Principle: Ultrasonic waves in the form of pulses are introduced by the probe and propagate through the inspected material. If there is a discontinuity (also more commonly called a defect in material) in the pulse’s path, it would cause part of the energy to be reflected back. The reflected wave signal is received by the probe and is displayed on a screen of ultrasonic flaw detector as an echo (indication) allowing to be gained information about type, size, location and orientation of the discontinuities.
Application: Ultrasonic inspection can be used for flaw detection/evaluation in casting, forging, rolled iron, welded joints, for measurement of wall thickness of metals and other sound conductive materials (metal-ceramic composites, concrete, ceramics, etc.)
Advantages: a wide range of materials and objects, high accuracy in detecting flat discontinuity, good scanning speed, fast response, permitting rapid and automated inspection, may be performed with access to only one side of the object.
Disadvantages: special requirements to scanning surface, too wide range of scanning techniques and need for various types of probes, requirements for good operator’s qualification and experience in evaluation of indications from discontinuities.
Principle: High energy ionizing radiation emitted by radionuclide source (Iridium-192, Selenium-75, etc.) or X-ray generator penetrates the material to be tested and depending on its density, only part of this energy passes through. The difference of radiation intensity is detected by an X-ray film, imaging plates, or semiconductor detectors and after appropriate processing or conversion provides an information about hidden flaws such as porosity, excessive penetration, undercuts, slag, gas and metal inclusions, cracks, lacks of fusion, etc. /as well as object thickness and macrostructure/
Application: Radiographic testing or Industrial Radiography is one of the most common methods of NDT in the main industrial areas – engineering, construction, and energy, chemical. In this method, the highest degrees of detection have the so-called volumetric defects whose dimensions have the same magnitude in all directions and registration does not depend on the beam direction. The possibilities of the method in terms of the thickness depend on the type of the source used and the inspected material.
Principle: Magnetic particle inspection as a part of NDT methods is based on the ability to be registered the magnetic properties and their changes in the inspected material. As a result of the interaction of the applied magnetic field with different surface and subsurface discontinuities (defects) distortion in the magnetic flux is caused at that place. The magnetic particles are attracted by the surface field in the area of the discontinuity and adhere to the edges of the discontinuity appearing the shape of the discontinuity – indication – providing the information about its type, location and size.
The magnetic-particle inspection is used only for ferrous materials and demagnetization may also be required after the inspection.
Principle: Liquid penetrant testing is based on the ability of a liquid to be drown into open surface discontinuities by capillary effect. Registration of the resulting indications is then possible because of the contrast dye of the used testing media. Liquid penetrant testing is applied to all types of non-porous materials which should not interact with chemicals used for this purpose. This NDT method provides significant assurance for the detection and registration of open cracks (welding, thermal, grinding, fatigue, deformation), pores and other surface defects. It detects many characteristics of the discontinuities: localization (single, group, distracted), orientation (parallel, perpendicular, disoriented) to one or more axes of the tested product, dimensions and shape. There are two main types of PT: fluorescent and dye (colored). The liquid penetrant testing can be applied in the following stages of the operation of a given object: control of raw materials, production and inter-operational control, final inspection, periodic or operational inspection.
Among its disadvantages is that it is not applicable to porous materials, as well as certain plastics and polymer parts /absence of capillary effect in these materials/
Visual inspection is one of the most common non-destructive tests. Visual and visual-optical inspections are used to determine the condition of the inspected object and for detection of surface imperfections, changes in shape, size, integrity and condition. There are two basic types of visual testing: direct visual testing and remote visual testing. The direct visual testing requires proper eye-sight of the tester, adequate illumination, contrast and relief of the test surface, and for this purpose it is often necessary to use additional directed light sources, mirrors and magnifying glasses as auxiliaries for increasing the test sensitivity. Remote visual testing requires using technical means such as borescopes, fiberscopes, videoscopes, photo cameras and others. The method has high performance but low sensitivity to micro defects. Visual non-destructive testing can be carried out after preparation of the surface to remove technological and operational dirt and deposits, which could reduce the sensitivity and reliability of control.
Hardness is defined by the resistance of solid matter to deformation by penetration of various kinds indentors. Measurements can be carried out directly on materials, objects, construction, tools without destruction and damage. Portable hardness testers are small devices which measure the hardness at the place of the work piece, which gives them many advantages over the stationary ones. The main measurement methods are three and they determine the type of probe to be used. The Leeb rebound hardness test (developed by Proceq, Switzerland) is one of the most used methods for testing metal hardness. The other methods are Rockwell (Rockwell probe – Equostat) and Ultrasonic Contact Impedance (UCI) methods.
Spectral analysis is the set of physical methods for qualitative and quantitative determination of the composition of substances by examining their spectrum in spark or arc excitation. The correlation between the composition of a substance and the light spectrum emitted by it in excitation allows optical emission spectral analysis to be used as a sensitive, accurate and rapid method in research, industry and others. Using spectral analysis the presence of very small amounts (0.001%) of a given element can be detected. It is widely used in metallurgy, in the analysis of chemical composition of alloys in the parts, castings, as well as for checking and elaboration of technology in casting process and others.
Leakage testing is used to detected pass-through defects, and also it allows the testing of products or systems at increased parameters /under pressure vessels/. Such testing is achieved by different methods that create defined pressure – lower or higher than the working pressure of a given installation
Eddy-current testing is based on the registration of the interference of eddy currents generated in the objects. Testing coil/probe (or system of such) is used for inducing and reading back these field interferences caused not only by defects/cracks, flaws/ but also surface roughness, discontinuities, material inclusions, differences in hardness (partial effects of differences in heat treatment), differences in permeability and structure. With the appropriate equipment differentiation of the interferences is possible and the determination of the size, position and orientation of the defects in the inspected object.
Testing of insulation coatings is done with corresponding equipment /holiday detectors/ and is used to identify pores and defects in the integrity of the insulation coatings. The principle of the method is based on the application of high voltage (~1 to 40 kV) between the metallic base and a mobile electrode positioned at the surface of the coating to be inspected. A pore/pinhole in the coating could be detected by an electric discharge that forms between the electrode and the metallic support, which is indicated by an audible and / or visual signal by the equipment.