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MAGNETIC TESTING

ALTERNATING CURRENT (AC) :

Alternating current is current that reverses its direction of flow at regular intervals. Such current is frequently referred to as AC.

FULL-WAVE RECTIFIED SINGLE-PHASE AC :

This rectified alternating current for which the rectifier is so connected that the reverse half of the cycle is “turned around”, and fed into the circuit flowing in the same direction as the first half of the cycle. This produces pulsating DC, but with no interval between the pulses. Such current is also referred to as single-phase full-wave DC.

FULL-WAVE RECTIFIED THREE-PHASE AC :

When three-phase alternating current is rectified the full-wave rectification system is used. The result is DC with very little pulsation – in fact only a ripple of varying voltage distinguishes it from straight DC.

HALF-WAVE RECTIFIED AC :

When a single-phase alternating current is rectified in the simplest manner, the reverse half of the cycle is blocked out entirely. The result is a pulsating unidirectional current with intervals when no current at all is flowing. This is often referred to as “half-wave” or as pulsating direct current.

SINGLE-PHASE ALTERNATIN CURRENT :

This term refers to a simple current, alternating in direction. Commercial single-phase current follows a sine wave. Such a current requires only two conductors for its circuit. Most common commercial frequencies are 25, 50 and 60 cycles per second.

THREE-PHASE ALTERNATING CURRENT :

Commercial electricity is commonly transmitted as three single-phase currents, that is, three separate currents following separate sine curves, each at 60 cycles (or other frequency) per second, but with the peaks of their individual curves one-third of a cycle apart. At least three (sometimes four) conductors are required for three-phase alternating current.

PULSED AND IMPULSE CURRENT – MAGNETIZATION :

A magnetization technique utilizing short circuited AC or condenser discharged DC. Very high magnetizing currents are possible for short durations. ( 1/100 to 1/1000 sec.) without the use of transformers. A Pulsed magnetization applies high fields for brief periods. A slight variation of pulsed magnetization is impulse magnetization. Betz called this flash magnetization. Associated with electrical methods of magnetization is the heating involved when current passes through a metal. Heating may limit the field intensity achievable. To overcome this difficulty a method of short duration high intensity fields was developed. Thin walled materials can be tested without the risk of heating at contact points. This is normally used for the residual method and allows for even higher field applications as they are applied once and briefly.

In the early days of MPI it was thought that DC from storage batteries provided best test results. Since then it has been shown that variations of the AC supplies are as effective and in some cases even more desirable. An important feature of alternating current is the skin effect.

The options of current available from AC and DC generators are all that is now required. In addition to the commutator that provides us with “full wave rectified” AC, there is electronic circuitry, called rectifiers, that provide us with the other modifications to the types of currents used. Only storage batteries (and Faraday’s Dynamo) provide true DC, all other sources we use are AC derived. Commutators and rectifiers limit or reverse negative flow so current, although its amplitude may fluctuate, flows only in one direction. For this reason rectified alternating current is considered direct and may sometimes be termed “half wave DC” and “full wave DC”.

ILLUSTRATIONS

Ampere Turns

This term refers to the product of the number of turns in a coil and the number of amperes of current flowing through it. This is a measure of the magnetizing or demagnetizing strength of the coil.

Ampere

This is the unit of electrical current. One ampere is the current which flows through a conductor having a resistance of one ohm, at a potential of one volt.

Ampere Turns

This tern refers to the product of the number of turns in a coil and the number of amperes of current flowing through it. This is a measure of the magnetizing or demagnetizing strength of the coil.

Black Light

Light energy just below the visible range of violet light, predominately of about 3650 Angstrom units. This wavelength reacts strongly on certain dyes to make them fluoresce in a range visible to the eye.

Black Light Intensity

Foot-candles of Black Light at any given point in a inspection area. Intensity of light is now more often given in units of Lux or micro-per square cm.

Central Conductor

A conductor that is passed through the opening in a ring or tube, or any hole in part, for the purpose of creating a circular or circumferential field in the tube or ring, or around then hole.

Circular Magnetism

When a electric current is passes through a solid magnetic conductor, a circular magnetic field is developed not only around the conductor, about also with conductor

Coil Shot

A “shot” of magnetizing current passed through a solenoid or coil surrounding a part, for the purpose of longitudinal magnetization is called a “coil shot” . duration of the passage of the current is usually very short often only a fraction of a second.

Decay (Magnetic)

As used in connection with electricity, decay is the falling off to zero of the current in an electrical circuit. Magnetic fields and electrical potentials can also decay in similar sense.

Paramagnetic

All materials are affected by magnetic fields. Those which are attracted are called Paramagnetic. Those which are repelled are called Diamagnetic. The reaction to a magnetic field of these two classes of substances is very slight indeed. The few materials that are strongly attracted by magnetic fields are called ferromagnetic.

Induced Current Magnetization

Passing an alternating current through a conductor will set up a fluctuating magnetic field. If a second conductor in the form of a closed loop is placed in this field, the action of the fluctuating field moving across the conductor will set up a second alternating current of the same frequency. This is an induced current.

Induced Current Magnetization

Passing an alternating current through a conductor will set up a fluctuating magnetic field. If a second conductor in the form of a closed loop is placed in this field, the action of the fluctuating field moving across the conductor will set up a second alternating current of the same frequency. This is an induced current.

YOKE

A magnet that induces a magnetic field in the area of a part that lies between the poles. Yokes may be permanent magnets or electromagnets.

