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...


Visual inspection of bonded part
With a visual inspection of the bonded part, it would be difficult to detect the defects. If the foreign material caused a change in the surface, geometry it may indicate a possible problem. In this case you could perform a radiographic inspection to verify foreign material, and this would also give you defect size information. The core crush may be evident on the surface of the part if the impression from the initial damage was still visible. This is not always the case with core crush, as the surface may show little damage and may have returned to its original location. This is one of the main problems with composite materials, in that internal damage is not always evident on the surface of the material; there may be hidden damage.

Tap Test Inspection
The tap test is the oldest and simplest method of inspecting adhesive bonds. The bonded item is lightly tapped with a suitable object to detect unbonds, and disbonds. Judgments are based on comparing the acoustic response from the material under test. A typical well bonded area will produce an even pitch sound compared to a disbonded area which usually produces a dead or dull sound. Tools used for the tap test can include such items as a tap hammer or a coin such as a quarter or a fifty-cent piece.

The tap test method should not be used when looking for deep lying unbonds or disbonds. Tap testing should be limited to near surface inspection of bondline defects.

Tap testing is one of the most misunderstood methods in testing of composites. Many people in the test industry believe this method is all that is need for the inspection of composites. Although tap testing is a viable method for the inspection of composite materials, it needs to be followed by another NDT method, such as ultrasonic and/or radiographic inspection to accurately report defect size and depth information. Tap testing is a good tool for NDT inspectors, but those using this method need to understand it's limitations. Some of the limitations involve the depth that it can be used for the type of materials that you can use it on. It is limited generally to testing bonded materials. One of the limitations, for example, is the thickness of materials down to a bond line. If it is only a few layers, it can be used, but generally, if you have over a quarter inch thick or more of material down to the bond line, don't use a tap test. I have been told that some people are using a tap test for areas that they shouldn't be using. For complex bonded assemblies using numerous materials in the assembly, it is not satisfactory. Tap testing is also not that accurate. You can be easily fooled as to the size of a defect as the lines distinguishing bonded and unbonded areas are blurred by variable pitched sounds.

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Tap testing of laminate
Tap testing of the laminate will not detect the defects. Tap testing should not be recommended for the inspection of laminate materials. Delaminations in a laminate are very difficult to detect due to the rigidity of the material and the inability to detect sound changes from a delam to good material.

Tap testing of bonded part
Tap testing of the bonded part may identify delams and disbonds. These are examples of where a tap tester can be a viable tool. If detection of a defect, such as a unbond/disbond, is identified using the tap test, an ultrasonic inspection should follow to determine the extent of the damage. In the bonded panel the unbonds/disbonds are located four plies from the surface. This is about the max depth where tap testing works well, and as you try to tap test bondlines with thicker laminates, the tap will test become less effective.

Ultrasonic Inspection
Ultrasonic inspection is the most common instrumental NDT method used on composites. Ultrasonics is based on the principle of transmitting high frequency sound into a test part and monitoring the received ultrasonic energy. Typical frequencies used to inspect composite materials include 1 Mhz to 5 Mhz because of the attenuative nature of this material.

The two common methods of ultrasonic inspection include pulse echo (see Figure 3) and through transmission. Pulse echo method is a single crystal ultrasonic test method that both generates ultrasonic pulses and receives the return echo. Through transmission is a test method using two transducers in which the ultrasonic vibrations are emitted by one and reviewed by another on the opposite side of the part. The ratio of the magnitudes of vibrations transmitted and received is used as the criterion of soundness.

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