Unsourced material may be challenged and removed. Cracks that would normally have been invisible are detectable magnetic particle inspection pdf to the magnetic particles clustering at the crack openings. The scale at the bottom is numbered in centimetres.
The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source.
To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form what is known as an indication, which is evaluated to determine its nature, cause, and course of action, if any. There are several types of electrical currents used in magnetic particle inspection. For a proper current to be selected one needs to consider the part geometry, material, the type of discontinuity one is seeking, and how far the magnetic field needs to penetrate into the part. Because the current alternates in polarity at 50 to 60 cycles per second it does not penetrate much past the surface of the test object. This means the magnetic domains will only be aligned equal to the distance AC current penetration into the part. The frequency of the alternating current determines how deep the penetration.
AC can not penetrate deep enough to magnetize the part at the depth needed. The amount of magnetic penetration depends on the amount of current through the part. DC is also limited on very large cross-sectional parts in terms of how effectively it will magnetize the part. DC, but allows for detection of surface breaking indications and has more magnetic penetration into the part than FWDC.
HWDC is advantageous for inspection process as it actually helps move the magnetic particles during the bathing of the test object. The aid in particle mobility is caused by the half-wave pulsating current waveform. In a typical mag pulse of 0. 5 seconds there are 15 pulses of current using HWDC. This gives the particle more of an opportunity to come in contact with areas of magnetic flux leakage. An AC electromagnet is the preferred method for find surface breaking indication.
The use of an electromagnet to find subsurface indications is difficult. An AC electromagnet is a better means to detect a surface indication than HWDC, DC, or permanent magnet, while some form of DC is better for subsurface defects. Using a similar machine, a U. Navy technician sprays magnetic particles on a test part under ultraviolet light. An automatic wet horizontal MPI machine with an external power supply, conveyor, and demagnetizing system.
It is used to inspect engine cranks. A wet horizontal MPI machine is the most commonly used mass-production inspection machine. The machine has a head and tail stock where the part is placed to magnetize it. Most of the equipment is built for a specific application. Mobile power packs are custom-built magnetizing power supplies used in wire wrapping applications.
Magnetic yoke is a hand-held device that induces a magnetic field between two poles. The draw back of magnetic yokes is that they only induce a magnetic field between the poles, so large-scale inspections using the device can be time-consuming. For proper inspection the yoke needs to be rotated 90 degrees for every inspection area to detect horizontal and vertical discontinuities. Subsurface detection using a yoke is limited.
These systems used dry magnetic powders, wet powders, or aerosols. After the part has been magnetized it needs to be demagnetized. This requires special equipment that works the opposite way of the magnetizing equipment. The magnetization is normally done with a high current pulse that reaches a peak current very quickly and instantaneously turns off leaving the part magnetized.
To demagnetize a part, the current or magnetic field needed has to be equal to or greater than the current or magnetic field used to magnetize the part. The current or magnetic field is then slowly reduced to zero, leaving the part demagnetized. Pull-through AC demagnetizing coils: seen in the figure to the right are AC powered devices that generate a high magnetic field where the part is slowly pulled through by hand or on a conveyor. The act of pulling the part through and away from the coil’s magnetic field slows drops the magnetic field in the part. AC decaying demagnetizing: this is built into most single phase MPI equipment. As AC is alternating from a positive to a negative polarity this will leave the magnetic domains of the part randomized. AC demag does have significant limitations on its ability to demag a part depending on the geometry and the alloys used.
Reversing full wave DC demagnetizing: this is a demagnetizing method that must be built into the machine during manufacturing. It is similar to AC decaying except the DC current is stopped at intervals of half a second, during which the current is reduced by a quantity and its direction is reversed. Then current is passed through the part again. The process of stopping, reducing and reversing the current will leave the magnetic domains randomized. This process is continued until zero current is passed through the part.