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. ASSESSMENT OF COMPOSITE MATERIAL 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. DAMAGE INSPECTION METHODS There are many methods available for locating and evaluating the damage. Ideally, the fastest method that will reliably find the appropriate type and size of defect should be employed since recurring costs will probably outweigh nonrecurring equipment procurement costs. Some of the inspection methods to be discussed are visual inspection, tap test, X-ray, and ultrasonic inspection methods. Visual Inspections Visual inspections are a methodical search for defects, checking for obvious damages. Be suspicious of any nick, dent, or paint chip because there may be underlying damage. Many types of defects, such as impact damage, corrosion, and delamination, cannot be detected by visual inspections alone. Tap Testing A tap test is used in conjunction with a visual inspection, and is an elementary approach to locating delaminations, disbonds, core damage, water, or corrosion. Tapping should be done with a small hammer about the weight of a US 50-cent coin. A dull or dead sound indicates that some delamination or disbond exists. A clear, sharp sound indicates a solid structure. Tap testing is limited to finding defects close to the surface, and is ineffective in areas of sharp contours and changes in shape. X-ray Inspections X-ray inspections use the same basic process as a dentist uses to X-ray teeth. The penetrating power of the radiation is used to reveal the interior of objects and to record it on film. Defects in material essentially change the thickness of the material, thus changing the degree of absorption of radiation. More radiation passes through the thinner area of a part, and shows up as a darkened area on the developed film. Ultrasonic Inspections Ultrasonic inspections use sound wave fre- quencies higher than the human hearing level, above 20,000 Hz, to penetrate the part. It measures the time the transmitted sound waves take to pass through the object and return to the receiver. The signals are changed into a display on a cathode-ray tube that provides a means of interpreting defects. Accurate results are dependent on an experienced operator, clean surface, known standards of part construction, and repeatability of indications. 14-23