Permanent Magnet Yoke – A body which possesses the ability to retain or hold a large amount of the applied magnet field after the active power of the field is removed.

The great advantage of magnetization using permanent magnets for MPI is the portability they allow. No source of electricity is needed and they are relatively light and compact. Apart from this permanent magnets have little to offer as an MPI magnetizing method. Permanent magnets can be used as single poles or as pairs (a north and south pole) in the form of a horseshoe magnet or two bar magnets arranged with a fixed spacing. In this form they are the permanent magnet yoke. Fields used when employing permanent magnet yokes are essentially only longitudinal in nature. The piece magnetized is given north and south poles at the points of contact on the test piece. Inspection at the points of contact is not possible due to the confusing mass of particles that gather at the poles.

Figure 1 shows permanent magnets and their associated fields contacting only one pole to the test piece. It is also shown the more common use of the yoke arrangement in that both a north and a south pole are made to contact the piece. Its somewhat easier in this method to arrange the field such that suspected defects are orientated at right angles to the field.

Other limitations to permanent magnets include; the inability to vary the field strength at will, large areas cannot be magnetized with sufficient field strength to locate indications and removal of the magnet from the part is difficult if the magnet is very strong.

Electromagnetic (EM) yokes, the same shape as the permanent magnet yokes, can also be used. These are U shaped cores of soft iron, usually laminated, with a coil wound around the base of the U. when we pass an electric current through the coil the result is a north and south pole at the ends of the core. This makes it look like a horseshoe magnet. Direct current yokes have some advantages over the permanent magnet yokes. It is possible to vary field strength using varying current and the EM yoke is also easy toplace onto and remove from the part as no field exists unit current is applied.

Operation of the EM yoke and of the coil method of longitudinal magnetization utilizes the field that results in a solenoid. In the EM yoke the field is concentrated and directed by the laminated steel core which extends to form legs (often flexible). Coil magnetization is performed using fixed coils having the conductors housed in a holder. Often coils are made without the convenience of a fixed support by merely wrapping the cables around the test piece. This is usually done for larger or irregular shaped parts.

Unlike permanent magnets, yoke and coil magnetizations can be varied in strength (as well as switched off and on). Sensitivity of the test relies not only on the amount of current and number of turns, but also on the ratio of the test part cross-sectional area to the coil area where the part is placed into the coil. The ratio is called the fill factor and it is recommended that the fill factor not exceed 10%. When using coils another consideration is length of coil with respect to the part length.

When testing a part, only a single orientation of defect may be expected, however other orientations might occur. An effective MPI test usually involves test in more than one direction. Weld inspections using yokes are usually done using an “X” pattern thereby inducing longitudinal magnetizations in 2 directions at right angles to each other.

ILLUSTRATIONS :

OPTICAL HOLOGRAPHY FOR SURFACE DEFORMATIONS

Introduction to Optical Holographic NDT

Optical Holographic techniques can be used for nondestructive testing of materials (HNDT). Non-optical Holography techniques include Acoustical, Microwave, X-Ray and Electron beam Holography. HNDT essentially measures deformations on the surface of the object. However, there is sufficient sensitivity to detect sub-surface and internal defects in metallic and composite specimens.

In HNDT techniques, the test sample is interferometrically compared with the sample after it has been stressed (loaded). A flow can be detected if by stressing the object it creates an anomalous deformation of the surface around the flaw.

Optical holography is an imaging method, which records the amplitude and phase of light reflected from an object as an interferometric pattern on film. It thus allows reconstruction of the full 3-D image of the object. In HNDT, the test samle is interferometrically compared in two different stressed states. Stressing can be mechanical, thermal, vibration etc. The resulting interference pattern contours the deformation undergone by the specimen in between the two recordings. Surface as well as sub-surface defects show distortions in the otherwise uniform pattern. In addition, the characteristics of the component, such as vibration modes, mechanical properties, residual stress etc. can be identified through holographic inspection. Applications in fluid mechanics and gas dynamics also abound.

The light used to illuminate the surface of the specimen must be coherent, which means that it must also be monochromatic, and the only practical source is a laser. Each type of laser emits a characteristic wavelength, e.g. a helium-neon laser emits 632.8nm; a ruby laser emits 6943nm. Laser diodes are nowadays an exciting and compact alternative. Indeed, holography using laser pointers have also been demonstrated. High-resolution films are another necessity for holography. With the advent of CCD and digital image processing, digital holographic interferometry offers tremendous flexibility and real-time visualization. Furthermore, image processing schemes can provide computerized analysis of patterns for automated defect detection and analysis, finally since intricate interferometric patterns have to be recorded, vibration isolation is also required. Novel schemes have been proposed, including use of pulsed lasers to record holograms in factory environments. Advances and developments in lasers, computers, and recording materials introduce new techniques such as electronic (or TV) holography, multi-wavelength recording, thermoplastic medium, time-averaged holography, dynamic holographic interferometry, cineholography, and digital holography with each new development. Methods that once held only academic interest often become practically viable with these developments in hardware and software.

HNDT is widely applied in aerospace to find impact damage, corrosion, delamination, debonds, and cracks in high performance composite aircraft parts as well as turbine blades, solid propellant rocket motor casings, tyres and air foils. But holography is also finding new applications in commercial and defense related industries to investigate and test object ranging from microscopic computer chips and circuits to cultural articles, paintings and restoration.

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