Drag Force Method (DFM)
Introduction
MagCanica has recently developed an innovative and effective method of non-destructive evaluation based on an entirely new physical principle and construction of the scanning device. This Drag Force Method (DFM) has the potential to provide significant benefits and is currently being developed for a variety of applications including void and crack detection, shot peening monitoring, and pipeline inspection gauges.

Conventional Magnetic Flux Leakage (MFL) Methods of NDE
Conventional Magnetic Flux Leakage (MFL) methods of non-destructive evaluation (NDE) are based on the fields which arise from the divergence of magnetization in inhomogeneous or physically discontinuous material. The inhomogeneity can be associated with either (or both) dimensional or permeability variation. It is easy to visualize that fields will arise in the vicinity of those "flaws" which can be characterized as "missing" material, e.g., holes, dents, corrosion, scratches, foreign matter inclusion, and the like. Conventional MFL techniques all rely on magnetizing local portions of the sample being examined (using permanent magnets or electromagnets), and sensing the fields in proximate space. Interpreting the signature features of these fields provides information on the location, size and other characteristics of the causative flaw. Conventional MFL techniques generally utilize Hall Effect field sensors, or pick-up coils when there is continuous motion of the sample or magnet.

MagCanica's Drag Force Method (DFM) of NDE
MagCanica's Drag Force Method (DFM) of non-destructive evaluation constitutes a significant innovation and offers a number of potential benefits. It employs force sensors to sense the interactive force between a permanent magnet (which does the magnetizing) and the dipole moments which arise with inhomogeneous magnetization of the underlying material being scanned for defects. If the material is homogeneous and free of defects, these forces will be equal and opposite (i.e., symmetrical) on either side of the magnet's neutral plane. While the material will be thereby stressed, there will be no net tangential (drag) force acting on either the material or the magnet. Inhomogeneity such as that caused by a crack, for example, clearly upsets this balance, and the resulting net force is sensed by the force sensor. Note that the force measured in the Drag Force Method is not the normal attractive force which acts to bring the material being scanned and the magnet closer together. In fact, the material being scanned and the magnet must each be supported in such fashion as to prevent motion in this direction.


Schematic diagram of the Drag Force Method (DFM). The permanent magnet (PM) scans over the sample under test (SUT), generating a net tangential drag force indicative of the level of underlying inhomogeneity in the sample, such as that which might be caused, for example, by a crack or a void.

Benefits of MagCanica's DFM and Development Plans
The key benefits of the Drag Force Method, i.e. sensing the reaction force on the magnet, compared with the conventional Magnetic Flux Leakage method, i.e. sensing the field associated with the flaws, are the following:
  • Conventional Magnetic Flux Leakage NDE systems utilize field sensors, especially galvanomagnetic (Hall effect or magnetoresistance) sensors, that are small and have highly localized sensing regions, and therefore require the use of substantial numbers (10-100) in arrays to simultaneously scan significant areas. Such an arrangement inherently implies significant complexity and processing capability.
  • Drag Force Method NDE systems utilize a magnet/force sensor combination that can scan an area an order of magnitude larger than the equivalent magnet/field sensor combination that would be used in an equivalent MFL system
MagCanica is in the relatively early stages of development of the DFM, and has recently begun to explore its range of applications. As part of this effort, MagCanica is evaluating a variety of magnet/force sensor arrangements, each configured for a specific application. Potential applications include void, crack, and corrosion detection for oil & gas pipelines or nuclear power plant transfer piping, shot peening monitoring and closed-loop control, fatigue and structural health monitoring, and electrical steel quality control.