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TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335-23CHAPTER 3MAGNETIC PARTICLE INSPECTION METHODSECTION I MAGNETIC PARTICLE (MT) INSPECTION METHOD3.1GENERAL CAPABILITIES OF MAGNETIC PARTICLE INSPECTION.NOTEThe terms MPI, MPT, and MT are used interchangeably in this chapter.3.1.1 Introduction to Magnetic Particle Inspection (MPI). Magnetic particle inspection is an NDT method used toreveal surface and near surface discontinuities in magnetic materials. This inspection method can only be used on materialsthat can be magnetized (known as ferrous). The MPI process, when properly performed, establishes a field leakage site on thesurface of the part below which the flaw lies. This chapter presents theory and practical guidance for the performance ofmagnetic particle inspection. Process control and basic inspection procedures are located in TO 33B-1- Benefit of Magnetic Particle Inspection. MPI is the method of choice on ferrous materials instead of liquidpenetrant because it is faster, requires less surface preparation, and in some instances is able to locate subsurface flaws.3.1.3 Basic Concept of Magnetic Particle Inspection. MPI relies on the principle of magnetism (paragraph 3.2.1). Verysmall ferrous particles, which are suspended in a bath of oil or water, are attracted to magnetic field leakage sites, just as ironfilings are attracted to the poles of a magnet. Cracks and similar types of discontinuities cause disruptions in the magneticfield of magnetized parts, in turn attracting these ferrous particles to the leakage site. This allows the inspector to visualizewhere the discontinuities are located in the part. The keys to a successful magnetic particle inspection are the correct amountof magnetization of the part, in an optimum direction with respect to flaws, and adequate contrast between the part’s surfaceand the particles used to identify the flaw. The particles used are precipitated soft iron, and are stained or dyed in variouscolors, usually with a fluorescent dye or a red dye. Fluorescent dyes on particles in a liquid suspension are used to find verytight surface flaws. Visible dyes on dry particles are less sensitive to tiny surface defects, but are better for finding subsurface flaws. The type of flaw and/or the inspection environment determines selection of the color or type of particles. The following paragraphs describe in detail the standard terminology used, the theory of magnetism, MPImagnetization and demagnetization techniques, process controls, and safety concerns.3-1

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335-23SECTION II MAGNETIC PARTICLE PRINCIPLES AND THEORY3.2PRINCIPLES AND THEORY OF MAGNETIC PARTICLE INSPECTION.3.2.1 Principles of Magnetization. When parts made of ferrous materials, such as iron, are placed in a strong magneticfield or have electric current flowing through them, they will become ''magnetized.'' The degree of magnetization is affectedby the strength of the magnetizing field or the amount of current flow. How strongly the ferrous part will be magnetized afterthe magnetizing force is removed is called ''retentivity.'' Permanent magnets have high retentivity and conductors normallyhave low retentivity. When a surface or near-surface discontinuity interrupts the magnetic field in a magnetized part, some ofthe field is forced into the air above the discontinuity resulting in a leakage field. The size and strength of the leakage fielddepends on the size and proximity of the discontinuity to the magnetic field. The discontinuity is detected by the use of finelydivided iron particles applied to a part’s surface and attracted to the leakage field. This collection of particles indicates thepresence and location of the discontinuity.3.2.2 Basic Terminology. The following terms and definitions are basic to an understanding of the MPI method.NOTELetters in parentheses refer to the hysteresis curve (Figure 3-17). Coercive Force. The negative or reverse applied magnetizing force (H) necessary to reduce the residual magnetizingforce (B) to zero in a ferromagnetic material, after magnetic saturation has been achieved. The line (O/G) represents themagnitude and direction of this force. Direct Contact Magnetization. Use of current passed through the part via contact heads or prods to produce amagnetic field. Ferromagnetic. A term that describes a material which exhibits both magnetic hysteresis and saturation, also whosemagnetic permeability is dependent on the magnetizing force present. In magnetic particle testing, we are concerned onlywith ferromagnetic materials. Circular Magnetic Field. A circular magnetic field is a magnetic field surrounding the flow of the electric current.For magnetic particle testing, this refers to current flow in a central conductor or the part itself. Longitudinal Magnetic Field. A longitudinal magnetic field is a magnetic field wherein the flux lines transverse thecomponent in a direction essentially parallel with its longitudinal axis. Magnetic Field. The term used to describe the volume within and surrounding either a magnetized part or a currentcarrying conductor wherein a magnetic force is exerted. Magnetic Leakage Field. The magnetic field outside of a part resulting from the presence of a discontinuity, achange in magnetic permeability, or a change in the part’s cross-section. Magnetic Flux Density (B). The strength of a magnetic field is expressed in flux lines per unit cross-sectional area. Flux Lines or Lines of Force. A conceptual representation of magnetic flux illustrated by the line pattern producedwhen iron filings are sprinkled on paper laid over a permanent magnet. Magnetic Hysteresis. The phenomenon exhibited by a magnetic system wherein its state is influenced by itsprevious history. Induced Current Magnetization. Use of an externally applied magnetic field to induce current in a part to produce amagnetic field having the flux direction needed for the inspection. Useful for parts where flowing current directly through thepart would risk damaging the part. Magnetizing Current (I). The electric current passed through or adjacent to an object that produces a magnetic fieldin the object.3-2

