In-situ X-ray computed tomography characterization of compaction-creep-recovery response and statistical void analysis of carbon/epoxy prepregs

Vacuum bag only (VBO) prepreg processing requires several time-consuming experimental trials in order to achieve a void volume fraction (V_v) of less than 2%. Alternatively, a limited number of in-situ X-ray computed tomography (XCT) compaction-creep experiments on prepregs under pressures that are equivalent to the VBO process can potentially reduce both time and cost whilst assisting in identifying and understanding the mechanisms associated with void formation during processing. In this study, compaction-creep tests at the first dwell temperature during the cure cycle were conducted at 0.1 MPa for different dwell periods (1, 2, and 4 h) using a specially-designed compression stage installed inside an XCT machine. Based on the thickness values measured at different stages of the test, the percentage of compaction-creep and permanent deformations were determined. Three-dimensional (3D) XCT-aided geometrical models were developed both before and after the compaction-creep tests, followed by V_v calculations at each stage. The results showed that higher levels of permanent deformation were recorded in samples tested using a dwell time of 4 h, due to a lower level of recovery compared to other dwell periods. Furthermore, the most suitable condition, in terms of the lowest V_v, was found in samples manufactured with a dwell time of 2 h, where relatively small voids, with a diameter less than 100 μm and a V_v of 0.57% were observed. These values are significantly lower than those observed in uncompressed samples which have many larger voids with a diameter greater than 200 μm and a V_v of approximately 15%.