Non-Destructive Testing of Materials and Electrical Devices

     Purpose of the discontinuity of the material and dimensions of the non-destructive testing is to detect it without destroying the material. Sizes and types of discontinuities compared with the standards. Discontinuities in excess of permissible limits that are considered errors, material is sent for repairs or is rejected. In this way, errors are detected clearly and it gives get rid of cost of destructive testing and necessary labor. In daily life people uses transportation vehicles, communication products, home stuffs, hand tools etc. For all these instruments some metallic and non-metallic materials are used. Using for these instruments, materials which are used in them should be tested many times clearly. Wide range of uses these materials directly affect the testing method which is non-destructive testing.

Why do you need to non-destructive testing?

• Prevention of Accidents
• Improvement of product reliability
• Determination of the adequacy
• Provide sufficient information on an issue

For this reason, the following elements are required in order to obtain the information;

• Trained and qualified personnel
• The application procedure of test
• A system for reporting results
• A standard for interpretation of results

Commonly Used 6 Main NDT Methods

1. Liquid penetrant inspection
2. Testing with magnetic particles
3. Eddy Current Inspection
4. Ultrasonic Inspection
5. Radiographic Inspection
6. Inspection with eye or optical instruments

Liquid Penetrant Inspection

    Liquid penetrant testing is a non-destructive testing that is used for detecting the capillarity or surface discontinuities which is based on capillary affect. In the liquid penetrant inspection, liquid is applied on surface of sample and waiting for the liquid reaches in surface discontinuities.

After the enough time which liquid penetrant will be reach into the capillary area, the surface of sample is cleaned. For to draw the liquid penetrant on discontinuity ,capillary affect also used as an absorbent. In any case which blocking the liquid to entering the discontinuities of the penetrant should be removed.

Discontinuous regions on the surface can be covered with silly or sand blasting method.

 The steps of liquid penetrant test will be explained using paintings below.

A. Applied to the surface penetrant

B. Time allowed leaking the penetrant into open areas

C. Clean the more penetrant on surface

D. The developer is applied on surface to take on the penetrant from open areas

E. The sample under visual inspection

F. Last cleaning

Penetrant Test Equipment

Depending on the process and type, following features are should be on stable penetrant test equipment.

• Cleaning station
• Penetrant station (tank)
• Unloading station (tank)
• The emulsification station (tank)
• Agitation station (tank)
• Developer station (tank)
• Dryer station (tank)
• Inspection station (Using black light)
• Last cleaning station (Far from the penetrant station)

Almost every liquid can named as penetrant. However easy cleaning, spreading homogeneously, carrying the color easily type features gives preferable property to liquid which want to be use as a penetrant.

Another important point is temperature of penetrant or temperature of sample can affect time required for penetration. Therefore, heating the sample up to 70 Fahrenheit or more temperature will accelerate the penetration. 

Testing with Magnetic Particles

Certain metals to be able to magnetization, using an agent which has magnetic attraction property like iron powder can uncover possible discontinuities. Magnetic particles, during or after the induction of the magnetic field is applied to the surface of the test sample.

Following figure shows, the structure of magnetic particle have active article on discontinuities which is on magnetized sample.

Magnetic particle testing is an easy and simple testing for magnetic materials and it can applied during the every steps of process and manufacturing.

The aim is to ensure the reliability of the magnetic particle test items by the following ways:

• To obtain a finding of a visual image of a material surface
• To determine the structure of discontinuities without damaging the material
• Distinguish to acceptable and unacceptable materials according to the predetermined standard

An object, the magnetic field all or a portion magnetized when placed north and south poles as shown in the figure below.

Pole is called to pull or push capability of the local focus of a magnet. North and South poles show pulling or pushing characterized as shown below.

Magnetic power lines form a closed loop and these lines cannot interrupt each other. All of these power lines create magnetic field. Magnetic power lines and own magnetic field are shown in following figure.

Other magnetized material is provided to attracting force to the magnetic poles named as magnetic flux. This flux forms from all magnetic power lines. 

In angular magnets magnetic field consist totally inside of angular shape. Changing cracks disrupts the flow of lines of force within an annular magnet and forms a flux leakage.

In real, flux leakages are a magnetic power line which leave the angular magnet and travels the one pole to another in air. When the stray field exposed to a force so as to get out of the track, iron particles is arranged so as to show a sign of discontinuities.

If the leakage field strong enough, as shown below, it can be determined even subsurface discontinuities.

