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.
During the process, carbon or epoxy laminates are usually scanned at around 5 MHz while honeycomb assemblies require lower frequencies (1 or 2.25 MHz) to penetrate the thicker structure. Foam filled structures require even lower frequencies with 250 kHz, 500 kHz or 1 MHz being typical.
As the ultrasonic beam passes through the composite, it is attenuated due to scattering, absorption and beam spreading. This loss or attenuation is usually expressed in decibels. Thicker laminates will attenuate more sound than thinner laminates. Though transmission ultrasonics is one of the two most common methods used to inspect fabricated composite laminates and assemblies.
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.
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.
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.
During the process, carbon or epoxy laminates are usually scanned at around 5 MHz while honeycomb assemblies require lower frequencies (1 or 2.25 MHz) to penetrate the thicker structure. Foam filled structures require even lower frequencies with 250 kHz, 500 kHz or 1 MHz being typical.
As the ultrasonic beam passes through the composite, it is attenuated due to scattering, absorption and beam spreading. This loss or attenuation is usually expressed in decibels. Thicker laminates will attenuate more sound than thinner laminates. Though transmission ultrasonics is one of the two most common methods used to inspect fabricated composite laminates and assemblies.
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.
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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