Magnetic Particle Testing · MT

The Skin Effect

← Exercise Hub
The Concept

AC hugs the surface. When alternating current runs through a part, the current doesn't spread through the whole bar — it crowds into a thin layer at the surface, like water flowing only along the outside of a pipe. That's the skin effect. It makes AC great at finding surface cracks (all the magnetism is right where the crack is), and blind to defects hiding deeper inside. To reach those, we rectify the current toward DC, which flows through the whole cross-section.

Part 1 — The Penetration Lab

Pick a current · watch where it flows · see which defects show up

Head shot — the bar is clamped between the bench head-stocks and current passes through it

circular magnetic field we cut here ↓ current in current out head-stock steel bar · 25 mm
↓ the slice at the cut, face-on — orange = where the current flows ↓
2.5 mm 5 mm 7.5 mm A B C D
current density particle indication discontinuity

Pick the magnetizing current

Reach into the part≈ 2 mm
surfacecenter (12.5 mm)
Particle mobility
What's happening

Discontinuities in this bar

Try this
  • Start on AC — only the surface crack shows strongly. The current (and flux) never reaches B, C or D.
  • Switch to HWDC — watch C light up. This is why we rectify the current for subsurface defects.
  • Try DC — it reaches just as deep, but the indications are duller: no pulse shaking the particles.
  • Nothing finds D reliably. MT is a surface and near-surface method.

Part 2 — Predict & Check

7 questions · instant feedback

Question 1 / 7 Score 0