Nondestructive Testing of Aircraft Composites
By John D. Register
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 consistency and reliability of these composites. Yet, relatively speaking, the technologies needed to inspect composite materials have lagged. Fortunately, that's quickly changing. Several sophisticated technologies have been introduced in the last five years which are quite good. Additionally, there has been a great deal of NDT technology transfer from the knowledge used on metals. Today's nondestructive testing (NDT) of composite materials typically involves the use of more than one inspection method, including both non-instrumental and instrumental methods. Non-instrumental methods include visual inspection and tap testing. Commonly used instrumental methods include ultrasonic inspection, radiography, and advanced methods such as thermography and laser shearography.
Understanding the limitations
The correct use of currently available NDT methods depends on the understanding of the capabilities and the limitations of each method. There also needs to be an understanding of the materials being tested and the defects that can occur. The metallics industry for many years has been a typical application for nondestructive test methods, but the inspection of composite materials involves using differing techniques and accessories than traditional metallics. In addition to traditional NDT methods, advanced methods of inspection can also be utilized to inspect composite materials. Following is an introduction of several of these methods, including ultrasonic, radiographic, visual inspection, and tap testing.
Sample materials to test
For the purposes of describing the proper methods to inspect composites, there are two types of materials we will discuss in this article. These represent the most common and significant challenges for inspection in aviation.
The first one is a laminated part that, in this case, is made up of a carbon pre-preg material and was made with a 4-ply to 24-ply thickness range. The material is a woven laminate and is hand laid and cured in an autoclave.
The second material is a bonded test part with a carbon prepreg laminate six plies thick and .5 inch honeycomb core bonded with film and foam adhesive.
A trained composite visual inspector typically is looking for resin starvation, edge delamination, fiber break-out, and other types of discontinuities present on the exterior of the item inspected. Tools used to aid visual inspection can include 10X magnification, boroscopes, and optical comparitors.
Visual inspection can be a stand alone inspection and also should be utilized before performing any other NDT inspection method. A good visual inspection may identify variables that will hinder the NDT inspection and can identify obvious defects accessible to the surface. In the aviation industry a visual inspector is generally the first person to assess damage, and after this initial inspection, other forms of inspection may be requested to determine the extent of hidden damage.
Visual inspection of laminate
Visual inspection of the laminate would detect resin starvation which appears as porosity on the surface of the laminate. The porosity would have to be further evaluated using ultrasonic inspection to give accurate sub-surface information and to evaluate per-sound attenuation criteria. The acceptable limits for porosity vary from material to material and with the processes involved. Generally, you have to refer to the manufacturer's specifications and procedures to determine if is within limits. For example, parts cured in an autoclave have a much different appearance than those cured on the shop floor.
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