Rate-dependent compaction and relaxation response of uncured prepregs under high-pressure conditions

The compaction-relaxation response at different compaction rates and fiber volume fractions plays a key role in understanding the viscoelastic response of uncured prepregs. Hence, this study characterizes the time-dependent behavior of un-cured 4-layer prepregs subjected to compaction-stress relaxation test at different displacement rates i.e., 0.1 mm/min, 1.0 mm/min, and 10 mm/min, at 0.65 fiber volume fraction and allowed to relax for two hours. In this study, the complete deformation history of the Hexply M26T multilayer prepregs is measured from a stress-free state to the cured state. The effects of rate-dependent compaction-relaxation at different rates on percentages of compaction, recovery, stress change during relaxation, and permanent deformation of prepregs are computed. It was concluded that the 0.1 mm/min displacement rate showed the lowest peak stress level and the lowest stress relaxation and permanent deformation. A viscoelastic model was used to fit the experimental data and the results showed a good agreement. The void content was determined analytically and from the XCT-aided geometrical model. It was observed that for a given test condition, the void content increases as the displacement rate increases, due to the high applied pressure. This study highlights the importance of rate-dependent compaction-relaxation behavior and the need to determine the suitable process parameters and models to manufacture high-quality aerospace composite structures.