(1) quality management. The performance and quality level of each product are usually clearly defined in its technical documents, such as technical conditions, specifications, acceptance standards, etc. , which is characterized by some technical quality indicators. One of the main purposes of nondestructive testing is to provide real-time quality control for raw materials and parts processed discontinuously (such as multi-process production) or continuously (such as automatic production line), such as metallurgical quality, processing quality, microstructure, coating thickness, size, orientation and distribution of defects, etc. In the process of quality control, the obtained quality information will be fed back to the design and process department, which in turn will further improve the design and manufacturing process of products, and the product quality will be consolidated and improved accordingly, thus achieving the effect of reducing costs and improving production efficiency. Of course, using nondestructive testing technology can also control the quality level of raw materials or products within the design requirements according to the acceptance criteria, without increasing the quality requirements indefinitely, and even use some defective materials without affecting the design performance, thus improving the utilization rate of social resources and improving economic benefits.
(2) On-the-job testing. Using nondestructive testing technology to monitor the equipment or components in operation, or to carry out regular inspection during maintenance, can find hidden dangers that affect the safe operation of equipment or components in time and prevent accidents. This is of great significance for important large-scale equipment, such as nuclear reactors, bridge buildings, railway vehicles, pressure vessels, pipelines, aircraft, rockets and so on.
The purpose of in-service inspection is not only to find and confirm the hidden dangers endangering the safe operation of equipment and eliminate them in time, but also to evaluate whether the equipment or components can continue to be used and its safe operation life on the basis of determining its orientation, size, shape, orientation and performance according to the early defects and their development degree (such as the initiation and development of fatigue cracks). Although nondestructive evaluation has just started in China, it has become an important development direction of nondestructive testing technology.
(3) Quality appraisal. For finished products (including materials and spare parts), final inspection should be carried out before assembly or put into use, which is quality appraisal. Its purpose is to determine whether the inspected object meets the design performance and can be used safely, that is, to judge whether it is qualified, which is not only the acceptance of the previous processing procedures, but also to avoid hidden dangers for future use. Non-destructive testing technology is used to detect whether materials or parts meet the requirements in every process (or one or more processes) of casting, forging, welding, heat treatment and cutting, so as to avoid ineffective processing of unqualified products. This work is generally called quality inspection, and it also belongs to the category of quality appraisal in essence. It is very necessary to check the quality of products before use, especially those products that will be used in complex and harsh conditions (such as high temperature, high pressure, high stress and high cyclic load). In this respect, the nondestructive testing technology shows the incomparable superiority of 100% inspection.
To sum up, nondestructive testing technology plays an extremely important role in production design, manufacturing technology, quality appraisal, economic benefit and work efficiency improvement. Therefore, nondestructive testing technology has been recognized and accepted by far-sighted enterprise leaders and engineers. Basic theory, test methods, analysis of test results, especially the analysis of some typical application examples, have become necessary knowledge for engineers and technicians.
It is worth noting that nondestructive testing technology is not the so-called "molding technology", so the expected performance or quality of products can only be achieved in product manufacturing, and it is impossible to complete it simply by product inspection.