Nondestructive testing or Non-destructive testing (NDT) is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage.^[1] The terms Nondestructive examination (NDE), Nondestructive inspection (NDI), and Nondestructive evaluation (NDE) are also commonly used to describe this technology.^[2] Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both money and time in product evaluation, troubleshooting, and research. Common NDT methods include ultrasonic, magnetic-particle, liquid penetrant, radiographic, remote visual inspection (RVI), eddy-current testing,^[1] and low coherence interferometry^[3] .^[4] NDT is a commonly-used tool in forensic engineering, mechanical engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and art.^[1]

  Methods

NDT methods may rely upon use of electromagnetic radiation, sound, and inherent properties of materials to examine samples. This includes some kinds of microscopy to examine external surfaces in detail, although sample preparation techniques for metallography, optical microscopy and electron microscopy are generally destructive as the surfaces must be made smooth through polishing or the sample must be electron transparent in thickness. The inside of a sample can be examined with penetrating electromagnetic radiation, such as X-rays or 3D X-rays for volumetric inspection. Sound waves are utilized in the case of ultrasonic testing. Contrast between a defect and the bulk of the sample may be enhanced for visual examination by the unaided eye by using liquids to penetrate fatigue cracks. One method (liquid penetrant testing) involves using dyes, fluorescent or non-fluorescing, in fluids for non-magnetic materials, usually metals. Another commonly used method for magnetic materials involves using a liquid suspension of fine iron particles applied to a part while it is in an externally applied magnetic field (magnetic-particle testing). Thermoelectric effect (or use of the Seebeck effect) uses thermal properties of an alloy to quickly and easily characterize many alloys. The chemical test, or chemical spot test method, utilizes application of sensitive chemicals that can indicate the presence of individual alloying elements.

  Applications

  Weld verification

  1. Section of material with a surface-breaking crack that is not visible to the naked eye.
2. Penetrant is applied to the surface.
3. Excess penetrant is removed.
4. Developer is applied, rendering the crack visible.

In manufacturing, welds are commonly used to join two or more metal surfaces. Because these connections may encounter loads and fatigue during product lifetime, there is a chance that they may fail if not created to proper specification. For example, the base metal must reach a certain temperature during the welding process, must cool at a specific rate, and must be welded with compatible materials or the joint may not be strong enough to hold the surfaces together, or cracks may form in the weld causing it to fail. The typical welding defects, lack of fusion of the weld to the base metal, cracks or porosity inside the weld, and variations in weld density, could cause a structure to break or a pipeline to rupture.

Welds may be tested using NDT techniques such as industrial radiography or industrial CT scanning using X-rays or gamma rays, ultrasonic testing, liquid penetrant testing or via eddy current. In a proper weld, these tests would indicate a lack of cracks in the radiograph, show clear passage of sound through the weld and back, or indicate a clear surface without penetrant captured in cracks.

Welding techniques may also be actively monitored with acoustic emission techniques before production to design the best set of parameters to use to properly join two materials.^[5]

  Structural mechanics

Structures can be complex systems that undergo different loads during their lifetime. Some complex structures, such as the turbomachinery in a liquid-fuel rocket, can also cost millions of dollars. Engineers will commonly model these structures as coupled second-order systems, approximating dynamic structure components with springs, masses, and dampers. These sets of differential equations can be used to derive a transfer function that models the behavior of the system.

In NDT, the structure undergoes a dynamic input, such as the tap of a hammer or a controlled impulse. Key properties, such as displacement or acceleration at different points of the structure, are measured as the corresponding output. This output is recorded and compared to the corresponding output given by the transfer function and the known input. Differences may indicate an inappropriate model (which may alert engineers to unpredicted instabilities or performance outside of tolerances), failed components, or an inadequate control system.

  Radiography in medicine

  Chest radiography indicating a peripheral bronchial carcinoma.

As a system, the human body is difficult to model as a complete transfer function. Elements of the body, however, such as bones or molecules, have a known response to certain radiographic inputs, such as x-rays or magnetic resonance. Coupled with the controlled introduction of a known element, such as digested barium, radiography can be used to image parts or functions of the body by measuring and interpreting the response to the radiographic input. In this manner, many bone fractures and diseases may be detected and localized in preparation for treatment. X-rays may also be used to examine the interior of mechanical systems in manufacturing using NDT techniques, as well.

