Prepreg Flow Mechanisms

Batch testing is carried out to verify prepreg properties, such as resin content, volatile level, and flow. The resin advancement (chemical reaction) is monitored via a Differential Scanning Calorimeter (DSC) and the formulation consistency by testing the Tg via DSC or Dynamic Mechanical Analyzer (DMA). The laminate properties are also determined. All are documented and quoted on a Release Certificate. 

As reviewed in conventional RTM, the preform is placed into a closed, matched tool and resin is injected under pressure about 100 to 200 psi (0.69 to 1.38 MPa). Early RTM processes lacked the consistency needed for aerospace components, in both dimensional tolerances and mechanical properties. Fiber volume fractions were significantly lower than the 60 to 65 wt% typical of prepregs. Problems with predicting flow fironts as well as flaws that were introduced into the preform when closing the matched metal molds often led to high void contents and dry spots. 

When the textile fabric reinforcement is combined with the matrix to form the composite prepregs, a set of deformation modes are introduced, which may be referred to as top-level deformation modes since they involve the movements of the reinforcement (macro- and micro-level fabric deformation mechanisms) and the matrix. They are in fact the conformation modes of the composite prepreg sheet or group of sheets, as is usually the case, during the forming process. The hierarchy of the top-level deformation modes consist of transverse flow, resin percolation, interply shear and intraply shear, as summarized by Cogswell and revisited by Martin et... 

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