Nondestructive testing technology using optical properties

1 Laser holographic testing

Laser holographic testing is a holographic interferometry method.

When the internal defects of an object are subjected to external forces, such as vacuuming (applying negative pressure), inflation and pressurization, heating, vibration, bending and other loading methods, the surface of the object corresponding to the defect will produce a local micro-deformation (displacement) different from the surrounding area. By using the laser holographic 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 utilizes the interference phenomenon of light. The figure on the right is a schematic diagram of the laser holography optical path system. As shown in the figure, part of the laser beam emitted by the laser generator 1 (such as helium-neon laser, ruby ​​laser, argon ion laser, etc.) is reflected by prism 2 to reflector 4 and then expanded by lens 5 to the surface of the specimen 6 (loading). The light wave reflected from the surface of the specimen is projected onto the photographic coherent plate 7 (object wave). The other part of the laser beam passes through prism 2 and is expanded by lens 3 to reflector 8, and then reflected and projected onto the photographic coherent plate 7 (reference wave). These two light waves will interfere (they come from the same laser source and have a fixed phase relationship). The result of the interference is the generation of interference fringes: in some areas, when the phases of the two waves are the same, constructive interference occurs to form bright fringes in the interference fringe image; when the phases of the two waves are opposite, destructive interference occurs to form dark fringes, thus forming an interference fringe image of alternating light and dark. When there are no defects in the specimen, the deformation of the specimen surface after loading is continuous and regular, and the changes in the interference fringe shape and the spacing between light and dark fringes are also continuous and uniform, coordinating with the changes in the specimen's outer contour. If there are defects in the specimen, the deformation of the surface area of ​​the specimen corresponding to the internal defect after loading is greater than the deformation of the surrounding area, and there will be a difference in the optical path. Discontinuous and sudden interference fringes will appear in the local area corresponding to the defect, that is, the shape and spacing of the fringes will be distorted, so the defects inside the specimen can be identified based on the interference fringe pattern.

The object wave carrying the information of the tiny deformation (displacement) of the specimen surface interferes with the reference wave to form a graphic that records all the information of the specimen in the form of contrast, shape and spacing changes of the interference fringes, which is a hologram.

The laser-ultrasonic holographic detection mentioned above is a hologram formed by using ultrasound as the object wave and laser beam as the reference wave.

Laser holographic detection can be used to detect cracks, debonding, and non-bonding defects in honeycomb structures, laminated adhesive structures, composite materials, and thin-walled components. Its advantages are that it does not require high processing accuracy for the specimen, is easy to install and debug, and can obtain a three-dimensional image of the object. Its disadvantages are that it has no penetration ability for opaque objects and can generally only be used for thin materials with small thickness. The equipment is expensive, and it is greatly disturbed by mechanical vibration, acoustic vibration (such as environmental noise), and ambient light during detection, so it needs to be detected in a quiet and clean darkroom.

2 Laser electronic speckle shearing technology

ESPI (Electronic Speckle Pattern Interferometry) is also called TV holography or digital holography. A laser beam is expanded by a lens and projected onto the surface to be measured. The reflected light interferes with the so-called reference beam projected directly from the laser to the camera, which records a series of spot images. Image comparison shows changes in the spot structure and generates related fringes, which are caused by surface displacements and deformations between the recorded images. Intelligent software automatically analyzes these fringes and calculates quantitative displacement values. Advanced ESPI systems use several laser irradiation directions or cameras to generate three-dimensional information on displacements and deformations as well as profile information (3D-ESPI systems). From these data, strains, stresses, vibration modes and more values ​​can be obtained.

ESPI systems provide information on deformations, displacements, strains and stresses. The materials industry uses this technology to measure Young's elastic modulus, Poisson's ratio, crack growth, true strain/true stress effects, and many other material parameters needed to describe new materials. High-speed measurement systems can also deliver dynamic material values ​​that can be used in crash tests and crash simulations.

The automotive industry uses ESPI in many ways: analyzing the fatigue behavior of chassis, drive trains, engines, gearboxes, wheels and many other components that are highly stressed and critical to vehicle safety. In addition, noise vibration harmness (NVH) problems can also be solved by pulsed ESPI technology. A pulsed laser emits two laser pulses with a variable time delay, and 1-3 high-speed ESPI cameras record the image. The measurement results show the deviation of the operation, which is used to eliminate the sound source, optimize the damping system, eliminate the sharp noise or vibration when braking, etc. The typical application of NVH is noise reduction. ESPI can also be used to optimize the sound quality, such as the collision test of closing the car door. Other advantages of pulsed ESPI technology include the analysis of impact events, such as showing 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 measurement technology, which is widely used in non-destructive testing or non-destructive inspection, but its optical setup has been improved. The reference beam is replaced, and the object image is double, sheared and layered in the camera. The resulting speckle image shows the gradient of the deformation of the surface being tested or analyzed. This information can be automatically analyzed by modern phase shift and edge opening techniques.

Since laser shear measurement obtains a unique deformation gradient that is not affected by the motion of the rigid object, this technique is typically used for defect identification in production lines or maintenance.

EPSI and Shearography are laser optical full-field measurement techniques that are based on the laser speckle effect, which occurs when a rough surface is irradiated with a laser.

Nondestructive testing and nondestructive inspection are the most widely used areas for shear measurement technology. In the production of modern composite materials, many different components are bonded together. The assembly process of these parts often requires manual operation. Therefore, the implementation of nondestructive testing at a certain stage in the production line is very important for product reliability and quality control. Shear measurement technology provides a very useful tool for all nondestructive testing applications.

The aviation industry uses shear measurement technology to test glass fiber reinforced plastics, carbon fiber reinforced plastics (CFRP) composites, smooth layers, foam plastics and aluminum composites. Fully automatic inspection systems have been installed for the inspection of ARIANE 5 and helicopter rotating blades. For maintenance inspection, portable shear measurement inspection systems have been used to detect defects using vacuum loading or heat loading. Recently, shear measurement technology has also been demonstrated for maintenance inspection of Concorde parts. Pratt & Whitney's jet engine wear seals have also been inspected using laser shear measurement systems using vibration excitation. Tire testing and panel inspection in the automotive industry are also well-known applications. (end)