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335- Magnetizing Force (H). The magnetizing field applied to a ferromagnetic material to induce magnetization. Magnetic Permeability (u). Magnetic permeability is the ease with which a ferromagnetic part can be magnetized.It is equal to the ratio of the flux density (B) produced to the magnetizing force (H) inducing the magnetic field. It changes invalue with changes in the strength of the magnetizing force. A metal easy to magnetize, such as soft iron or low carbon steel,has a high permeability or is said to be highly permeable. Residual Magnetism. This is the magnetic field that remains in the part when the external magnetizing force hasbeen reduced to zero. Retentivity. The property of a metal that remains magnetized after the magnetizing force has been removed. Ametal, such as hard steel has a high percentage of carbon, and will retain a strong magnetic field after removal of themagnetizing current. Hard steel has high retentivity, or is said to be highly retentive. Magnetic Saturation. This is the level of magnetism in a ferromagnetic material where the magnetic permeability isequal to one. This is characterized as that level where an increasing in magnetizing force (H) results in no greater increase inmagnetic field (B) than would occur in a vacuum or air.3.2.3 Magnetic Field Characteristics. Horseshoe Magnet. A familiar type of magnet is the horseshoe magnet (Figure 3-1). Like a bar magnet, this is apermanent magnet and possesses residual magnetism. It will attract iron filings to its ends where a leakage field occurs. Byconvention, these ends are commonly called ''north'' and ''south'' poles, indicated by N and S on the diagram. Continuousmagnetic flux lines, or lines of force in leakage fields, flow from the north to the south pole. In an ideal horseshoe magnet,the flux lines leave only at the poles and consequently an external magnetic force capable of attracting magnetic materialsexists only at the poles. This action provides an example of a longitudinal magnetic field. In a real horseshoe magnet verysmall discontinuities are distributed throughout creating small, weak, localized leakage fields distributed over the surface ofthe magnet.Figure 3-1.Horseshoe Magnet3. If the shape of an ideal horseshoe magnet is changed (Figure 3-2), the ends will still attract iron filings. However,if the ends of the magnet are fused or welded into a continuous ring as shown (Figure 3-3), the magnet will no longer attractor hold exterior magnetic materials. This is because the north and south poles no longer exist; thus a leakage field does notexist. The magnetic field will remain as shown by the arrows, but no iron filings are attracted.3-3

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335-23Figure 3-2.Horseshoe Magnet With Poles Close TogetherFigure 3-3.Horseshoe Magnet Fused Into a Ring3. A transverse crack in the fused magnet or circularly magnetized part Figure 3-4) will create a leakage field withnorth and south poles on either side of the crack. Some of the magnetic flux (lines of force) will exit the metal and form aleakage field. The leakage field created by the crack, forming an indication of the discontinuity in the metal part, will attractferrous particles. This is the principle whereby magnetic particle indications are formed.Figure 3-4.Crack in Fused Horseshoe Magnet3.2.3.2 Bar Magnet. If a horseshoe magnet is straightened, a bar magnet is created (Figure 3-5). The bar magnet has polesat either end and the magnetic lines of force flow through the length, returning around the outside. Magnetic particlesSHOULD be attracted only to the poles (in the ideal case). Such a part is said to have a longitudinal field, or is longitudinallymagnetized.3-4

