Triens

Liquid Penetrant Testing (PT)

Liquid Penetrant Testing is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw where a visible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used - fluorescent or non-fluorescent (visible).

Visible penetrant testing uses a penetrant that can be seen in visible light. The penetrant is usually red, so that resultant indications produce a definite contrast with the white background of the developer. Visible penetrant indications must be viewed under adequate white light.

Fluorescent penetrant testing utilizes penetrants that fluoresce brilliantly when excited by black light (UVA). The sensitivity of fluorescent penetrants depends on their ability to be retained in the various size discontinuities during processing, and then to bleed out into the developer coating and produce indications that will fluoresce. Fluorescent indications are many times brighter than their surroundings when viewed under appropriate black light illumination.

Advantages

  • The method has high sensitive to small surface discontinuities.
  • The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, conductive and nonconductive materials may be inspected.
  • Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.
  • Parts with complex geometric shapes are routinely inspected.
  • Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
  • Aerosol spray cans make penetrant materials very portable.
  • Penetrant materials and associated equipment are relatively inexpensive.

Disadvantages

  • Only surface breaking defects can be detected.
  • Only materials with a relative nonporous surface can be inspected.
  • Pre-cleaning is critical as contaminants can mask defects.
  • Metal smearing from machining, grinding and grit or vapor blasting must be removed prior to PT.
  • The inspector must have direct access to the surface being inspected.
  • Surface finish and roughness can affect inspection sensitivity.
  • Multiple process operations must be performed and controlled.
  • Post cleaning of acceptable parts or materials is required.
  • Chemical handling and proper disposal is required.

Magnetic Particle Testing (MT)

Magnetic particle testing is a method to perform nondestructive testing (NDT) of ferromagnetic material. Ferromagnetic is defined in ASME Section V as “a term applied to materials that can be magnetized or strongly attracted by a magnetic field.” MT is an NDT method that checks for surface discontinuities but can also reveal discontinuities slightly below the surface.

There are several methods of magnetizing the test parts.

A current flow method through contact heads, encircling coil magnetizing, threaded bard magnetizing are the examples of magnetizing methods on a magnetic particle bench. The most common method utilized in general industries is the magnetic flow method using electromagnetic yoke. The particles are often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light (black light). The test method using fluorescent-coated particles is called as Fluorescent Magnetic Particle Inspection or test (FMPI) and the usage of other coloured particles is termed as color contrast Magnetic Particle Inspection or test (MPI). Magnetic Particle Inspection (MPI) is the economical and comparative faster non-destructive test method used widely in Aerospace, Locomotive, automotive, power generation, nuclear, petrochemical industries. The most common examples are testing of crank shafts, cam shafts, connecting rods, engine gears, landing gear, bearing caps, engine blocks, motor shafts, engine bolts, nuts, washers, threaded bars, studs, piping joints (fabricated joints, welds) in power generation and petrochemical industries, etc. Magnetic particle test (MT) is very sensitive test method. It can detect tight in-service fatigue cracks in rotating parts or creepcracks on steam piping. Magnetic Particle Inspection cannot be used for non-ferrous materials and non-magnetic ferrous materials such as austenitic stainless steels.

Advantages

  • Test method process is quick and simple in principle and application
  • Highly sensitive to the detection of surface and slightly subsurface linear indications
  • Indications appear on the actual test part
  • Test method process may often work through several layers and coating thickness
  • The method lends itself to automation and high volume production inspection
  • Less expensive than other more sophisticated methods of quality assurance

Disadvantages

  • Test material must be ferrous
  • Provides limited and variable potential for detection of subsurface indications
  • Care is required to avoid burning and arcing of test part surface at points of electrical contact
  • The magnetic field direction must intercept the major dimension of the discontinuity
  • Complex test part geometry may sometimes pose problems with proper amperage determination and magnetic field intensity
  • Demagnetization of test part following the inspection is often necessary

Radiographic Testing (RT)

