Thermo-Mechanical Compaction-Creep and Void Analysis of Prepregs Using XCT-Aided Geometrical Models

An investigation of the effect of simultaneous thermal and mechanical loads on the compaction-creep-thickness recovery response of uncured prepreg system is essential to have a better understanding of the conditions applied during autoclave and out-of-autoclave processes. Compaction response mimics the mold closure effect, creep response mimics the dwell effect and thickness recovery response of the prepregs mimics the spring-back effect. However, there are almost no studies available to compare the void transport within prepregs for pristine (before compaction) and compacted (after spring-back) states using X-ray computed tomography (XCT) aided geometrical models. This study aims to analyze the thermo-mechanical compaction-creep-recovery response of 4- and 8-layers glass fiber uncured prepreg system. The target compaction pressures used were 0.1 and 0.3 MPa, applied at four different temperatures (25, 50, 80 and 100°C). The thickness of the specimens was measured at different time and based on these values, percentage of compaction, creep and permanent deformation were determined. XCT aided geometrical models were generated for the pristine and tested states to determine the void volume ratio and void counts. The decrease in thickness values at the target load is observed to be lower at 25°C than at 100°C, which confirms the higher initial compaction occurs at high temperatures. At room temperature, 4-layer specimens showed higher creep percentage at both pressures and offered more fibrous level changes compared to 8-layers. The creep percentage was found to decrease as the number of layers increased at low temperatures whereas the opposite trend was found at higher temperatures. Void maps indicated that friction between the fabric layers increases at higher temperatures (80 and 100°C) during resin bleed out which leads to low creep percentage in the 4-layer specimens as compared to 8-layer specimens.