Green lasers find metal faults invisible to the naked eye
If someone handed you an airplane part, could you tell if it was ready to fail at any second? If not, don't feel bad. In most cases, even the engineers can't. That's why they develop testing techniques. One of the latest comes from a team at Brigham Young University led by James E Patterson that uses green lasers to examine metals for signs of damage that would otherwise be invisible to the naked eye.
In his 1948 novel and the 1951 film based on it, Nevil Shute's No Highway related the unpleasant predicament of fictional aerospace engineer Mr Theodore Honey on a transatlantic flight. His abstract knowledge of metal fatigue told him that the plane was in danger of losing its tail and crashing at any second, but he couldn't get anyone to believe him because there was no visible evidence of the fault.
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The tragic ironic of this bit of entertainment was that in 1954 one of the first jet airliners, a de Havilland Comet on BOAC flight 781, broke up over the Mediterranean through undetected metal fatigue at the corners of the poorly designed rectangular windows. This sort of episode is why the aerospace industry operates on a policy of better very, very safe than very, very sorry.
It's a policy that has made air travel statistically safer than taking a bath, but it's also very expensive to maintain. As Patterson relates, a US Air Force plane once unexpectedly turned upside down and suffered stress beyond its specification. Though there was no obvious sign of problems, there was no way to test the plane's parts without ripping it apart, so the multi-million dollar airframe was scrapped as a write off.
To try to prevent this sort of expensive decision, engineers relentlessly test parts – both before assembly and during service. Unfortunately, this sometimes means dismantling the craft in question to get at a buried component, which can be as damaging as the allegedly faulty part itself.
Worse, much testing involves subjecting components to stress until they fail completely. So, in practice, parts are replaced according to a schedule based on average wear, whether there's anything demonstrably wrong with them or not. It helps to ensure against catastrophic failure, but it's also very wasteful.
The Brigham Young team hopes to minimize this with a form of Non-Destructive Testing (NDT). That is, instead of removing a component or damaging it by stressing it or cutting holes in it for samples, it's evaluated by looking for cracks or other imperfections caused by stress, chemicals, heat, radiation, or other factors.
It's already the basis of a billion-dollar industry, but far from perfect. Most NDT processes use X-rays, neutrons, or magnetic fields to seek out flaws in metals, but most of these need heavy shielding and a complex arrangement of emitters and sensors, plus they're often expensive, require expert interpretation, or cannot give results in the field.
Patterson's team is working on a new spectroscopic method called Second Harmonic Generation (SHG) based on how the surface of a material alters the wavelengths of reflecting light. Shawn Averett, a graduate student of Patterson's, suggested that the phenomenon could be used to detect internal damage in metals. In practice, shining a green laser on a metal sample will cause some of the incoming light to convert to ultraviolet light. How much is converted indicates the properties of the metal and how stresses have changed it over time.
According to the team, the new technique is not only non-intrusive and portable, but it can distinguish between an irreversibly damaged component and one that is still sound to a degree that provides a much earlier indication of danger than comparable NDT methods, therefore avoiding needless and expensive replacements.
In addition, Patterson says that the technique is of interest to the US Navy, which uses aluminum/magnesium alloys in its ships. The magnesium tends to migrate through the crystalline structure of the metal and may not be noticeable until a chunk falls away, and by then the damage is irreparable. With the green lasers, it may be possible to detect the problem in time to reverse it.
The team is currently working on making their apparatus more portable, so that one could simply wave a fiber optic wand and look for damage even in tight confines, like plane wings, pipelines, buildings, and bridges.
The research results were presented at the 253rd National Meeting & Exposition of the American Chemical Society (ACS).
The video below illustrates the new green laser inspection technique.
Source: American Chemical Society