Radiography is used in a very wide range of applications including medicine, engineering, forensics, security, etc. In NDT, radiography is one of the most important and widely used methods. Radiographic testing (RT) offers a number of advantages over other NDT methods, however, one of its major disadvantages is the health risk associated with the radiation. In general, RT is method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials. The intensity of the radiation that penetrates and passes through the material is either captured by a radiation sensitive film (Film Radiography) or by a planer array of radiation sensitive sensors (Real-time Radiography). Some of the common uses are detection of surface and subsurface features of interest in welded parts, castings, forgings, wall thickness measurement, corrosion mapping, detection of blockages inside sealed equipment, detection of reinforcing material in concrete slabs, measuring bulk density of materials, measuring porosity in concrete, etc. The conventional film radiography is the most sensitive test method. On the contrary digital or computed radiography have certain advantages over conventional film radiography. Film radiography is the oldest approach, yet it is still the most widely used in NDT.

Advantages

  • Can easily locate internal structural discontinuities using visual comparison with known geometric features of the test object.
  • Is applicable to most classes of materials.
  • Is considered by many to be the most universal approach to volumetric examination.
  • Yields a visual rendition of internal voids and fabrication errors that is readily interpretable.
  • Provides a permanent record of the inspection, including evidence of the sensitivity of the test when the image quality indicators (IQIs) are used.
  • Is capable of detecting local changes in thickness and density on the order of 1%, as measured along the line of sight of the X-ray beam.
  • Can provide a digital record of the test object for subsequent display on a computer monitor.

Disadvantages

  • It is a relatively expensive method of nondestructive testing.
  • It is impractical to use on specimens of complex geometry.
  • Isolated local discontinuities, with in-line dimensions much less than 2% of the total thickness, are usually not detected.
  • The specimen must lend itself to two-sided accessibility.
  • Laminar type discontinuities are often undetected by radiography because they are aligned transverse to the path of radiation.
  • It requires highly trained and skilled personnel for evaluation.

Ultrasonic Testing (UT)

Ultrasonic testing is one of the more common non-destructive testing methods for detection of volume discontinues in materials. This testing utilizes high frequency mechanical energy i.e. sound waves, to conduct examinations and measurements on a test area.

Typically the UT inspection system consists of a transducer, pulser/receiver, and display unit. A pulser/receiver is an electronic device that can produce high voltage electrical pulses to the transducer. When driven by the pulser, the transducer generates high frequency ultrasonic sound energy into the material in the form of sound waves. When there are discontinuities such as inclusions, porosity, cracks, etc. in the sound path, part of the mechanical energy will be reflected from the discontinuities' (reflectors') surface. The reflected sound waves signal received by the transducer is then transformed back into an electrical signal and its intensity is shown on the display unit. The sound waves travel time can be directly related to the distance that the signal has travelled. From the signal, information about reflector location, size, orientation and other features can be determined.

Advantages

  • 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.
  • It provides instantaneous results.
  • Detailed images can be produced with automated systems.
  • It is nonhazardous to operators or nearby personnel and does not affect the material being tested.
  • It has other uses, such as thickness measurement, in addition to flaw detection.
  • Its equipment can be highly portable or highly automated.

Disadvantages

  • 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.

Eddy Current Testing (ET)

Eddy Current Testing is an electromagnetic technique used for conductive materials to detect surface and near surface defects. This inspection method is sensitive to small cracks and other flaws. It is used to inspect parts of complex shape and size, gives immediate results and is widely used within the commercial, military, petrochemical, aircraft and aerospace industries. In Eddy Current testing, a small coil of wire is excited by an alternating current (AC) signal and held in close proximity to the metallic object. The magnetic field generated by the coil of wire enters the metallic object and causes a small electrical current to flow inside the metal. These are called eddy currents. The eddy currents flowing in the metal will change their flow depending on the “material structure” of the metal. The presence of cracks or flaws will influence how the signals flow, as will differing heat treat conditions, case depth, alloy content, and physical properties. Comparing the eddy current flow in a metallic object “under test” to “known good” object will indicate whether there are flaws or a different material structure in the component. Testing at multiple frequencies facilitates the finding of structural defects.

