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Nondestructive testing (NDT) is the process of testing and evaluating materials without destroying them, and the field is evolving.
Testing methods used to evaluate materials for their adaptability and toughness, including impact resistance, are used, such as magnetic particle testing, liquid penetrant testing, radiographic testing, ultrasonic testing, electromagnetic testing, and visual testing.
New Applications of Non-Destructive Testing
Roll-to-Roll Manufacturing
Researchers at the National Institute of Standards and Technology (NIST) Gaithersburg, MD, created a process of taking optical measurements during roll-to-roll manufacturing - the continuous processing methods involved in tire making or nanotechnology to ensure parts have desired qualities.
Standard test methods could mean a product has to be handled, but NIST physicists want to avoid this because there will be more parts in the future that are nano-sized and sensitive. These parts could be used as device screens or wearable electronics. Rather than handling them, the films were passed through a metal box known as a microwave cavity and measured. An electrical circuit measured the films’ alterations to changes in electromagnetic resonance frequency.
The measurements included: size, electrical resistance, and dielectric constant, the latter of which is the measure as its ability to store energy in an electric field. Specifically, in experiments lasting less than a second, 12- and 15-centimeter-long films of carbon nanotubes deposited on sheets of plastic were run through the cavity.
The process is translated to an industrial roll-to-roll set-up, the researchers predict it will have speeds beyond 10 meters per second to allow real-time measurements along the film and not just on a sample of the film. The idea is that the entire manufacturing process can be tuned quickly if needed, without it having to be shut down or making poor batches that are discarding.
The types of products that can be created using the roll-to-roll technique include films of carbon nanotubes and graphene, which are now becoming bulk manufacture items for composite airplane materials, for example.
Reparing Cracks in Concrete
As part of HealCON, an international team of researchers are working towards developing concrete that can repair itsel. Christian Grosse, PhD., Chair of the Non-destructive Testing Laboratory from the Technical University of Munich (TUM), is working towards way to repair from the corrosive effects of water and salts. Grosse is using acoustic emission technology to assess how well healing agents for concrete such as hydrogels, epoxies, or even clay mixed with microorganisms that actually close cracks in days are actually working in concrete.
The materials inserted into the concrete perform different functions. For example, hydrogels will expand to ten or even 100 times their original size when in into contact with moisture to prevent the areas for the cracks in the first place. Capsules of epoxy resins or polyurethane, on the other hand, work by breaking once a crack forms, spilling out polymer that then hardens to seal the crack and actually fortifying the concrete more than it previously was.
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Finally, there is the healing agent of certain bacteria as a metabolic product. The scientists soak balls of clay with the spores of bacteria that produce calcium carbonate before mixing them into the concrete. Once water penetrates the concrete, the microorganisms release calcium carbonate, one of the main components of concrete, and actually promote the closing of the concrete cracks of up to a few millimeters in width in just a few days.
As part of the acoustic emission process to test these materials in the laboratory, sensors measure the acoustic waves that are generated from cracks in concrete subjected to pressure to tell where and when the cracks occurred. The results of tests are a comparison of the healing methods used.
Once the concrete is in place, ultrasound is applied for on-site testing by measuring the time it took for the ultrasound waves to move through the concrete. Because cracks interrupt the sound waves and prolong their transmittal, this time and the lessening strength of the signal are assessed to see if the agents mixed into the concrete really worked to prevent or heal cracks.
References and Further Reading
Recent Trends in Industrial and Other Engineering Applications of None Destructive Testing: A Review
Introduction to Nondestructive Testing
Fleet and Fast Test for Nanomanufacturing Quality Control Invented
Materials with Self-Healing Powers
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