Figure 14-24.-Standard ply orientation clock.

80 percent composites. As a structural mechanic, you will be required to maintain these new types of aircraft. To be proficient, you must be able to recognize the types of damage, understand the processes involved in damage assessment, inspection, and repair of composite materials. As new materials are introduced, new repair procedures will be required. It will be your responsibility to keep abreast of these developments.

Composite materials damage may be categorized as either environmental or physical. Environmental damage includes crazing and cracking caused by solar and ultraviolet radiation, water absorbed through humidity and rain, and lightning strike damage. Lightning strikes can cause holes to be burned in the structure, puncturing and splintering, and it has been known to weld bearings and hinges. Physical damage is caused by an applied force or deficiency in fabrication, such as dents, scratches, cracks, cuts and abrasions, pits, voids, disbonds, delamination, core crush on sandwich structures, and impact damage.

The task of repair begins when you determined that the structure has been damaged and that the damage is sufficient to require the structure to be repaired. The existence of damage may be obvious, such as a skin penetration, a gouge, or a dent. Con-versely, the proper identification and classification of the damage may be difficult. Because of the brittle, elastic nature of composite laminate materials, for example, the fibers may break upon impact, but then spring back, leaving little visible indication of damage.

There are three distinct steps involved in damage assessment. The first step is to locate the damage. The second step is to evaluate the defect to determine such information as the defect type, depth, and size. This information is important because the method of repair will vary, depending on this information. The third step is to re-evaluate, after defect removal (as applicable), the area being repaired.