(a) Polymer and curing agent prior to reaction

(b) Curing initiated with size of molecules increasing

(c) Gellation with full network formed

(d) Full cured and crosslinked

(a) Polymer and curing agent prior to reaction

(b) Curing initiated with size of molecules increasing

(c) Gellation with full network formed

(d) Full cured and crosslinked

Fig. 7.6. Crosslinking Reaction for Thermoset Resin4

The first consideration in selecting a resin system is the service temperature required for the part. The glass transition temperature Tg is a good indicator of the temperature capability of the matrix. The glass transition temperature (Tg ) is the temperature at which a polymer changes from a rigid glassy solid into a softer, semi-flexible material. At this point, the polymer structure is still intact but the crosslinks are no longer locked in position. A resin should never be used above its Tg unless the service life is very short (e.g., a missile body). A good rule of thumb is to select a resin in which the Tg is 50° F higher than the maximum service temperature. Since most polymeric resins absorb moisture that lowers the Tg, it is not unusual to require that the Tg be as much as 100° F higher than the service temperature. It should be noted that different resins absorb moisture at different rates, and the saturation levels can be different. Therefore, the specific resin candidate must be evaluated for environmental performance. Most thermoset resins are fairly resistant to solvents and chemicals.

Although fiber selection usually dominates the mechanical properties of the composite, matrix selection can also influence performance. Some resins wet-out and adhere to fibers better than others, forming a chemical and/or mechanical bond that can improve the fiber-to-matrix load transfer capability. The matrix can also microcrack during cure or in-service. Resin rich pockets and brittle resin systems are susceptible to microcracking, especially when the processing temperatures are high and the use temperatures are low (e.g., -65° F), since this condition creates a very large difference in thermal expansion between the fibers and the matrix. Toughened resins can help in preventing microcracking but often at the expense of elevated temperature performance. The selection of a matrix material has the largest effect on the fabrication and processing conditions.

7.1.3 Product Forms

There are a multitude of material product forms used in composite structures. The fibers can be continuous or discontinuous. They can be oriented or disoriented (random). They can be furnished as dry fibers or preimpregnated with resin (prepreg). Since the market drives availability, not all fiber or matrix combinations are available in all product forms. In general, the more operations required by the supplier, the higher the cost. For example, prepreg cloth is more expensive than dry woven cloth. While complex dry preforms may be expensive, they can translate into lower fabrication costs by reducing or eliminating hand lay-up costs. If structural efficiency and weight are important design parameters, then continuous reinforced product forms are normally used because discontinuous fibers yield lower mechanical properties.

Rovings, tows, and yarns are collections of continuous fiber. This is the basic material form that can be chopped, woven, stitched, or prepregged into other product forms. It is the least expensive product form and available in all fiber types. Rovings and tows are supplied with no twist, while yarns have a slight twist to improve their handleability. Some processes, such as wet filament winding and pultrusion, use rovings as their primary product form.

Continuous thermoset prepreg materials are available in many fiber and matrix combinations. A prepreg is a fiber form that has a predetermined amount of uncured resin impregnated on the fiber by the material supplier. Prepreg rovings and tapes are usually used in automated processes, such as filament winding and automated tape laying, while unidirectional tape and prepreg fabrics are used for hand-lay up. Unidirectional prepreg tapes (Fig. 7.7) offer better structural performance than woven prepregs, due to absence of fiber crimp and the ability to more easily tailor the designs. However, woven prepregs offer increased drapeablility. With the exception of predominantly unidirectional designs, unidirectional tapes require placement of more individual plies during lay-up. For example, with cloth, for every 0° ply in the lay-up, a 90° reinforcement is also included. With unidirectional tape, a separate 0° ply and a separate 90° ply must be placed onto the tool.

Prepregs are supplied with either a net resin (prepreg resin content ~ final part resin content) or excess resin (prepreg resin content > final part resin content). The excess resin approach relies on the matrix flowing through the plies and removing entrapped air, while the extra resin is removed by impregnating bleeder plies on top of the lay-up. The amount of bleeder used in the lay-up will dictate

Fig. 7.7. Unidirectional Prepreg Tape

the final fiber and resin content. To insure proper final physical properties, accurate calculations of the number, and areal weight, of bleeder plies for a specific prepreg are required. Since the net resin approach contains the final resin content weight in the fabric, no resin removal is necessary. This is an advantage because the fiber and resin volumes can more easily be controlled.

