What is holographic nondestructive testing?

Application and development trend of laser holographic nondestructive testing technology

Author: updated on July 20, 2005

I. Introduction

Non-destructive testing is to detect some physical and mechanical parameters of components or products without damaging them, so as to determine the nature of defects and their influence on structural performance. It can predict whether components or products meet the requirements of engineering use, or monitor during production to ensure that products meet the design requirements.

Laser holographic nondestructive testing is a new branch of nondestructive testing technology, which was developed in the late 1960s and is one of the important applications of holographic interferometry. Over the years, great progress has been made in the theory, technology, photographic system and image processing system of laser holographic nondestructive testing. In the aerospace industry, the detection of composite materials, honeycomb sandwich structures, laminated structures, aviation tires and high-pressure pipeline containers has some unique features, which have solved problems that cannot be solved by other methods. After the appearance of pulsed laser, the requirement of vibration isolation in holographic interference is eliminated. This makes it possible to apply laser holographic nondestructive testing technology to industrial production site. At present, due to the rapid development of video reproduction and computer image processing technology, CGD camera can quickly and accurately input holographic interference fringe images into computer for digital image processing, which meets various requirements of nondestructive testing technology. It can even be transmitted over a long distance through information expressway, and distorted holographic interference fringe images can be transmitted to experts' offices, so that they can diagnose defects. It can be predicted that in the near future, holographic nondestructive testing will be combined with CCD camera and computer digital image real-time processing technology, and this technology will be pushed to a new development climax through information expressway transmission.

Second, the current situation of domestic development

The application of laser holographic nondestructive testing technology in China began with 1974. At that time, Tianjin University cooperated with Nanchang Hongdu Machinery Factory to develop a JD-II holographic interferometer with He-Ne laser as light source, which was used to detect the honeycomb sandwich structure of aluminum plate on Qiang -5 aircraft. Therefore, some large factories and universities in the aerospace field set off a research upsurge of laser holographic nondestructive testing. Some parts that are difficult to be detected by conventional methods have been detected by laser holographic interferometry in Eni Machinery Factory, Songling Machinery Factory, 303, Xigong University, Harbin Institute of Technology, 606, 62 1, 703, 529, Nanchang Institute of Aeronautics and Astronautics and other units. Such as carbon fiber size panel metal honeycomb sandwich structure, helicopter rotor rear section, glass fiber bonded conical radome, boron fiber composite material, carbon fiber horn inner wall pure gold coating, sealing rubber oil pad, solid rocket motor propellant column coating, launch vehicle attitude engine combustion chamber, high-pressure pipeline, pressure vessel, printed circuit board solder joint defect detection, etc. , has made gratifying progress. Some projects have been used in production practice, such as glass fiber conical radome, aluminum honeycomb sandwich plate, solid rocket propellant coating and bonding quality inspection. , has been included in the product quality inspection process specification by the manufacturer, which meets the needs of military production and has received good economic benefits. In the aspect of aircraft tire testing, SJQL- 1500 1 holographic tire nondestructive testing instrument jointly developed by Zhengzhou Institute of Technology and Guilin Shuguang Rubber Research Institute was put into practical use in the mid-1980s. On this basis, SJQL- 1500E tire laser holographic nondestructive detector has been successfully developed. The improved instrument can detect the quality of the whole tire only by one double exposure, and there is no detection blind spot. It has reached the international advanced level of similar products in the late 1980s.

In order to get rid of the dependence on vibration reduction test-bed, Beijing Institute of Optoelectronic Technology and Xi 'an Institute of Optics and Mechanics have developed double-pulse holographic interferometers and multi-pulse holographic interferometers respectively, which have been used in production practice. Xi Institute of Optics and Mechanics has also successfully developed a high-resolution transient holographic interferometry system to measure the jet characteristics of Chang 'e-1 and Dongfeng-2 solid rocket motors. The system can complete the shooting of single-pulse, double-pulse and multi-pulse holographic interference images, and provide hardware equipment for holographic nondestructive testing to go out of the laboratory for on-site real-time monitoring.

