TECHNICAL INFORMATION
NONDESTRUCTIVE TEST METHODS
VISUAL TEST
Scientific Background
It is testing of parameters such as fugacities, structural disorders, surface condition etc. on the surface of a product, which affect quality, with or without the help of an optical tool (magnifier, etc.).
Although visual testing can be considered a simple method, it has its own delicacies. Generally, it is necessary work before implementation of another nondestructive testing method. As a matter of fact, most of the implementation standards prepared for other nondestructive testing methods also require primarily visual test and recording of findings.
Fields of Application
It can be applied to all materials, metallic or non-metallic. Supplementary tools like endoscopes can also be used when necessary in view of accessibility of the test surfaces.
Application
In most cases, no surface cleaning is required as preparation of the test surface. In other words, expected imperfection should be in the best visible form. Test should be made under adequate lighting from suitable perspectives.
PENETRANT TEST
It is atest method used for detection of surface imperfections. Imperfections should be open to the test surface, therefore, this method is unable to find out imperfections under the surface or whose opening to the surface is blocked for any reason.
Scientific Background
Penetrant liquid penetrating through capillary effect within fugacities exposed to the test surface is drawn back to the surface by the developer to acquire symptoms of fugacity. Lineal symptoms and round symptoms will be acquired if fugacities are cracks and pores respectively.
Fields of Application
It can be used for detection of surface imperfections expected in all materials, either metallic or not.
Limitations
In the event that the test part is multiporous, then sound implementation of the method is unlikely. Only surface breaking imperfections can be detected. If surface cleaning is not made properly, this will affect the outcome directly. An additional final cleaning process may be necessary after the test. Utilization of chemical substances require special diligence.
Application
1. Preliminary cleaning on test surface
2. Penetrant Application
3. Wait for Penetrasyon
4. Intermediate cleaning
5. Development
6. Examination
7. Evaluation and report preparation
8. Final cleaning
TEST WITH MAGNETIC PARTICLES
It is an test method used for detection of surface imperfections. Imperfections are not required to be open to the test surface
Scientific Background
Stray flux occurs on the fugacities on the surface if magnetic flux is applied on the test surface, depending on its location on the surface. If ferromagnetic powders are spattered onto the test surface meanwhile, those powders are drawn by stray fluxes and collected on fugacities. Thus, the places of the fugacities can be detected.
Fields of Application
It can be applied to all ferromagnetic materials.
Limitations
It cannot be applied to any non-ferromagnetic material. Fugacity cannot be determined if not position in proper angle with the direction of the applied magnetic field. High magnetizing fluxes may be necessary for big parts. Too rough test surfaces have adverse effects on the result. Paints or coatings on the test surface, if any, will have direct effect on the test result. Generally, elimination of magnetizing and final cleaning etc. additional processes are necessary after the test.
Application
1. Preliminary cleaning on test surface
2. Elimination of magnetizing, if necessary
3. Application of magnetizing current
4. Spraying of ferromagnetic powders
5. Cutting of magnetizing current
6. Examination
7. Evaluation and report preparation
8. Elimination of magnetizing and final cleaning
VORTEX CURRENTS TEST
Scientific Background
A magnetic field occurs around a coil when alternative current (AC) is passed through. When the coil is approached to the surface of an electrically conductive material, the alternative magnetic field of the coil causes induction currents on the material's surface. These currents flow in a closed circuit and are called vortex currents. Vortex currents create their own magnetic fields. That secondary magnetic field can be measured to find superficial imperfections, and to determine parameters of the material such as conductivity, permeability.
Fields of Application
This method is used in all electrically conductive materials (copper, aluminum etc.) for detection of surface and sub-surface fugacities. It is also possible to classify materials via vortex currents test method based on features such as electrical conductivity or magnetic permeability etc.. Furthermore, it is possible to measure coating thickness or the thickness of fine metal plates.
Limitations
It is not applicable for materials that are not electrically conductive. Special procedures are necessary for test of ferromagnetic materials. Permeability depth is limited. Since the result of the test depends on the direction of scan, some imperfections may not be observable as a result of scanning in wrong directions. It depends on the training and experience of the user more compared to other methods. Surface conditions have high effect on the test outcome. Special reference standard blocks are needed for the mechanism.
