NDT and its importance

  • NDT and its importance

    Posted by NDT-Inspect on 31/01/2022 at 8:11 am

    The importance of Nondestructive Testing (NDT) as a Quality Control/ Quality

    Assurance tool in the industrial domain cannot be over-emphasized. With the rapid

    advancement in research and technology, the NDT field is becoming larger and more

    sophisticated day by day. Innovative research in materials science and digital technology

    is paving the way for more and more new methods in NDT technology.

    Although the NDT technology has improved over the years, the basic ‘human factor’

    underlying the success of the NDT field remains the same. There are two major factors

    that influence the ‘Quality Assurance in NDT’. First, knowledgeable and skilled NDT

    Operators are the most important factor in assuring the reliable test results. Second, the

    Management oversight of the NDT operations plays a major role in assuring the overall

    quality of NDT.

    Whether the NDT operations are performed in-house or by a contractor, periodic

    Management Self-assessments should include the following question: How can the

    Management assess and improve the ‘Quality Assurance in NDT’?

    This paper attempts to answer the above question. Some practical examples are provided

    to illustrate the potential quality incidents that could lead to costly failures, and the role of

    NDT Operator and the Management in preventing such quality incidents. Also, some

    guidelines are provided on how the Management can apply the elements of Quality

    Assurance to NDT in order to assess and improve the ‘Quality Assurance in NDT’.

    Nondestructive testing (NDT) plays a vital role in assuring the quality and reliability of

    several critical products whose integrity is of paramount importance for safety. As an

    inspection, condition monitoring and diagnostic tool in a variety of applications, NDT

    plays a key role in the safety of our lives. Billions of parts in the manufacturing, power

    generation and transportation industries throughout the world are being inspected on a

    daily basis using one or more NDT methods. Many potential accidents are avoided due

    to the timely detection and elimination of the defects in materials and products through

    careful application of the principles of ‘Quality Assurance in NDT’.

    Most of us agree that Quality and Safety go hand in hand. In other words, a quality

    product is a safe product. In this context, quality is defined as ‘freedom from defects’.

    There is a considerable risk associated with the inspection of the pressure boundary

    structures, systems and components. However, the importance of NDT is often not

    realized until serious accidents occur that lead to personal injury and/or economic losses.

    A single major accident in the field could seriously damage the reputation of the

    organization responsible for the product and/or testing service. On the other hand,

    undesirable quality incidents in the manufacturing plants or research laboratories could

    also lead to costly rework and delayed shipments.

    “Quality Assurance in NDT” means all those planned and systematic actions needed to

    provide adequate confidence to the customers, regulators and other stake holders that the

    NDT operations were performed and documented in accordance with the specified

    requirements.

    There are two major factors that influence ‘Quality Assurance in NDT’- the qualified

    NDT personnel who perform the tests, and the Management oversight on NDT

    operations.

    The purpose of this Paper is to review the key elements of Quality Assurance (QA)

    relevant to NDT and emphasize the role of the Management in implementing a Quality

    Management System (QMS) that focuses on NDT. It is beyond the scope of this Paper to

    discuss the technical details of the essential variables that affect the reliability of NDT.

    The Role of the NDT Operator in Quality Assurance

    Despite the automation of several NDT techniques, NDT technology today is very much

    operator dependant. There is no doubt that the reliability of the test results depends on

    the experience, knowledge and skill of the NDT Operator. The testing environment,

    accessibility and other site conditions during the test have major influence on the operator

    performance. Therefore, the ‘human factor’ plays a major role in the reliability of NDT.

    From this point of view, it is possible that a critical defect might be missed even by an

    experienced and skilled NDT operator. On the other hand, the root cause of a component

    failure could be traced to the inadequate NDT technique applied, poor sensitivity of the

    test setup or incorrect procedure employed during the test, which are operator dependent.

