Composite materials consist of a combination of materials that  are mixed together to achieve specific structural properties.  The individual materials do not dissolve or merge completely  in the composite, but they act together as one. Normally, the  components can be physically identified as they interface  with one another. The properties of the composite material  are superior to the properties of the individual materials from  which it is constructed. An advanced composite material is made of a fibrous material  embedded in a resin matrix, generally laminated with fibers  oriented in alternating directions to give the material strength  and stiffness. Fibrous materials are not new; wood is the most  common fibrous structural material known to man. A matrix supports the fibers and bonds them together in the  composite material. The matrix transfers any applied loads  to the fibers, keeps the fibers in their position and chosen  orientation, gives the composite environmental resistance, and  determines the maximum service temperature of a composite. Structural properties, such as stiffness, dimensional stability,  and strength of a composite laminate, depend on the stacking  sequence of the plies. The stacking sequence describes  the distribution of ply orientations through the laminate  thickness. As the number of plies with chosen orientations  increases, more stacking sequences are possible. For  example, a symmetric eight-ply laminate with four different  ply orientations has 24 different stacking sequences. The strength and stiffness of a composite buildup depends  on the orientation sequence of the plies. The practical range  of strength and stiffness of carbon fiber 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. Proper  selection of ply orientation in advanced composite materials  is necessary to provide a structurally efficient design. The  part might require 0° plies to react to axial loads, ±45° plies  to react to shear loads, and 90° plies to react to side loads.  Because the strength design requirements are a function of  the applied load direction, ply orientation and ply sequence  have to be correct. It is critical during a repair to replace  each damaged ply with a ply of the same material and ply  orientation. An isotropic material has uniform properties in all directions.  The measured properties of an isotropic material are  independent of the axis of testing. Metals such as aluminum  and titanium are examples of isotropic materials. The fibers in a unidirectional material run in one direction  and the strength and stiffness is only in the direction of the  fiber. Pre-impregnated (prepreg) tape is an example of a  unidirectional ply orientation. A fiber is the primary load carrying element of the composite  material. The composite material is only strong and stiff in  the direction of the fibers. Unidirectional composites have  predominant mechanical properties in one direction and are  said to be anisotropic, having mechanical and/or physical  properties that vary with direction relative to natural reference  axes inherent in the material. Components made from fiber- reinforced composites can be designed so that the fiber  orientation produces optimum mechanical properties, but  they can only approach the true isotropic nature of metals,  such as aluminum and titanium. The fibers in a bidirectional material run in two directions,  typically 90° apart. A plain weave fabric is an example of  a bidirectional ply orientation. These ply orientations have  strength in both directions but not necessarily the same  strength.  [Figure 7-1] The plies of a quasi-isotropic layup are stacked in a 0°, –45°,  45°, and 90° sequence or in a 0°, –60°, and 60° sequence.  [Figure 7-2]  These types of ply orientation simulate  the properties of an isotropic material. Many aerospace  composite structures are made of quasi-isotropic materials.  • Fairings •  Flight control surfaces •  Landing gear doors •  Leading and trailing edge panels on the wing and  stabilizer •  Interior components •  Floor beams and floor boards •  Vertical and horizontal stabilizer primary structure on  large aircraft •  Primary wing and fuselage structure on new generation  large aircraft •  Turbine engine fan blades • Propellers Major Components of a Laminate Fiber Orientation  7-2 Strength Characteristics Laminated Structures Applications of composites on aircraft include: 