Development of residual stresses during electron beam processing

Abstract

In order to further advance the state of the art in the electron beam processing of polymers and composites, it is necessary to better understand the development of cure-induced residual stresses. In situ measurement of electron beam cure–induced stress development is challenging due to the bombardment of specimens with intense beams of very high-voltage electrons. In this study, a custom fixture was designed to measure the deformation of an electron beam–cured specimen during processing and thereby to assess specimen stress state during the curing process. The unbalanced composite specimen consisted of two cured unidirectional carbon–epoxy laminates separated by a thin woven fibreglass carrier, which was infused by a layer of electron beam curable epoxy resin systems (Tactix 123 and CAT B). The out-of-plane specimen deformations were monitored during various electron beam irradiation schedules and subsequent thermal post-cure. The preliminary results confirmed that the apparatus was not affected by the irradiation and was capable of accurately measuring the specimen warpage during the cure. It was also shown that the electron beam–cured specimens exhibit a reduced residual stress state compared to equivalent thermally cured specimens. It was observed that the irradiation dose rate applied to a specimen is the main factor in the difference in the way residual stresses develop during electron beam curing. 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).