A team of researchers at Brigham Young University (BYU) (Provo, Utah) believes a spectroscopic method known as second harmonic generation (SHG) can be adapted to look for signs of internal damage in metals and other material components.
While the research remains in its early stages, the BYU team says its studies show the SHG method—which uses a green laser light to spot damages—can detect warning signs earlier than traditional nondestructive testing (NDT) methods, while also being more portable and less expensive. Depending on surface conditions and dislocation density, the SHG process converts a portion of the laser light into ultraviolet (UV) light, which bounces back from the metal. The extent of this conversion can help identify surface areas with high stress.
“Commercially available NDT methods often involve some form of imaging that relies heavily on technicians to either interpret the results or look for microfractures,” says Scott Smith, a BYU student researcher. “These fractures form after a period of time of applied stress. You might get a small crack. However, the small crack is the immediate precursor to failure. So current methods like x-ray imaging or magnetic particle inspection only tell you if a component is likely to fail right before it fails. With our technique, we can see these changes far before this point.”
Other methods besides NDT have also been used historically to predict the failure of various components. However, non-NDT methods typically involve testing a component by waiting for it to break under certain conditions, such as exposure to colder temperatures or repeated bending.
“Engineers test components used to build things and determine an average lifetime for a given component,” Smith says. “However, in determining those lifetimes, they often have to break a lot of these components in order to do so. Ideally, we’d like to be able to test a component without breaking it, so that we could continue using it.”
Green Laser Light
The SHG method begins by shining a green laser light onto a metal sample. Through SHG, the metal converts some of the incoming light into UV light, which bounces back from the metal.
“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,” says James. E. Patterson, a BYU professor leading the research in his lab.
Specifically, the amount of green light converted into UV light depends on the surface conditions, most notably the dislocation density. Because dislocations migrate under stress, the researchers are able to identify surface areas experiencing high stress.
“These microfractures correlate to an increase in the dislocation density,” Smith says. “Dislocations are irregularities within the material lattice. As stress is applied, they tend to move toward the surface.”
The detection process involves a standard photomultiplier tube. “As photons come in, we create a feedback loop, and it runs through a standard oscilloscope,” Smith explains.
Overall, the system is small enough to be portable and used in the field, according to the team.
“You need a strong enough laser, but lasers are very compactible these days,” says Alex Farnsworth, another BYU student involved in the research.
Farnsworth says his team uses a laser typically about 20 in (508 mm) in height, adding that that size is still bigger than what the researchers actually need.
“It’s easy enough to say you could turn this into a mobile device that a technician could wear,” he explains.
Tests to date have shown the technique can successfully distinguish between metal parts that are still intact and those that have been irreversibly damaged and require replacing.
The marine and aerospace industries could be two immediate beneficiaries of the technique.
The team is exploring applications with the U.S. Navy, which is partially funding their work. For example, the aluminum/magnesium alloy used in Navy vessels can often undergo corrosion when transitioning from its original alpha phase to a beta phase. The beta phase transition can occur if the material is cooled too quickly, which can happen in Navy vessels due to water exposure.
The magnesium will migrate through the crystalline lattice and congregate on grain boundaries, Farnsworth explains. That congregation will change the local type of aluminum to beta phase.
The beta phase has different electrochemical potentials and is anodic to the alpha phase, making it more likely to corrode. Additionally, this beta phase makes the overall structure more brittle.
“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,” Patterson says. “Cracks also form in walls. And once visible cracks form, it’s often too late to reverse the damage.”
Although the Navy has developed a method to reverse this type of corrosion, processes such as the laser light method that can detect the corrosion early are critical to reversing the corrosion before it is too late.
“We know it happens, we know how to fix it, but we lack a method that can reliability detect it without destroying or having to cut out a part of the sample,” Farnsworth says.
That’s where the green laser light system could provide a solution.
“In principle, you could go around with a wand and some other fiber optics and scan large areas of a ship for hidden damage,” Patterson says.
Additionally, because the SHG method is more precise, it could be used to determine whether a component is actually worn out or if it still has useful life ahead of it. In turn, this could save resources associated with unnecessary replacements. For example, airplane parts are routinely replaced after a certain amount of use to avoid failure. The replacement time is based on the average performance of similar components, rather than the actual condition of an individual component.
Other potential structures that could be evaluated with such a system include oil pipelines, building components, and bridges, Patterson adds.
The researchers say they are very excited by the preliminary results of their studies, but they understand they need more test runs to build statistical significance for the method.
The extreme sensitivity of their laser-based technique, however, complicates that goal because it can also detect surface changes caused by factors other than dislocations. As a result, a priority for the team moving forward is better correlation of signal changes to their precise physical causes.
“There are so many variables that go into the SHG signal that is generated, and we have trouble working out the noise,” Smith says. “So if anything, the technique is too sensitive. The trouble wouldn’t be developing statistical significance, because you can do these runs fairly rapidly. However, it’s just ensuring your conditions are such that from one sample to the next, it’s reproducible.”
Based on the unit’s portability, the researchers are confident they can easily test enough samples to eventually refine the model and better filter out the statistical noise. Additionally, the versatility of the system offers its own set of benefits. For example, the team believes the enhanced sensitivity of the method could also make it a useful technique on composites including carbon fiber and industrial steel.
“We’ve seen there’s a lot of potential for further applications,” Smith says.
The research is being funded jointly by BYU and the U.S. Office of Naval Research (Arlington, Virginia). Initial results were presented earlier this year at a meeting of the American Chemical Society (ACS) (Washington, DC).
Source: American Chemical Society, acs.org.