Some magnetic materials:

• Diamagnetic metals: Magnetic sensitivity negative and small. For example, copper, silver and gold.

• Paramagnetic metals: Magnetic sensitivity is positive and small. For example, magnesium, molybdenum, lithium and tantalum.

• Ferromagnetic metals: Gravitational force is very large, and even retains the magnetization magnetic field is removed. Iron, cobalt and nickel are an example ferromagnetic material. 

Eddy Current Inspection

Eddy current is a suitable method for the detection of surface defects. Method can be applied to all electrically conductive metals and alloys. However, it is more effective in the non-ferromagnetic material. The principle of the variable magnetic field generated by an alternating current coil, the circular surface of the material is based on the induction of eddy currents. This induced eddy current constitutes a second magnetic field in the opposite direction to the magnetic field in the coil. If there is a discontinuity in the region of formation of eddy currents in the test piece due to the difference in electrical resistance between the test material and transient current will follow a different trajectory. This difference is detected by the coil.

Eddy current test is applied in the following purposes different from error testing:

•         Coating thickness measurement

•         Electrical conductivity measurement

•         Classification of metals according to their chemical composition

Within acceptable limits over the entire surface of the material thickness are tested to determine whether there is a thin layer of aluminum. In this case the coil is placed on a bridge offset by the correct standard. Operators of the same type of aluminum and upper and lower limits acceptable according to what standards should ensure that rolled.

Eddy current inspection coils are designed according to specific application areas. So far the material surface or control coils placed near the coil was known until now. Which are :

• Surface coil
• Peripheral coil
• Inner coil 

The coil is used as a wear-mounted probe tip over a surface it has epoxy coating. The coil spring supported in a housing can be mounted. The spring continuously tested by preventing the detachment of the coil allows applying a constant pressure. Slots may be designed to fit the geometry of the material being tested in other words, to meet any particular requirement. Thus surface probe, can be in an unlimited number of different ways. given all the coils belonging to this class is known as surface coils.

Eddy Current Orientation

They tend to coil creates eddy currents by themselves, the gap between the coil and test piece influence and penetration depth are among these factors. their positions are changed according to the eddy current coil. Mites follow circular paths in parallel Eddy current inspection winding. The nature of the control process, have contributed greatly to the determination of the bobbin type to be used. According to a vertical discontinuity in the flow path of the eddy currents as much as possible to get the maximum response time of the control of discontinuities must follow. If eddy currents in a parallel flow path is followed discontinuity current distortion factor will disappear completely or very low level and will result in very little reaction will take place over the control coil.

      A seam discontinuity in the structure of the test piece surface coils can be used to detect the presence and location of the seam. According to permanently bar to reveal cracks in the surface coil circular bar-shaped test pieces need to remain on the same geometric position. In this case, so that the same radius in the bar will require a curved equipment layout.

Coil Shape and Sizes

Control coils, are produced in various sizes and shapes. Coil diameter is one of the major factors that contributed to the discovery of the discontinuity. For best results, a smaller diameter and a shorter circumferential surface coils and the inner coils are obtained when used, but such coils, which may be more susceptible to changes in lift-off and filling factor.

A long coil and then the best sensitivity for detecting longitudinal cracks is controlled with a short coil will be obtained when the cut off a large portion of the vortex flow created fractures.

Depth of Penetration

Eddy processing inside of another dimensional eddy current in the current control factors is depth. Magnetic field of the coil surface will fully penetrate the thick test pieces. Weight limits and possible factors that influence the penetration depth is available. Eddy currents are not distributed and properly controlled on a test piece. Located just below the surface of the coil intense eddy currents, right below the surface, it decreases in inverse proportion to the increase in the distance. 

When electrical resistivity  and relative permeability are known.

d	=	Standard Depth of Penetration (mm or inch)
r	=	Electrical Resistivity(mohm-cm)
f	=	Test Frequency (Hz)
mr	=	Relative Magnetic Permeability (dimensionless)

Ultrasonic inspection

The principle of ultrasonic testing, the high frequencies generated by the probe (0.1 to 20 MHz), propagation of elastic waves in materials testing environment and then hit a discontinuity is based on the reflection of the probe again. Waves that are detected by the probe is converted into electrical signals and cathode ray tube echoes a harbinger of the internal structure of the material in the screen it can be seen into. Where the error location and magnitude of the observed echo on the screen give information about the location and size.

