Why non-destructive testing is needed.
Non-destructive testing is a basic tool of industry. As a powerful weapon for reducing costs, improving product quality, and maintaining quality levels, it can play an important role in improving the competitive position of the manufacturer. In fact, managers who early recognized its potential, gained considerably through the intelligent use of non-destructive tests. Today, their companies employ such tests as a basic means of implementing management policies.
On the other hand, failure to use non-destructives tests effectively can be disastrous in corporate management, particulary in industries whose products are judged by their safety features. Experiences in the aircraft industry clearly illustrates this. Twenty years ago, some aircraft manufacturers, pioneered the development and application of new non-destructive tests. Others lagged behind in this effort, considering such tests as an added expense to be avoided as long as possible. Today, those who neglected non-destructive testing are no longer in the aircraft business. Still others limited their use of non-destructive tests to military air vehicles where such tests were required by military specifications. These manufacturers are no longer in the civilian aircraft market.
Similar techniques can often be devised to control the work of personnel in inplant operations. The results of immediate feedback by incontrovertible evidence from non-destructive tests (rather than a supervisor's opinion) gain immediate respect by skilled personnel.
Of course, such systems must be properly introduced and explained to production personnel, but in most instances, they will prefer such fair and impartial judgment to the possibly biased opinions of immediate supervisions.
Here is one example: In the critical production of brazed honey comb stainless steel structures, utmost cleanliness in assembly is essential. When the X-ray images of the actual fingerprints of a specific worker (shown by failure of brazing alloy to wet the material) brough a reaction from his fellow workers, the percentage of defective products from the department immediately decreased. Where incentive group income depends upon quality of output, such measures can greatly influence productivity.
Supermarkets for Testing
Today, the situation in non-destructive testing is changing with amazing rapidity. Supermarket services, supplying all forms of non-destructive test equipment and services, are being developed. Firms with a complete line of non-destructive testing equipment and processes can advise the optimum available test method. The customer's problem can be solved more effectively by the correct test method or group of test methods for his requirement.
This trend is fully justified by the rapid development of non-destructive testing from the status of occasional applications in special product inspection to that of an essential component of mechanization and automation of production. To illustrate, no longer does the customer buy his own isotope and ionization gage, and attempt to develop some form of test devise on his own. For the most part, he selects a proven firm which can provide a complete system to be integrated with his production-line operations. These well-engineered systems have been remarkably successful in many applications.
In conclusion, the measure of management's achievement through proper use of non-destructive tests will be evident in terms of the market retained or developed, the reputation of the firm and the degree of customer satisfaction with its products, and the continuing profits of the company. Their origin can often be proved to be related to an intelligent management application of non-destructive testing - the greatest unexploited roof remaining fully under management control.
In the United States, isotope radiationgaging systems have been developed with measure metallic sheet thickness while the material passes through rolling mills as high speeds. Such systems sense deviations from desired gage, and change screwdowns on rolls rapidly enough to result in increased percentages of within-tolerance product. Many other examples could be cited where non-destructive measurements provide greater accuracy and speed than sampling tests involving direct measurement or destructive evaluations.
Modern production equipment is often an integral part of a continous production line. Here, equipment can cost far more, in lost production time, than the cost of repair of the specific machine that has failed. Non-destructive test systems can monitor equipment either by directly measuring the deterioration of critical components, or by continuous automatic testing of machine parts in action. They can also oversee machine deterioration and warn of impending failure by measuring tell-tale properties of the machine's products. And spread in dimensions or properties of output parts, or trends towards one tolerance limit, can indicate that the machine is about to go out of control. With advance warning, the machine can be shut donw before costly failure can occur, or corrective action can be taken during the next maintenance period.
Monitoring the Labour Force
Since the cost of skilled labour is a major factor in many types of production and assembly processes, control of quality requires constant indoctrination of personnel and supervision of output. An example is in the welding of pipelines. Welds are X-rayed primarily to control the performance of skilled pipeline welders, rather than to predic the serviceability of the pipeline welds. Each welder knows that the X-ray crew is following closely behind him and can surely detect his operational failures - this is a driving stimulant to careful welding. In fact, the story is reliably told (by the manager of X-ray inspection of a major pipeline) that, on one occasion their inspection crews ran out of film on a pipeline job. When they reported to the construction boss that they would have to stop radiography until more film arrived, the supervisor replied vehemently (and profanely) that they were not to interrupt radiography for a single minute. They were instructed to go through all of the operations of radiography (without using film) just as usual, since the welders would instantly recognize the stoppage in inspection and relax their care in welding.
As another example, checks of machined parts (directly after machining) can reveal dimensional error or inadequate surface finish. Then, if the process is tending to go out of control, it can be immediately corrected. Rapid non-destructive tests can also weed out welding defects, and poor welding procedures can be adjusted. In addition, if the part is tested immediately after the defect is initiated, defective materials can be scrapped. This eliminates further costly processing.
As a further advantage of on-the-spot testing, immediate feedback of information from inspection often leads to detection of the causes for the defects while the causative condition is still apparent. This can lead to prompt improvement in process or control, eliminating many defectives from future production. (Changes in product design also can help). By indicating processs changes must insure product quality, many non-destructive tests often work themselves out of a job. This seeming contradiction can usually be considered as the ultimate measures of success of a given test.
