Development of residual stresses during electron beam processing

In order to further the comprehension of the electron beam (EB) processing of polymers and composites, it is necessary to better understand the development of residual stresses during processing. In-situ measurement of stress development during EB irradiation is a challenging task. The instrumentation has to be adequately shielded from the high-energy electron beam. In this work, a special fixture was designed specifically to measure the warpage of a specimen throughout the EB curing process. The specimen consisted of a thin layer of epoxy resin (Tactix 123 or CAT B) embedded in a layer of glass scrim cloth, sandwiched between layers of steel and aluminum plates and also antisymmetric unidirectional carbon-epoxy laminates. The warpage of the specimens was monitored during and after irradiation at different constant dose rates. The results confirmed that the experimental instrumentation was not affected by the EB exposure and that it was possible to monitor the specimen warpage during the EB process. The results show that the EB cured specimens have lower stress-free temperature compared to equivalent thermally cured specimens. It was also shown that an increase in dose rate increased the level of residual stresses. Furthermore, the results suggest that there is a direct relation between the stress free temperature (TSF) and the temperature of the specimen at gelation (TGEL). Also, the stress free temperatures for CAT B specimens were 5°C higher than the Tactix 123 specimens cured under the same curing conditions. Finally, a cure kinetics model for Tactix 123 was used to predict the degree of cure (∼0.32) at onset of residual stress development and the results proved to be valid when compared to parallel plate rheology results.