All errors with ultrasonically tested within accuracy limits. Errors are detected when they are in the best shape in the direction perpendicular to the ultrasonic beam. Coarse-grained structures are particularly difficult Ultrasonic testing of austenitic materials

Material is sent to ultrasonic vibrations called vibrations. It described the backscatter the sound vibrations and must be interpreted. Answers desired properties it does not provide any material to be used in other methods of Non-Destructive Testing

Vibrations, many examples given environment; the fingers touched the sound of a radio cab moment felt a sensation in the fingers. This means that the energy is detected. Two facts should be noted that about vibrations as fit for purpose. These are ;

• Vibration forwards and backwards,
• Energy is a form of vibrational motion.

Pass through the material of the ultrasonic vibration, it is related to the material elasticity. Elasticity enables the ultrasonic waves pass through the material energy. "Vibrations" movement is now known, each one of these movements "cycle" is called. When applied blow to the metal surface, interior surface toward collapse. The inner surface of the distance toward the collapsed portion of "displacement" is called. Two displacement creates a cycle. Metal, because it has the elasticity back down the surface so the original (static) position is the future. Surface is the right way to take the maximum from the move and in the opposite direction from its original position. The maximum distance from the "displacement" is called. As a result, the surface again returns to its center position or stagnant. so that the sequence of all the action "cycle" is determined.

Shaking the globe continues in different numbers. consisting completed within the given time limit cycle 's total number of "frequency" is called. The only time measure used sec. Thus, in the case of a second cycle, "cycles per second" are accepted. Unit as a term used universally accepted and frequency measurement "Hertz" (Hz) are used.

Transducer will collect its response to the ultrasonic vibration material. The energy may also be pulsed or continuously sent by the transducer. Sometimes a transducer energy, material in a very short period of time (micro seconds) transmits energy and creates space, pausing briefly before taking it again.

This is called "pulsed sound" and there was no vibration before and after the short pulse group is treated as other groups.

Radiographic Inspection

Test records permanently and widely applied the method to be highly sensitive radiography.

The test piece is irradiated with a radiation beam emanating from a source. It reaches the film disposed on the rear surface of the part after the radiation passes through the material.

In order to be absorb different radiation intensity of the radiation passing through the region by discontinuities where the blackout will create discontinuities and will be different in the film. After the film's bath process becomes seen as a symptom of errors tarnish on the film. It can be detected with radiography methods as long as they are planar defects parallel to the radiation beam.

X-ray film evaluation room light intensity should be 20Lux( 2 Foot candle)

Inspection with eye or optical instruments
(Visual inspection)

Visual examination of surface discontinuities light control used in quick, cheap, simple and oldest non-destructive testing methods. cracks can be visibly accessible surface open sections of the piece to be controlled and monitored by this method in terms of the general structural discontinuities such as corrosion is checked.

Noticeably reach the surface to be checked in the inspection, cleaning of the surface, so that it must be large enough to be seen from error are factors that limit this examination. Eye examination in a good light source, available aids such as a magnifying glass and mirrors. Opinion opportunities in areas that are difficult to spot the type of light source used. What eyes where you see one of the most important elements of such an examination is obliged to identify discontinuities that good looking.

Eye examinations are performed using optical instruments in other matters will be examined within the scope of the examination. Discontinuities in small enough to be invisible to the human eye baroscope endoscope and made visible to the human eye level with the use of optical instruments such as microscopes and thereby control the presence of the discontinuity.

Shape of baroscope

According to the location of the property to be controlled are used in many types of borescopes. Some of these fixed some flexible structure, which can be extended in length, illuminated in various ways (right angled left angled, angled backward) and may be listed as fiber optic baroscope. If the combustion chambers of aircraft engines will use examples to place the turbine and compressor control panel and the inside of the cylinder.

Borescope and be seen with the naked eye as small discontinuities micro-optical comparator can be 5 to 500 times larger object and can be identified by the use of a microscope. Microscope examination of the grain structure of the material, cracks formed in the micro-level, and monitored the progress of the cracks and can be examined.

Eye examination is seen illuminated magnifying glass used in the application help. Additionally cavities, holes and various recesses of the pieces are inaccessible or invisible places of examination purpos shows a flexible fiber optic cable baroscope equipment. This type of display device can be transferred to the screen via a camera and can display photos taken or modified on.

Of course, today, thanks to increased use of advanced technological means and image processing techniques to use for this purpose and finger miniature camera.