Determining the direction of the field is important when conducting a magnetic particle inspection because the defect must produce a significant disturbance in the magnetic field to produce an indication. It is difficult to detect discontinuities that intersect the magnetic field at an angle less than 45o. When the orientation of a defect is not well established, components should be magnetized in a minimum of two directions at approximately right angles to each other. Depending on the geometry of the component, this may require longitudinal magnetization in two or more directions, multiple longitudinal and circular magnetization or circular magnetization in multiple directions. Determining strength and direction of the fields is especially critical when inspecting with a multidirectional machine. If the fields are not balanced, a vector field will be produced that may not detect some defects.
Ferrites Commonly known as Ceramics, have been in production since the 1950’s. They are primarily made from Iron Oxide (FeO) and the addition of Sr and Ba through a calcining process. They are the least expensive and most common of all magnet materials. Primary grades are C1, C5 and C8. They are mostly used in motors and sensors.
To make a magnet “magnetic” it must be exposed to a strong external magnetic field. This field reorganizes the magnet’s domain structure and leaves the magnet with a remanent magnetization (Br). If a magnet is isotropic, the remanent magnetization has the same direction as the external field. Meanwhile, an anisotropic magnet can only be magnetized in its anisotropy direction.
Magnetic field is a physical phenomena induced by the motive electric charges in their surrounding space. Magnetism of materials arises from two atomic sources: the spin and orbital motions of electrons. Therefore, the magnetic characteristics of a material may change as a function of alloying constitution. Magnetic field strength is the measure of the vector magnetic current, or a magnetic body, to induce a magnetic field at a given point, denoted as H, measured in A/m (amperes per/meter, SI electromagnetic unit system) or Oe (oersteds, CGS electromagnetic unit system). One unit of A/m is defined as the field strength value induced by an infinite long linear conductor bearing one unity ampere electric current that locates 1/(2Л) meters away from the measured position. 1Oe=1/(4Л)×103A/m.
There’s no escaping magnetic fields—they’re all around us. For starters, the Earth itself is like a giant magnet. A spinning ball of liquid iron in our planet’s core generates the vast magnetic field that moves our compass needles around and directs the internal compasses of migrating birds, bats, and other animals. On top of that, ever-industrious humans have produced artificial magnetic fields with power lines, transport systems, electrical appliances, and medical equipment.
Permanent magnets are those which create their own persistent magnetic field when magnetized by an external magnetic field. The function of permanent magnets in different appliances and instruments is usually one of the following: