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Composite materials and nondestructive testing


As a new engineering material, compared with the traditional metal materials, composite materials have good anti-fatigue resistance, shock absorption, and heat resistance, and is easy to be processed and molded. It is widely used in the aircraft manufacturing industry.


Due to the dynamic load and foreign object impact, the composite materials are prone to have problems, such as water accumulation, delamination, debonding, fracture, and other damages during the service process. X-ray, ultrasonic and vibroacoustic testing are mostly used for the conventional detection of composite materials. However, due to the harm of X-ray to the human body, the irrelevant personnel on-site must leave the radiation area when X-ray detection is carried out. X-ray detection will occupy the maintenance cycle alone. Moreover, the X-ray detection efficiency is low, the operation is difficult and the cost is high. The ultrasonic and vibroacoustic method is a kind of point detection, which is low efficiency and easy to be missed when testing large area composite parts. In contrast, infrared thermal imaging detection is surface scanning, which can complete large area detection at one time. Compared with conventional ultrasonic, vibroacoustic and X-ray detection, it is not only accurate, fast, efficient, but also safe, and has no impact on the human body and environment. Therefore, infrared thermal imaging detection stands out and becomes the main detection method.


Infrared thermal imaging detection


1. The detection principle of infrared thermal imaging

Infrared thermal imaging detection is based on the principle of infrared radiation, using infrared radiation measurement analysis method and technology to measure and analyze the distribution and change of temperature field distribution on equipment, material and object surface. By receiving the infrared radiation from the object and displaying it in the form of thermal imaging, and judging from the temperature distribution of the object surface, it determines the internal change of the object.


2. The operation method of the infrared thermal imaging testing

Infrared thermal imaging testing is based on the principle of infrared radiation, through scanning records, observing the temperature changes caused by defects on the surface of the workpiece to be detected, so as to detect the surface and near-surface defects.


A fixed heat (usually heated by a blower) is uniformly blown into the surface of the workpiece, and the speed of its diffusion into the interior of the workpiece is affected by the internal properties of the workpiece. If there is a defect inside, the uniform heat flow will be blocked by the defect, also known as thermal resistance. After a time delay, there will be heat accumulation in the defected part, and the temperature on the surface of the workpiece will be abnormal. When the workpiece surface is scanned by an infrared instrument, the infrared instrument will record the temperature field on the surface of the workpiece, and defects can be judged under the abnormal temperature.


According to its detection methods, infrared thermal imaging detection can be divided into the active type and the passive type. The active method is to heat the workpiece manually before the infrared thermal imaging detection. During the heating process or after the heating is stopped, when the internal temperature of the workpiece has not reached a uniform and stable state, the detection is generally carried out during the heating process, and the temperature distribution on the surface of the workpiece can be observed by scanning and recording, which is suitable for the detection of static parts. The passive method is to use the difference between the temperature of the workpiece itself and the ambient temperature and display the internal defects of the workpiece in the process of heat exchange between the two, which is suitable for the quality control of the equipment in operation.


The main equipment of infrared detection is a thermal imaging device, which uses thermal imaging technology to display the temperature and temperature distribution of the measured target with visible thermography. Any object with temperature will send out infrared rays. The thermal imager receives the infrared radiation from the object and displays the temperature distribution on the surface of the measured object with color pictures. According to the temperature difference, it can find out the temperature abnormal points, so as to realize the purpose of inspection and maintenance. It is also called infrared thermal imager.


The infrared thermal imager uses the infrared detector and optical imaging objective lens to receive the infrared radiation energy of the object to be measured, reflects it to the photosensitive element of the infrared camera detector, and obtains the infrared thermography, which corresponds to the thermal distribution field of the measured object surface. In fact, the infrared thermal imager is infrared temperature measurement, which transforms the invisible infrared energy emitted by the object into a visible thermal image. The different colors displayed on the thermography represent different temperatures. When using the infrared thermal imager, the temperature measurement range, target size, optical resolution, and response time should be determined according to the actual situation.


3. The application of infrared thermal imaging detection

The infrared thermal imaging detection can be used to detect the delamination and debonding and other defects in the framework skin, integral panel, carbon fiber reinforced plastic, and multilayer sandwich structure bonded by adhesives. Infrared thermal imaging detection can also be used to check the water accumulation in a honeycomb structure, but it cannot correctly distinguish which cell contains water. Further judgment can be achieved by an X-ray examination or ultrasonic examination. At present, airbus companies have incorporated infrared inspection into the maintenance manual to detect composite components such as rudder and elevator, and infrared thermal imaging detection has also been involved in Boeing series aircraft maintenance.


4. The advantages and disadvantages of infrared thermal imaging detection

The infrared thermal imaging detection is surface scanning, which can complete a large-area detection at one time; the detection is accurate, fast, efficient, safe, and has no impact on the human body and environment and the detection results are intuitive. However, the detection sensitivity decreases rapidly with the increase of the depth of the defect, and it cannot locate the defect accurately.

Due to the advantages of infrared thermal imaging, it is widely used in the inspection of large composite components. It is necessary to understand and master the knowledge of infrared thermal imaging detection.