NDT Consultants Magnetic Flux Strip

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NDE Information Consultants is the exclusive distributor of the NDT Consultants Magnetic Flux Strip for the entire Western hemisphere. Contact our Sales Department for information about these products.

These strips are useful in determining whether similar sized surface imperfections in parts will give a magnetic particle indication.  Two types of strip are available with similar thicknesses but differing test flaw widths. Each has three milled slots in high-m material as shown. For both types, the top and bottom brass plates are 0.0020-in. thick. The steel sandwich is also 0.0020" thick.  The bottom brass layer acts as a lift-off of 0.002-in. from the part, and thus places the active steel element a small fixed but unavoidable distance above the part. The brass is non- ferromagnetic, and thus plays no other role than protection of the slots, and lift-off.

Type G: (General Use)
The three slots have widths 0.0075", 0.009" and 0.010".  The strip is most sensitive when the narrowest slot is seen under MPI.

Type A: (Aerospace)
The three slots have widths 0.003", 0.004" and 0.005"

As is well known, (Bray & Stanley - Nondestructive Evaluation - A Tool in Design, Manufacturing and Service) in an active magnetic field, wider slots of the same depth have larger amounts of magnetic flux leakage than narrower slots, and therefore give bigger magnetic particle indications.   Use of these flux indicators enables magnetizing field levels to be set for a variety magnetizing conditions such as internal (central) conductors, coils, yokes, cable wraps, etc.

flux diagram

When used in conjunction with a hall-effect gaussmeter, (Teslameter) measuring the tangential surface field strength, they can provide assurance that magnetizing current and field levels set by the gaussmeter do in fact provide sufficient field for MPI without the over-magnetization that often occurs when using outdated formulae.

Taking the widths of the G slots as twice those of the A slots, and knowing that the particle-holding ability of the leakage field from a slot is proportional to (HgLg)2 where Hg is the field in the gap, and Lg is the width of the slot, we see that the thinner slots provide for higher sensitivity to tighter flaws. Taking a special case where the field Hg is the same for both types, then the MFL from type G slots is roughly 4 times that from type A slots (from Lg2), and so will more easily hold powder indications.

flaw diagram

The strips are sold in packages of 5.  

The picture at right shows a flux strip on 1.75-in steel tubing with a weld seam defect using AC magnetic flux and dry magnetic particles.

Notes: The G strips have proved to be extremely popular for the detection of residual circular flux in oilfield tubulars, since they show that a surface imperfection of the same width and depth will give a similar indication as the one seen on the strip. Should a brass layer peel off, it may be re-attached with super-glue.

The above information is available as a PDF file: Download Sales Brochure

NDE Information Consultants

last updated November 23, 2006