Introduction to Non-Destructive Testing (NDT)

History of Non-Destructive Testing

The art and science of Non-destructive Testing (NDT) are very old. Probably one of the most famous and well known examples is that of Archemedes and Hiero’s Crown. In performing a test to determine if the king had been defrauded by the silversmiths, Archemedes discovered the principle that now bears his name. The art of NDT is used in many fields of endeavour without even being considered in the realm of NDT. To give an example, the fruit vendor who can tell if a watermelon is ripe by ‘thumping’ or if a cantaloupe is ripe by shaking and listening for the ‘rattle’ of the seeds is using NDT. Since, 1920, the art of NDT has developed from a laboratory curiosity to an indispensable tool of production (1). However, the real revolution in NDT took place during World War II. The progress in materials engineering in identifying new and improved materials subsequent to a number of catastrophic failures in World War II like the brittle fracture of Liberty ships, necessitated the requirement to test and improve material properties. This requirement resulted in a wider application of the then existing NDT methods and techniques and also paved the way for development of new methods and techniques. Though in the beginning, NDT was used primarily for process control and secondarily for quality control, subsequently, the use of NDT was recognised by management as a means of meeting consumer demands for better products, reduced cost and increased production. NDT tests were used world-wide to detect variations in structure, minute changes in surface finish, the presence of cracks or other physical discontinuities, to measure thickness of materials and coatings and to determine other characteristics of industrial products. NDT became a vital ingredient of modern engineering practice to achieve the required standards of quality in manufacturing and fetched reputations and profits to many industries. This traditional role changed steadily and NDT was relegated to the role of an inspection tool, popularly known as Non-destructive Inspection (NDI) and Non-destructive Evaluation (NDE), catering to the safety needs of components in aircraft, nuclear rectors, offshore installations, petrochemical plants, gas turbines, bridges etc. It is not uncommon to find the use of ultrasonics to detect submarines, schools of fish, and as navigational aids; eddy currents for baggage control at air ports as well as metal detectors and infrared techniques for detecting heat losses from buildings, hot spots in electrical equipments, defects/stresses in metals. NDT has become a vital ingredient of modern engineering practice contributing significantly to overall safety, reliability and confidence at economic cost (2).

Role, Benefits and Components of NDT

NDT is a branch of the materials sciences that is concerned with all aspects of the uniformity, quality and serviceability of materials and structures. Essentially, NDT refers to all the test methods which permit testing or inspection of material without impairing its future usefulness. The science of NDT incorporates all the technology for detection and measurement of significant properties (3). In other words, from an industrial viewpoint, the purpose of NDT is to determine whether a material or a component will satisfactorily perform its intended function. By use of NDT methods and techniques, it is possible to decrease the factor of ignorance about material without decreasing the factor of safety in the finished product (4). In general, the purpose of NDT will fall into one of the following categories:
1. Determination of material properties
2. Detection, characterisation, location and sizing of discontinuities/defects
3. Determining quality of manufacture or fabrication of a component/structure
4. Checking for deterioration after a period of service for a component/structure

The benefits derived from NDT to the industry are many (5). The contribution which NDT tests can make to the industry can de divided into four categories:
1. increased productivity
2. increased serviceability
3. safety
4. identification of materials.

By detecting faulty material and thus preventing loss of material, manpower and shop time non-destructive tests will increase productivity, and hence the economic gains. NDT techniques can be used to as an aid in new process and manufacturing techniques. Preventive maintenance tells if parts are still satisfactory for use, it pays off in dependable predictable production, fewer repairs, less accidents and lower over-all operating costs. Increased serviceability of equipment and materials will result through the application of NDT methods and techniques by finding and locating defects which may cause malfunctioning or breakdown of equipment (6). In the field of safety proper use of NDT will aid in the prevention of accidents, with their possible loss of life, property, and vital equipment. The identification of materials differing in metallurgical, physical or chemical properties can often be done by using NDT methods.

