NDI – Non destructive inspection techniques Flashcards
(9 cards)
What are non destructive inspection techniques?
NDT, NDI and NDE; non-destructive testing, non-destructive inspection and non-destructive evaluation. They are further categorized into passive and active techniques, based on the observation of the component and on the detection of the change of one or more properties of the energy introduced into the component, respectively. Some notable examples are optical method, dye penetrants, acoustic methods, and passive thermography (passive); as well as magnetic and electromagnetic methods (magnetic particles and eddy currents), radiography, ultrasounds, and active thermography (active). They can further be classified into surface and sub-surface techniques, or volumetric techniques, which deal with only certain regions of the component as implied by their names. Optical methods, dye penetrants, magnetic particles and eddy currents are surface and subsurface, whereas acoustic methods, radiography, ultrasounds and thermography are volumetric methods. There are therefore post-process and in-process (alternatively off-line and on-line) measurements and inspections. Optical, radiography, ultrasounds and thermography are in-process, whereas dye penetrants, eddy currents, magnetic particles and acoustic methods are post-process.
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What is metrology and what are key aspects of it?
Metrology is the discipline that studies the theoretical and practical rules which leads to the measurement of physical quantities. Measurement, in turn, is the procedure which leads to the determination of a physical quantity; a measure is the result of the measurement procedure. Accuracy defines the deviation between average measure and theoretical values, that is, it takes into account systematic errors. Precision on the other hand defines the repeatability of the measurement, that is, takes into account random errors.
Explain dye penetrants:
Dye penetrant inspection is based on the principle of penetration by capillarity of liquids having low surface tension and low viscosity. To perform a dye penetration the following steps are followed: degreasing, application of the dye penetrant, washing and drying, and application of revealing powder. In order to work, the liquid must be low surface tension and low viscosity even at room temperature. It must also be easily removable and create a high contrast with the revealing powder. For applicability it must allow spraying, brushing and/or dipping. Different techniques exist depending on different types of penetrants. Red dye penetrant, fluorescent dye penetrants, and non-emulsifiable fluorescent react to UV lights. Fluorescent dye penetrants are characterized by dipping in an extra fine powder which does not adhere to the component surface but only to the dye penetrant. This allows application without preliminary removal of fluorescent dye penetrant, reducing inspection times. Non-emulsifiable fluorescent dye penetrants are those where liquid cannot be directly removed by means of water, but must be preliminarily treated with an emulsifying agent. It then becomes more easily removable than other dye penetrants and allows detection of really small dimensions.
Advantages include: suitability to detect surface defects, objective (unbiased) results without requiring skilled/experienced operators, suitable to most types of materials, very in-expensive, is easy to use, easily portable (suited to field inspections). Disadvantages however, include: inability to measure the depth of the defect, in-useability in highly porous or rough materials, and inability to detect internal or sub-surface defects.
Explain the method of magnetic particles:
The technique can only be adopted in magnetized ferromagnetic materials. It is based on the appearance of necking of flow lines of the magnetic field inside the component and the release of flow lines in the air in correspondence of voids, flaws and cracks. The procedure is as follows: magnetization of the component, defect detection, demagnetization of the component. Changes in flow lines distribution can be visualized by applying magnetic particles that will be more attracted to those areas. Cracks parallel to flow lines, therefore, cannot be detected.
To magnetize the components an electric current is used. Direct current has flow lines penetrating in depth, whereas alternating current has lines which remain confined on the surface but grant a better distribution of magnetic particles due to the frequency of the current (50Hz). A pulsating current allows advantages of both configurations.
Defects are detected through deposition of magnetic particles on the surface of the component. These particles consist of ultra-fine iron powders applied directly through liquid solutions. Sometimes pigments or fluorescent agents are used to improve detectability. An high granulometry increases the contrast but reduces the ability to detect defects of small dimensions. Similarly to dye penetrant: an accurate preliminary de-greasing and cleaning of the component is mandatory. The material is then demagnetized by increasing temperature beyond the curie temperature in a series of thermal cycles. Gradually the remaining residual magnetization is lesser, until it finally vanishes. If the component is already going to be subjected to thermal treatments, demagnetization is therefore not needed.
