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...

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

April 1998

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.

Image Image

A = Front Surface of Part
B = Defect within Part
C = Back Surface of Part

Figure 4 A-Scan and C-Scan Displays

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
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.

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