Woven fabric, shown in Fig. 7.8, consisting of interlaced warp and fill yarns, is the most common continuous dry material form. The warp is the 0° direction as the fabric comes off the roll and the fill, or weft, is the 90° fiber. Typically, woven fabrics are more drapeable than stitched materials; however, the specific weave pattern will affect their drapeablility characteristics. The weave pattern will also affect the handleability and structural properties of the woven fabric. Many weave patterns are available. All weaves have their advantages and disadvantages, and consideration of the part configuration is necessary during fabric selection. Two of the more widely used weave patterns are shown in Fig. 7.9. The plain weave has the advantage that it has good stability and resists distortion, while the satin weaves have higher mechanical properties, due to less fiber crimp, and are more drapeable. Most fibers are available in woven fabric form; however, it can be very difficult to weave some high modulus fibers due to their inherent brittleness. Advantages of woven fabric include drapeablility, ability to achieve high fiber volumes, structural efficiency, and market availability. A disadvantage of woven fabric is the crimp that is introduced to the warp or fill fiber during weaving. Finishes or sizings are typically put on the fibers to aid in the weaving process and minimize fiber damage. However, it is important to insure that the finish is compatible with the matrix selection when specifying a fabric.

Fig. 7.8. Dry Woven Carbon Cloth
Plain Weave Satin Weave
Fig. 7.9. Plain and Satin Weave Cloth5

A stitched fabric consists of unidirectional fibers oriented in specified directions that are then stitched together to form a fabric. A common stitched design includes 0°, +45°, 90°, and -45° plies in one multi-directional fabric. Advantages include: (1) the ability to incorporate off-axis orientations as the fabric is removed from the roll. Off-axis cutting is not needed for a multi-directional stitched fabric and, when compared to conventional woven materials, can reduce scrap rates (up to 25%); (2) labor costs are also reduced when using multi-ply stitched materials, because fewer plies are required to be cut and handled during lay-up; and (3) due to the z-axis stitch threads, ply orientation remains intact during handling. A disadvantage is the availability of specific stitched ply set designs. Typically, a special order is required due to the tailoring requested by the customer, such as fiber selection, fiber volume, and stitching requirements. In addition, not as many companies stitch as weave and drapeablility characteristics are reduced. However, this can be an advantage for parts with large simple curvature. Careful selection of the stitching thread is necessary to insure compatibility with the matrix and process temperatures.

Hybrids are material forms that make use of two or more fiber types. Common hybrids include glass/carbon, glass/aramid, and aramid/carbon fibers. Hybrids are used to take advantage of properties or features of each reinforcement type. In a sense, a hybrid is a "trade-off" reinforcement that allows increased design flexibility. Hybrids can be interply (two alternating layers), intraply (present in one layer), or in selected areas. Hybridization in selected areas is usually done to locally strengthen or stiffen a part.

A preform is a pre-shaped fibrous reinforcement that has been formed into shape, on a mandrel or in a tool, before being placed into a mold. As shown in Fig. 7.10, the shape of the preform closely resembles the final part configuration. A simple multi-ply stitched fabric is not a preform unless it is shaped to near its final configuration. The preform is the most expensive dry, continuous, oriented fiber form; however, using preforms can significantly reduce fabrication labor. A preform can be made using rovings, chopped, woven, stitched, or

Fig. 7.10. Fiberglass Preform unidirectional material forms. These reinforcements are formed and held in place by stitching, binders or tactifiers, braiding, or three-dimensional (3-D) weaving. Advantages include reduced labor costs; minimal material scrap; reduced fiber fraying of woven or stitched materials; improved damage tolerance for 3-D stitched or woven preforms; and the desired fiber orientations are locked in place. Disadvantages include high preform costs; fiber wetability concerns for complex shapes; tackifier or binder compatibility concerns with the matrix; and limited flexibility if design changes are required.

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