    Also, there are several important essential variables that affect the test results. For

    example, the test sensitivity and resolution are the most important parameters, which

    depend on the capability of the equipment, selection of the appropriate techniques,

    calibration standards, surface preparation and the procedure employed. Such factors can

    be controlled by a qualified NDT Operator to the best of his ability by applying the

    correct techniques and following the approved procedures. Hence, NDT Operator is

    taken into consideration while determining the Probability of Detection (POD) during the

    qualification of the Ultrasonic Testing (UT) examination system.

    Thus the NDT Operator plays a key role in the reliability of the test results. It is the

    responsibility of the qualified NDT Operator to follow the Code of Ethics and apply

    his/her technical knowledge and skills to ensure the best results possible out of the tests.

    Non-destructive testing is one of the most important means to detect and verify the quality of items. As an organization providing NDT services, it is essential to assure the correctness and accuracy of the test results. To assure the quality of the NDT inspections, it is necessary to have not only the advanced equipments and material, but also well trained and qualified inspectors and good inspection procedures, as well as adequate attentions paid to the environmental impacts on the inspection results. The article describes the practices of the quality assurance for the non-destructive testing service organization by carrying out ISO 9000 quality performance in its service activities.

    It seems to be the trend that the non-destructive testing of material, semi- and final products or installed components are designated to the special servicing companies providing NDT services other than the manufacturer or the owner himself to do the work. Therefore special NDT servicing companies are becoming popular. It is the responsibility for the service company to provide reliable testing and provide confidence to the customer of its good quality of test results. The practices of SNPI, Suzhou Nuclear Power Research Institute, which provides NDT services, shows that quality assurance provide us an effective tool to reach this goal.

    The main objective of a NDT service organization is to meet the requirements of customers and regulatory bodies for the safety of industry applications, for example, pressure vessels, electrical components. The customer’s requirements are normally related to the high quality of the NDT service, i.e. accurate and reliable test results, good service, as well as low cost. The regulatory requirements are normally related to the test method, personnel qualification and classification of test results. The practices of SNPI is to set up and maintain a quality system with reference to ISO 9000 family standards, so that a good quality culture are created and maintained by the whole staff within the institute.

    The quality system is maintained so that all the customer demands and regulatory requirements are satisfied during all our site applications of non-destructive testing activities.

    3.0 Quality assurance measures for NDT activities

    All aspects required by ISO 9001 standard are controlled by the quality system, which includes the organizational structure, procedures, processes and resources for implementing quality management. The main idea of setting up ISO 9001 quality system is to prevent occurrence of any non-conformance through various of preventive measures. Despite of different kinds of such systematic measures, the main QA measures for controlling site NDT activities are: controlling NDT inspector qualifications, validness of test equipment and test material, effectiveness of test procedures, impact of environmental conditions to test results, and monitoring actual NDT performance by quality surveillance inspectors.

    3.1 Personnel qualifications
    Inspector qualification is essential to a reliable inspection, and it is also mandatory required by regulatory bodies. China has such a NDT personnel qualification system that each industrial sector has its specific requirements in addition to the basic NDT society requirements. Therefore, in order to fulfill different requirements for different industrial applications, our NDT personnel hold different qualification certificates, including foreign certificates, to satisfy different customers for different requirements. Although much expenses are spent for the training and qualifications, our NDT personnel are well qualified in such a way.

    3.2 Test equipment and material
    Test equipment and material are very important for NDT inspections. As an inspection service organization, we developed and purchased all necessary equipment and materials for our services, including ultrasonic testing, radiographic testing, eddy current testing, magnetic particle testing, liquid penetrate testing, and visual testing as well as acoustic emission testing. These equipment and materials are mainly used in power generating industry and chemical industry for pressure vessel and related component inspections.

    Periodic calibration of test equipments are carried out and documented so that their accuracy and tolerance are identified and controlled.

    Test materials are controlled so that they are valid for the tests, and have no harmful to the component tested, which is extremely important to stainless steel component testing. For example, when stainless steel components for nuclear power plants are tested by ultrasonic testing method, the coupling material should contain very little amount of chlorinate, sulphate, phosphate materials, and should be qualified.

