Ultrasonic inspection by ultrasonic test equipment is the most valuable technique for aerospace composite material inspection. The two most prevalent fabrication defects in solid laminates are porosity and foreign objects. Porosity is detectable because it contains solid-air interfaces that transmit very little and reflect large amounts of sound. Inclusions, or foreign objects, are detectable if the acoustic impedance of the foreign object is sufficiently different than that of the composite material.
Pulse echo is more sensitive to transducer alignment than through transmission. Since there is a large impedance mismatch between air and a solid interface, ultrasound does not propagate well through air; therefore, a couplant is used to more effectively transmit the sound from the transducer to the part. For hand inspection, glycerin compounds are frequently used while all automated systems use water.
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.
Through transmission is excellent at detecting porosity, unbonds, delaminations and some types of inclusions. However, this method cannot detect all types of foreign objects and it cannot detect the depth of any defects. Mylar film and nylon tapes are particularly difficult to detect with through transmission. Through transmission is usually conducted in water tank immersion systems or by using water squitter systems.
Baselines and thresholds are determined by conducting effects-of-defects test programs in which known good laminates are compared with laminates of varying porosity levels. Both photomicrographs and mechanical property testing is used to establish the threshold levels. Part zoning can also be used to reduce cost. Areas that are highly stressed would be zoned to lower threshold values than non-critical lower stressed areas. This is enough to show that ultrasonic inspection is there to stay.
Pulse echo is more sensitive to transducer alignment than through transmission. Since there is a large impedance mismatch between air and a solid interface, ultrasound does not propagate well through air; therefore, a couplant is used to more effectively transmit the sound from the transducer to the part. For hand inspection, glycerin compounds are frequently used while all automated systems use water.
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.
Through transmission is excellent at detecting porosity, unbonds, delaminations and some types of inclusions. However, this method cannot detect all types of foreign objects and it cannot detect the depth of any defects. Mylar film and nylon tapes are particularly difficult to detect with through transmission. Through transmission is usually conducted in water tank immersion systems or by using water squitter systems.
Baselines and thresholds are determined by conducting effects-of-defects test programs in which known good laminates are compared with laminates of varying porosity levels. Both photomicrographs and mechanical property testing is used to establish the threshold levels. Part zoning can also be used to reduce cost. Areas that are highly stressed would be zoned to lower threshold values than non-critical lower stressed areas. This is enough to show that ultrasonic inspection is there to stay.
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