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The fibers in a continuous fiber reinforced composite may not actually be “continuous” in the strictest definition of the word. In actuality they vary from as short as several feet in length to several thousand feet in length. Everything in this wide range is generally called continuous because the length of the fibers tends to be orders of magnitude larger than the width or thickness of the raw composite material.
Image #1: Continuous Carbon Fiber Reinforced Thermoplastic Composite Tape
Raw continuous fiber composites can come in many forms (pultruded rods, woven mats, etc.), but we will concentrate on “tape”. This tape can be thermoplastic or thermoset. It is created by impregnating the continuous fibers with the desired matrix and forming this combination into a thin, wide and long geometry. Continuous fiber composite tapes tend to be 0.005″ to 0.030″ thick, 0.063″ to several feet wide and up to several thousand feet long. The wide spools of tape are generally slit down to narrower widths for final processing. Image #1 below is a picture of 1.0″ wide tape that was slit from a spool that started at 14″ wide.
When continuous fibers are processed into the composite tape they are unidirectional and run longitudinally. In other words all of the fibers run in the long direction of the composite tape. This unidirectional fiber orientation creates a raw composite material that is highly anisotropic, as the properties (mechanical, thermal, electrical, etc.) are significantly different when evaluated in the direction of (parallel with) the fibers as opposed to across (perpendicular to) the fibers. This anisotropy has some significant advantages and disadvantages. The biggest advantage is that by controlling the orientation of the raw composite tape in the final structure, important design criteria (strength, modulus, CTE, etc.) can be altered and customized. The major disadvantage is that it is very challenging to get effective fiber reinforcement in all three dimensions. While it is possible to get fibers in all dimensions (e.g. woven preforms), in general structures made this way are unable to have high loading in any of the three directions and these applications are usually better serviced by isotropic materials.
As a group, continuous fiber reinforced composites tend to be more expensive than short or long fiber reinforced composites but offer significantly improved performance.