    3.3 Test procedure
    Test procedures define the responsibilities, techniques, operational steps and control means for specific method of testing and test objects. SNPI establishes a series of test procedures for different tasks, which we consider is necessary for carrying out quality services of testings.

    3.4 Environmental conditions
    Environmental conditions affect the test results seriously on some occasions. Attention should be taken for the temperature, humidity, radioactive, and/or surrounding materials to the test objects. For example, the ultrasonic transmission speed in the steel changes about 8 m/s when the temperature varies 1°; radioactive background of test objects affect the radiographic testing results.

    3.5 Monitoring of NDT processes
    As an important quality assurance measure, quality surveillance inspectors are assigned to verify the effectiveness of actual performance of NDT activities. The surveillance inspectors have very good background knowledge of NDT, they check the prerequisite of testing for a specific task, including NDT examiner’s personnel qualification, status of test equipment and materials to be used, test procedures adapted, and the environmental conditions presented. They make sample checks to major test performance to verify if the test procedures are followed by the NDT examiners. They check also the test results to verify if the results are correct and the regulatory requirements are met.

    Of course, the responsibility of testing is still on NDT examiner shoulders; the quality surveillance inspector is responsible for the verifications. Such verifications are normally carried out by means of implementation of witness and hold points inspections of quality plans for NDT testing.

    high praise of the quality provided and reached as well as the quality management system implemented.

    All feedback information are analyzed and used for continuous improvement of our quality of NDT services and our quality management system.

    5.0 Conclusion

    Quality management system should be set up and improved continuously to assure the quality of NDT services. Effective quality assurance measures should be taken to prevent any non-conformance occurring during NDT activities.

    Basic Principles of Ultrasonic Testing

    Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. To illustrate the general inspection principle, a typical pulse/echo inspection configuration as illustrated below will be used.

    A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

    Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include:

    • It is
      sensitive to both surface and subsurface discontinuities.
    • The
      depth of penetration for flaw detection or measurement is superior to
      other NDT methods.
    • Only
      single-sided access is needed when the pulse-echo technique is used.
    • It is
      highly accurate in determining reflector position and estimating size and
      shape.
    • Minimal
      part preparation is required.
    • Electronic
      equipment provides instantaneous results.
    • Detailed
      images can be produced with automated systems.
    • It
      has other uses, such as thickness measurement, in addition to flaw
      detection.

    As with all NDT methods, ultrasonic inspection also has its limitations, which include:

    • Surface
      must be accessible to transmit ultrasound.
    • Skill
      and training is more extensive than with some other methods.
    • It
      normally requires a coupling medium to promote the transfer of sound
      energy into the test specimen.
    • Materials
      that are rough, irregular in shape, very small, exceptionally thin or not
      homogeneous are difficult to inspect.
    • Cast
      iron and other coarse grained materials are difficult to inspect due to
      low sound transmission and high signal noise.
    • Linear
      defects oriented parallel to the sound beam may go undetected.
    • Reference
      standards are required for both equipment calibration and the
      characterization of flaws.

    A customer (also known as a client, buyer, or purchaser) is usually used to refer to a current or potential buyer or user of the products of an individual or organization, called the supplier, seller, or vendor. This is typically through purchasing or renting goods or services. However, in certain contexts, the term customer also includes by extension any entity that uses or experiences the services of another. A customer may also be a viewer of the product or service that is being sold despite deciding not to buy them. The general distinction between a customer and a client is that a customer purchases products whereas a client purchases services

    Queueing theory is the mathematical study of waiting lines, or queues. The theory enables mathematical analysis of several related processes, including arriving at the (back of the) queue, waiting in the queue (essentially a storage process), and being served at the front of the queue. The theory permits the derivation and calculation of several performance measures including the average waiting time in the queue or the system, the expected number waiting or receiving service, and the probability of encountering the system in certain states, such as empty, full, having an available server or having to wait a certain time to be served.