  Notable events in early industrial NDT

• 1854 Hartford, Connecticut: a boiler at the Fales and Gray Car works explodes, killing 21 people and seriously injuring 50. Within a decade, the State of Connecticut passes a law requiring annual inspection (in this case visual) of boilers.
• 1880 - 1920 The "Oil and Whiting" method of crack detection is used in the railroad industry to find cracks in heavy steel parts. (A part is soaked in thinned oil, then painted with a white coating that dries to a powder. Oil seeping out from cracks turns the white powder brown, allowing the cracks to be detected.) This was the precursor to modern liquid penetrant tests.
• 1895 Wilhelm Conrad Röntgen discovers what are now known as X-rays. In his first paper he discusses the possibility of flaw detection.
• 1920 Dr. H. H. Lester begins development of industrial radiography for metals.
• 1924 — Lester uses radiography to examine castings to be installed in a Boston Edison Company steam pressure power plant [1].
• 1926 The first electromagnetic eddy current instrument is available to measure material thicknesses.
• 1927 - 1928 Magnetic induction system to detect flaws in railroad track developed by Dr. Elmer Sperry and H.C. Drake.
• 1929 Magnetic particle methods and equipment pioneered (A.V. DeForest and F.B. Doane.)
• 1930s Robert F. Mehl demonstrates radiographic imaging using gamma radiation from Radium, which can examine thicker components than the low-energy X-ray machines available at the time.
• 1935 - 1940 Liquid penetrant tests developed (Betz, Doane, and DeForest)
• 1935 - 1940s Eddy current instruments developed (H.C. Knerr, C. Farrow, Theo Zuschlag, and Fr. F. Foerster).
• 1940 - 1944 Ultrasonic test method developed in USA by Dr. Floyd Firestone.
• 1950 The Schmidt Hammer (also known as "Swiss Hammer") is invented. The instrument uses the world’s first patented non-destructive testing method for concrete.
• 1950 J. Kaiser introduces acoustic emission as an NDT method.

(Source: Hellier, 2001) Note the number of advancements made during the WWII era, a time when industrial quality control was growing in importance.

  Applications

NDT is used in a variety of settings that covers a wide range of industrial activity, with new NDT methods and applications, being continuously developed. NDT services ^[6] are not only integrated with Asset Integrity Management (AIM) solutions, but also with Material Testing laboratories and seamlessly fit into Supply Chain services.

• [Rehabilitation].
• Automotive
  • Engine parts
  • Frame
• Aviation / Aerospace
  • Airframes
    • Spaceframes
  • Powerplants
    • Propellers
    • Reciprocating Engines
    • Gas turbine engines
  • Rocketry
• Construction
  • Structures
  • Bridges
  • Cover Meter
• Maintenance, repair and operations
  • Bridges
• Manufacturing
  • Machine parts
  • Castings and Forgings
  • Fabrication Inspection
• Industrial plants such as Nuclear, Petrochemical, Power, Refineries, Pulp and Paper, Fabrication shops, Mine processing and their Risk Based Inspection programmes.
  • Pressure vessels
  • Storage tanks
  • Welds
  • Boilers
  • Heat exchangers
  • Turbine bores
  • In-plant Piping
  • Full Storage tank Assessment
  • Shutdown Inspections
  • In-service Equipment Inspections
• Miscellaneous
  • Pipelines
    • In-line Inspection using "pigs"
    • Pipeline integrity management
    • Leak Detection
    • Pipeline Open Data Standard
    • ASME Pressure Vessel and Piping as-built Inspections
    • Piping and Pressure Vessel Corrosion Monitoring
  • Railways
    • Rail Inspection
    • Wheel Inspection
  • Tubular NDT, for Tubing material
  • Corrosion Under Insulation (CUI)
  • Amusement park rides
  • Submarines and other Naval warships
  • Medical imaging applications (see also Medical physics)

  Methods and techniques

  An example of a 3D replicating technique. The flexible high-resolution replicas allow surfaces to be examined and measured under laboratory conditions. A replica can be taken from all solid materials.

NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.

