DOC) for one-stage and two-stage cure cycles. The mechanical properties of interest for specimens cured at one-stage cure cycles were short beam shear (SBS) strength, combined loading compression (CLC) strength, CLC modulus, CLC Poisson’s ratio, open-hole compression (OHC) strength, and OHC modulus. The SBS, CLC, and OHC tests were performed at room temperature to obtain the mechanical properties. For the one-stage cure cycles studied, it was observed that the mechanical properties, except SBS strength, did not vary significantly; therefore, no correlation with the viscoelastic properties or the DOC was found for them. Moreover, the failure mode for OHC specimens cured at different one-stage cure cycles was similar. Likewise, the failure mode for CLC specimens cured at different one-stage cure cycles was the same. However, the failure mode for the least-cured SBS specimens was different from that of other SBS specimens. Also, the SBS strength of the least-cured specimens was significantly less than that of other specimens. The complex viscosity of the specimens cured at one-stage cure cycles in the ESR showed a similar drop-off trend for the least-cured specimens. As such, SBS strength showed a good correlation with the complex viscosity. SBS strength showed a weaker correlation with the Tg and DOC for vii the same cure cycles. The Tg had a strong correlation with the DOC for all one-stage cure cycles. No correlation between gel time and other material properties was found. A considerable improvement in SBS strength, final complex viscosity, Tg, and DOC of the least-cured specimens was observed after the dwell time was increased enough to ensure that no further curing occurred. It was also observed that for the two-stage cure cycles, faster heat-up rates and higher first-stage dwell temperatures resulted in faster curing. The DOC for the entire cure cycle was modeled using the Springer-Loos cure kinetics model for one-stage and two-stage cure cycles. The complex viscosity up to the gel time was modeled using the Kenny viscosity model for one-stage and two-stage cure cycles. The modeling results agreed well with the experimental data. The results presented in this dissertation suggest that the ESR can be used as an ex-situ cure-monitoring instrument to mimic autoclave/oven curing and, hence, eliminate the need for multiple measurement instruments. The cure time-temperature data, provided by thermocouples attached to the composite part in the autoclave/oven would be the only input to the rheometer for cure monitoring. The complex viscosity as measured by the ESR was shown to be the best viscoelastic property for monitoring the state of the material during cure for the following reasons: (a) it could be precisely measured throughout the cure and post-cure cycles using the rheometer, (b) it could reveal the important changes in the material state during cure, (c) it could be modeled by sophisticated viscosity models, and (d) it could be correlated to the mechanical properties of the composite material. Utilizing the ESR as the main ex-situ cure-monitoring instrument makes it possible to offer a new approach to curing composites. In this new approach, called Material State Management (MSM), the acceptance of cured composite materials is based on the materials’ viscoelastic properties as measured by the ESR during cure and post-cure monitoring. Moreover, knowledge of the material’s viscoelastic properties during cure can be used to improve the current cure specifications. In the MSM approach, cure process confidence limits can be prescribed based on the viscoelastic properties of the material, thus addressing the shortcomings of the current time-temperature approach to curing.
Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering