Laser ultrasonic C-scan images of aircraft component.

PURPOSES OF NONDESTRUCTIVE TESTING

Since the 1920s, nondestructive testing has developed from a laboratory curiosity to an indispensable tool of production. No longer is visual examination the principal means of determining quality. Nondestructive tests in great variety are in worldwide use to detect variations in structure, minute changes in surface finish, the presence of cracks or other physical discontinuities, to measure the thickness of materials and coatings and to determine other characteristics of industrial products.

Modern nondestructive tests are used by manufacturers (1) to ensure product integrity, and in turn, reliability; (2) to avoid failures, prevent accidents and save human life; (3) to make a profit for the user; (4) to ensure customer satisfaction and maintain the manufacturer's reputation; (5) to aid in better product design; (6) to control manufacturing processes; (7) to lower manufacturing costs; (8) to maintain uniform quality level; and (9) to ensure operational readiness.

Ensuring the Integrity and Reliability of a Product

The user of a fabricated product buys it with every expectation that it will give trouble-free service for a reasonable period of usefulness. Few of today's products are expected to deliver decades of service but they are required to give reasonable unfailing value. Year by year the public has learned to expect better service and longer life, despite the increasing complexity of our everyday electrical and mechanical appliances.

America has always been a nation on the move. Today our railroads, automobiles, buses, aircraft and ships carry people to more places faster than ever before. And people expect to get there without delays due to mechanical failure. Meanwhile factories turn out more products, better, faster and with more automatic machinery. Management expects machinery to operate continuously because profits depend on such sustained output. The complexity of present-day products and the machinery which makes and transports them requires greater reliability from every component. If a product has one part that has a probability of failure of 1 in 1,000 before it has served a reasonable life, it may be satisfactory. This seems to be a very low chance of failure. Now suppose that a product is assembled from 100 critical parts of various kinds and that each part has a failure possibility of 1 in 1,000. What then is the possibility of failure of the assembled item? The overall reliability of any assembly is the mathematical product of the component reliability factors. Overall reliability of this example is then:

R = 0.999¹⁰⁰ = 0.9057   (Eq. 1)

The possibility of failure of the assembly is then:

1.00 – 0.9057 = 0.0943   (Eq. 2)

or almost 1 in 10. It is certain that the user of this product will be highly dissatisfied if 1 out of every 10 units fails prematurely. The point is that component integrity and, in turn, reliability must be immensely greater than the required reliability of the assembled product. Consider the ordinary V-8 automobile engine. It has only one crankshaft but eight connecting rods, sixteen valve springs and hundreds of other parts. Theoretically, failure of any one of these could make the motor useless. Yet how frequently does the car owner experience a part failure? This amazingly low incidence of service failure during the normal life of an automobile is a great tribute to the ability of the automotive engineers to design well, of metallurgists to develop the right materials, of production personnel to cast, roll, forge, machine and assemble correctly, and of inspectors and quality control staff to set standards and see that the product meets those standards.

Preventing Accidents and Saving Lives

Ensuring product reliability is necessary because of the general increase in performance expectancy of the public. A homeowner expects the refrigerator to remain in uninterrupted service, indefinitely protecting the food investment, or the power lawnmower to start with one pull of the rope and to keep cutting grass for years on end. The manufacturer expects the lathe, punch press or fork lift to stand up for years of continuous work even under severe loads.

But reliability merely for convenience and profit is not enough. Reliability to protect human lives is a valuable end in itself. The railroad axle must not fail at high speed. The front spindle of the intercity bus must not break on the curve. The aircraft landing gear must not collapse on touchdown. The mine hoist cable must not snap with people in the cab. Such critical failures are rare indeed. And this is most certainly not the result of mere good luck. In large part it is the direct result of the extensive use of nondestructive testing and of the high order of nondestructive testing ability now available.

Ensuring Customer Satisfaction

While it is true that the most laudable reason for the use of nondestructive tests is that of safety, it is probably also true that the most common reason is that of making a profit for the user. The sources of this profit are both tangible and intangible.