• Acoustic emission testing (AE or AT)
• Blue Etch Anodize (BEA)
• Dye penetrant inspection Liquid penetrant testing (PT or LPI)
• Electromagnetic testing (ET)
  • Alternating current field measurement (ACFM)
  • Alternating current potential drop measurement (ACPD)
  • Barkhausen testing
  • Direct current potential drop measurement (DCPD)
  • Eddy-current testing (ECT)
  • Magnetic flux leakage testing (MFL) for pipelines, tank floors, and wire rope
  • Magnetic-particle inspection (MT or MPI)
  • Remote field testing (RFT)
• Ellipsometry
• Guided wave testing (GWT)
• Hardness testing
• Impulse excitation technique (IET)
• Infrared and thermal testing (IR)
  • Thermographic inspection
• Laser testing
  • Electronic speckle pattern interferometry
  • Holographic interferometry
  • Low coherence interferometry
  • Profilometry
  • Shearography
• Leak testing (LT) or Leak detection
  • Absolute pressure leak testing (pressure change)
  • Bubble testing
  • Halogen diode leak testing
  • Hydrogen leak testing
  • Mass spectrometer leak testing
  • Tracer-gas leak testing method Helium, Hydrogen and refrigerant gases
• Magnetic resonance imaging (MRI) and NMR spectroscopy
• Metallographic replicas ^{[7] [8]}
• Near-infrared spectroscopy (NIRS)
• Optical microscopy
• Positive Material Identification (PMI)
• Radiographic testing (RT) (see also Industrial radiography and Radiography)
  • Computed radiography
  • Digital radiography (real-time)
  • Neutron radiographic testing (NR)
  • SCAR (Small Controlled Area Radiography)
  • X-ray computed tomography (CT)
• Scanning electron microscopy
• Surface Temper Etch (Nital Etch)
• Ultrasonic testing (UT)
  • ART (Acoustic Resonance Technology)
  • Electro Magnetic Acoustic Transducer (EMAT) (non-contact)
  • Laser ultrasonics (LUT)
  • Internal rotary inspection system (IRIS) ultrasonics for tubes
  • Phased array ultrasonics
  • Time of flight diffraction ultrasonics (TOFD)
  • Time of Flight Ultrasonic Determination of 3D Elastic Constants (TOF)
• Vibration Analysis
• Visual inspection (VT)
  • Pipeline video inspection
• Corroscan/C-scan
• IRIS - Internal Rotary Inspection System
• 3D Computed Tomography
  • Industrial CT Scanning
• Heat Exchanger Life Assessment System
• RTJ Flange Special Ultrasonic Testing

  Personnel training, qualification and certification

Successful and consistent application of nondestructive testing techniques depends heavily on personnel training, experience and integrity. Personnel involved in application of industrial NDT methods and interpretation of results should be certified, and in some industrial sectors certification is enforced by law or by the applied codes and standards.

  Definitions

The following definitions for qualification and certification are given in ISO 9712^[9] and EN 473^[10]:

• Certification: "Procedure, used by the certification body to confirm that the qualification requirements for a method, level and sector have been fulfilled, leading to the issuing of a certificate".

• Qualification: "Demonstration of physical attributes, knowledge, skill, training and experience required to properly perform NDT tasks".

In US standards and codes, while a very similar definition of qualification is included in ASNT SNT-TC-1A, certification is simply defined as: "Written testimony of qualification".

  Training

Non-Destructive Testing (NDT) training is provided for people working in many industries. It is generally necessary that the candidate successfully completes a theoretical and practical training program, as well as have performed several hundred hours of practical application of the particular method they wish to be trained in. At this point, they may pass a certification examination.^[11] Further, NDT training has recently become available online. WorldSpec.org is one of the innovative companies that helped pioneer this new "era" in NDT Training.

  Certification schemes

There are two approaches in personnel certification:^[12]

1. Employer Based Certification: Under this concept the employer compiles their own Written Practice. The written practice defines the responsibilities of each level of certification, as implemented by the company, and describes the training, experience and examination requirements for each level of certification. In industrial sectors the written practices are usually based on recommended practice SNT-TC-1A of the American Society for Nondestructive Testing.^[13] ANSI standard CP-189 outlines requirements for any written practice that conforms to the standard.^[14]
2. Personal Central Certification: The concept of central certification is that an NDT operator can obtain certification from a central certification authority, that is recognized by most employers, third parties and/or government authorities. Industrial standards for central certification schemes include ISO 9712,^[9] EN 473.^[10] and ACCP.^[15] Certification under these standards involves training, work experience under supervision and passing a written and practical examination set up by the independent certification authority.

In the United States employer based schemes are the norm, however central certification schemes exist as well. The most notable is ASNT Level III (established in 1976-1977), which is organized by the American Society for Nondestructive Testing for Level 3 NDT personnel.^[16] NAVSEA 250-1500 is another US central certification scheme, specifically developed for use in the naval nuclear program.^[17]

Central certification is more widely used in the European Union, where certifications are issued by accredited bodies (independent organizations conforming to ISO 17024 and accredited by a national accreditation authority like UKAS). The Pressure Equipment Directive (97/23/EEC) actually enforces central personnel certification for the initial testing of steam boilers and some categories of pressure vessels and piping.^[18] European Standards harmonized with this directive specify personnel certification to EN 473. Certifications issued by a national NDT society which is a member of the European Federation of NDT (EFNDT) are mutually acceptable by the other member societies ^[19] under a multilateral recognition agreement.

