Role of NDT during Manufacture

Role of NDT during Manufacture

NDT has become an essential element in the vital quality control of manufactured goods. Without effective means of NDT, it would probably be impossible to build many of the major high integrity structures that are successfully tackled today. There is no doubt that quality is now a far better understood and a much more respected term than it was 20 years ago. Control is a basic concept in manufacturing industry. Metallurgists, inspectors, operators and production personnel know the problems of keeping any manufacturing process under control.

The material being manufactured or fabricated must be controlled. when any element of a manufacturing operation gets out of control, quality drops and waste may be produced. Any NDT method applied in one way or other to control processes, makes profit for the manufacturer. A non-destructive test can reduce manufacturing costs when it locates undesirable characteristics of a material or component at an early stage, thus saving the money that would be spent in further processing or assembly. An example of testing of forging blanks before the forging operation, illustrates this principle.

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 labour and forge hammer time, involved working the defective material into the product. The profits gained by performing NDT on these blanks, prior and during working are unimaginable. In some instances, non-destructive tests may produce desirable information at lower cost than some other destructive or non-destructive tests, thus reduce manufacturing costs

(15).In general, quality of manufactured goods is accomplished by measuring dimensions, materials properties or other characteristics of a part, comparison of the measurements with predetermined standards and modifying the manufacturing process to control accordingly to control these characteristics. This is possible either by destructive or non-destructive methods. Often, direct measurements can be accomplished only by destroying the parts. The commercial impact of this is twofold- costs were incurred to make the product, yet no profit can be made from the product.

However, the same information is obtained without destroying the part, even if only as an indirect measurement, then the part can be sold for a profit after it has been tested. The commercial incentive to test indirectly i.e. non-destructively is large when small quantities and large profit margins are involved, and is crucial with one-of-a-kind products. In most cases, the objectives of NDT techniques during manufacture fall into one of three categories as follows:1. measurement of physical/mechanical properties of materials or manufactured geometry2. information on flaws/discontinuities in the materials3. information about the condition of material which may have deteriorated or changed with time.

Level of quality to be achieved by using NDT techniques is very important in a manufacturing process. In a competitive marketplace, the quality of a product directly affects its success and may carry additional far-reaching consequences. Quality below the optimum can ruin sales and reputation (4). On the other hand, quality above the optimum can swallow up profits through excessive production and scrap losses. Hence, the true function of testing is to control and maintain the quality level that management decides for the particular product and circumstances. A successful product is one which does the intended task reliably and at minimum cost. Design and associated NDT must consider not only the user's needs, but also the ease and associated cost of manufacture and cost of maintenance and repair.

It goes without saying that Well designed and thoroughly inspected products, in theory, start life in a sound and utterly reliable condition. Further, indeed, with very well inspected components, it may be argued that the pre-service inspection eliminates the need for inspection in-service (15).Considering this aspect, NDT has now become an essential part of quality assurance of many areas of manufacturing industry. Also, use of NDI has become necessary as a means of meeting certain legal and contractual requirements affecting the production and sale of a wide variety of manufactured products.

Modern non-destructive tests are used by the manufacturers for various reasons including:1) To ensure product reliability2) To prevent accidents and save human life3) To make profit for the user To ensure customer satisfaction and to maintain the manufacturer’s reputation To aid in better product design To control manufacturing processes To lower manufacturing costs To maintain uniform quality level Successful application of NDT methods to the inspection of manufactured goods requires that a) the test system and procedure be suited to both inspection objectives and types of flaws to be detected, b) the operator have sufficient training and experience, and c) the standard for acceptance appropriately define undesirable characteristics of a nonconforming part.

If any of these pre-requisites are not met, there is a potential for error meeting quality objectives. It is necessary that the types of flaws that can be induced by each manufacturing operation be understood. Only then is it practical to define the NDI that should be used. In the routine NDI of parts, there are four possible results:1) A flaw is indicated where there is a flaw2) No flaw is indicated where there is a flaw3) A flaw is indicated where there is no flaw and4) No flaw is indicated when there is no flaw

The first result is the successful detection of a flawed part, and leads to correct rejection.

The second result is known as a miss, leads to the acceptance of a nonconforming part(type-I error). the third result is known as a false indication or false detection and leads to the rejection of a flaw-free-part (type-II error) . The fourth result is the successful detection of a flaw-free part, and leads to correct acceptance. The frequency of type-I errors (acceptance of flawed parts) can be reduced by lowering the specified value for maximum acceptable response. Unfortunately, this often increases the frequency of type-II inspection errors (rejection of sound parts). A reasonable balance between type-I and type-II inspection errors must be achieved for most practical inspection procedures.