A variety of NDT techniques have been developed to detect and characterise the above types of defects. All the NDT techniques are based on physical principles. Nearly every form of energy has been utilised in NDT. Likewise nearly every property of the materials to be inspected has been made the basis for some method or technique of NDT. Nearly all methods of NDT involve subjecting the material being examined to some form of external energy source and analysing the detected response signals. The essential parts of any NDT test are
1) application of a testing or inspection medium
2) modification of the testing or inspection medium by defects or variations in the structure or properties of the material
3) detection of this change by suitable detector
4) conversion of this change into a suitable detector
5) interpretation of the information obtained.

For example, in the case of X-ray film radiography, 1) the X-rays are the testing or inspecting medium, 2) any defects in the material being radiographer will modify the intensity of the radiation reaching the film on the opposite side of the specimen, 3) certain silver bromide emulsions are sensitive to X-rays and can be used as a detector, 4) the emulsions are capable of recording variations in X-ray intensity and by the proper developing procedures can be made to give a permanent record, and 5) interpretation is then a process of explaining variations in density of the radiograph. In most of the instances, NDT results are indirect measurements. Hence, it is essential that the interpretation be made by an experienced or skilled person. The person interpreting the results sometimes determines the success or failure of a test method or technique.

Popular NDT Techniques

Ultrasonic Testing,
Radiography(X, Gamma, Neutron)
Eddy Current testing,
Potential Drop
Liquid Penetrant Testing,
Magnetic (particle, flux leakage, Barkhausen)
Acoustic Emission Testing,
Infrared Thermography
Visual Testing (Optical)
Leak Testing

Defects: Definition, Types and Origin

Often, in NDT and Quality control anomaly, discontinuity, defect, flaw, imperfection, non-conformance are the terms used when the material/component tested deviates from requirement/ideality. Though all of them look similar, there exists a vast difference in their meaning and interpretation.

The term ‘flaw’ means a detectable lack of continuity or a detectable imperfection in a physical or dimensional attribute of a part. The term ‘nonconforming’ means only that a part is deficient in one or more specified characteristics. In many instances, a non-conforming part is entirely capable of performing its intended function, even in its non-conforming form. In other instances, a non-conforming part can be reworked to make it conform to specifications. Hence, it should not be automatically assumed that a non-conforming part is unfit for use.

The types of defects that NDT is called upon to find, can be classified into three major groups:
1. Inherent defects - introduced during the initial production of the base or raw material.
2. Processing defects - introduced during processing of the material or part.
3. Service defects - introduced during the operating cycle of the material or part.

Some kind of defects or structural variations which may exist in these three groups are, cracks, surface and subsurface, arising from a large number of cases; porosity; tears; machining, rolling and plating defects; laminations; lack of bond; inclusions; segregation; lack of penetration in welds; pipe; fatigue defects; seams; blow holes, dross shrinkage etc.

The origin of defects in a material can take place during manufacturing stage, or during assembly, installation, commissioning or during in-service (7). We can broadly categorise these steps into two viz. pre-service and in-service. In the pre-service scenario, the defects may be present in the raw material stage or may be introduced during machining, fabrication, heat treatment, assembling. The pre-service quality can be achieved essentially by good engineering practice i.e. by way of selecting suitable quality raw materials and by ensuring that harmful defects are not produced during the subsequent stages of fabrication and assembly, prior to putting the part/component into service.

However, even with the highest quality of materials and workmanship, the occurrence of some form of imperfections during manufacture is inevitable and there will be a typical distribution of imperfection sizes associated with a particular manufacturing process and quality. The ideal situation is where the inherent distribution of initial imperfection sizes is well separated from the distribution of critical defect sizes which may cause failure. Hence, the role of NDT is not only to detect the defects but also to give information about the distribution.

There are little benefits derived out of repairing the parts/components with defects for their delivery to the customer. Here, the industry should aim at produce parts / components without defects. In the subsequent section, it is shown how to achieve this objective.

On the other hand, in the in-service scenario, defects will be generated due to deterioration of the component/structure as a result of one or combination of the operating conditions like elevated temperature, pressure, stress, hostile chemical environment and irradiation leading to creep, fatigue, stress corrosion, embattlement, residual stresses, microstructural degradation etc. which, in turn, result in deterioration of mechanical properties, crack initiation and propagation, leaks in pressurised components and catastrophic failures (8).

