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Microwave testing (MW) uses the same principle that is used in radar devices such as police speed guns and adaptive cruise control in modern vehicles. The use of microwaves as a testing method was described in literature as early as the s but only recently has it become more common in industry. This is mainly the result of its effectiveness at inspecting plastic and composite materials, particularly complex composite materials. As the uses for these composite materials in industry expands, so does the need for inspection of those materials and, thus, the growth of MW. Microwave inspection techniques have been used successfully for inspection of fiberglass wind turbine blades, fiberglass vessel hulls, and high-density polyethylene (HDPE) piping butt fusions and electrofusions, as shown in the figure below at left.Microwave testing (MW) uses the same principle that is used in radar devices such as police speed guns and adaptive cruise control in modern vehicles. The use of microwaves as a testing method was described in literature as early as the s but only recently has it become more common in industry. This is mainly the result of its effectiveness at inspecting plastic and composite materials, particularly complex composite materials. As the uses for these composite materials in industry expands, so does the need for inspection of those materials and, thus, the growth of MW. Microwave inspection techniques have been used successfully for inspection of fiberglass wind turbine blades, fiberglass vessel hulls, and high-density polyethylene (HDPE) piping butt fusions and electrofusions, as shown in the figure below at left.MW is like UT, except it is air-coupled, and is best described as follows: electromagnetic radiation in the microwave frequency range is introduced into the part being inspected and if the wave encounters an area with a different complex permittivity (dielectric and loss tangent), some or all of the electromagnetic energy will be reflected back to the transmitter. This reflected signal is recorded and can then be presented on a visual display and otherwise processed and interpreted for size and type of reflector. Additional knowledge about the location of the reflector, such as its depth from the top surface of the part, can be discerned by post processing the data. In its simplest form, the depth can be determined by knowing the speed of light in the part (based on the refractive index) and the time required for the signal to return to the transmitter, commonly referred to as &#;time of flight.&#; In the case of a swept frequency transmitter, the reflected data is captured in the frequency domain that can be converted to the time domain by using an inverse fast Fourier transform. Then the distance to the reflector can be determined like a time of flight device, as shown at right.The most common definition for microwaves is electromagnetic waves in the frequency range of 1 to 100 GHz. Usually, the lower frequencies have greater penetrating power but less resolution capability because of their longer wavelengths, while the higher frequencies do not penetrate as deeply but can resolve smaller reflectors. Microwaves are totally reflected from regions that are metallic or conductive (such as many carbon fiber materials) and cannot be relied on for a volumetric inspection. They can prove useful for detection of small surface imperfections or vibration of metallic objects.Recent improvements in the technology and methods include specialized larger bandwidth for multifrequency inspections, inspection antennas that improve the focus and directionality of the microwave beam, and synthetic aperture radar focusing of the data to improve spatial resolution of flaw depth location. It is anticipated that MW equipment and techniques will continue to improve as the use of the method becomes more prevalent for plastic and composite inspection.
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