The intangible source of profit is ensured customer satisfaction. Its corollary is the preservation and improvement of the manufacturer's reputation. To this obvious advantage may be added that of maintaining the manufacturer's competitive position. It is generally true that the user sets the quality level. It is set in the market place when choosing among the products of several competing manufacturers. Certainly the manufacturer's reputation for high quality is only one factor. Others may be function, appearance, packaging, service and price. But in today's highly competitive markets, actual quality and reputation for quality stand high in the consumer's mind.

Aiding in Product Design

Nondestructive testing aids significantly in better product design. For example, the state of physical soundness as revealed by such nondestructive tests as radiography, magnetic particle or penetrant testing of a pilot run of castings often shows the designer that design changes are needed to produce a sounder casting in an important section. The design may then be improved and the pattern modified to increase the quality of the product. This example is not academic; it occurs almost daily in manufacturing plants the world over.

Somewhat outside the scope of discontinuity detection are nondestructive tests to determine the direction, amount and gradient of stresses in mechanical parts, as applied in the field of experimental stress analysis. These play a very important part in the design of lighter, stronger, less costly and more reliable parts.

Controlling Manufacturing Processes

Control is a basic concept in industry. Engineers, inspectors, operators and production personnel know the problems of keeping any manufacturing process under control. The process must be controlled, and the operator must be trained and supervised. When any element of a manufacturing operation gets out of control, quality of the affected product is compromised and waste may be produced.

Almost every nondestructive testing method is applied in one way or another to assist in process control and so ensure a direct profit for the manufacturer. As one example of thousands which could be cited, consider a heat treating operation. The metallurgist sets up a procedure based on sound material of a given analysis. One nondestructive test, applied to all parts or to a few from each batch of parts, tells whether the chemical analysis of the material is so erratic that the procedure will fail to produce the desired hardness or induce cracking. A second test may show when and where cracking has occurred. Another test may show that the desired hardness has not been developed. If so, process variables may be corrected immediately. In these ways, cost and processing time are saved for the manufacturer.

Lowering Manufacturing Costs

There are many other examples of both actual and potential cost savings possible through the use of nondestructive tests. Most manufacturers could cut manufacturing costs by deciding where to apply the following cost reduction principle: a nondestructive test can reduce manufacturing cost when it locates undesirable characteristics of a material or component at an early stage, thus eliminating costs of further processing or assembly. An example of this principle is the testing of forging blanks before the forging operation. The presence of seams, large inclusions or cracks in the blanks may result in a woefully defective product. Using such a blank would waste all the labor and forge hammer time involved in forming the material into the product.

Another profit making principle is that a nondestructive test may save manufacturing cost when it produces desirable information at lower cost than some other destructive or nondestructive tests. An example of this principle is the substitution of a magnetic particle test for acid pickling to detect seams or cracks. As it has in many plants, a straightforward economic study of comparative costs of the two methods may show the cost saving advantage of the nondestructive test over the pickling examination.

Maintaining Uniform Quality Level

Improved product quality should be an aim of and a result of nondestructive testing. Yet this is not always the case, for there is such a thing as too high a quality level. The true function of testing is to control and maintain the quality level that engineers or design engineers establish for the particular product and circumstances.

Quality conscious engineers and manufacturers have long recognized that perfection is unattainable and that even the attempt to achieve perfection in production is unrealistic and costly. Sound management seeks not perfection but pursues excellence in management of workmanship from order entry to product delivery. The desired quality level is the one which is most worthwhile, all things considered. Quality below the specified requirement can ruin sales and reputation. Quality above the specified requirement can swallow up profits through excessive production and scrap losses. Management must decide what quality level it wants to produce and support. Once the quality level has been established, production and testing personnel should aim to maintain this level and not to depart from it excessively either toward lower or higher quality.

In blunt language, a nondestructive test does not improve quality. It can help to establish the quality level but only management sets the quality standard. If management wants to make a nearly perfect product or wants at the other extreme to make junk, then nondestructive tests will help make what is wanted, no more and no less. In preparing a drawing for a part, the designer sets tolerances on dimension and finish. If a drawing specifies a certain dimension as 32 mm (1.25 in.) but fails to specify the tolerance, the machine shop supervisor rejects the drawing as incomplete or assumes the standard tolerance. In nondestructive testing, a quality tolerance (the tolerance on the characteristic being tested) or criteria for acceptance or rejection must also be specified.

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