Magnetic Particle Inspection (MPI)
Definition
A non-destructive testing method that reveals magnetic-domain contrast between cold-worked and unstressed zones in ferromagnetic metals using a ferrofluid under UV illumination; must precede any chemical etching.
- Method type
- Non-destructive testing
- Suitable for
- Ferromagnetic metals (steel, iron, etc.)
- Sequence rule
- Must precede chemical etching
Common questions
What makes MPI useful for restoring erased serial numbers?+
When a serial number is stamped into metal, it creates a cold-worked zone that retains magnetic properties even after the stamp marks are removed. MPI magnetizes the surface and applies iron-oxide particles that collect along the residual strain field, making the erased number visible without damaging the metal.
What substrates can you use MPI on?+
MPI works on ferromagnetic metals. Firearms and other stamped metal objects are common forensic targets.
Can MPI damage the evidence?+
No. It is a non-destructive testing method, so it reveals the erased mark without altering the surface. This is why it must be done before any chemical etching, which is destructive and cannot be undone.
Related terms
- Electrolytic Etching
- Electrochemical alternative to chemical etchants. The part is the anode in an electrolyte bath; controlled current selectively dissolves the strained metal at...
- Fry's Reagent
- Classical etchant for plain carbon and low-alloy steel: 40 g cupric chloride (CuCl2) + 90 mL concentrated HCl + 100 mL water....
- ASTM E2885-13
- ASTM International standard practice for restoration of obliterated vehicle identification numbers; the reference protocol for VIN casework in US and internationally aligned...
- Backscattered Electron (BSE) Imaging
- An SEM imaging mode sensitive to atomic number and crystallographic texture; can reveal grain-boundary elongation patterns in the sub-surface cold-worked zone after...
- Cold-Working Stress Field
- Residual plastic deformation and elevated dislocation density in a metal lattice created by stamping; persists below the visible surface even after surface...
- Davis Reagent
- Variant etchant for steel using cupric ammonium chloride (Cu(NH4)2Cl4) in dilute HCl. Slower acting than Fry's, sometimes preferred when contrast on Fry's...
- IMEI (International Mobile Equipment Identity)
- A unique 15-digit number permanently assigned to a mobile device's hardware. Used by networks to identify and block stolen devices, and by...
- INTERPOL iARMS
- INTERPOL's Illicit Arms Records and tracing Management System. Multilateral database enabling law enforcement in INTERPOL member countries to trace recovered or seized...
- Neutron-Induced X-Ray Fluorescence (NIXRF)
- A synchrotron-based method using neutrons to probe deep obliterations; can detect digit geometry buried several millimetres beneath a machined surface, entirely non-destructively.
- Obliteration
- Covering of existing writing with an opaque layer (ink, correction fluid, or correction tape) so that the underlying text is invisible in...
- Profilometry
- White-light interferometry or confocal laser scanning to detect nanometre-scale height differences on a metal surface; can reveal topographic ghost images of obliterated...
- Stamping Depth
- The depth to which the original punch deformed the metal grain. Typically two to three times the visible indentation depth. Restoration works...
Explained in these topics
- Restoration of Erased Serial NumbersA non-destructive testing method that reveals magnetic-domain contrast between cold-worked and unstressed zones in ferromagnetic metals using a ferrofluid unde...
- Serial Number and Erased Mark RestorationNon-destructive method for ferromagnetic substrates. The surface is magnetised, iron-oxide particles are applied, and they collect along the residual strain fi...