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.
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.
Since the ability to detect defects suffers at lower frequencies, parts are generally scanned with the highest frequency that can penetrate the part. This being said, air coupled ultrasonics are occasionally used for materials with low acoustic impedance (lower density materials) such as honeycomb assemblies. Air coupling has been used to inspect honeycomb materials up to eight inches thick.
It should be noted that while through transmission is good at detecting porosity, it cannot tell the difference between scattered porosity and planar voids if the defect densities are similar. In addition, other defects, such as ply wrinkling, can often appear to be porosity. C-scan units can be programmed to print out the changes in sound levels as varying shades of gray or can be set in a go-no go mode where only rejected areas are printed.
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.
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.
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.
Since the ability to detect defects suffers at lower frequencies, parts are generally scanned with the highest frequency that can penetrate the part. This being said, air coupled ultrasonics are occasionally used for materials with low acoustic impedance (lower density materials) such as honeycomb assemblies. Air coupling has been used to inspect honeycomb materials up to eight inches thick.
It should be noted that while through transmission is good at detecting porosity, it cannot tell the difference between scattered porosity and planar voids if the defect densities are similar. In addition, other defects, such as ply wrinkling, can often appear to be porosity. C-scan units can be programmed to print out the changes in sound levels as varying shades of gray or can be set in a go-no go mode where only rejected areas are printed.
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.
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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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. USMGo
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