Without components made from lightweight composite structures, modern aircraft would not be able to achieve their current levels of fuel efficiency. However, composite parts in the fuselage, wings, radomes, tail assemblies, and other areas of airplanes or helicopters pose new challenges that apply to the whole product life cycle. Due to the often complex internal structure of the materials, it is important to monitor the formation of correct bond lines between the individual layers during production. If subsurface disbonds pass quality control undiscovered, they can become the starting point of serious problems such as delamination as soon as the part is loaded during service.
This continues to be a challenge during the aircraft’s service life. The structural integrity of the composite material has to be regularly checked by inspecting components for potential damages. During this, the remaining strength and need for repair work have to be determined. The reasons for damage or defects are many. They range from bird strike to hail and include such seemingly harmless incidents as tool drop.
What makes inspection of plastic composite parts so tricky is that damage can be practically invisible to the eye. Take a structure with a glass-fiber reinforced plastic as face sheet, bonded to one or more layers of structural foam or honeycomb underneath—a frequently used material these days. If the top layer is impacted by hail, for instance, the material’s elasticity and strength will allow elastic deformation, which leaves hardly any visible trace. However, the energy from the impact is passed on to the foam layer underneath, and this can result in hidden plastic deformation of the foam structure, which in turn can mean a loss of strength.
An important set of instruments to warrant long-term quality of composite parts are the methods of nondestructive testing (NDT). Inspection procedures based on X-raying, ultrasonic inspection, magnetic measurements, eddy current measurement, liquid penetration, and others are essential to air safety.
To make sure that there is a good level of knowledge about NDT, the American Society for Nondestructive Testing (ASNT) and the Collaboration for NDT-Education work to raise engineers’ awareness of the growing need for NDT. However, even the current introduction to NDT developed by the Collaboration for NDT-Education only briefly mentions laser interferometry and thus omits what could be the fastest and most efficient method for composite material inspection—electronic shearography.
This is an interferometric imaging method, based on exposing a component to different types of stress, including vibration, thermal, vacuum, and pressure. During this loading, the surface is filmed by a CCD (charge coupled device) camera, and the known light path from the unloaded component state is compared to the loaded state by superimposing of the two images to form a shear image—hence the name. This image reveals out-of-plane areas that indicate subsurface material or bonding defects. In 2005, shearography was added to the ASNT’s SNT-TC-1 A list of laser methods fit for level III certification.
According to John W. Newman, president and founder of Laser Technology Inc. (LTI) and co-inventor of electronic shearography, this NDT technology is particularly fast and efficient. Just prior to the Munich Aerospace 09 show, the world’s largest shearography installation went into operation, says LTI: The U.S. Air Force uses it at its Ogden, UT, site for aircraft component inspection. This vacuum shearography chamber system, developed and installed by LTI, is 34 ft deep, 14 ft tall, and 18 ft wide. Its gantry can hold large flap or wing panels and check them for defects. “We have been told by the Air Force that our shearography NDT system is at least 45 times faster than the previously used method of ultrasonic scanning,” said Newman during an interview at Munich.
Within the vacuum shearography test chamber, the component surface is exposed to single-frequency green laser light under operating pressure conditions that can reach a maximum differential of up to 0.3 bar. Out-of-plane distortions of the surface, caused by hidden defects or disbonds, cause a phase shift of the reflected laser light. This effect is recorded by a high-definition CCD camera. Large structures are examined patch by patch and the individual images are later stitched together for a total view of the component. At the end of the inspection, the test engineer can activate an integrated laser pointer that outlines all the defects in the aircraft panel. Thus, areas that need repair work can easily be marked by pen.
Alternatively, shearography can also be used without disassembling an aircraft. Mobile systems are used to check whatever part of the structure. The type of stress, which the component is exposed to, depends on the material: Vacuum stress shearography is particularly suited to reveal the depth of impact damage. Among other applications, it is currently used to inspect the Space Shuttle Orbiter payload bay doors and is also an instrument of choice for radome maintenance inspection. “The Royal Air Force uses it on their E-3D Awacs,” said Newman.
Portable vacuum shearography systems, on the other hand, are used to reveal disbonds on the Boeing 787 fuselage, which is manufactured from a co-cured composite laminate material. Another big application for portable vacuum shearography is inspecting the Airbus A310 rudder, made from a honeycomb sandwich structure. During this type of inspection, shearography shows both the location of damages and disbonds. The resulting image even reveals whether the problem is a near-side or far-side disbond. In this context, vacuum shearography has yet another benefit to offer over ultrasonic inspection in particular. “Damage and repairs result in differentiable patterns during shearography,” explained Newman. “It is, therefore, easy to judge prior repair work and to distinguish it from new damage. With shearography, customers see defects they would not see otherwise.”
Within the manufacturing process, inline shearography is a means “to economically scrap at the earliest possible point,” according to Newman. “By early elimination of defects, the finished part that goes to the customer is defect-free.” For the up-and-coming technology of resin transfer molding, for instance, shearography could be used to identify resin-lean areas, according to the expert.
While the principle is well suited for inspecting honeycomb sandwich panels, thin to medium thickness multiple laminate bonds and non-visible impact damage, turbine components, and rubber-metal bonds, shearography is less suited for very thick laminates. However, it is noncontacting and noncontaminating because it does not require a testing agent such as the water that is usually needed for ultrasonic inspection.