A hole in a core lamination has a manufacturing history before a bolt or fixture applies any load. Punching or other hole-making processes may leave a changed local condition. Later assembly can introduce another stress distribution through contact and restraint. A magnetic result obtained only after assembly combines the two.
Separate these contributions with a defined sequence. Record the hole geometry and material condition, observe the unloaded state, apply the selected assembly load, and repeat after unloading. A matched unperforated reference can help establish the background, provided its magnetic geometry is treated appropriately.
Map distance from the boundary
Choose observation regions referenced to the actual hole edge. Use several surrounding bands or sectors where the measurement method can resolve them. These are sampling regions, not an assumed damaged-zone width. Sector direction relative to rolling direction and the applied load matters.
Document whether the magnetic measurement can isolate each region or averages over a larger volume. A whole-strip result cannot directly identify a narrow local annulus without an additional model or measurement.
| State | Manufacturing history retained | Assembly load | Question addressed |
|---|---|---|---|
| Unperforated reference | Matched material and handling | Defined reference restraint | Background response |
| Hole present, unloaded | Hole-making process | No applied assembly load | Persistent local manufacturing condition |
| Hole under controlled loading | Same hole and material | Measured load/contact state | Additional assembly effect |
| Hole after unloading | Same history | Re-established reference | Recovery or remaining change |
The difference between these states is meaningful only if the magnetic preparation and measurement boundary are comparable.
Do not import an isotropic textbook factor
A simple hole stress-concentration factor from an ideal isotropic plate does not automatically represent a textured, thin lamination in a multilayer assembly. Bearing contact, friction, edge distance and neighboring layers can change the local state. Use the simple solution as a clearly identified reference, then test the assumptions against the actual geometry.
NIST’s applied-load diffraction work describes sheet-stress measurements around features such as holes and in cases where force divided by area is inadequate. It supports the need for local evidence, while a GOES-specific stress conversion still requires qualification.
Link the load path to the magnetic observation
Record bolt or fixture contact position and the actual load evidence. Torque alone is not a complete description of lamination stress. If loading changes hole-edge contact or sheet alignment, retain those geometric observations as separate possible explanations.
Compare in-load and unloaded magnetic responses before calling a change residual damage. A recovered response suggests a different question from one that persists after the reference state is restored. Inspect the original coating and steel where contact can alter them.
The completed spatial record shows whether the relevant magnetic change follows the hole-making history, the installed load path, or both. That finding can guide a process or assembly revision without attributing every local effect to the visible cut edge.
Technical references
- Materials / MDPI. Magnetostrictive Properties of the Grain-Oriented Silicon Steel Sheet under DC-Biased and Multisinusoidal Magnetizations.
- NIST. NCAL: Diffraction Stress Measurement Under Applied Load.