SAN FRANCISCO, April 5, 2017 -- Imagine being able to check the structural integrity of an airplane, ship or bridge, without having to dismantle it or remove any material for testing, which could further compromise the structure. That's the promise of a new laser-based technique that chemists are developing to reveal hidden damage in metals.
The researchers will present their work today at the 253rd National Meeting & Exposition of the American Chemical Society (ACS). ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features more than 14,000 presentations on a wide range of science topics.
A brand-new video on the research is available at http://bit.
"Metals are often subjected to mechanical stress or fatigue that can weaken them structurally, but you can't tell that just by looking at them," James E. Patterson, Ph.D., says.
One real-world example is a U.S. Air Force plane that was unintentionally turned upside down in flight, a feat it wasn't designed for. The maneuver exceeded the specifications for the plane's stress tolerance, Patterson says, but there was no way to know if the inversion had actually damaged components enough to cause the plane to crash during a future flight. So the entire multimillion-dollar plane had to be scrapped.
"That's where nondestructive testing comes in," says Patterson, who is at Brigham Young University. NDT, as it's known, is already a billion-dollar industry, he notes. Current techniques for inspecting materials without harming them include X-ray imaging, which can detect microscopic cracks in metals. But the method is expensive, requires shielding from the X-rays and is hard to adapt for use in the field. Other NDT techniques give equivocal results and require highly trained technicians, he says.
His team is instead relying on a spectroscopic method known as second harmonic generation (SHG), which alters the wavelength of light. One of Patterson's graduate students, Shawn Averett, realized that the technique could be adapted to look for signs of internal damage in metals. Averett and undergraduates Scott D. Smith and Alex Farnsworth are working with Patterson on the project.
They begin by shining green laser light onto a metal sample. Through SHG, the metal converts some of the incoming light into ultraviolet light, which bounces back from the metal along with the remaining green light. "The amount of conversion depends on the properties of the metal, and if those properties have been changed by some form of stress, we can detect that in the converted light," Patterson explains. Tests to date indicate the technique could distinguish between metal parts that are still intact and those that have been irreversibly damaged and require replacing. The researchers say their method is more sensitive than existing NDT techniques and could thus give earlier warning of danger.
With some further refinements, the method could have applications in the aerospace industry, where plane parts are routinely replaced after a certain amount of use to avoid catastrophic failure, Patterson says. The replacement schedule is based on the average performance of several of the same components, rather than the actual condition of that individual component. The SHG method could be used to check whether a particular component is really worn out or still has useful life, leading to savings in time, money and material.
Patterson's team is also exploring applications with the U.S. Navy. The aluminum/magnesium alloy used in Navy vessels can undergo invisible corrosion with serious consequences. "There are stories of someone walking along a metal deck and stepping in the wrong spot, and a big chunk falling through to the deck below," he says. "Cracks also form in walls. And once visible cracks form, it's often too late to reverse the damage."
The researchers hope to develop their technique into a portable system that would indicate whether a scanned object is in good shape. "In principle, you could go around with a wand and some fiber optics and scan large areas of a ship for hidden damage," Patterson says. Other potential structures that could be evaluated with the technology include oil pipelines, building components and bridges.
A press conference on this topic will be held Wednesday, April 5, at 10:30 a.m. Pacific time in the Moscone Center. Reporters may check-in at the press center, South Building, Foyer, or watch live on YouTube http://bit.
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Nondestructive testing with second harmonic generation
By 2020, non-destructive testing (NDT) will be a 6.8-billion-dollar industry. NDT is valuable because it either mitigates the replacement of costly mechanical parts, or it prevents catastrophic failure, such as the collapse of a bridge or mechanical failures in an airplane. NDT seeks to assess the integrity of a component without further compromising it, enabling the identification and replacement of only those components most likely to fail.
Current NDT methods probe for microscopic cracks, which are the immediate precursor to catastrophic failure and indicators of extensive fatigue. However, by testing for fractures current NDTs are limited in their sensitivity because they cannot detect the small-scale plastic deformations which cause them. Plastic deformations are caused by an accumulation of internal irregularities within a material's lattice, known as dislocations. An abundance of dislocations correlates to an increase in the number of minute intrusions and extrusions on the surface of the material and that could potentially be detected with Second Harmonic Generation (SHG). In this regard SHG is a promising candidate for NDT because it is very responsive to the characteristics of the surfaces that it probes, and can be applied to many materials such as metals and composites. Furthermore, SHG signal takes only nanoseconds to generate, and can be done with relatively inexpensive commercial lasers. These factors make SHG a prime candidate to be adapted into a fast and affordable method for NDT.
Our research has shown SHG is sensitive to plastic deformation, and suggests that SHG signal may be a viable method of NDT. Our current goal is to utilize SHG signal to extrapolate the extent to which a sample has been stressed. SHG's sensitivity to plastic deformation is currently unprecedented in the field of NDT, and SHG may provide a means to detect weaknesses in materials far earlier than any other NDT method currently available today.
Noninvasive detection of aluminum sensitization using SHG
Structural failure of metals can result from mechanical, chemical, or thermal stress. For example, naval grade aluminum when exposed to temperatures of 50-200 °C will have Mg2+ ions migrate and congregate on the grain boundaries. This magnesium segregation forms an intermetallic compound: Mg2Al3. This new composition called beta phase aluminum acquires distinct physical properties becoming more susceptible to corrosion than the native alloy. These temperatures can be easily reached during normal functions of marine machinery. Results indicate that second harmonic generation, a branch of non-linear optics, could be utilized as a noninvasive method for detecting beta phase in naval grade aluminum alloys.