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 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.

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 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.