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Magnetic sawing

Stan Zurek, Magnetic sawing, Encyclopedia-Magnetica.com, {accessed on 2019-03-19}
Illustration of magnetic sawing1) Copyrights © A.J. Sitzman

Magnetic sawing, magnetic cutting or non-contact magnetic cutting2) - a damage mechanism occurring in electrical conductors exposed to high-amplitude pulses of electric current.3)

Extremely high current amplitudes generate heat which can locally melt the metal of the conductor. At the same time large magnetic forces are generated because of the associated magnetic field.

The forces push away the softened or molten metal creating a cut, which further concentrates the current and forces. As a result, a runaway process can develop and a deep magnetic cut can be produced.

The phenomenon was first described in 1957 by Furth et al.4) The defects appeared “usually at the coil edge or or other especially vulnerable place”.

Magnetic sawing can occurs at a sharp change or transition between two metals 5)

by S. Zurek, E. Magnetica, CC-BY-3.0

Magnetic sawing can be considered as the worst case of melt-wave erosion, which works in a similar way (molten metal removed by magnetic force), but over smaller volume. Magnetic sawing produces a narrower and deeper crack, which can cut through the whole thickness of the conductor.6)

Melton et al. demonstrated an experiment with aluminium bars with cross-section 44 mm x 16 mm (40 cm long) covered with thinner copper strips. A current of 1700 kA was applied for 2.1 ms or 4.39 ms. Such excitation was sufficient to create deep cuts (several mm) around the ending of the copper strip.7)

Sitzman experimented with cutting of aluminium and copper plates. The electromagnetic excitation was applied from a 13-turn coil made from copper ribbon 0.5 mm x 6.3 mm. The coil had OD 25.4 mm and ID 10.7 mm. The Al and Cu plates exposed to cutting were 0.8 mm thick, with 51 mm x 51 mm area.

The electromagnetic pulses were produced by discharging capacitors into the coil, with a specially constructed pulsed power supply. Pulses up to 80 kA could be produces, with estimated current density in the sample at the level of 105 A/mm². The pulses lasted tens of μs, and were repeated every 1 s.8)

Photomicrographs of magnetic cuts at various excitation levels used by Sitzman9) Copyrights © A.J. Sitzman
Experimental setup for investigation of magnetic cutting used by Sitzman10) Copyrights © A.J. Sitzman
3D drawing of a coil used for magnetic cutting by Sitzman 11) Copyrights © A.J. Sitzman
Clean blowhole and bifurcation produced by Sitzman for different excitation conditions 12) Copyrights © A.J. Sitzman
Magnetic cutting can propagate at a speed of 25 m/s (Sitzman)13) Copyrights © A.J. Sitzman
Magnetic cut in Al plate produced by 200 pulses (Sitzman)14) Copyrights © A.J. Sitzman

Magnetic sawing can be used also as a useful effect, for instance for removing of unwanted metal objects.15)

In theory it can be also used for producing a cut in metal sheet. Such procedure can produce fine cuts with arbitrary length and shape, and can be referred to as non-contact magnetic cutting.16)

See also

References


2), 8), 16) A.J. Sitzman, Investigation on Cutting Metals using Induced Currents, PhD thesis, The University of Texas at Austin, 2014, Austin, Texas, USA
4) H.P. Furth, M.A. Levine, R.W. Waniek, Production and use of high transient magnetic fields II, Rev. Sci. Instrum., Vol. 28 (1), p. 949, 1957
magnetic_sawing.txt · Last modified: 2017/01/30 15:08 by Stan Zurek

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