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Magnetic Field Testing
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  What are the properties of dry powder and wet suspension magnetic particles? A:  They are iron oxide particles, finely divided in sizes varying between 0.125 and 60 microns with a high permeability (easily magnetized) and low retentivity (ability to stay magnetized). Dry particles are chemically dyed to provide contrast against the background of the test surface. Stock colors are gray, red, black and yellow. Application is either with a squeeze bulb or spray gun. Wet suspension particles are suspended in an oil-based or water-based liquid vehicle or carrier. With water, operators must add conditioners in addition to a wetting agent and corrosion inhibitors. Q: What is the sensitivity comparison between wet vs. dry methods? A:  Wet method provides improved sensitivity for the detection of very fine surface flaws. Dry method provides improved sensitivity for the detection of subsurface flaws. Glossary    Alternating Current (AC). Electric current flows through a conductor in a back and forth direction at specific intervals. Note: It provides the best sensitivity for the detection of surface discontinuities only.    Direct Current (DC).  Electric current flows through a conductor in only one direction at all times. Note: DC from a battery source has been phased out in favor of rectified forms of AC for surface and subsurface flaw detection.    Flux Density (B).  Magnetic field strength per unit volume within a ferromagnetic test part measured in gauss.    Flux Field Penetration.  The ability to establish and drive high-density magnetic lines of force deep into the test part.    Full-Wave Rectified (FWDC).  Electric current flows through a conductor in one direction only with an increased rate of pulsating surges and drops at specific intervals. Note: FWDC is recommended for effective surface and subsurface flaw detection when using the wet method of inspection.    Half-Wave Rectified (HWDC).  Electric current flows through a conductor in one direction only with pulsating surges and drops at specific intervals-hence the name half wave.  Note: It is most effective for surface and subsurface flaw detection when using the dry powder method of inspection.    Linear Indication.  Any indication having a length dimension at least three times greater than its width.    Magnetic Domains.  Ferrous material atoms or molecules normally illustrated as small bar magnets with north and south poles.    Magnetism.  A form of energy directly associated with electrical current, and characterized  by fields or lines of force.    Magnetizing Force (H).  Magnetic field strength per unit volume in air, measured in oersteds.     Particle Mobility.  The ability to establish activity or motion to the magnetic particles applied to the test part surface.    Permeability.  The ease with which a material can be magnetized. The ability of a material to conduct magnetic lines of force.    Reluctance.  The opposition of resistance by a material to conduct magnetic lines of force.    Retentivity.  The ability of a material to retain magnetic lines of force following magnetization.    Right Hand Rule.  Magnetic lines of force will always travel perpendicular or 90 degrees to the direction of electrical current flow.    Magnetic Writing .     Relevant indications Relevant indications are discontinuities or flaws, which in turn are un-designed imperfections. When it is considered that a relevant indication will affect the fitness for purpose of a test specimen, it is classified as a defect, but not all defects are cracks. Product and process knowledge (a knowledge of product technology and the processes that a test specimen has been through) is necessary to define and interpret defects more closely. It is  perhaps safer, without that knowledge, to categorise indications by their:    Size    Shape    Orientation Spurious indications Indications which are not held on the surface by a flux leakage are called spurious. Lint, scale, dirt, hairs, drainage lines, etc, are examples. However, one spurious indication, called magnetic writing, is a little different. If two pieces of steel touch when one of them is in a magnetised condition, local poles are created at the areas of contact. If magnetic particles are then sprinkled on the surface the local poles become visible as fuzzy lines.  Non-relevant indications  Non-relevant indications are true magnetic particle patterns actually formed and held in place  by leakage fields. However, they are caused by design features and the structure of the specimen and only exceptionally will they affect the fitness for purpose of the specimen. Below is a non-exhaustive list:    Scores and scratches    Key ways    Internal splines and drillings    Abrupt changes of section    Fine threads    Force fits    Dissimilar magnetic material (HAZ or heat treated material)    Forging flow lines    Grain boundaries    Brazed joints    Cold working  NOTES 1.   Guss meter is not used for Multidirectional and Longitudinal Magnetized Fields. 2.   Head Shot and Prod Techniques are Direct Magnetized Fields and Coil (Solenoid), Yoke, and Central Conductor Techniques are In-direct Magnetized Techniques. 3.   At and above curie point the ferromagnetic materials become paramagnetic so demagnetize. 4.   AC tends to flow the surface of conductor this property of AC is called Skin Effect and so AC current detect surface discontinuities. 5.   Longitudinal techniques: Coil (Solenoid) and Yoke (No North and South Pole and easy to Demagnetized) 6.   Circumferential Techniques: Head ---, Central Conductor & Prod Techniques 7.  
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