Ultrasonic nondestructive testing technology for t

2022-08-25
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Ultrasonic nondestructive testing technology for composites

ultrasonic nondestructive testing technology is a very important testing method for composites, and its detection frequency is usually 0.5 ~ 25MHz. Ultrasonic pulses are transmitted into the material to be tested through the probe, and the reflected and penetrated signals are analyzed to obtain the relevant information of the material structure. Although manual detection methods are still widely used, more and more aviation manufacturing enterprises begin to use automated detection systems to produce intuitive scanning. Do not easily disassemble the main pendulum images, such as projection images and cross-sectional images, that is, the so-called C-scan and B-scan imaging

many aviation materials and structures can be imaged and displayed in this way, and various types of defects can be detected. For composite materials, it is necessary to detect bonding defects, delamination defects, porosity and foreign matters between delaminations

detection technology

the equipment used for automatic detection usually uses three coupling methods to make the ultrasonic emitted from the probe enter the parts to be inspected effectively. They are contact method, water immersion method and water spray method respectively

contact method, that is, the detection probe is in direct contact with the piece to be detected. The advantage of this method is that the mechanical tracker can be used in the detection of curved parts, but the detection speed is limited

in most cases, water immersion or water spray methods are used for automatic detection, and the parts to be tested are completely immersed in water, or the sound beam reaches the surface of the parts through the ejected water column. The water immersion detection method usually uses the pulse echo technology to detect from one end, and the same probe both emits and receives. Recently, the water immersion system installed by USL has been upgraded to operate in two modes, depending on the inspection requirements and the shape of the parts. In the first mode, the complex shape parts use a single probe, the scanning line speed is 500mm/s, and the reciprocating motion interval is 1mm, that is, the scanning area per hour is about 2M2; The system can also scan in the second more efficient way, which uses a 100mm wide phased array probe to combine phased array scanning with mechanical movement. This method can achieve the output of 1m2 per minute when detecting flat parts and single curved parts, and the production capacity is greatly improved compared with that of a single probe

a phased array probe contains 128 independent chips, which emit ultrasonic pulses in sequence at very small intervals. It is usually 20000 times per second, that is to say, when the phased array probe covers the whole part with the mechanical motion mechanism, a scan such as completing 128 wafers of the whole array can be completed hundreds of times per second. Each independent chip can be controlled to produce very detailed images in a small area of the test material

composite material detection usually uses water spray detection method. The acoustic beam emitted by one probe passes through the ejected water column and is received by the second probe on the other side. During scanning, the water column angle on both sides must be accurately controlled and kept coaxial, otherwise the ultrasonic signal will be lost. This puts forward very strict requirements for the mechanical control system, especially when testing hyperboloid parts. At this time, it will be very difficult to improve the production capacity. However, for USL system, even if the parts with complex surfaces are scanned, the scanning speed can still be fast, and the detection quality will not be reduced

composite parts used in military and commercial aircraft usually have very complex shapes. Ultrasonic testing systems need to integrate multiple mechanical motion axes to scan these complex shaped parts. A typical system will have 10 ~ 12 axes, which are linked at the same time to track the contour of the part. USL system has two different structures: one is horizontal operation arm structure, and the other is vertical operation arm structure. How to choose depends on the wishes of customers and the scope of parts to be tested. Sometimes both water spray and water immersion methods are required, in which case the vertical operation arm structure is more suitable. In general, the multi surface tracking and detection of composite materials is much more complex than the water immersion detection system. Most manufacturers in the world start from water immersion, but there are not many manufacturers that can truly realize the multi surface tracking and detection system with 10 ~ 12 axis scanning. USL has nearly 20 years of experience in manufacturing this type of system, and there are more than 10 sets of this type of system, which has accumulated rich experience

CATIA model

for water immersion and water spray detection systems, the main potential bottleneck is how to program complex shaped parts. By directly obtaining the contour of complex shaped parts from CATIA (computer-aided three-dimensional design system), the problem of part programming can be solved

however, CATIA models, especially those of composite parts, are not always consistent with the real shape of the parts to be tested. This is partly because the parts will rebound plastically when they are removed from the tooling, and also because these parts are not self-supporting, but supported by 2 or 3 points, which will cause sagging, distortion and other deformation. In this way, the parts on the testing equipment are slightly different from CATIA models

although this difference is not great, it is enough to affect the effect of ultrasonic system in detecting complex shaped parts. USL has developed a software program to automatically measure the position and real shape of parts, which is used before scanning. It will adjust the scanning contour in the three-dimensional direction and recalculate the position of up to 12 linked axes, so that each axis tracks the real part contour rather than the theoretical contour. This process avoids repeating the scanning process, because usually it will show whether the actual contour is consistent with the theoretical contour only after the scanning is completed

in addition, the equipment also needs to use ultra-high specification ultrasonic equipment, which has high signal-to-noise ratio and excellent ability to resist external noise. However, this is not enough to ensure 100% detection of small defects. All these urge USL to develop systems that integrate many special functions to meet the requirements of detection. These systems have been widely used all over the world

control of sound beam

in any ultrasonic testing, it is necessary to strictly control the angle between the sound beam and the part surface. If the material to be tested is completely straight, the situation is much simpler, but in actual testing, such situations are rare. The key to the increase of the market share of polyurethane insulation materials in China is that the contour of the part surface is changing, and may be random and unpredictable

in order to achieve high productivity of plate detection, it is necessary to use multiple probe groups and be equipped with multiplexers. Typically, it is equipped with 7 and 15 rectangular probes, which are staggered and cover the width of 80mm and 150mm. Since the material is tested in the water immersion tank, all probes also measure the surface position of the plate while looking for defects. The measured distance from the plate surface to the probe surface, that is, the water path, can display the middle angle between the probe and the surface. And use this data to adjust the probe operator in real time, so that the detection angle is maintained at the correct value (usually 90 °). In this way, it can ensure that the probe is perpendicular to the plate surface and conduct reliable detection

after scanning, possible defects will be marked on the C-scan diagram. The scanning device automatically moves to each defect position, and the operator confirms it, including two groups of faults: a defect or a label or bubble on the surface. Because the rectangular probe is designed to find defects, but it can not accurately determine the size of defects, so here UL unremittingly promotes the future development of the safety field and uses a circular focusing probe to evaluate defects. In some cases, phased array scanning mentioned above is also used, but multi probe group scanning has higher productivity

when designing parts, nondestructive testing is usually the last thing to be considered, so it will make the problem more complicated. Nowadays, many advanced assembly methods, such as diffusion welding, friction welding and friction stir welding, are increasingly used in daily production, and the production of very complex metal parts requires special forms of nondestructive testing. This puts forward new requirements for the manufacturers of testing equipment and the actual operators of nondestructive testing. Similarly, for aerospace composites, due to the increasing use of resin transfer molding process, the same problem also occurs. These problems need to be solved, and can not cause bottlenecks and subsequent problems in the manufacturing process, so it is necessary to constantly innovate instruments, machinery and software

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