NDT techniques are increasingly applied to components/systems for the detection and characterisation of defects, stresses and microstructural degradation to ensure the continued safety and performance reliability of components in industry. NDT techniques improve the performance reliability of components through periodic in-service inspections, by way of preventing premature and catastrophic failures. (9,10)

NDT also provide valuable inputs to plant specification and design i.e. to determine which components are the most likely to fail and then to ensure that those have easy maintenance access for repair or replacement. In in-service scenario, it is rather difficult to stop the formation of defects and the growth of defects already formed.

Role of Fracture Mechanics

From the above sections, it is clear that, in spite of utmost care by ensuring pre-service quality, optimum operating conditions and in-service inspection programme, the degradation of components/structures does take place and is unavoidable just as the ageing of human beings. Sooner or later the inspection of any large engineering structure is likely to result in the identification of a possible defect. It is essential to know whether the detected defect is likely to impair the life or performance of the structure. A common approach is to estimate the fatigue life of the structure in the presence of the defect. This is the number of fatigue cycles the structure can withstand before the defect grows to a critical size and rapid fracture ensues (Paris Law). Further, the application of fracture mechanics helps identifying whether the defect is harmful or not (11).

Fracture mechanics is the applied mechanics of crack growth. It helps to quantify the rather elusive concept of a material's toughness which is the resistance to crack growth under a static load. This is measured in terms of a critical value of the stress intensity factor, a material property.

According to fracture mechanics, defects present in materials lead to failure by growing to a critical, self propagating size. The fracture mechanics concepts allow one to calculate the critical sizes of defects as a function of their depth, length, active stress system and stress intensity and such properties of the material as its elastic modules, yield strength and fracture toughness. Therefore, by knowing the dimensions of defects present in a component, it is possible to estimate both remaining life of the component and extent of degradation using the fracture mechanics concepts.

In particular, the size of the defect, its nature, its location, the stress to which it is subjected to and the local properties of the material in which it is embedded will play a major role in determining its rate of growth. It is common to place a limit on the acceptable height or depth of the defects to be accepted in a structure. It is the task of the NDT operator to determine the size of the defect that is used in future behaviour.

NDT Technique Vs. Fracture Mechanics

The use of fracture mechanics concepts places a premium on the ability of NDI to detect small cracks and on the need to determine the practical reliability of a particular inspection process when that process is used to detect defects of a specific type and size. If the design is such that the critical crack size based on design loads is greater than the smallest defect that can be reliably detected, the inspection process can be used. The difference between the critical size and the smallest detectable size is the factor of safety.

Any measurement technique will result in experimental errors. Since this error affects the fracture mechanics calculations, there have been concerted efforts in NDT to reduce the errors in defect sizing. For example, with conventional ultrasonic techniques, the error may well exceed 5 mm. Clearly there is some considerable benefit if precision is improved and with the Time-of-flight-diffraction (TOFD) technique, the error is unlikely to exceed 1.5 mm so the classification of defects can be more exact. Despite a small echo from the defect tip, an effective reduction in the size of the minimum detectable defect may be provided by the use of TOFD (12).

It is certain that the greater majority of the defects considered potentially dangerous will, if monitored, turn out to grow less rapidly than assumed and many will not grow to failure in the design life of the structure. Monitoring might then result in savings in repair and shut-down times without impairing the essential safety of structures.

Selection of NDT Technique

One of the primary considerations when selecting an NDT technique, whether it is capable of detecting the existing discontinuities with sufficiently high probability. This establishes the threshold rejection criterion, or the decision point of whether the component is fit for service. Sophisticated techniques are required for finding tight surface cracks and internal discontinuities. Further, not all NDT techniques are physically capable of detecting all discontinuities. Each on has its own limitations. The capability of a method depends on the inherent limitations of the method, technique or procedure used. For example, ultrasonic beam inspection can not reliably detect discontinuities very near to the surface due to the erratic effects of the probe’s near field and ring down. Conversely, eddy current techniques could not be expected to reliably detect discontinuities more than a few millimetres below the inspection surface. On the other hand, magnetic techniques can detect both surface-breaking discontinuities and volumetric discontinuities provided they are of sufficient size. So, care is required while selecting a technique for a definite inspection task. Though in many instances single NDT technique is sufficient to solve an inspection problem, more than one technique are also employed. No doubt, they provide additional information and are expected to enhance inspection reliability (13).