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335-23Figure 3-5.Horseshoe Magnet Straightened to Form a Bar Magnet3. A transverse slot or discontinuity in the bar magnet that crosses the magnetic flux lines will create north and southpoles on either side of the discontinuity (Figure 3-6). The resulting leakage field will attract magnetic particles. In a similarmanner, a crack, even though it is very fine, will create magnetic poles as indicated in (Figure 3-7). These poles will alsoproduce a leakage field that can attract magnetic particles. The strength of the leakage field will be a function of the numberof flux lines (e.g., the strength of the internal field), the depth of the crack, and the width of the air gap at the surface. Thestrength of this leakage field, in part, determines the number of magnetic particles gathered to form indications. Clearindications are found at strong leakage fields, while weak indications are formed at weak leakage fields.Figure 3-6.Slot (Keyway) in Bar Magnet Attracting Magnetic ParticlesFigure 3-7.Crack in Bar Magnet Attracting Magnetic Particles3-5

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335- Electricity and Magnetism. Electric current can be used to create or induce magnetic fields in parts made offerromagnetic materials. Magnetic lines of force are always aligned at right angles (90 ) to the direction of electric currentflow. It is possible to control the direction of the magnetic field by controlling the direction of the magnetizing current. Thismakes it possible to induce magnetic lines of force so they intercept defects at right angles. Magnetic Attraction. Magnetic attraction can be explained by using the concept of flux lines or lines of force.Each flux line forms a closed continuous loop, which is never broken. For a circularly magnetized object, the flux lines arewholly contained in the object (ideal case). No external magnetic poles are present and therefore there is no attraction forother ferromagnetic objects. For a longitudinally magnetized object, the flux lines leave and enter at magnetic poles. Theyalways seek the path of least resistance (e.g., maximum permeability and minimum distance). When a piece of soft iron isplaced in a magnetic field it will develop magnetic poles. These poles will be attracted to the poles of the magnetic object thatcreated the initial field. As it approaches closer to the source of the original field, more flux lines will flow through the pieceof iron, thus creating stronger magnetic poles and further increasing the attraction. This concentrates the lines of flux into theeasily traversed high permeability (iron path) rather than the alternative low permeability (air paths). This is magneticattraction and is the reason magnetic particles concentrate at leakage fields. The leakage field is established across an air gapof relatively low permeability at the discontinuity. Since they offer a higher permeability path for the flux lines, the magneticparticles are drawn to the discontinuity and bridge the air gap to the extent possible. Effects of Flux Direction. The magnetic field must be in a favorable direction, with respect to a discontinuity, toproduce an indication. When the flux lines are parallel to a linear discontinuity, the indications formed will be weak. The bestresults are obtained when the flux lines are perpendicular (at right angles) to the discontinuity.NOTEWhen an electrical current is used for magnetizing, the best indications are produced when the path of themagnetizing current is parallel to and in-line with the discontinuity. Circular Magnetization. A circular magnetic field always surrounds a current carrying conductor, such as a wireor a bar (Figure 3-8). The direction of the magnetic lines of force (magnetic field) is always at right angles to the direction ofthe magnetizing current. Field orientation and magnitude are based on the direction and amount of current flow.Figure 3-8.Magnetic Field Surrounding an Electrical Conductor3. Since metals are conductors of electricity, an electric current passing through a metallic part creates a magneticfield (Figure 3-9). The magnetic lines of force are at right angles to the direction of the current. This type of magnetization iscalled circular magnetization because the lines of force, which represent the direction of the magnetic field, are circularwithin the part.3-6

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335-23Figure 3-9.Magnetic Field in a Part Used as a Conductor3. Circular Magnetization with Inspection Equipment. One method of creating or inducing a circular fieldwithin a part with stationary MPI equipment is to clamp the part between two contact plates and pass current through the partas indicated in (Figure 3-10). If a longitudinally aligned crack or discontinuity exists within the part, a leakage field will beestablished at the site of each crack or discontinuity. The leakage field will attract magnetic particles to form an indication ofthe discontinuity.Figure 3-10.Creating a Circular Magnetic Field in a Part3. For hollow or tube-like parts, it is often important to inspect both the inside and outside surfaces. When suchparts are circularly magnetized by passing the magnetizing current through the part ends, the magnetic field on the insidesurface is smaller and opposite than what is produced on the outside surface. To produce a stronger magnetic field on boththe inner, and outer surface of the part, a separate conductor, such as a copper rod, is positioned inside the hollow part (seeFigure 3-11 and Figure 3-12). Since a circular magnetic field surrounds such conductors when an electric current is passedthrough them, it is possible to induce a satisfactory magnetic field on the inside surface and depending on the thickness of thepart, the outside surface as well.Figure 3-11.Using a Central Conductor to Circularly Magnetize a Cylinder3-7