    Queueing theory has applications in diverse fields,[1] including telecommunications[2], traffic engineering, computing[3] and the design of factories, shops, offices and hospitals.[4]

    Queueing theory is generally considered a branch of operations research because the results are often used when making business decisions about the resources needed to provide service. It is applicable in a wide variety of situations that may be encountered in business, commerce, industry, healthcare,[5] public service and engineering. Applications are frequently encountered in customer service situations as well as transport and telecommunication. Queueing theory is directly applicable to intelligent transportation systems, call centers, PABXs, networks, telecommunications, server queueing, mainframe computer queueing of telecommunications terminals, advanced telecommunications systems, and traffic flow.

    Networks of queues are systems which contain an arbitrary, but finite, number m of queues. Customers, sometimes of different classes,[11] travel through the network and are served at the nodes. The state of a network can be described by a vector , where ki is the number of customers at queue i. In open networks, customers can join and leave the system, whereas in closed networks the total number of customers within the system remains fixed.

    Limitations of queueing theory

    The assumptions of classical queueing theory may be too restrictive to be able to model real-world situations exactly. The complexity of production lines with product-specific characteristics cannot be handled with those models. Therefore specialized tools have been developed to simulate, analyze, visualize and optimize time dynamic queueing line behavior.

    For example; the mathematical models often assume infinite numbers of customers, infinite queue capacity, or no bounds on inter-arrival or service times, when it is quite apparent that these bounds must exist in reality. Often, although the bounds do exist, they can be safely ignored because the differences between the real-world and theory is not statistically significant, as the probability that such boundary situations might occur is remote compared to the expected normal situation. Furthermore, several studies show the robustness of queueing models outside their assumptions. In other cases the theoretical solution may either prove intractable or insufficiently informative to be useful.

    Alternative means of analysis have thus been devised in order to provide some insight into problems that do not fall under the scope of queueing theory, although they are often scenario-specific because they generally consist of computer simulations or analysis of experimental data. See network traffic simulation.

    Monte Carlo methods (or Monte Carlo experiments) are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in simulating physical and mathematical systems. Because of their reliance on repeated computation of random or pseudo-random numbers, these methods are most suited to calculation by a computer and tend to be used when it is unfeasible or impossible to compute an exact result with a deterministic algorithm.[1]

    Monte Carlo simulation methods are especially useful in studying systems with a large number of coupled degrees of freedom, such as fluids, disordered materials, strongly coupled solids, and cellular structures (see cellular Potts model). More broadly, Monte Carlo methods are useful for modeling phenomena with significant uncertainty in inputs, such as the calculation of risk in business. These methods are also widely used in mathematics: a classic use is for the evaluation of definite integrals, particularly multidimensional integrals with complicated boundary conditions. It is a widely successful method in risk analysis when compared with alternative methods or human intuition. When Monte Carlo simulations have been applied in space exploration and oil exploration, actual observations of failures, cost overruns and schedule overruns are routinely better predicted by the simulations than by human intuition or alternative “soft” methods.[2]

    In statistics, signal processing, econometrics and mathematical finance, a time series is a sequence of data points, measured typically at successive times spaced at uniform time intervals. Examples of time series are the daily closing value of the Dow Jones index or the annual flow volume of the Nile River at Aswan. Time series analysis comprises methods for analyzing time series data in order to extract meaningful statistics and other characteristics of the data. Time series forecasting is the use of a model to forecast future events based on known past events: to predict data points before they are measured. An example of time series forecasting in econometrics is predicting the opening price of a stock based on its past performance. Time series are very frequently plotted via line charts.

    Time series data have a natural temporal ordering. This makes time series analysis distinct from other common data analysis problems, in which there is no natural ordering of the observations (e.g. explaining people’s wages by reference to their education level, where the individuals’ data could be entered in any order). Time series analysis is also distinct from spatial data analysis where the observations typically relate to geographical locations (e.g. accounting for house prices by the location as well as the intrinsic characteristics of the houses). A time series model will generally reflect the fact that observations close together in time will be more closely related than observations further apart. In addition, time series models will often make use of the natural one-way ordering of time so that values for a given period will be expressed as deriving in some way from past values, rather than from future values (see time reversibility.)