It needs to distinguish between effectiveness and efficiency of inspection. An effective inspection is one which finds all the required defects with the required probability of success. On the other hand, an efficient inspection is one which is not only effective for defects concern but also avoids the unnecessary rejection of minor imperfections. It can be therefore be deduced that inspection reliability includes both effectiveness and efficiency.

The choice of inspection equipment is a function of several important considerations. The equipment must have sufficient capability yet be simple to calibrate, maintain and operate, it must withstand the field conditions of the inspection, it should allow ease of signal interpretation and recording, it should be portable (14).

Indications obtained during NDI need to be interpreted and evaluated. Any indication that is found is called a discontinuity. As discussed earlier, discontinuities are not necessarily defects, but need to be identified and evaluated to decide whether the part is at or below specifications. The following definitions would help in categorising the NDI data:

False: Indication not due to the testing procedure. It may be due to improper processing, incorrect procedure, also known as a ‘ghost’, an artefact, ‘spurious’ or ‘electrical interference’.

Nonrelevant: An indication which has no relation to a discontinuity that is considered a defect in the part being tested; a defect within acceptable tolerance levels.
Discontinuity: An interruption, intentional or unintentional in the configuration of the part.
Indication: Observation of a discontinuity that requires interpretation, for example, cracks, inclusions, gas pockets.
Interpretation: Determination whether an indication is relevant, Nonrelevant or false.
Evaluation: Assessment of a relevant indication to determine whether specifications of the serviceability of the part are met.
Defect, flaw: One or several discontinuities that do not meet specifications.

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 geometry
2. information on flaws/discontinuities in the materials
3. 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 reliability
2) To prevent accidents and save human life
3) 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 flaw
2) No flaw is indicated where there is a flaw
3) A flaw is indicated where there is no flaw and
4) 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.

Advances in NDT

In many types of NDT, the sensitivity of the technique depends upon the ability to distinguish the significant part of the signal from the general background due to electronic noise, or inherent background signal from the material being examined.. With the power and speed of modern computers, signal processing methods and modelling numerical methods, remarkable developments took place in research on NDT techniques Significant improvements have been made both in the NDT equipment and in the specific techniques used (16). Thus, defects that may not have been revealed by NDT performed five, ten or 40 years may be detected by more sophisticated NDT equipment or techniques currently available.

The results of reliability studies indicate that the probability of detecting a defect with ultrasonics increases with the degree of sophistication of the system. (manual ultrasonics, without sophistication, can be expected to reject an equal or greater percentage of the discontinuities present than will radiography). Manual ultrasonic systems relaying on 20dB or 6dB drop are known to be inaccurate. The incorporation of computer assisted processing into ultrasonic systems has allowed the easy implementation of potentially better methods for defect detection and sizing such as SAFT, ALOK, TOFD etc. (17)

Similarly, use of multiple NDT sensors, NDT techniques and computer assisted processing in modern NDI systems have reduced costs by increasing both the speed and reliability of inspection

Human Component

NDT inspections are performed over extended periods of time. In addition to keeping track of areas inspected and to be inspected, the operator must remain alert to possible signs of a discontinuity. As a consequence, inspection reliability depends significantly on the operator.

The three main components of human performance are the person, the activity and the environment. One can expect ideal performance when the person is highly skilled and motivated, the activity is familiar and satisfying and the environmental conditions are favourable. These three requirements are not likely to be satisfied simultaneously (18).

Manual scanning requires skill and probe movement control. In addition to scanning, the human operator must exhibit vigilance in observing the flaw detector screen for over long, unbroken, periods of time to detect small changes in the information. Maximum human performance can not be expected to persist over time. As a consequence, inspection reliability depends on the performance and judgement of the operator carrying out that inspection.

One question that arises is that why not just remove the human from the inspection routine ? In many instances this is possible, especially in manufacturing processes. An interesting example is quality assurance of hardened components. One industrially important heat treatment is case hardening of steel, which is carried out to improve surface properties. For consistent quality it is important to ensure a constant case depth. This is in general is measured by conventional optical microscopy which slows down the production process and does not ensure consistency of the case depth quality. The main effect of the treatment is to produce a hard, highly strained martensitic layer which, unlike the ferrite/pearlite, is unable to support the 90° domain wall motion that generates MAE. Hence, MAE measurements can be used to measure the case depth on-line. In many instances, the case depth deduced from the MAE measurements was in good agreement with the case depth deduced by optical microscopy.

