ON THE IMPROVEMENT OF NONDESTRUCTIVE TESTING SYSTEMS

V. I. Gorbunov, B. N. Epifantsev

Until recently, the overwhelming majority of research in the field of nondestructive testing methods was devoted to improving methods and means for detecting discontinuities in products or media; as a result, flaw detectors suitable for industrial use were created, with the aid of which it is possible to obtain satisfactory and even good results in solving the corresponding production problems.

There is, however, another side to the question. The continuing intensive introduction of nondestructive testing systems into the technological process has revealed a contradiction between the desire to intensify the latter and the potential necessity of spending time on an additional operation. The requirement of obtaining a one-hundred-percent guarantee of the quality of manufactured products has transformed this contradiction into one of the most important problems of modern industrial production, the solution of which is unthinkable without the use of computer technology.

Indeed, at present there exist devices that make it possible practically instantaneously to register a disturbance in the structure of products. Examples of such devices are various kinds of introscopes, which give a visible image of the state of the macrostructure of an object on the screen of a cathode-ray tube. Owing to the presence of a number of shortcomings, however, they have not yet become widespread. But, taking into account the fact that industry places great hopes on reducing inspection time with the advent of improved introscopes, let us estimate the time that will actually be required for inspection.

According to data from engineering psychology [1, 2, 3], “operator time” consists of the following components: the process of signal reception (searching for changes on the indicator and direct perception of the signal), signal detection (selecting, from the set of possible signals, the one that carries the information needed at the given moment), determination of the message (i.e., its interpretation), the process of solving the problem, formulation of the decision (translation of the decision into the language of output signals), search for means of implementing the command, and implementation of the decision (preparatory movements, the process of directly performing the necessary operation). Figure 1 presents the experimental dependence of “operator time” on the amount of information received.

It is known that the area of simple geometric figures is estimated at 2.6 bits of information. Therefore, taking the average number of figures on the screen subject to analysis as three (which is far from the limiting value), we find that the “operator time” reaches 6 sec. For a screen with an aspect ratio of 2 : 1 and with 2048 information points, the simplest calculation shows that the maximum speed of the items being inspected should not exceed 0.28 m/sec. In practice, however, the value of the maximum speed must be still smaller.

Indeed, one of the main tasks of introscopy is to establish the type of defect (for example, lack of fusion, slag inclusions, etc.). Analyzing the stated problem from this point of view, one should regard as probable the superposition of images of two defects and the resulting need to distinguish slight brightness gradations. Figure 2 presents experimental data relating the speed of information processing by a person to the magnitude of the difference between images. Assuming that the brightnesses of adjacent points at the boundary of contours differ by less than a factor of two, we arrive at the conclusion that the operator’s time expenditure doubles. This means that the maximum speed of motion of the items must have a value of less than 0.14 m/sec, so that the operator can react to the incoming information with sufficient reliability. It should also be borne in mind that the probability of defects being missed by the operator rises sharply after approximately thirty minutes of intensive work.

Fig. 1

Fig. 2

From these very approximate considerations one can draw the quite obvious conclusion that the operator must be replaced by a machine in those cases where significant inspection speeds are required. This conclusion can in principle be implemented in practice, since the operator’s functions are amenable to automation, while “machine time” is several orders of magnitude less than “operator time.”

What qualitatively new contribution does the use of computing technology make to defectoscopy?

1. The acquisition of multielement information requires the presence of expensive sensors that make it possible to obtain data on the state of the structure of items at a large number of elementary points. At the same

at the same time it is in principle possible to solve the same problem with the aid of a single sensor (for example, a photomultiplier with a NaJ crystal) and a storage device of appropriate speed, i.e., to obtain the same information by means of a more reliable and less expensive system.

1. Calculations show that, when the operator is replaced by a machine, it is possible to inspect the structure of articles moving at a speed of 6–10 m/sec; in other words, it will no longer be the time required for processing information that determines the possibility of inspecting high-speed processes, but rather the time required for the sensors to register the presence of defects.

2. Using the information obtained from the flaw detector, the machine can, without time delay, carry out optimal cutting, sorting of articles and, moreover, control the technological process of production according to the criterion of minimum defectiveness; and, finally, there are no fundamental difficulties, on the basis of the static theory of decisions, in obtaining, with the aid of existing sensors, an increase in the signal-to-noise ratio by orders of magnitude. The implementation of this consideration is also based on the use of machines.

Thus, analyzing the trends in the development of nondestructive testing systems, one may say that their improvement is possible through the use of computing machines.

REFERENCES

1. W. Woodson. Handbook of Engineering Psychology for Engineers and Artists-Designers. “Mir,” Moscow, 1968.

2. B. F. Lomov. Man and Technology. “Sov. Radio,” Moscow, 1966.

3. D. Howard. Electronic Systems for Displaying Information. Publishing House of the USSR Ministry of Defense, Moscow, 1966.