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2.6 Non-destructive testing techniques using optical properties
§2.6.1 Laser Holographic Detection
Laser holographic inspection is a holographic interferometry method. When the defect inside the object is subjected to an external force, such as vacuuming (applying negative pressure), inflation pressure, heating, vibration, bending, etc., the surface of the object corresponding to the defect will produce a local slight deformation different from the surrounding. (Displacement), using the laser holography method, the wavefronts of the two light waves before and after the deformation are recorded for comparative observation, so that the internal defects of the object can be judged and detected.
Laser holography is the use of light interference phenomenon, the right picture is a schematic diagram of the laser holographic optical path system, as shown in the figure, laser beam 1 (such as 氦-氖 laser, ruby ​​laser, argon ion laser, etc.) A part of the light is reflected by the prism 2 to the mirror 4 and then expanded by the lens 5 to be projected onto the surface of the test piece 6 (loading), and the light wave reflected from the surface of the test piece is projected onto the photograph dry plate 7 (object wave), and the other part of the laser beam passes through the prism. 2 is then projected into the mirror 8 by the lens 3, and then reflected and projected onto the camera dry plate 7 (reference wave), the two light waves will interfere (they come from the same laser source, have a fixed phase relationship), The result of the interference is the generation of interference fringes: when the phases of the two waves are the same in some regions, constructive interference occurs, forming a bright stripe in the interference fringe image, and when the phases of the two waves are opposite, destructive interference is generated, forming a dark The stripes form an image of interference fringes between light and dark. When there is no defect in the test piece, the deformation of the surface of the test piece after loading is continuous and regular, and the variation of the shape of the interference fringe and the distance between the light and dark stripes is also continuous and uniform, which is in harmony with the change of the outline of the test piece. If there is a defect in the test piece, the deformation of the surface part of the test piece corresponding to the internal defect after loading is larger than the surrounding deformation, and the optical path is different, and the interference fringe with discontinuous mutation will appear corresponding to the defective local area. That is, the stripe shape and the pitch will be distorted, so that the defect inside the test piece can be discriminated based on the interference fringe pattern.
The object wave carrying the information of the small deformation (displacement) on the surface of the test piece interferes with the reference wave to form a pattern in which all the information of the test piece is recorded in the form of the contrast, shape and pitch of the interference fringe, which is a hologram.
The aforementioned laser-ultrasonic holographic detection is a hologram in which ultrasonic waves are used as object waves and laser beams are reference waves.
Laser holographic inspection can be used to detect defects such as cracks, debonding and unbonding of honeycomb structures, laminated bonded structures, composite materials and thin-walled members. The advantage is that the processing accuracy of the test pieces is not high, and the installation and debugging are convenient. It can obtain three-dimensional images of objects. The disadvantage is that it has no penetrating ability for opaque objects. Generally, it can only be used for thin materials with small thickness. The equipment is expensive, and it is subject to mechanical vibration and acoustic vibration (such as environmental noise) during detection. As well as disturbances such as ambient light, etc., it is necessary to perform detection in a quiet, clean darkroom.
§2.6.2 Laser electronic speckle cutting technology
ESPI (Electronic Speckle Pattern Interferometry) is also known as TV Holography or Digital Holography. A laser beam is expanded by the lens and projected onto the surface to be measured. The reflected light is interfered with a combination of a reference beam directly projected from the laser to the camera, and the camera records a series of speckle images. Image comparisons can show changes in the spot structure and produce related margins, which are caused by surface displacement and deformation between the recorded images, and the intelligent software automatically analyzes these margins and calculates the quantitative displacement values. The advanced ESPI system utilizes several laser illumination directions or cameras to generate three-dimensional information on displacement and deformation as well as contour information (3D-ESPI system). Based on these data, strain, stress, vibration modes, and more can be obtained.
The ESPI system provides information on deformation, displacement, strain and stress. The material industry uses this technique to measure Young's modulus of elasticity, Poisson's ratio, crack growth, true strain/true stress, and many other new materials. Material parameters required. High-speed measurement systems can also submit dynamic material values ​​for crash tests and collision simulations.
The automotive industry uses ESPI in many ways: analyzing chassis fatigue behavior, drive trains, engines, gearboxes, wheels and many other components, which are high stress and critical components for automotive safety. In addition, the NVH-Noise Vibration Harshness problem can also be solved with pulsed ESPI technology. A pulsed laser emits two laser pulses with variable time delay, and images are recorded by 1-3 high-speed ESPI cameras. The measured results show operational deviations, which are used to eliminate the sound source, optimize the damping system, and eliminate the brakes. Sharp noise emitted or eliminated. A typical application for NVH is to reduce noise, and ESPI can also be used to optimize sound quality, such as collision tests for closing doors. Other advantages of pulsed ESPI technology are the ability to analyze shock events, such as the propagation and reflection of Raleigh waves in metal or underground.
In addition to the automotive industry, all transportation industries, such as railways, shipping, aviation, etc., can take advantage of this ESPI with full field of view, three-dimensional, non-contact measurement capabilities.
Laser shearography is also a speckle interferometry technology, which is widely used in non-destructive testing or non-destructive testing, but its optical settings have been improved, the reference beam is replaced, and the object image is double. It is laterally sheared and layered in the camera. The resulting speckle image reveals a gradient of the surface deformation being tested or analyzed, which can be automatically analyzed by modern phase shift techniques and margin opening techniques.
Since the laser shear measurement yields the only deformation gradient that is unaffected by the motion of the rigid object, this technique is typically applied to defect identification in production lines or repairs.
EPSI and Shearography are laser optical full field measurement techniques based on the laser speckle effect, which occurs when a laser is used to illuminate a rough surface.
Non-destructive testing and non-destructive testing are the most widely used areas of shear measurement technology. In the production process of modern composite materials, many different components are bonded together. The assembly process of these parts often requires manual operation. Therefore, it is very important to implement non-destructive testing in a certain stage of the production line for product reliability and quality control. Shear measurement technology provides a very useful tool for all NDT applications.
The aerospace industry uses shear measurement techniques to test glass fiber reinforced plastics, carbon fiber reinforced plastic (CFRP) composites, smooth layers, foams, and aluminum composites. The fully automated inspection system has been installed for inspection of the ARIANE 5 and for helicopter rotary blade inspection. For maintenance inspection, portable shear measurement inspection systems have utilized vacuum or thermal loading for probing defects. Recently, shear measurement techniques have also proven to be useful for maintenance inspection of Concorde aircraft parts. Pratt & Whitney's jet engine wear seals have also been tested with vibration excitation using a laser shear measurement system. Tire testing and table inspection in the automotive industry are also well known applications.
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