Advantages

  • Sensitivity to surface defects. It is able to detect defects of 0.5mm in length under favorable conditions.
  • Can detect through several layers. It has the ability to detect defects in multi-layer structures, without interference from the planar interfaces.
  • Can detect through surface coatings. It is able to detect defects through non-conductive surface coatings in excess of 5mm thickness.
  • Accurate conductivity measurements. Dedicated conductivity measurement instruments operate using eddy currents.
  • Can be automated. Relatively uniform parts can be inspected quickly and reliably using automated or semi-automated equipment, e.g. wheels, boiler tubes and aero-engine disks.
  • Little pre-cleaning required. Only major soils and loose or uneven surface coatings need to be removed, reducing preparation time.
  • Portability. Portable test equipment is very small and light, some of the latest equipment being as small as a portable computer and weighing less than 2kg.

Disadvantages

  • It is very susceptible to magnetic permeability changes. Small changes in permeability have a pronounced effect on the eddy currents, especially in ferromagnetic materials. This makes testing of welds and other ferromagnetic materials difficult, but with modern digital flaw detectors and probe design, not impossible.
  • It is only effective on conductive materials. The material must be able to support a flow of electrical current.
  • Will not detect defects parallel to surface. The flow of eddy currents is always parallel to the surface. If a planar defect does not cross or interfere with the current then the defect will not be detected.
  • Not suitable for large areas and/or complex geometries. Large area scanning can be accomplished, but needs the aid of some type of area scanning device, usually supported by a computer, both of which are not inexpensive. The more complex the geometry becomes, the more difficult it is to differentiate defect signals from geometry effect signals.
  • Signal interpretation required. Due to the many factors, which affect eddy currents, careful interpretation of signals is needed to distinguish between relevant and non-relevant indications.
  • No permanent record (unless automated). Normally the only permanent record will be a paper print out or computer file when using automated systems.

Visual Testing (VT)

Visual testing is one of the most common and most effective means of non-destructive testing. Visual testing requires adequate illumination of the test surface and proper eyesight of the tester. To be most effective visual inspection does however, merit special attention because it requires training (knowledge of product and process, anticipated service conditions, acceptance criteria, record keeping, for example) and it has its own range of equipment and instrumentation. It is also a fact that all defects found by other NDT methods ultimately must be substantiated by visual inspection. VT can be classified as Direct visual testing, Remote visual testing and Translucent visual testing. Often the equipment needed is simple for internal inspection, light lens systems such as bore scopes allow remote surfaces to be examined. More sophisticated devices of this nature using fiber optics permit the introduction of the device into very small access holes and channels. Most of these systems provide for the attachment of a camera to permit permanent recording.

Advantages

  • Visual inspection is relative easy to performed.
  • It is quick, and relative inexpensive.
  • Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.
  • Parts with complex geometric shapes are easily inspected.
  • It can be time saving when used to identify incorrect welds/parts before other more expensive or time-consuming NDT methods are performed.

Disadvantages

  • Pre-cleaning is critical as rust, scale, grease and other elements can mask defects.
  • Only “some” surface breaking defects can be detected.
  • The inspector must have direct access to the surface being inspected.

Positive Material Identification (PMI)

PMI (XRF-analyzer) is not directly an NDT method, but is normally used by NDT personnel in industrial services. Positive Material Identification equipment sends low, level radiation to the material and the energy levels reflected back from each element. Each element is measured and identified. PMI works by exposing the material to a flux of x-rays. The atoms then absorb the energy and become temporarily excited and they fluoresce, or emit x-rays. The x-rays emitted by the sample’s atoms possess clearly defined energies that are unique to the elements present in the sample. By measuring the intensity and energy, the XRF instrument can provide qualitative and quantitative analysis. In other words, it can identify the elements, measure the concentration of each and display them on the unit. PMI are easy to use, the units are light and small in size, and the sample to be measured does not require more preparation then ordinary cleaning. But, there are limitations on the number of elements that XRF units can measure. Traditional methods of generating X-rays have used radioactive isotopes, but the latest generation of portable XRF analyzers has been replaced by small X-ray tubes.

When to use PMI

  • When working with a part that is part of an assembly or that is too large for shipping.
  • When a sample cannot be cut for routine testing.
  • When a mixed lot is suspected.
  • When material identification/documentation has been misplaced.
  • When there are questions about samples too costly to destroy.
  • When testing on weld consumables, both before and after welding.
  • When suspicion on contaminate of two, or more, different materials.