Nondestructive testing of aircraft composites

Nondestructive Testing of Aircraft Composites By John D. Register April 1998 Composite materials have been used in aviation for many years. In the last decade, many new processes and materials have been introduced to improve the...

Bond testers have some advantages over high frequency ultrasonic equipment. One advantage is no couplant is required for the pitch/catch and MIA modes. The bond testers also have the ability to test two bondlines at one time from a single side.

Bond testing of laminate
Bond testing of the laminate will detect delaminations, and foreign material in the laminate. The resonance method of testing is recommended for delamination detection, and this method requires couplant. Porosity would be difficult to detect using the resonance method.

Bond testing of bonded part
Bond testing of the bonded part would detect unbonds/disbonds, foreign material (if disbonded) and core crush. The MIA mode works well on unbonds/disbonds, and core crush. The pitch catch swept and impulse mode works well on unbonds/disbonds and core crush and generally can detect defects as small as .5 inches in diameter.

Radiographic Inspection
Radiography is used to detect the features of a component or assembly that exhibit a difference in thickness or physical density as compared to the surrounding material.

Radiographic testing usually requires exposing film to x-rays or gamma rays that have penetrated a specimen, processing the exposed film, and interpreting radiograph.

Radiography can be used as gamma, neutron, and x-ray. The most common to the inspection of aircraft composite components is x-ray.

Radiographic inspection of the laminate
T his inspection method will detect the foreign material if material density is different from the graphite. Radiography will not detect delaminations because there is no difference in density between a delam area and clean (defect free) area. Radiography may be able to detect a crack in a composite material, but this is difficult due to the orientation of the crack.

Radiographic inspection of bonded part
Radiographic inspection of the bonded part will detect an unbond if there is a lack of adhesive condition (adhesive missing) because this would cause a density change. Radiographic inspection can also detect foreign material, and core crush if the damage to the core is extensive. Radiographic inspection is commonly used in conjunction with ultrasonic inspection for bonded components.

Radiography of composite materials is generally done at lower energy levels to obtain the required contrast and definition. Lower KV and smaller portable systems such as 160 KV units are very practical for performing radiographic tests on aircraft.

Standards used for nondestructive testing are an important part of the calibration process. The reference standards used for composite inspection need to be manufactured in the same process and using the same materials as the part under test. Differing environments such as autoclave cures compared to a room cure can change the characteristics of the reference standard enough to make it an in-effective reference standard for a nondestructive test. Part thickness, core dimension and weight, material type, and curing process can all effect the characteristics of the reference standard and should be as similar to the part as possible.

Choosing the correct method that will detect composite defects requires not only the knowledge of available NDT technology and knowledge of the materials under test; it requires a commitment to understanding definitions and processes as well.

One of the biggest problems in the industry in the past has been the lack of standardization of definitions and processes. In the past, you didn't know who to go to when you ran across a problem — the manufacturer or the NDT experts.

Fortunately, that's slowly changing. Composites today are being more and more recognized by the NDT industry and as a result, more standards and specifications are being written specifically for aviation. Additionally, many groups are working to bring the manufacturers together with the NDT companies to work on better information transfer to the industry. Although there is still a long way to go, there has been a lot of progress made in the aviation industry and the future for composites looks bright.

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