Advantages include: particular suitability to detect surface and sub-surface defects, objective (unbiased) results without requiring skilled/experienced operators, in-expensive, easy to be used, easily automated (e.g. can be used as on-line control technique during production). However, disadvantages include: unsatisfactory results in case of internal defects, unability to provide depth of the defects, functionality only in case of ferromagnetic materials, accurately cleaned surface requirement(mandatory), and additional demagnetization process once the procedure is completed.
Explain the method of radiography:
Highly penetrant X or gamma rays are shot into the component under examination, which absorbs them to different degrees depending on density and thickness. The radiation beyond the component reaches the film in different quantities, allowing evaluation of defects when porosities and delamination show as dark spots in the regions of low radiation absorbance.
X-rays as a source are artificially produced, whereas γ rays are naturally emitted by radioactive isotopes. To generate X-rays, a tube of Coolidge is used. The Coolidge tube is a type of X-ray tube that generates X-rays by accelerating electrons toward a metal target. The tube consists of a sealed glass enclosure with a vacuum inside, preventing electron collisions with air molecules. Electrons are emitted from a heated filament, usually made of tungsten. As the filament temperature increases, more electrons are released due to thermionic emission. A high voltage is applied between the cathode and anode, creating an electric field that accelerates the emitted electrons toward the anode. The accelerated electrons collide with the tungsten plate embedded in a copper cylinder. Upon impact, two phenomena occur: Bremsstrahlung Radiation, where electrons are decelerated due to the interaction with the tungsten atoms, causing the emission of X-rays; and characteristic X-rays, where some electrons knock inner-shell electrons out of the tungsten atoms, leading to the emission of X-rays with specific wavelengths. Since around 99% of the energy from electron collisions is converted into heat rather than X-rays, the copper structure helps dissipate excess thermal energy. The emitted X-rays exit through a designated window in the tube and can be directed toward an object for imaging purposes.
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Radiographic films are similar to photographic films, consisting in several layers of transparent polyester in a thin layer, and gelatine layers embedding the emulsion (silver halides). Each grain of silver halide is independently hit by electromagnetic waves, causing chemical reaction in each grain emulsion. Deposited grains darken while non-reacted halides are removed during development. The darkening is the logarithm of the opacity (ratio between input and output light intensity), and can be described as follows:
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Radiographic quality depends on contrast and definition (or sharpness), where contrast is made from two contributions: photographic contrast (depending on the characteristics of the film) and absorbance contrast (ratio between intensity of emerging beam from 2 contiguous points, depending on the nature of the material); and definition is the inverse of shading, shading being the blurring or loss of sharpness in X-ray images, which can affect the clarity of defect detection. Shading is caused by different factors that contribute to image degradation, making it harder to distinguish fine details. There are three primary types of shading: geometric, which occurs due to the finite size of the Xray source rather than perfect point source; film shading, which occurs due to intrinsic properties of radiographic film; and kinetic shading, resulting from motion during exposure.
Advantages include: high penetration capability - it is able to detect voids or presence of foreign materials in depth, radiographic techniques are suited for the majority of materials, in case of composite materials, opaque-enhanced dye penetrants can be used, in order to improve detection capability, it is possible to detect layers orientation by embedding special boron fibres coated with a fluorescent liquid, which distinctly stood out against radiography background, the use of dyes which penetrates among the fibres improves the contrast of delaminated regions without influencing their mechanical performances. Disadvantages, however, include: it is unable to detect defects through-the-thickness position, personnel safety concerns arise heavy shielding systems are needed.
Stereo-radiography is when two or more radiographies are taken at different inclinations and then overlapped. This causes a parallax, allowing us to establish the defect depth through simple trigonometry.
Explain the method of radioscopy:
Radioscopy differs from radiography in the type of image detector used. Instead of the X-ray film, a fluorescent screen based on zing sulphide and cadmium is adopted. Under action of X-rays the fluorescent screen illuminates with a yellow-green light that is clearly visible to the human eye. This gives the advantage of dynamic acquisitions and no need of developing, but disadvantages of lower power inspection capabilities and image based on intensity of radiation rather than dose.
Explain the method of ultrasounds:
The technique is based on the propagation of high-frequency mechanical vibrations. They propagated well through solid materials but will be reflected in presence of discontinuities, allowing the establishing of locations of defects. Different types of waves can propagate differently, allowing one to obtain information about thickness, physical nature and defectology of the material itself.
