Experimental and Numerical Studies on the Effects of Environmental Conditions on Defect Formation during Autoclave Processing of Prepreg

Abstract

Primary load-bearing composite structures such as aircraft wings, rotor blades and sailing masts require high laminate quality. This is commonly achieved by autoclave prepreg processing where, in addition to the vacuum pressure, high external pressures during the resin cure stage enhance consolidation. As a result, parts of simple geometry can be manufactured with relative ease having fibre volume fractions typically above 60%, with void content less than 1%. However, defects such as wrinkles and voids are typically found within more complex structures despite the high applied external consolidation pressure. This research investigates defect formation within the layup, debulking and consolidation stages of prepreg processing, and is focused on female moulded corners with high laminate thickness to corner radius ratios. Experimental studies were carried out to investigate the effect of humidity exposure to uncured prepreg on defect formation during the steps of autoclave processing. It is hypothesized that moisture uptake influences uncured prepreg characteristics that affect laminate consolidation behaviour, and therefore achieved quality following consolidation. A detailed prepreg characterisation program establishing the influence of relative humidity was conducted. In addition, the effects of changing prepreg characteristics due to relative humidity, laminate thickness to corner radius ratios, and laminate stacking sequences on the pressure distribution in sharp corner areas was investigated using the Tekscan pressure mapping system. Lastly a novel numerical consolidation model for toughened prepreg systems in complex geometries was implemented and applied to several parametric studies. It is concluded that defects already occur during the layup and debulking process, which then affect defect evolution in the subsequent autoclave curing process. It is also shown that humidity affects the uncured prepreg characteristics and thereby the pressure distribution and defect formation in corner areas. The results obtained from numerical simulation show similar trends compared to experimental values, and a strong dependency of the applied vacuum pressure on the corner pressure distribution.