Primarily in a manufacturing/quality control/quality assurance setting, when inspections can be automated, a great deal of consistency gained over human operators, provided engineering factors are given the appropriate emphasis. However, automated inspection, by definition, require each and every component to be identical. When each component is different from the other, the automated inspection becomes impractical (19).

The first step in the development of an inspection procedure is to anticipate the types of discontinuities that may be present and determine whether they may ultimately interfere with the service requirements of the test piece. Discontinuities may arise from (raw) material selection, manufacturing process, handling, geometric configurations, service loads and environmental conditions (20). They may be localised or they may span a larger volume of the test piece.

During manufacturing stages, NDT can be used to detect defects at various stages during production, by means of off-line measurements, for example, before and after each set of welds, on the parent plate before welding, on the billets before rolling or forging or on the steel slabs after casting etc. While there are clear benefits in this approach, it has the drawback that off-line measurements interrupt the process stream and reduce the productivity (21). A better way would be monitor for incipient defect formation on-line. On-line monitoring and control of the welding process has the potential to improve weld quality and increase productivity in the automated welding. Weld monitoring and control can be achieved by the integration of real time non-destructive evaluation techniques with the welding process. In-production weld inspection can improve weld quality and may provide a significant cost reduction. The welding parameters can usually be adjusted to prevent defects from forming. Furthermore, if welding defects do occur the flaws can be found and repaired before they are covered by subsequent welding passes, leading to a decrease in the level of post-weld inspection and repair. Good quality welds rely on the correct weld pool size, geometry and position relative to the weld preparation. NDE sensors provide information on the state of the weld pool. This information, together with critical welding parameters such as current, voltage, torch position and travel speed, is used to adjust the welding process to maintain desired stable process with little operator intervention.

The major advantages of the in-process method are cost and speed. Conventional NDT is relatively slow, because of the need to position the sensor on the spot and make measurements for each spot weld. As a result, 100% testing may be more time consuming than manufacturing. For example, for components with large numbers of spot welds, in-process techniques provide a measure of weld quality in fractions of a second and can be automated.

For on-line material monitoring, when the parameters affecting the NDT are limited and identified, it is possible to establish a viable empirical approach, provided experimental relationships can be established between readily measured NDT parameters and the desired materials properties. The final step is process control, preferable by means of a closed feed-back loop in real-time. Provided a reliable relationship is found between measured NDT parameter and process variables, the technique can be used to improve the product quality and process efficiency (22).

The on-line approach offers the earliest possible warning of problems so that a remedial action can be taken in the most cost effective manner, with hopefully minimum scrap. It will be also possible to use the information about the presence of defects to adjust the process parameters or the feed-stock, in order to minimise or even prevent further defect formation either open-loop or closed loop control. It has been demonstrated that NDT techniques such as ultrasonics and magnetics are sensitive not only to defects but also to certain material properties such as such as homogeneity, grain size, texture, elastic modules, plasticity, hardness, stress and temperature. For example, If there exists 1:1 relationships between measured parameters such as ultrasonic velocity and material properties and in turn, between material properties and mechanical properties such as strength, fracture toughness, then the application would be straight forward. A reliable on-line inspection methodology can be accomplished with a sensing heat suitable for application to the production line without adversely affecting product quality or productivity. Ideally, the NDT sensing head should be non-contact, robust and capable of processing the data sufficiently rapidly to guide the remedial action. This is especially important if the goal is closed-loop control, when a high degree of automation is desired.

Fortunately, a wide range of NDT technologies developed for more traditional post-manufacture NDT, can be used, in principle, during the manufacturing process and also in-service

It is likely that when an automated process indicates an anomaly, human inspectors will be sent for manual verification before rejecting the component. At this moment it is essential to note that the human inspector has two key qualities than an automated system does not: The ability to adopt to the requirements of the individual items being inspected, and the ability to judge whether an indication is in fact a discontinuity or not.