Improvement in Process Control
Non-destructive test devices can serve as basic components of feedback process control systems since all such tests are based upon measurements which do not damage the materials or parts being inspected. In several instances, such indirect measurements have become more accurate and reproducible than the direct measurements they have replaced. For example one facility has reported that ultrasonic thickness testing (of nuclear fuel elements) from one side only proved more precise and reproducible than the most careful micrometer measurements by carefully trained personnel.
It has been reported that eddy current tests monitor the entire output of ball-bearing factories in Germany. In a matter of milliseconds per measurement balls are thoroughly tested as they roll through four tests coilds. In one coil, the ball diameter is measued to the order of one micron, the balls being separated into 13 classes by diameters. The next coil rates eccentricity down to one micron on groups of balls which similar diameters. Hardness is measured more precisely than by Rockwell "C" hardness testers as balls pass through the third coil. For the last step in this particular setup, balls containing defects are detected and segregated as they pass through another coil.
For each such measurement statistical quality control recordsw are obtained automatically in the form of requency distributions. These can show the presence of two or more populations, or indicate deviation trends that can lead to defective production if not corrected. Also feedback controls can be actuated by the statistical trends the automatic equipment reveals.
When key personnel are denied information by management restrictions they are partially crippled in terms of their potential value to the corporation. Needless security or commercial restrictions can be a very short-sighted management policy leading to excessive test costs, duplication of developments, and repetition of errors. And in the end, the conservative company will be far behind its competitors in the field of non-destructive testing. There is probably no cheaper or more effective means of training key personnel than active participation in technical society committee activities and extensive personal contacts throughout industry.
Reduction of Materials Costs
In using non-destructive tests to lower costs, one of the first points of attack is at the level of raw materials control and coordination with design. To start with, designers, materials engineers, and non-destructive test engineers can study possible areas in which materials savings could be made if the quality of the raw material were controlled adequately at a reasonable cost. Then, the designer can use less material in each component. As an extra benefit, such reductions can often be coupled to a design improved through stress analysis. The aircraft industry has long employed non-destructive tests to eliminate incoming defective materials from the production line. Obviously, both machine and labour time can be saved if imperfect materials are rejected before any time or effort has been spent on production. By expending all labour only upon good materials, overall productivity can be improved.
Monitoring the Production Line
If raw materials are carefully controlled, the next point of attack in applying non-destructive tests comes in processing. After each basic operatin which is critical to the service performance of the final product, the aprt is tested. In a nutshell, the optimum point at which to check for processing defects is directly after the process in which they originate. For example, after heat treating, parts can be checked for hardness, tensile strength, and many other cracks, decarburization, and many other factors which might be affected by the treatment. In some instances, such tests can be made automatic to provide feedback controls for the process (if continuous) as well as statistical qality control charts for later evaluation by management.
A Profit is Essential
However, the threat of corporate failure is not the prime reason why management should use non-destructive tests. Today, and in the future, a basic responsibility of management is to operate at a profit. Under competitive conditions, profits can be assured only by an effectiver programmed leading to reduced material and production costs, elimination of wasted labour and machine time, assurance of adequate product quality that eliminates final rejections, and the improvement of existing products and development of new products that receive wide customer preference. Non-destructive tests can play an effective part in such programme.
A vital responsibility of management is to study all appropriate factors and to evaluate them on a dollar and penny basis for specific areas within the corporation. Furthermore insofar as is feasible, all factors of costs and benefits should be evaluated, including the incidental advantages of sales and advertising costs, and promotional and competitive advantages accrued or lost by management decisions in this area.
Generally, casual or occasional observations alone are not enough to estimate the costs of quality control. All factors must be carefully analyzed. If there is a possibility that hidden production costs are being wasted on improper or unnecessary non-destructive tests, management consideration is indicated. As an example, if a costly non-destructive test (such as film radiography) is being used where a much cheaper test (such as fluoroscopy) could be employed with equal reliability, the latter should be used.
Individual items used in non-destructive testing vary in cost from less than USD 300 for simple liquid-penetrant test kits used in shop or field inspection to investments of perhaps USD 10-15,000 for X-ray test facilities. Very expensive test systems, such as multimillion volt X-ray equipment, television fluoroscopy, or ultrasonic immersion test units with automatic bridge scanning, can range in price from perhaps USD 30,000-300,000 or more. Despite these costs, such systems find practical application in industry.
Making Test Personnel Effective
A few words about personnel are in order here. Key test personnel have responsibilities comparable to those of higher management in that their decisions can be equally important to the manufacturer. Consequently, such personnel should have considerable freedom within the plant and be encouraged to keep abreast of all new developments. It must be emphasized that free interchange of technical information on a personal basis has been the cahracteristic of this field.
The industrial and technical world is becoming increasingly testminded. Generally, architects, builders, engineers, and industrial designers and managers are familiar with the idea of testing and are coming to rely more and more on tests as a basis for making many important decisions. In view of the important place that the making of tests has attained in the technical world, it seems appropriate to devote some time to the study of testing as a subject by itself.
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|Author:||Mirza, Khurram M.|
|Date:||Mar 1, 1990|
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