Engineers Demonstrate Pulsed Thermography Inspection for GLOBUS II

  • Published
  • By Mindy Cooper
  • AFRL Materials and Manufacturing Directorate
Wright-Patterson Air Force Base, Ohio --  Engineers with the Air Force Research Laboratory's Materials and Manufacturing Directorate (AFRL/RX) performed an onsite pulsed thermography inspection of the GLOBUS II radar cover, demonstrating the capability of this new inspection technique.

GLOBUS II is a radar system located at Vardo, Norway, that is operated solely by Norwegian personnel, but which was developed by the United States and serves as part of the 29-sensor, global space surveillance network that provides data to the US Strategic Command. The radar system can identify more than 10,000 man-made objects orbiting the earth. The radar is sheltered by a composite fabric cover, manufactured by St. Gobain, of Merrimack, NH. The cover is fabricated in sections, which are joined together using a hot bonding process. The cover is suspended over the radar using pressurized air (similar to a balloon) and secured to the structure which houses the GLOBUS II radar.

"In 2005, the GLOBUS II suffered a cover failure due to a tear in the fabric, resulting in extensive damage to the cover and rendering it unserviceable," explained Mr. Kenneth LaCivita the senior nondestructive evaluation engineer that traveled to Norway and trained the team to perform the inspection. "The Structural Materials Evaluation Team (AFRL/RXSA) was contacted to conduct a failure analysis to determine the cause and develop a nondestructive method of inspecting the cover to prevent future failures."

Specimens of the failed cover were evaluated both nondestructively as well as destructively by the team. Nondestructive inspection (NDI) engineers used pulsed thermography inspection to characterize the extent of damage on the specimens and provide valuable information to support the failure analysis activity. The analysis determined that the bonding procedure used on previous repairs may have been the primary contributor to the cover failure.

According to Mr. LaCivita, as a result of the evaluations performed by RXSA engineers, a new radar cover was developed using a heavier and more durable Kevlar-based material. The engineers continued to provide support by developing and conducting a test program which resulted in a robust NDI procedure for evaluating bonded areas of the radar cover. The procedure utilized a Themoscope II Pulsed Thermography inspection system manufactured by Thermal Wave Imaging Incorporated, of Ferndale, MI.

"The system is comprised of an inspection hood containing an infrared camera and high intensity flash lamps, connected directly to a ruggedized laptop computer," explained Mr. LaCivita, "The inspection hood is placed directly onto the surface of the area of interest, heat is applied using the flash lamps, and real-time infrared thermal data (up to 300 frames per second) of the surface is collected, post-processed and displayed on the laptop."
The data can identify subsurface areas that are not bonded, which must then be dispositioned by the responsible engineering authority for either repair and/or recurring inspections. The flash thermography method is the state-of-the-art in thermography inspections. Conventional thermography also uses an infrared camera, but the heat source is typically a hand-held heat gun or heat lamp. In general, flash thermography inspection, as compared to conventional thermography, can detect deeper subsurface defects and provide higher resolution data, while being a more repeatable and less operator dependent inspection technique.

Mr. LaCivita traveled to Vardo, Norway, to perform onsite inspections on the new radar cover.He performed inspections on much of the radar cover, and also trained certified NDI technicians (who were also certified rope climbers) to use the thermography inspection system. The technicians assisted in many of the inspections and then went on top of the radar cover (which is over 100 feet high) and performed inspections of additional areas of interest. Indications of unbonded areas were identified in the new cover, but ITT of Colorado Springs, CO, which is the prime contractor for the radar, was able to determine that the size and location of the defects would not affect the cover's integrity. The defects will continue to be monitored and the cover will be inspected annually.

Peterson Air Force Base, in Colorado Springs, CO, the home of the HAVE STARE Integrated Product Team (850th ELSG/NSH ) which manages the GLOBUS II program, has decided to purchase a pulsed thermoscope inspection system in order to perform reoccurring inspections. RXSA engineers are working with Peterson personnel and Thermal Wave to develop a customized system that will meet their specific needs.