Permeability (m) on the other hand is a magnetic property. Not all metals have significant levels of permeability. The permeability value of a metal suggests how it will alter a magnetic field moving through it. Metals with high permeability values (carbon steels) restrict the inspectable area to the upper skin layer with a given probe. Fortunately, the largest percentage of aircraft structures and surfaces are made from nonferromagnetic materials. Their relative permeability is assumed to be one. This means that as the primary magnetic field moves through these materials their inherent permeability does not alter the magnetic field energy level.
The third major test variable, geometry, depends on the type of probe being used. Standard terminology for surface scanning applications include lift-off and edge effect. For either bobbin or encircling coil probe styles the terminology addresses fill factor and end effect. Geometry responses are considered to be a limiting factor in our ability to detect true material changes. This type of response could lead to a poor signal-to-noise (S/N) ratio. Anything that we can do to limit geometry responses from our field of view will improve the quality of the test.
Primary / Secondary Force Relationships
The coil becomes the tool that determines how well we can link our coil's energy into a test piece. The choice of coils becomes critical to how well we can perform a test. The magnetic field produced about a coil is directly proportional to the applied current magnitude (amperage), the rate of current change (frequency), as well as the coil's physical parameters. Coil parameters include factors such as inductance, diameter, length, thickness, number of turns of wire, and core material.
With a primary alternating (AC) current (Ip) flowing through the coil, a primary electromagnetic field (fp) is produced about the coil. By placing this excited test coil on (or near) an electrically-conductive test object we will begin to generate eddy currents (IE) in the test object.
The distribution of eddy currents in a test object varies exponentially with depth from the surface. The current density in the test object is most dense near the test coil. Using a simple calculation, it becomes apparent that eddy current testing is best for surface or near surface detection issues. A typical inspection range would be about the first 0.2 inches from the inspection surface. The energy would be concentrated closer to the upper surface as frequency, conductivity, or permeability increases.
A term can be developed to describe the distribution of current density in the test object. Consider the eddy current generated at the surface (right under the test coil) to be 100 percent of the available current. The point in the test object thickness where this current is diminished to 37 percent of its original value is known as "one standard depth of penetration" (d, delta). Figure 2 (on previous page) shows how the available energy is rapidly decreasing as we move deeper and deeper into the test sample. As the energy level decreases we are less likely to detect a change in the material at that point.
Frequency Choice Frequency is going to be the main tool that we use to define the "field of view" during our inspection. The practical field of view is considered to be about 2.5 to 3d (three times the calculated depth of penetration). If our test is designed to look for small stress cracks on the upper surface, then our decision is probably going to be for a smaller coil diameter with a fairly high frequency range. We would want to keep our energy in the area of interest to maintain good flaw sensitivity. If we know that we need to put more energy deeper into a material to detect back surface or second layer conditions, then we are going to have to select a lower inspection frequency. How do we decide what is a reasonable frequency choice for this type of test?
A simple formula using the material's electrical properties (p) and its thickness (t), where "f" is in hertz, "t" is in inches, and "r" is in micro-ohm centimeters (mWcm):
f = 4(p) / t2
Because of the way the formula has been arranged we can disregard the issue of mWcm being in the metric system. We simply look up the resistivity value on that scale and plug in the number. I normally round off the resistivity for all aluminum to about 5. If we assumed that we wanted to detect a back wall crack on a piece of aluminum 0.2-inch thick we would get:
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