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Non-Destructive Testing (NDT) is the application of measuring techniques in order to identify damage and/or irregularities in materials. NDT often provides the only method of obtaining information about the fitness for purpose of safety critical components. NDT is essential in ensuring the safe operation as well as in quality control and assessing plant life.
The correct selection and application of NDT processes can provide confidence that a piece of plant or individual components do not contain defects.
Types of Non-Destructive testing (NDT)
NDT methods can be adapted to automated production processes as well as to the inspection of localised problem areas.
Ultrasonic Testing (UT) a type of Non Destructive Testing uses high frequency sound energy to conduct inspections and take measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization etc. To illustrate the general inspection principle, a typical pulse/echo inspection configuration.
A typical UT inspection system consists of several functional units,firstly the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that produces high voltage electrical pulses during the inspection. 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. 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.
This NDT method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Where there are surface and near-surface flaws it will disrupt the flow of the magnetic field within the part or equipment and force some of the field to leak out at the surface. Iron particles are attracted and concentrated at sites of the magnetic flux leakages. This produces a visible indication of defect on the surface of the material.
With this type of Non Destructive Testing, the test object is coated with a solution that contains a visible or fluorescent dye. The next step is for the excess solution is then to be removed from the surface of the object but is left in surface breaking defects. A developer is then applied to draw the penetrant out of the defects. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen and recorded accordingly. With visible dyes, a vivid color contrast between the penetrant and developer makes the bleedout easy to see.
Radio Graphic inspections involve using penetrating gamma – or X-radiation on materials and products to look for defects or examine internal or hidden features. An X-ray generator or radioactive isotope is utilised to produce the rays. Radiation is directed through a part and onto film or other detector. This would then result in a shadowgraph showing the internal features and soundness of the part. Material thickness and density changes are indicated as lighter or darker areas on the film or detector. The darker areas in a radiograph would represent internal voids in the component.
Phased array ultrasonic testing (PAUT) probes are composed of several piezoelectric crystals that can transmit/receive independently at different times. To focus the ultrasonic beam, time delays are applied to the elements to create constructive interference of the wavefronts, allowing the energy to be focused at any depth in the test specimen undergoing inspection.
This principle is where delay laws have been computed to focus the acoustic beam at a specified depth and angle. Each element radiates a spherical wave at a specific interval. The superposition of these wavelets results in an almost planar wavefront at the specified location.
Before and after the targeted spot, wavefronts are spherically converging and diverging, respectively. The configuration illustrated in b results in the same ultrasonic beam that would be generated by a conventional flat transducer used in conjunction with a wedge.
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