Airplanes endure thousands of takeoffs and landings over the course of their lifetimes, and the microfissures that come from wear aren't always visible until they become problematic or nearly so.
The straightforward answer is to make stressed parts more robust, but that adds weight—not an ideal situation for a metal tube that flies through the sky. But a new metal imaging technique can see these kinds of microfissure earlier. This technology might not only catch problems before they become catastrophic, it could lead to lighter airplanes in the future.
A good, if terrifying, example of cracks affecting airplanes is the story of Chalk's Ocean Airways flight 101, in which aGrumman Turbo Mallard seaplane built in 1947 crashed into a shipping channel adjacent to the Port of Miami on December 19, 2005. The problem had deep roots, but in short, metal fatigue led to a structural failure that led to a wing falling right off the airplane not long after take-off.
Passenger are designed to be extremely robust, but even in the 787 Dreamliner, over-building means more weight.
Again, these types of failures are extremely rare, thanks much in part to rigorous maintenance schedules and strict reporting required by the FAA and airline industry. But cracks aren't always easy to find.
Engineers looks for cracks (in airplanes as well as other large structures like bridges) using traditional linear acoustic imaging techniques that use transducers to send sound waves into a material. Those waves are reflected back, and their patterns can indicate a crack inside the material.
This works if the crack is big and at the correct angle to the sound waves. Some cracks, like hairline fractures and edge-on cracks, aren't visible with linear imaging. And some cracks might actually be bigger than they seem. These types of cracks can go unnoticed until they cause a major failure.
The new imaging method for comes from a team at the University of Bristol in the UK, who published their work in Physical Review Letters. They've developed a nonlinear acoustic imaging technique that uses standard equipment but combines two methods, parallel and sequential imaging.
The "parallel" method uses sound pulses sent from a series of transducers such that each sends its sound waves slightly after the last to focus the waves on one specific point in the metal. The "sequential" method has each transducer fire in sequence. Reflection data is received for each individually and then combined.
Both methods give identical results when the material is linear, but not when it's nonlinear. And that's the key: the difference in the amplitudes of the responses gathered by both methods can be interpreted as a measure of nonlinearity. Aiming at different points in a metal gives a survey of the material's overall nonlinearity. In testing, this method revealed fatigue cracks that were totally invisible with traditional linear imaging.
So how could this lead to lighter, thinner airplanes in the future? A better understanding of how fatigue cracks develop and a better ability to find them would mean airframes could be thinner while still supporting the same flight loads. Better imaging is also obvious from a safety standpoint, be it for airplanes, bridges, or anything other large metal structure.
Bringing new imaging and potentially new structures into aircraft design is a long way off. But refined methods for finding fissures and cracks as they form will certainly be an aid to the industry.