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.
This method can also screen the placement and dimensions of defects within, or be used to find out the thickness of supplies. The benefits of this method are apparent. The penetrating capability of the technique is fantastic. And also, the flaw inspection sensitivity is large, especially for plane kind defects including, cracks, sandwich and different others.
The waves are produced when an electrical signal generator sends a burst of electrical energy to a piezoelectric crystal in the transducer causing the crystal to vibrate and convert the electrical pulses into mechanical vibrations. The piezoelectric crystal will also convert the returning sound waves back into electrical energy when the sound is received back from the part. A single crystal can be pulsed to send and receive sound waves, or two crystals can be used with one sending and the other receiving the pulse.
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.
Surface temperatures are normally restricted to a given temperature. An additional benefit of laser ultrasonic inspection is that the ultrasound propagates perpendicular to the surface somewhat independent of the laser angle of incidence. The transmitters and receivers can be off axis to normal at a specified angle without loss of performance. However, since the part must have a thin layer of resin on the surface for effective sound generation, resin starved or machined surfaces may limit the success of the technique.
This method can also screen the placement and dimensions of defects within, or be used to find out the thickness of supplies. The benefits of this method are apparent. The penetrating capability of the technique is fantastic. And also, the flaw inspection sensitivity is large, especially for plane kind defects including, cracks, sandwich and different others.
The waves are produced when an electrical signal generator sends a burst of electrical energy to a piezoelectric crystal in the transducer causing the crystal to vibrate and convert the electrical pulses into mechanical vibrations. The piezoelectric crystal will also convert the returning sound waves back into electrical energy when the sound is received back from the part. A single crystal can be pulsed to send and receive sound waves, or two crystals can be used with one sending and the other receiving the pulse.
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.
Surface temperatures are normally restricted to a given temperature. An additional benefit of laser ultrasonic inspection is that the ultrasound propagates perpendicular to the surface somewhat independent of the laser angle of incidence. The transmitters and receivers can be off axis to normal at a specified angle without loss of performance. However, since the part must have a thin layer of resin on the surface for effective sound generation, resin starved or machined surfaces may limit the success of the technique.
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