Application
If crack-type symptoms are sought on the test surface, then scanning is generally made via probes called differential probes, but it is also possible to detect crack-type imperfections via absolute probes. Absolute probes are used mostly for material characterization and coating thickness measurement. "Lift-off" effect is used for measurement of thickness. The device should be calibrated first for the same. Thicknesses to be used for calibration should be selected close to and somewhat bigger or smaller than the thickness to be measured. If characterization is to be made via measurement of the magnetic permeability or electrical conductivity of the test part, then absolute probes shall still be used, and the device values should be calibrated before measurement with the help of known reference blocks.
RADIOGRAPHIC TEST
Scientific Background
High-energy electromagnetic waves (radiation) can penetrate into several materials. Radiation penetrating into a certain material may affect radiation-sensitive films placed on the other side of the material. When these films are exposed to developing process later, the image of the inner side of the material which the radiation passes through shall appear. The image is formed due to cavities inside the material or changes in thickness / intensity. Such imaging of inside the material is called Radiography. If a detector is placed behind the material instead of film and the radiation from the material is collected to be transferred to a monitor, that technique is called radioscopy.
Fields of Application
It can be used for detection of volumetric and superficial imperfections expected in all materials, either metallic or not.
Limitations
The thickness of the test part may not exceed certain values based on the type of the radiation source to be used. Material thickness ranges suitable for various radiation sources have been given in application standards. It can be applied to all kinds of materials except for the limitation of thickness. Operator's education and experience are very significant. The test part should be accessible from both surfaces. The hardware to be used for test is more expensive compared to other methods. Radiation safety is the matter requiring most careful work.
Application
Various radiation sources can be used for radiographic test. The sources can be X-ray tubes or Gamma beam producing isotopes. The X-ray energy range used in industrial radiography is generally between 50 kV -350 kV. Radiation energy depends on the type and thickness of the material to be beamed. The most known and used gamma sources are Ir 192, Co 60. Furthermore, isotopes such as Se 75, Yb 169 Tm 170 are used in the field of industrial radiography.
Tests should be made in accordance with the standards for sound and reliable results. The standards have been prepared in accordance with the type of the material and/or product. Besides, there are application standards for performance of the test and those providing acceptance levels. Test is conducted having specified standards suitable for the characteristics of the test part.
ULTRASOUND TEST
Scientific Background
The high-frequency sound waves sent within the material reflect if they come across an obstacle on the path of the sound. The signal reflecting based on the angle of impact may or may not be received by the receiver probe. The reflecting signal reaching the receiver probe forms an echo sign on the screen of the ultrasonic test device. The coordinates of the reflector within the test part can be calculated in view of the position of the echo. The height of the echo also provides an opinion on the size of the reflector. It is possible to comment on the type of the reflector in view of the form of the echo signal.
Fields of Application
It can be used for detection of volumetric and crack-type superficial imperfections expected in metallic or non-metallic materials.
Limitations
Right evaluation gets harder if speed of sound and leptophania features show strong regional changes. In materials where sound weaking is very high due to big grain size or absorption, test can be sometimes impossible. Probes specially designed for hot test surfaces should be used. An adequately wide acceptable surface should be prepared for the test. The surface condition directly affects examination parameters. Test of fine parts is relatively hard. It is not possible to detect planar fugacities parallel to the acoustic beam axis. In general, reference standard blocks are needed.
Application
High frequency sound waves are produced by a piezoelectric crystal within a part called probe. The frequency range of the metallic materials used in ultrasound test can be between 500 kHz and 10 MHz. Frequency in compliance with the microstructure features of the test part is determined. A suitable contact fluid (oil, grease, water etc.) should be used for penetration of sound waves inside the material when the probe is contacted to the test surface. The probe is moved on the surface of the test surface to observe if there are echoes besides the ones due to the (scan) part geometry, and if any, the position and heights of those echoes are evaluated to analyze for errors.
The most commonly used wave types for ultrasound test are longitudinal (pressure) and transverse (shearing) waves. Waves moving inside the material when working with probes with zero introduction angle called normal probe are longitudinal waves. As to angled probes, they generally send transverse waves with an entry angle of 45 °, 60 ° and 70 ° in general (values for steel materials).
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