As airline fleets age, maintenance issues inevitably arise and aircraft inspection becomes more critical. Travelers suffering through recent airport delays associated with these issues surely understand the impact of aircraft maintenance and may appreciate the value of the inspection tools used—especially those that rely on imaging technology.
At the SPIE Defense & Security Symposium (Orlando, FL, USA), Markus Tarin, president of MoviMED (Irvine, CA, USA) presented an infrared (IR) system for nondestructive testing of aircraft fuselages. Developed by Automation Technology (Trittau, Germany), the JetCheck system uses a variant of IR thermography known as “lock-in” thermography to analyze samples under test. In this method, the sample to be tested is heated at its surface by an intensity-modulated source. This heat penetrates into the interior of the sample as a thermal wave, and any change in the properties of the sample material, such as cracks, cause a change in the propagation characteristics of the wave.
After the thermal wave is reflected to the surface of the sample, it becomes superimposed (or locked in) onto the initial wave such that any defect is revealed by a local change in the phase angle. By recording the surface temperature of the sample with an IR camera, the temperature-time function of the image can be analyzed on a host computer. By calculating a phase image, the internal structure of the component and its interior defects are revealed. Depending on the modulation parameters of the heat source during the measurement, different depth ranges within the image can be displayed.
A variety of materials are used in aircraft hulls, including aluminum and rivets, carbon-fiber-reinforced plastics, honeycomb structures, and other composite materials. Constant temperature changes, pressurization and depressurization, vibration, and high wind loads lead to material fatigue, which can cause deriveting, impact damages, cracking, and hull delamination. Other factors such as condensate and water inclusion can also cause cracks at high altitudes.
The JetCheck system inspects several square meters of an aircraft within minutes. An array of high-power halogen lights is used to heat the exterior of the aircraft to no higher than 40°C, so as not to cause damage or deformation to the fuselage. The halogen-lamp array is modulated with a sinusoidal waveform. Instead of directly measuring the temperature of the object, the system measures and evaluates differences in the temporal behavior of the heat at the object surface, and defects appear with excellent contrast. The advantage of using lock-in thermography with phase-angle image evaluation is that external effects, such as emissivity of the material and other radiation effects such as sunlight, do not affect the measurements.
In the JetCheck system, thermal images are captured by a ThermoVision SC 6000-HS cooled focal-plane-array (FPA) IR camera from FLIR Systems (N. Billerica, MA). Using a 640 × 512 InSb FPA, the camera transfers 120 frames/s, 14-bit digital images to a PCI-based Camera Link frame-grabber board resident in a host PC running Windows XP. Developed by Automation Technology, the IRFlashLink frame-grabber board also contains the circuitry for generating the sinusoidal stimulus signal to the halogen-lamp array and performs the synchronized IR-image recording.
Infrared nondestructive-testing application software, also developed by Automation Technology, running on the PC allows for test setup and phase and amplitude thermal-image analysis. As well as controlling the heat source and IR camera, the software stores live images from the camera in real time and calculates the phase image from the IR data.
The increasing number of aging commercial jetliners has led to more stringent inspection and maintenance procedures. After approval by the U.S. Federal Aviation Administration, lock-in thermography is now being used to perform aircraft fuselage inspection in the USA and abroad by companies such as Lufthansa Technik (Hamburg, Germany), which first studied the technique in 2001.
Since gaining acceptance by Boeing and the FAA, the system has been used for inspecting the bonds between the skin panels and the inner structures of the fuselage on the Boeing 737. One complete inspection takes approximately 100 hours. Compared to the old procedure, this represents a time saving of approximately 1000 hours.