Discontinuity signals are taken from the tape by an

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Discontinuity signals are taken from the tape by an array of tape recorder heads. Elongated magnetic balloons also exist for the testing of the inside surface of tubes. Scale, dirt or oil on the test surface can contaminate the tape. Surface roughness can tear the tape. Magnetic Particles Magnetic particles are finely ground high permeability magnetic material, sometimes dyed for visible contrast with the test surface. Ideal test conditions occur when a fine spray of such particles is intercepted by a magnetic flux leakage field and some of them stick to the field. An advantage over other forms of magnetic indicators is that the particles have zero liftoff from the discontinuity field. In a simple approximation, the force F mag holds the particles in the leakage field: (20) where is the vector differential operator (gradient operator), H is the local flux leakage field intensity (ampere per meter), V vol is the volume (cubic meter) of the particle, α is a factor related to the demagnetization factor of the particle and μ 0 is the permeability of free space ( μ 0 = 4 π × 10 –7 H·m –1 ). The force that holds the particle to the discontinuity leakage field is proportional to the result of a vector calculus operation on the leakage field. This force can be computed for simple leakage fields by using approximations such as those by Förster 28 and by Zatsepin and Shcherbinin 29 or more accurately by using finite element techniques such as those described elsewhere, in this volume’s chapter on modeling. The force is also F V mag vol = ⋅∇ ( ) α μ 0 H H 158 Electromagnetic Testing F IGURE 45. Response of magnetodiode is linear up to about 40 kA·m –1 (500 Oe) at ambient temperature of 25 °C (77 °F) and potential of 6 V. 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 –1.2 –1.4 –1.6 –200 –120 –40 0 40 120 200 (–2.5) (–1.5) (–0.5) (0.5) (1.5) (2.5) Magnetic field intensity H , A·m –1 (kOe) Potential (V) F IGURE 46. Frequency response of magnetodiode at ambient temperature of 25 °C (77 °F). Output potential, Δ V (V) Magnetic field frequency (kHz) 0 –3 –6 –9 –12 0.02 0.04 0.1 0.2 0.4 1 2 4 10 20 40 F IGURE 47. Temperature dependence of magnetodiode. Output potential Δ V (V) Center potential V m (V) Ambient temperature, °C (°F) 1.5 1.0 0.5 –40 –20 0 20 40 60 80 (–104) (–54) (54) (104) (140) (176) V m Δ V E = 6 V H = + 80 kA·m –1 ( + 1 kOe) 3.0 2.0 1.0
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related to the shape of the particle, which is implied by the term α . For spherical particles, α = 3. For particles with a prolate ellipsoidal shape and an axis ratio of 2:1, α = 5.8. As particles become more elongated, α increases and the magnetic force on them in a given leakage field also increases. Elongation of the particle decreases the demagnetization factor for the particle. Additional decreases in demagnetization factor (or increases in α ) are achieved when particles stick together, end to end.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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