Non-Destructive testing (NDT) encompasses many different testing methods and techniques. As the name suggests, NDT refers to tests and methods that do not damage or alter the material being tested. Non-Destructive test methods include X-ray inspection, ultrasonics, optical imaging and nuclear magnetic resonance (MNR). By contrast, some test equipment including mass spectrometry and gas chromatographs require portions of sample to be consumed in the test procedure.
No single test method can answer all questions about a sample of material. NMR testing may reveal information about chemistry while X-ray testing may show variations in density for example. It's important to choose technologies and methods that provide meaningful information. When looking for disbonds in a carbon composite structure, an ultrasonic solution may be preferable to an X-ray or NMR system. If the application is to inspect containers then X-ray or gamma ray sources may be the best option.
There will often be overlap between the various available NDT methods. Usually the best choice of technology is determined by practical considerations including speed, accuracy, cost and complexity. For example an NMR method may be accurate and reliable for laboratory analysis of oil and fat samples but may be either too slow or too expensive for on-line inspection of the same materials. In this case the NMR method could be used with an ultrasonic system in a complementary solution.
Ultrasonics have been used for many years for such applications including thickness gauges and weld inspection. Usually these are contact ultrasonic methods. Applying contact gel to a green ceramic or paper material may actually damage the material and therefore is no longer considered non-destructive. The other disadvantage of contact ultrasonics is the difficulty in applying it to a production line. Air coupled ultrasonics opens up ultrasonic inspection to a wide range of products and materials and is truly non-destructive. Ultrasonics is often a good choice when looking at disbonds, delaminations, cracks, internal defects as well as other mechanical properties including density and porosity.
X-rays and the related gamma ray testing is often used to look for changes in internal structures. Soft X-rays (X-rays with relatively low energy) are used for thickness gauges and for X-ray fluoresces to determine chemical composition. Hard X-rays or gamma rays can be used to penetrate relatively thick structure. X-ray techniques are used to look for changes in density. The total amount of material tends to determine the relative attenuation of an X-ray signal. While X-rays are good at detecting the presence or absence of material, they may not be as useful at detecting bonds or disbonds since the total material is nearly identical whether a bond exists or not. Especially with hard X-rays or gamma ray sources there are also health and regulatory issues to consider.
Nuclear Magnetic Resonance
NMR methods rely on a strong DC magnetic field and a high frequency radio wave that is used to probe the chemical composition of a sample. NMR units can be relatively small and simple, designed to look at test tube sized samples, all the way up to full body NMR imaging systems used in hospitals. NMR methods for industrial applications are used to monitor the chemical composition of samples. Usually test samples are removed from a production batch for analysis. NMR methods are well proven and reliable, however they may not provide instantaneous feedback in an on-line process. The time required to remove and analyze a sample may be costly.
Infrared techniques are often used to characterize moisture content in samples. The advantages are that the methods can often be used with infrared imaging systems to collect fast and efficient data regarding a fairly large area of sample. Infrared methods usually depend on a calibration method to detect a chemical property. One limitation of the technique is that only thin portions of a volume can be measured. The infrared light only penetrates through very thin layers of material in most cases.
Optical imaging is commonly used for such diverse applications as validating dimensions of parts or products, assessing the correct placement of labels on a package or inspecting samples for defects. Optical methods tend to be limited to surface features.
Spectroscopy could be thought of as an extension of optical imaging. Provided a test sample can be illuminated properly or the sample can be made to emit light, spectroscopy can reveal a wealth of information about the chemical structure of the sample. As with other optical methods a primary limitation is the ability to look mainly at surfaces rather than volumes.
resonance methods rely on placing samples on a mount that vibrates the sample
using a broad spectrum of sound. The
noise emitted from the sample is recorded.
Known good samples are used to teach the system what a good part sounds
like. Any parts deviating from the
'good' sound are rejected. This
solution requires special handling and does require an acoustically quiet
environment with little or no external vibration from machines or equipment.
Ultrasonics and Other NDT Methods
Since no single technology can answer all possible applications, it's better to think of the various methods as complimentary. For example NMR has been successfully used for many years to characterize fat content in oils and food products. Preparations necessary to set up the samples for measurements take some time. The process of acquiring samples and making measurements works well in a QC lab environment. Ultrasonics has recently been demonstrated as a potential application to detect solid fat content. The ultrasonic solution does not necessary provide the same information as the NMR but it can be implement on-line and in production sized batches of material. The idea is to use the NMR in the lab while relying on the ultrasonics systems to provide real time feedback for process control.
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