Discovering More About Ultrasonic Inspection

By Paulette Short

Attenuation of the ultrasound is affected by internal defects and the time delay of the pulse is related to the depth of the defect. Pulse echo ultrasonic inspection will detect just about all types of foreign objects but is not as capable of determining porosity levels as thoroughly as through transmission. If appropriate reference standards are available, pulse echo can be used to measure laminate thickness and the depth of defects.

The technique operates on the principle of transmitted and reflected sound waves. An ultrasonic wave traveling through a composite laminate that encounters a defect will reflect some of the energy at the interface while the remainder of the energy passes through the porosity. The more severe the porosity, the greater the amount of reflected energy and the lesser it is transmitted through the defect.

Automated systems can either be squirter systems or submerged reflector plate systems. Squirter systems, the most frequently used in production, are usually large gantry systems with as much as a 7 axis scanning bridge. They are computer controlled to track the contour of the part and keep the transducers normal to the surface. They also index at the end of each scan pass.

Flaws are detectable since they alter the amount of sound returned to the receiver. The test equipment conducts inspection in the frequency range of 1 to 30 MHz, although most composite material inspection is usually tested at 1 to 5 megahertz. High frequencies are more sensitive to small defects, while low frequencies or longer wavelengths can penetrate to greater depths.

There are also special units for cylindrical parts that contain turntables that rotate during the scanning operation. The output from these automated units is displayed as a C-scan, which is a planar map of the part, where light (white) areas indicate less sound attenuation and are of higher quality than darker areas (gray to black) that indicate more sound attenuation and are of lower quality. The darker the area, the more severe sound attenuation is and the poorer the quality of the part.

The transducers are placed close to the part surface (within an inch) and frequencies of 50 kHz to 5 MHz are employed. A relatively new inspection technology is laser ultrasonics. It provides essentially the same information as conventional inspection except that it is faster than conventional methods, especially for highly contoured parts. Two lasers are used. The first laser, generally a carbon dioxide laser, generates ultrasound in the part by causing thermoelastic expansion, while the second laser, normally a neodymium: yttrium-aluminum garnet laser, detects the sound signal as it returns to the top surface.

Part manufacturers usually establish baseline attenuation in decibels for each part. When the attenuation level exceeds the baseline by a predetermined dB, that area of the part is rejected. For example, if the baseline for a good laminate is 25 dB and the rejection threshold is 18 dB, then any indication over 43 dB would be rejected.

As other techniques cannot be relied on of detecting all types of foreign objects and the depth of defects, pulse echo ultrasonic inspection is frequently used in conjunction with through transmission ultrasonics to inspect parts. In the pulse echo method, the sound is transmitted and received by the same transducer. Thus, it is an excellent method when there is access to only one side of the part. The amplitude of a echo received from the back surface is reduced by the presence of defects in the structure.

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