    Methods for time series analyses may be divided into two classes: frequency-domain methods and time-domain methods. The former include spectral analysis and recently wavelet analysis; the latter include auto-correlation and cross-correlation analysis

    Abrasion Resistance

    The ability of a material to withstand mechanical action such as rubbing, scraping, or erosion, that tends progressively to remove material from its surface. Such an ability helps to maintain the material’s original appearance and structure.

    Forge welding is a solid-state welding process[1] that joins two pieces of metal by heating them to a high temperature and then hammering them together.[2] The process is one of the simplest methods of joining metals and has been used since ancient times. Forge welding is versatile, being able to join a host of similar and dissimilar metals. With the invention of electrical and gas welding methods during the Industrial Revolution, forge welding has been largely replaced.

    Forge welding between similar materials is caused by solid-state diffusion. This results in a weld that consists of only the welded materials without any fillers or bridging materials.

    Forge welding between dissimilar materials is caused by the formation of a lower melting temperature eutectic between the materials. Due to this the weld is often stronger than the individual metals.

    The temperature required to forge weld is typically 50 to 90 percent of the melting temperature. Steel welds at a lower temperature than iron. The metal may take on a glossy or wet appearance at the welding temperature. Care must be taken to avoid overheating the metal to the point that it gives off sparks from rapid oxidation (burning).

    Acoustic
    emission

    Acoustic Emission (AE) is a naturally occurring phenomenon whereby external stimuli, such as mechanical loading, generate sources of elastic waves. AE occurs when a small surface displacement of a material is produced. This occurs due to stress waves generated when there is a rapid release of energy in a material, or on its surface. The wave generated by the AE source, or, of practical interest, in methods used to stimulate and capture AE in a controlled fashion for study and/or use in inspection, quality control, system feedback, process monitoring and others.

    Acoustic Emission phenomena

    AEs are commonly defined as transient elastic waves within a material caused by the release of localized stress energy. Hence, an event source is the phenomenon which releases elastic energy into the material, which then propagates as an elastic wave. Acoustic emissions can be detected in frequency ranges under 1 kHz, and have been reported at frequencies up to 100 MHz. Rapid stress-releasing events generate a spectrum of stress waves starting at 0 Hz and typically falling off at several MHz.

    AE is related to an irreversible release of energy, and can be generated from sources not involving material failure including friction, cavitation and impact. Additionally, events can also come quite rapidly when materials begin to fail, in which case AE activity rates are studied as opposed to individual events. AE events that are commonly studied among material failure processes include the extension of a fatigue crack, or fiber breakage in a composite material.

    Use in Non-destructive testing

    The application of AE to non-destructive testing of materials in the ultrasonic regime, typically takes place between 100 kHz and 1 MHz. Unlike conventional ultrasonic testing, AE tools are designed for monitoring acoustic emissions produced within the material during failure, rather than actively transmitting waves then collecting them after they have traveled through the material.[1] Part failure can be documented during unattended monitoring. The monitoring of the level of AE activity during multiple load cycles forms the basis for many AE safety inspection methods that allow the parts undergoing inspection to remain in service.

    The technique is used, for example, to study the formation of cracks during the welding process, as opposed to locating them after the weld has been formed with the more familiar ultrasonic testing technique.[1] In a material under active stress, such as some components of an airplane during flight, transducers mounted in an area can detect the formation of a crack at the moment it begins propagating. . [1] A group of transducers can be used to record signals then locate the precise area of their origin by measuring the time for the sound to reach different transducers.[1] The technique is also valuable for detecting cracks forming in pipelines transporting liquids under high pressures.[1] Also this technique is used for estimation of corrosion in reinforced concrete structures.[2]

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