Figure 7-50.  Examples of the effects caused by nonsymmetrical  laminates. Saturation Techniques Fabric Impregnation With a Brush or Squeegee Fabric Impregnation Using a Vacuum Bag Mixing Resins  The strength and stiffness of a composite buildup depends  on the ply orientation. The practical range of strength and  stiffness of carbon epoxy extends from values as low as  those provided by fiberglass to as high as those provided by  titanium. This range of values is determined by the orientation  of the plies to the applied load. Because the strength design  requirement is a function of the applied load direction, ply  orientation and ply sequence must be correct. It is critical  during a repair operation to replace each damaged ply with  a ply of the same material and orientation or an approved  substitute. Warp is the longitudinal fibers of a fabric. The warp is the  high-strength direction due to the straightness of the fibers.  A warp clock is used to describe direction of fibers on a  diagram, spec sheet, or manufacturer’s sheets. If the warp  clock is not available on the fabric, the orientation is defaulted  to zero as the fabric comes off the roll. Therefore, 90° to zero  is across the width of the fabric. 90° to zero is also called  the fill direction.  Epoxy resins, like all multipart materials, must be thoroughly  mixed. Some resin systems have a dye added to aid in seeing  how well the material is mixed. Since many resin systems do  not have a dye, the resin must be mixed slowly and fully for  three minutes. Air enters into the mixture if the resin is mixed  too fast. If the resin system is not fully mixed, the resin may  not cure properly. Make sure to scrape the edges and bottom  of the mixing cup to ensure that all resin is mixed correctly. Do not mix large quantities of quick curing resin. These types  of resins produce heat after they are mixed. Smoke can burn  or poison you when the resin overheats. Mix only the amount  of material that is required. Mix more than one batch if more  material is needed than the maximum batch size.  For wet layup repair, impregnate the fabric with resin. It is  important to put the right amount of resin on the fabric. Too  much or too little resin affects the strength of the repair. Air  that is put into the resin or not removed from the fabric also  reduces the repair strength.  The traditional way of impregnating the fabric is by using  a brush or squeegee. The technician puts a mold release  compound or a release film on a caul plate so that the plies  will not adhere to the caul plate. Place a sheet of fabric on  the caul plate and apply resin in the middle of the sheet. Use  a brush or squeegee to thoroughly wet the fabric. More plies  of fabric and resin are added and the process is repeated  until all plies are impregnated. A vacuum bag will be used  to consolidate the plies and to bleed off excess resin and  volatiles. Most wet layup processes have a room temperature  cure but extra heat, up to 150 °F, are used to speed up the  curing process.  [Figure 7-51] The vacuum-assisted impregnation method is used to  impregnate repair fabric with a two-part resin while enclosed  inside a vacuum bag. This method is preferred for tight- knit weaves and when near optimum resin-to-fiber ratio is  required. Compared to squeegee impregnation, this process  reduces the level of entrapped air within the fabric and offers  a more controlled and contained configuration for completing  the impregnation process. Vacuum-assisted impregnation consists of the following  steps: 1.  Place vacuum bag sealing tape on the table surface  around the area that is used to impregnate the material.  The area should be at least 4 inches larger than the  material to be impregnated. 2.  Place an edge breather cloth next to the vacuum bag  sealing tape. The edge breather should be 1–2 inches  wide. 3.  Place a piece of solid parting film on the table. The  sheet should be 2-inches larger than the material to  be impregnated. 4.  Weigh the fabric to find the amount of resin mix that  is necessary to impregnate the material. 5.  Lay the fabric on the parting film. 6.  Put a piece of breather material between the fabric  and the edge breather to provide an air path. 7-30 Flat, constant midplane  stress Induces curvature Induces twist Induces twist and  curvature Symmetrical,  balanced Nonsymmetrical,  balanced Symmetrical,  nonbalanced Nonsymmetrical,  nonbalanced (+45,  – 45, 0, 0,  – 45, +45) (90, +45, 0, 90, –45, 0) (–45, 0, 0, –45) (90, –45, 0, 90, –45, 0) Type Comments Example