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.
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.
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 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.
Information from a ultrasonic inspection can be in the form of an A-scan, which is a display typically utilized on portable equipment, and in the form of a C-scan, which is a data presentation method yielding a plan view through the scanned surface (see Fig. 4). Another form of presenting the information from a ultrasonic inspection is a B-scan, in which the display shows the location of a discontinuity as it would appear in a vertical section view through the thickness direction of the material.
Ultrasonic inspection can be accomplished on small areas by portable contact inspection or on large items using automated inspection systems. Ultrasonics can be complemented by other methods of inspection such as radiographic inspection. It is common to use ultrasonic inspection in conjunction with radiographic inspection when inspecting bonded structures.
Nondestructive Testing of Aircraft Composites
By John D. Register
Ultrasonic inspection of laminate
Ultrasonic inspection of the laminate would most likely be accomplished using the pulse echo method. The pulse echo method works well on laminates because they are not as attenuating as bonded structures. Pulse echo inspection of the laminate will detect the delaminationss, provided the size of the transducer diameter is small enough.
A = Front Surface of Part
B = Defect within Part
C = Back Surface of Part
A general rule of thumb is if you need to detect a .25 inch diameter delamination, you would use a .25 inch diameter crystal. Delay line transducers are also helpful in improving the near surface resolution of the pulse echo method. Ultrasonic inspection of composites is normally done using longitudinal sound transmission, which means the sound travels into the part perpendicular to the surface. With most of the defects occurring parallel to the surface of the material, this method works very well for defining defects in laminate. Gross porosity on the surface of the laminate would be detectable with ultrasonic inspection and will be evaluated by its sound attenuation and compared to an applicable specification or procedure for acceptance.
The foreign object may be detectable in the laminate if it causes a delamination, or if the material velocity is not similar to the laminate, and a sound will reflect from the interface of the FM and the laminate. One advantage with the ultrasonic pulse echo method is that pulse echo is a single-sided test method, so when access to the opposite side is not available, you can inspect the laminate from one side. On another note, this can be a disadvantage also because if you detect a near surface defect, you will not have the ability to evaluate the material under that defect. The near surface defect will not allow the transmittal of sound through it, so inspection below that level is not possible, unless there is access to the back side of the panel, and in aviation this is usually not the case. In the example of the laminate, if testing from the top side, half of the defect it would not be detectable.
Ultrasonic inspection of bonded part
Ultrasonic inspection of bonded parts usually requires the through transmission method due to the attentive nature of the adhesive bonds and the core materials. Through transmission also gives the ability of inspecting both the top and bottom laminate and both the upper and lower bond line at the same time. In some cases, both sides of the material may not be accessible, and in this case, the pulse echo method should be performed, but this is only testing the laminate and bondline nearest the transducer. Through transmission testing will detect unbonds/disbonds/foreign material, if it causes a delamination, and core crush.
Bond Testing is a form of lower frequency ultrasonic inspection and is generally classified as a separate category for inspection methods. Bond testing has been around for many years and can be a useful tool in inspecting laminates and bonded test parts.
One example of a typical bond tester is Staveley's Bondmaster. It is both a low and high frequency ultrasonic instrument that has four test methods of operation. The modes of operation include: mechanical impedance analysis (MIA), pitch/catch swept, pitch/catch impulse, and the resonance mode. The basic principle for MIA is measuring the impedance (stiffness) of structure.
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.
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.