Canada also implements an ISO 9712 central certification scheme, which is administered by Natural Resources Canada, a government department.^[20][21][22]

The aerospace sector worldwide sticks to employer based schemes.^[23] In America it is based mostly on AIA-NAS-410 ^[24] and in the European Union on the equivalent and very similar standard EN 4179 ^[25]

  Levels of certification

Most NDT personnel certification schemes listed above specify three "levels" of qualification and/or certification, usually designated as Level 1, Level 2 and Level 3 (although some codes specify roman numerals, like Level II). The roles and responsibilities of personnel in each level are generally as follows (there are slight differences or variations between different codes and standards):

• Level 1 are technicians qualified to perform only specific calibrations and tests under close supervision and direction by higher level personnel. They can only report test results. Normally they work following specific work instructions for testing procedures and rejection criteria.

• Level 2 are engineers or experienced technicians who are able to set up and calibrate testing equipment, conduct the inspection according to codes and standards (instead of following work instructions) and compile work instructions for Level 1 technicians. They are also authorized to report, interpret, evaluate and document testing results. They can also supervise and train Level 1 technicians. In addition to testing methods, they must be familiar with applicable codes and standards and have some knowledge of the manufacture and service of tested products.

• Level 3 are usually specialized engineers or very experienced technicians. They can establish NDT techniques and procedures and interpret codes and standards. They also direct NDT laboratories and have central role in personnel certification. They are expected to have wider knowledge covering materials, fabrication and product technology.

  Terminology

The standard US terminology for Nondestructive testing is defined in standard ASTM E-1316.^[26] Some definitions may be different in European standard EN 1330.

Indication 

The response or evidence from an examination, such as a blip on the screen of an instrument. Indications are classified as true or false. False indications are those caused by factors not related to the principles of the testing method or by improper implementation of the method, like film damage in radiography, electrical interference in ultrasonic testing etc. True indications are further classified as relevant and non relevant. Relevant indications are those caused by flaws. Non relevant indications are those caused by known features of the tested object, like gaps, threads, case hardening etc.

Interpretation 

Determining if an indication is of a type to be investigated. For example, in electromagnetic testing, indications from metal loss are considered flaws because they should usually be investigated, but indications due to variations in the material properties may be harmless and nonrelevant.

Flaw 

A type of discontinuity that must be investigated to see if it is rejectable. For example, porosity in a weld or metal loss.

Evaluation 

Determining if a flaw is rejectable. For example, is porosity in a weld larger than acceptable by code?

Defect 

A flaw that is rejectable — i.e. does not meet acceptance criteria. Defects are generally removed or repaired.^[26]

Penetrant testing 

Non-destructive test typically comprising a penetrant, a method of excess removal and a developer to produce a visible indication of surface-breaking discontinuities.^[27]

  Reliability and statistics

Defect detection tests are among the more commonly employed of non-destructive tests. The evaluation of NDT reliability commonly contains two statistical errors. First, most tests fail to define the objects that are called "sampling units" in statistics; it follows that the reliability of the tests cannot be established. Second, the literature usually misuses statistical terms in such a way as to make it sound as though sampling units are defined. These two errors may lead to incorrect estimates of probability of detection. ^{[28] [29]}

  See also

Underwater diving portal

  References

  Bibliography

  External links

Wikimedia Commons has media related to: Nondestructive testing

• The International Committee for Non-Destructive Testing (ICNDT)
• European Federation for Non-Destructive Testing (EFNDT)
• American Society for Nondestructive Testing
• Inspectioneering Journal, Trade Journal that publishes NDT news in the oil and gas and chemical industries
• NDT Services, http://www.udc.net/NDE-Boiler-Inspection.html
• NDT Education, University of Iowa Repository for Nondestructive Education Material
• NDT.net, Nondestructive Online Journal
• NDT.org NDT Jobs, News, and Social Network
• NDTWiki.com, NonDestructive Testing wiki for professionals
• SimplyNDT.com, NDT Employment, News, Blog Website
• "MIL-STD-271, Requirements for Nondestructive Testing Methods". United States Department of Defense. 27 June 1986. http://www.everyspec.com/MIL-STD/MIL-STD-0100-0299/MIL-STD-271F_21047/. 
• "TO 3B-1-1, Nondestructive Test Methods, Basic Theory, Technical Manual". United States Army. 15 June 2007. http://www.everyspec.com/USAF/USAF-Tech-Manuals/TO_33B-1-1_2007_5641/. 
• Non-Destructive Testing, http://www.sgs.com/~/media/Global/Documents/Brochures/SGS-IND-NDT-A4-EN-10.pdf