Fiber tows

Commercially, stabilization is accomplished by controlled heating in air at temperatures of 200—300°C. A variety of equipment has been proposed for continuous stabilization. One basic approach is to pass a fiber tow through heated chambers for sufficient time to oxidize the fiber. Both Mitsubishi and Toho patents (23,24) describe similar continuous processes wherein the fiber can pass through multiple ovens to increase temperature and reaction rate as the thermal stabiUty of the fiber is increased. Alternatively, patents have described processes where the fiber passes over hot roUs (25) and through fluidized beds (26) to provide more effective heat transfer and control of fiber bundle temperature. 

EoUowing sizing, the continuous carbon fiber tows ate wound onto precision packages of various sizes from 1 to 6 kg depending on customer preference. The final bobbins are relatively stable and can be stored at room temperature for periods of one year or more. With longer storage times epoxy sizes are prone to advancement resulting in a much stiffer tow with reduced spreadabUity. 

Fiber tows are impregnated with Epon 828 (cured with m-phenylene diamine) and the prepreg cut into plies and compression molded, with 16 plies to a composite. Thin Teflon strips are inserted between plies 8 and 9 prior to molding, which initiate an interlaminar crack when the bar is loaded as a double cantilever beam. 

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. 

Evaluating composite performance requires test methods for mechanical properties of the constituent materials (fibers, tows, filaments, and matrices) as well as the composite materials themselves. The types and quantities of tests to be performed, and the selection of testing parameters, depends on the information desired. For material development, the tests may be much simpler and less numerous than those that would be chosen for design qualification, but may encompass a greater range of test parameters than would be expected in service. In the case of materials development, it is the trends in the data and the mechanisms by which failures occur which are most important, and it is crucial to examine the extremes of behavior. For component design and qualification, it is more important to know the reliability and reproducibility of the material under conditions which resemble the expected service conditions. 

Suitable for Continuous Production The equipment enables a robust production system which has been shown to operate without failure around the clock for several days. Substrates that have been coated include fiber tows, pins, industrial rollers, wire, radiators, and roll sheet material. 

Al, Brass, Ag, Cu, Pt, Ni, Stainless and C-Steel, AI2O3, Fiber Tows, Glass, Graphite, LaAlOs, MgO, NAFION , NiCr, Optical Fibers, Polycarbonate, Silica, Si, Si-Ti/Pt Wafers, SiC, Si3N4, Superalloys, Teflon , Ti, TiAl alloy, YSZ. 

A continuous CVD fiber coating process is being explored for the preparation of a low cost, high strength, thermally stable silicon carbide fiber tow. By depositing a 5 pm thick layer of stoichiometric SiC onto each filament of a carbon fiber tow it is possible to prepare fibers that are 89 vol.% SiC which have twice the tensile strength of the commonly employed Nicalon fiber. In addition, the CVD fiber has superior resistance to creep. An economic analysis indicates that the fibers could be produced for 50/lb compared to 300/lb for Nicalon. 

The box statistical study was designed and completed to find the optimal conditions for SiC deposition. The temperature was varied from 1050 to 1400°C, H2 flow rate was varied from 1.5 to 3.5 L/min, and MTS flow rate was varied from 0.15 to 1.35 L/min. The pressure inside the furnace and the fiber speed were kept constant at 13.3 kPa and 10 cm/min, respectively. During each experiment, a fiber vibration device was used which cyclically applied and released tension, at one second intervals, along the axis of the fiber tow in an attempt to spread the tow and to minimize agglomeration of the individual filaments within the tow. 

Figure 6-7. Schematic of the pneumatic device used to spread the fiber tow.

Multiple regression analysis (Table 6-4) showed, with greater than 99% confidence, that the weight gain of coated fiber tows had a large positive correlation with both tem-... 

---