The acoustic impedance Z characterizes the resistance of the medium to US wave propagation. Z=R+iX,where R=resistance,X=reactance. Coefficient of transmission is t, with r+t=1, where r is the coefficient of reflection r=((Z_1-Z_2)/(Z_1+Z_2 ))^2
The intensity of US beam decreases along its path owning to two effects: absorption (heat dissipation) and diffusion (scattering). In case of composite materials the diffusion phenomenon is even more remarkable, since the intrinsic material heterogeneity is increased by the nature of the defects: diffuse voids and porosities, gas entrapments, micro-cracks, delaminations, foreign bodies inclusions.
To generate ultrasonic waves the piezoelectric effect is exploited, with quartz crystals or polarized ceramics. Several types of probes exist, including contact or immersion probes, straight or inclined probes (fixed or variable angle), and flat or focused (cylindrical beam or beam which converges on a set focal point).
To perform the analysis, one can use the reflection technique/pulse-echo method. A single probe emits and receives the possibly reflected US beam. The position of the echo allows one to determine the through-the-thickness location, whereas the amplitude and shape determine the type and dimension of the defect. For a single probe one must necessarily use a pulse-echo method, but this generates a dead zone near the emitter probe. One can also use the transmission technique, where a receiving probe is opposite the emitting probe. As such, if there is no defect, one expects the echo pulse to arrive with the same intensity, whereas a defect would decrease the energy (reflecting some) before it reaches the receiving probe. This method however cannot detect through-the-thickness position of defects, and can only be used as an immersion-technique. However, it is not affected by the dead-zone, has higher penetration, enables continuous emission, works on thin, thick and high fading materials, and crosses the material once at a time.
Readings can be presented as an A scan, B scan or C-scan; that is, a time domain representation of the received ultrasound signal, a representation of the A scan along a scanning path, or a top down planar image of the part being inspected (equivalent to a slice of the object).
Ultrasonic coupling is often required or beneficial. This is when the physical coupling between probe and material allows propagation without passing through air (high attenuation medium). Gel or vaseline are used as coupling liquids, immersion probes (where the coupling medium is de-mineralized water) and laminar flow jet of water are options to achieve this.
Advantages include: high penetration capability -it is able to detect voids and presence of foreign materials in depth, suited to the majority of materials, the most popular composites NDI technique, being able to detect delaminations, widely used to detect de-bondings, easily-automated technique, it is widely used as on-line quality control method (total quality approach). Disadvantages, however, include: needing the interpretation of results (highly skilled and experienced personnel), ultrasonic coupling is require (immersion tank: bulky non-portable equipment), requires a pretty smooth surface, affected by a pretty low scanning velocity.
Explain the thermography technique:
It can be defined as the acquisition and representation of the thermal image of an object; it is based on infrared radiation. Thermographic techniques are mainly based on the measurement of the thermal radiation emitted by the objects under analysis.
The detector to use is a thermal camera, there are two main types: piro-electric detectors, a crystal of ferro-electric material releases electrical charges (electrons) once heated, electrons beam is then converted into a thermal image; and photonic detectors, where thermal radiation excite the atoms of a semi-conductor which emits electrons and creates a measurable electric current.
There also exist several analysis techniques: thermal imaging technique, where emission of the IR is measured on a body subject to external loading; transient thermography, where the body is stimulated by a heat source for a given duration and temperature spots reveal internal defects (cooler regions for transmission, hotter regions for reflection); photothermal, where time-variable stimulation is given and amplitude and time-shift of the heating signal generates differences with respect to intact material; vibrothermography, where mechanical loading at low amplitude and high frequency or high amplitude and low frequency is imposed on the part, and thermal camera detects the heating of the body which happens near defects.
This method, however, is really sensitive to primers, paints and coatings, as well as surface roughness, as these methods all affect absorption, reflectivity and emissivity.
Advantages include: optimality in case of thermal mapping (e.g. heat leakages), suited to the majority of materials, recently the technique is becoming very popular for composite materials NDI delamination detection, suited for the detection of onset and growth of fatigue cracks, the most advanced techniques allow to perform thermoelastic stress analysis, to a certain extent portable. However, disadvantages include: need for preliminary calibration and tuning, requirement for skilled and experienced personnel, needs a smooth surface and is influence by the presence of surface primers, paints, contamination, oxidation, etc.
Comparison among different NDI techniques:
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