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335-23Figure 3-12.Using a Central Bar Conductor to Circularly Magnetize Ring-Like Parts3.2.3.7 Longitudinal Magnetization. Electric current can also be used to create a longitudinal magnetic field in a test partwith a current carrying encircling coil. Based on the perpendicular direction of magnetism to current direction, any segmentof a coiled conductor will show the field within the coil consists of contributions from each turn of the coil and is alignedlengthwise as indicated (Figure 3-13).Figure 3-13.Magnetic Lines of Force (Magnetic Field) in a Coil3. If a part is placed inside a coil (Figure 3-14), the magnetic lines of force created by the coil are aligned along thelongitudinal axis of the coil. If the part is ferromagnetic, the high permeability concentrates the lines of flux within the partand induces a strong longitudinal magnetic field.Figure 3-14.Longitudinal Magnetic Field Produced in a Part Placed in a Coil3. Longitudinal Magnetization with Inspection Equipment. Inspection of a solid bar part using longitudinalmagnetization is shown (Figure 3-15). When a transverse discontinuity exists in the part, as in the illustration, a magneticleakage field is formed at the crack location. This attracts magnetic particles, forming an MPI indication of the transversediscontinuity. Compare (Figure 3-15) with (Figure 3-10), and note in both cases, a magnetic field has been induced in the partat right angles to the defect. This is the most desirable condition for reliable inspection.3-8

TO 33B-1-1NAVAIR 01-1A-16-1TM 1-1500-335-23Figure 3-15.Longitudinal Field Produced by the Coil Generates an Indication of Crack in Part3.2.3.8 Multi-Directional Magnetic Field. Two separate fields, having different directions, cannot exist in a part at thesame time. However, two or more fields in different directions can be imposed upon a part sequentially in rapid succession.When this is done, magnetic particle indications can be formed when discontinuities are located favorably with respect to thedirections of any of the applied fields, and will persist as long as the rapid alternations of field direction continue. Indicationscan only be formed if the part is pre-wetted with magnetic particles. This enables the detection of defects oriented in anydirection in one operation. The indications must be viewed when the fields are being applied because they are weakly heldafter the current is discontinued and can be easily dislodged. Parallel Current Induced Magnetic Field. If a ferromagnetic bar is placed alongside, and parallel to, a conductorcarrying current, a magnetic field will be set up in the bar more transverse than circular (Figure 3-16). Such a field is of verylittle use for magnetic particle testing. Operators have tried to use this method as a substitute for a headshot for the purpose ofproducing circular magnetization, but the field produced is not circular and is extremely limited in the transverse directionwhen inspecting for defects such as seams. Furthermore, the external field around the conductor and the bar can attractmagnetic particles and produce confusing backgrounds.Figure 3-16.Field Produced in a Bar by a ''Parallel'' Current3.2.4 Currents Used to Generate Magnetic Fields. There are several types of current used in MPI. These are StraightDirect Current (DC), Single-Phase Alternating Current (AC), Three-Phase AC Current, Half-Wave Rectified AlternatingCurrent (HWRAC or HWDC), Full-Wave Rectified AC Current, and Three-Phase Full-Wave Rectified AC Current(commonly known as DC). Of these, three types of magnetizing current are most often used in magnetic particle inspection.Only one type of current is best suited for each type of inspection to be performed. Alternating current (AC) is preferred forthe detection of surface discontinuities. Direct current (DC), full-wave direct current (FWDC), or half-wave direct current(HWD

In magnetic particle testing, we are concerned only with ferromagnetic materials. Circular Magnetic Field. A circular magnetic field is a magnetic field surrounding the flow of the electric current. For magnetic particle testing, this refers to current flow in a central conductor or the part itself. Longitudinal Magnetic Field.

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