Prepregs surfaces

The occurrence of voids has been thoroughly documented in thick laminates [2], In almost all cases, they are apparently associated with the prepreg surface. The exact mechanism of void formation depends on the system, but in the most general case it can include mechanical entrapment as well as nucleation of stable voids in the resin phase. 

Lee and Springer [19] modeled the resin-rich prepreg surface roughness by a series of uniform rectangular elements, as shown in Figure 7.5. At the beginning of the consolidation... 

Techniques that have been used to measure the prepreg surface roughness were mentioned in Section 7.2.1 and include direct measurement with a micrometer and photo-micrographs of the prepreg cross section. Li and Loos [22,23] showed that a surface topology characterization machine (Talysurf 4) can be used to measure the waviness or roughness of the resin rich... 

Figure 13.11 Idealization of the uneven thermoplastic prepreg surface as a series of rectangles with relative heights and spacing as shown...

The mould is cleaned and a release agent or release film is applied. The backing film is removed from the prepreg surface and successive layers of prepreg are placed on the mould and rolled to conform to the mould surface. The layers are applied in a predetermined order and angle to achieve the laminate properties required. 

Treatment metrics Carbon/epoxy prepreg surface treatment ... 

Fig. 9. The effect of voids due to poor wetting on adhesive strength, (a) The zippering effect of voids aligned in the plane of shear, (b) Macro-voids in the resin formed during the manufacture of a carbon fiber reinforced prepregs. (c) Micro-voids caused by axial crenulations along carbon fiber surfaces.

Models of the intimate contact process that have appeared in the literature are commonly composed of three parts or submodels. The first submodel is used to describe the variation in the tow heights (surface waviness or roughness) across the width of the prepreg or towpreg. The second submodel, which is used to predict the elimination of spatial gaps and the establishment of intimate contact at the ply interfaces, relates the consolidation pressure to the rate of deformation of the resin impregnated fiber tow and resin flow at ply surface. Finally, the third submodel is the constitutive relationship for the resin or resin-saturated tow, which gives the shear viscosity as a function of temperature and shear rate. 

Equations 7.2 and 7.3 represent expressions for the degree of intimate contact of a single prepreg ply in contact with a smooth rigid surface (Fig. 7.5). 

APC-2 is a semi-crystalline thermoplastic prepreg. The surface roughness characterization for two different batches of APC-2 prepreg are reported in Table 7.2. Batch I is a 152.4 mm (6in.)-wide prepreg sheet, whereas batch II is 304.8-mm wide. In addition, the... 

The intimate contact data shown in Figure 7.16 were obtained from three-ply, APC-2, [0°/90o/0o]7- cross-ply laminates that were compression molded in a 76.2 mm (3 in.) square steel mold. The degree of intimate contact of the ply interfaces was measured using scanning acoustic microscopy and image analysis software (Section 7.4). The surface characterization parameters for APC-2 Batch II prepreg in Table 7.2 and the zero-shear-rate viscosity for PEEK resin were input into the intimate contact model for the cross-ply interface. Additional details of the experimental procedures and the viscosity data for PEEK resin are given in Reference 22. 

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