Laser holographic nondestructive testing technology is opening up a new application way. Nanchang Institute of Aeronautics and Astronautics and Chengdu Dianzi University used geomechanics similar structure model theory to photograph double exposure interference fringes and real-time interference fringes under axial pressure, which provided a useful method for analyzing the deformation characteristics and failure laws of similar structure models. Nanchang Institute of Aeronautics and Astronautics also used this method to analyze the whole process of initiation, propagation and failure of corrosion cracks in high strength steel under stress corrosion environment, and achieved phased results, which found a new path for the experimental study of stress corrosion cracking.

Third, the current situation of foreign development

As early as the end of 1960s, PRPA (Planning Bureau of Forward-looking Research Department of the US Department of Defense) made a plan and allocated special funds for Coo-leg Electronic Laboratory of the University of Michigan to conduct research in this field. They also use this new technology to solve these special problems, so their research direction is similar to that in China. The objects of inspection are honeycomb structure, laminated structure, composite material, powder column coating, turbine blades, tires and so on. In addition, this method has been used to detect some large components. For example, the spacecraft antenna with a diameter of 2.7 m is detected by thermal loading method; A large wing laminated structure with an area of 1.80m×6.6m was inspected by high-power pulse holographic interferometry, and GCO company made special equipment for this inspection.

Foreign scholars attach great importance to the research of holographic interferometry and instrument serialization. They studied several compensation techniques to eliminate and reduce the dependence of continuous output laser holographic interferometry system on vibration isolation, and promoted the application of shear speckle and electronic speckle interferometry in nondestructive testing. Due to the large investment in foreign scientific research, we pay special attention to the instrumentalization of principle experiments as soon as possible, and constantly improve, adjust and perfect them to meet the actual needs of production, that is, to transform scientific and technological achievements into productive forces as soon as possible. Since the nondestructive testing of aircraft tires by laser holographic double exposure method, after continuous improvement, the seventh generation of aircraft tire detector came out. Its instrument products are updated very quickly.

Fourth, the development trend.

Laser holographic nondestructive testing technology has experienced more than 30 years of development, and there are not many projects that are actually applied to production, and it is limited to aerospace industry. There are many reasons for this. The first is the limitation of laser holographic nondestructive testing technology itself. So far, this technology still relies on vibration isolation platform, silver salt dry plate recording and darkroom work. Moreover, the detection system is complex, the investment is large, and the operation technology is skilled, so it is difficult to popularize and apply it to production practice. It can only be used in the military production department to solve some parts that cannot be solved by conventional testing methods but must be nondestructive tested.

Secondly, the investment is not enough. Due to the limitations of this technology itself, it can not arouse the interest of entrepreneurs and the attention of the government. For a long time, there has been little capital investment, and it is entirely dependent on scientific and technological workers to conduct research spontaneously, resulting in outdated equipment. Third, it is not closely integrated with related disciplines. For example, it is urgent to solve the problem that the silver salt recording medium can not record transient continuous deformation in real-time holographic interferometry. Fourthly, new technologies such as CCD application and computer digital image processing are developing rapidly, which requires the rapid update of laser holographic nondestructive testing equipment. The author thinks that laser holographic nondestructive testing technology should first solve the problem that the recording medium adapts to the continuous change of displacement in real-time interference recording, and only in this way can its application range be expanded. Secondly, it should be combined with new technologies such as optical fiber, CCD and microcomputer digital image processing as soon as possible to form a miniaturized non-contact remote control detection instrument, get rid of the constraints of the laboratory, expand its application scope and solve more detection problems in production practice. This can not only improve the speed of holographic nondestructive testing, but also improve the quantitative analysis of testing. Thus, the problems of microcracks, residual stress, fatigue damage and safety life estimation in reliability analysis of large components are solved.

In a word, although laser holographic nondestructive testing technology has its unique advantages, it can solve some conventional testing methods or problems. But it also has its own limitations, which affect its popularization and application. For example, the detection of composite materials, honeycomb sandwich structure, laminated structure and aircraft tires has obvious advantages, which are incomparable to conventional methods such as ray, ultrasonic, magnetic powder, eddy current and infiltration. Therefore, laser holographic nondestructive testing technology will play an important role after continuous development and improvement.