Magnetic permeability is the aspect of a material which illustrates the degree a material is attracted to a magnet (from an engineering standpoint, magnetic permeability is a material’s ability to support an internal magnetic field induced by an external magnetic field). We learned in the “Magnetic Permeability” blog posting that the permeability of a material is dependent upon several factors and that one of these factors is the applied drive-field strength presented to the steel.
The ability to induce a pole in a material is not infinite and is limited as a material’s magnetic permeability decreases. When the magnetic permeability of a “workpiece” drops to “1” the material is said to be saturated. This is the point where no more external applied field (H) will give rise to any additional magnetic induction (B). No additional magnetic induction, therefore no additional attractive force; the system has reached its maximum limit and it is said to be at saturation.
Saturation usually occurs in thinner workpieces where there is less volume of material to act on. Oftentimes, the entire workpiece does not saturate and there is actually a gradient within the workpiece with only portions being driven into saturation. Even though the entire workpiece does not saturate, the force is still limited because the non-saturating areas are not contributing to the attractive force.
Directly measuring the degree of saturation within a material can be difficult as this is an internal phenomenon. There are, however some first order methods which may indicate a system is in saturation. One option is to inspect the non-working face of the workpiece (the face which is opposite of the externally applied magnetic field) and try to observe any field “leaking” through. The degree of the leakage is a good indicator of saturation and suggests the workpiece would benefit from being thicker. The other option is to increase the workpiece’s thickness and see what effect it has. If one doubles the thickness of the workpiece and receives a measurable increase in attractive force, then the original workpiece was saturating, however, there is a point of diminishing returns. Increasing the thickness of a workpiece is generally always beneficial, but it will impact cost, mass, and possibly ease of manufacturing.