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Original Date: 04/26/1999
Revision Date: 01/18/2007
Best Practice : Thermography Non-Destructive Evaluation
Thermography is a non-destructive analysis technique where a material is thermally excited by a high-energy source (e.g., quartz lamps, high intensity flash). As the material cools, emitted infrared (IR) radiation can be analyzed with a thermal-imaging camera. Different materials absorb and release IR energy at different rates as heat propagates through, thereby creating a thermal image that can progressively penetrate deeper layers of the material. Thermography is particularly useful for examining composite materials because many are nearly invisible to x-rays. Delamination (voiding) inside the composite will create air-filled pockets that act as insulators. The area inside and around this insulated area will cool at a different rate than the remainder of the material, thus creating a slightly altered thermal image.
In the past, Marshall Space Flight Center (MSFC) used analog methods to collect thermal images, and then stored them on VCR tapes. Heating was done through manual means. This approach had several limitations because the resulting image was affected by the scan rate of the camera, and the camera could only scan the image in a left-to- right, top-to-bottom fashion. Also, the temperature sensitivity of the camera was relatively low (less than 0.1o C), and post-processing of the analog image was limited. As a result, engineers had difficulty in enhancing images and identifying potential problems.
In 1996, MSFC upgraded its imaging system by installing a more sensitive camera capable of resolving temperature differences down to 0.025o C and collecting images in a digital format. Other new capabilities include scanning the entire picture frame at one time; automating and synchronizing a predictable heating source with data acquisition; and easily storing and enhancing the resulting images. The upgraded system can also quantify the size and severity of lamination and porosity anomalies, which aids in determining the size of a delamination (both interply unbonding and core/facesheet unbonding) and the porosity of the bonding material. This feature was achieved by developing defect standards. MSFC intentionally created delamination on coupons by inserting Teflon tape between bonding layers to simulate delamination, and used microballons and variations in vacuum bagging operations to simulate porosity during the fabrication process. The Center fabricated defects in various sizes, and took thermographic images of them. These images were then examined to determine the limitation of the technique, and to correlate the size of the defect on the image with the actual defect size.
By upgrading to a thermographic, non-destructive evaluation process, MSFC can now analyze composite materials for potential delamination problems and quantify delamination/porosity within composite material. This process was also used to qualify the nose cone for the space shuttle’s external fuel tank, as well as the space shuttle’s main engine nozzle and the Bantam RP-1 fuel tank.
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