Nondestructive testing

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The need for NDT

It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.

During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:

• Aircraft skins need regular checking to detect cracks;
• Underground pipelines are subject to corrosion and stress corrosion cracking;
• Pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
• Reinforced concrete structures may be weakened if the inner reinforcing steel is corroded;
• Pressure vessels may develop cracks in welds;
• The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.

Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing.

Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique.

Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.

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.
• 1895 Wilhelm Conrad Röntgen discovers what are now known as X-rays. In his first paper he discusses the possibility of flaw detection.
• 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.
• 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 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.

• Automotive
  • Engine parts
  • Frame
• Aviation / Aerospace
  • Airframes
    • Spaceframes
  • Powerplants
    • Propellers
    • Reciprocating Engines
    • Gas Turbine Engines
      • Rocketry
• Construction
  • Structures
  • Bridges
• Maintenance
  • Bridges
• Manufacturing
  • Machine parts
  • Castings and Forgings
• 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
• Miscellaneous
  • Pipelines
    • In-line Inspection using "pigs"
    • Pipeline integrity
    • Leak Detection
  • 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

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.

An example of Ultrasonic Testing (UT) on blade roots of a V2500 IAE aircraft engine.
Step 1: The UT probe is placed on the root of the blades to be inspected with the help of a special borescope tool (video probe).
Step 2: Instrument settings are input.
Step 3: The probe is scanned over the blade root. In this case, an indication (peak in the data) through the red line (or gate) indicates a good blade; an indication to the left of that range indicates a crack.

RepliSet: 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.

• Liquid penetrant testing (PT or LPI)
• Radiographic testing (RT) (see also Industrial radiography and Radiography)
  • Digital radiography (real-time)
  • Computed radiography
  • SCAR (Small Confined Area Radiography)
  • Neutron radiographic testing (NR)
  • Computed tomography (CT)
• Impulse excitation technique (IET)
• Ultrasonic testing (UT)
  • Phased array ultrasonics
  • Time of flight diffraction ultrasonics (TOFD)
  • Time of Flight Ultrasonic Determination of 3D Elastic Constants (TOF)
  • Internal Rotary Inspection System (IRIS) ultrasonics for tubes
  • EMAT Electromagnetic Acoustic Transducer (non-contact)
  • laser ultrasonics (LUT)
• Electromagnetic testing (ET)
  • Alternating Current Field Measurement (ACFM)
  • Alternating Current potential drop measurement (ACPD)
  • Direct Current potential drop measurement (DCPD)
  • Eddy-Current Testing (ECT)
  • Remote field testing (RFT)
  • Magnetic-particle inspection (MT or MPI)
  • Magnetic flux leakage testing (MFL) for pipelines, tank floors, and wire rope
  • Barkhausen testing
• Acoustic emission testing (AE)
• Positive Material Identification (PMI)
• Hardness testing (Brinell) (HT)
• Infrared and thermal testing (IR)
  • Thermographic inspection
• Laser testing
  • Profilometry
  • Holographic interferometry
  • Electronic Speckle Pattern Interferometry
  • Shearography
• Leak testing or Leak detection (LT)
  • Tracer-gas method testing Helium, Hydrogen and refrigerant gases
  • Bubble testing
  • Absolute pressure leak testing (pressure change)
  • Halogen diode leak testing
  • Mass spectrometer leak testing
• Magnetic resonance imaging and NMR spectroscopy
• Ellipsometry
• Visual inspection (VT)
  • Pipeline video inspection

Terminology

Indication 

The response or evidence from an examination, such as a blip on the screen of an instrument.

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.

(Source: ASTM E1316 in 'Vol. 03.03 NDT)

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. [2] [3].

Further reading

• Bray, D.E. and R.K. Stanley, 1997, Nondestructive Evaluation: A Tool for Design, Manufacturing and Service; CRC Press, 1996.
• Chuck Hellier, Handbook of Nondestructive Evaluation, McGraw-Hill Professional; 2001
• Peter J. Shull, Nondestructive Evaluation: Theory, Techniques, and Applications, Marcel Dekker Inc., 2002.
• ASTM International, Annual Book of ASTM Standards Volume 03.03 Nondestructive Testing
• ASNT, Nondestructive Testing Handbook

See also