UT Vs Automation

Ultrasonic waves have great potential for use in-process NDT (23). First UT velocity and attenuation give important information on material property changes during processing. Secondly, they give indirect information about temperature, pressure, flow etc. Thirdly, ultrasonic waves can be used on-line for inspection of part quality. Measurement of process parameters and materials properties makes possible the control process variables to achieve the required material properties.

RT Vs Automation

The advantages of real-time radiography are on-line testing of defect formation in the weld and study of metal fusion, filler-metal-to-base metal interaction, metal transfer and mass flow in the welding pool and the application of this information to welding process control (24). The welding current can be automatically controlled as a function of defect-feature extraction from computer processing of weld images. While the infrared and optical sensing and control systems have the disadvantage that the information on weld quality is indirect, closed-loop intelligent process controls have been demonstrated to characterise the weld penetration through the depth of the weld pool. The information extracted from real-time radiographic images about weld quality, supplemented by sensor data on weld current and voltage, is used for weld power-supply control (25).

Pros & Cons of Automation

Automation is desirable when high confidence, highly repeatable inspections must be performed in a timely fashion while recording and analysing a larger amount of raw data. Today, successful completion of an automation task can be strongly influenced by rapidly changing computer technology and the human element involved in the transition from manual to automated procedure. For these reasons, the development and integration of automated inspection technology into routine use should proceed on a phased basis. The pioneering status of current technology means that the development and application of an automated inspection system incurs significant costs and risks. Therefore, the decision to automate should be made after assuring that it is the optimum solution for the particular problem of concern. Listed below are the reasons that often justify pursuing the automated approach:

1) Full coverage of the item to be inspected is demonstrated, recorded and repeatable
2) Automated data acquisition and analysis permits working at higher sensitivity because the consequent increase amount of data can be handled rapidly with modern computational hardware.
3) Collecting and storing position annotated inspection signal information in a computer compatible format greatly increases signal interpretation options.
4) For nuclear systems one large benefit is the substantial reduction in radiation exposure.

The elements which must be fully integrated to produce an automated inspection system are

1) Electronic hardware
2) Software
3) Transducer position
4) Signal transmitter and receiver
5) Transducer

The challenge to the developer is to combine these elements into a well integrated system that performs inspection rapidly, efficiently and with great reliability. Two useful aspects for successful development of automated inspection systems are 1) involving the end user early and periodically during the development cycle and 2) system performance verification by someone independent of both developer and end user.

Quality of NDT

Quality of an NDT operation is defined as its performance in unambiguously revealing and reporting flaws of prescribed characteristics, in a cost effect way that safeguards the component's integrity and inspection's repeatability. This definition is helpful for the identification of the most important elements of a relevant quality assurance system.

It is apparent that special managerial measures are demanded since the ultimate quality is decided by the day-to-day performance and not by the theoretical capability of procedures, equipment or operators.

Quality assurance is the establishment of a program to guarantee the desired quality level of a product from raw materials through fabrication, final assembly and delivery to the customer. Quality control is the physical and administrative actions required to ensure compliance with the quality assurance program. These functions include physical and chemical tests and non-destructive tests. Growing concern for product quality calls for this kind of quality assurance program.

The important requirements for an effective QA system for ensuring reliable NDT are (26)

1. General requirements: quality policy, responsibilities, independence, confidentiality, corrective actions, cost-effectiveness
2. Qualification operators: competence of the operator and care to guarantee continuing effectiveness
3. Qualification of procedure: inspections must be conducted as per the procedures
4. Qualification of equipment and materials: qualification, calibration, maintenance etc.
5. Supervision and surveillance of operators: operator dependence, human errors minimisation
6. Records: test records, results, reports and necessary visible evidence
7. Quality audit: re-examination of methods, audit, review of procedures and system

The leading companies now recognise the need for 'total quality management', which is defined as the optimisation of all parts of an organisation to achieve conformance to customer requirements and expectations. A total Quality Management is best defined as a world class competitive organisation and characterised by:

+ A focus on customer needs
+ Universal participation
+ The realisation that every process contributes to quality
+ The use of continuous improvement mechanisms
+ An ongoing program of training

The setting up of co-located, multi-disciplinary, project teams for the development of both products and business process, appears to be the most effective route towards total quality. Total quality involves teamwork and commitment.

References

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