Yoke (Magnetic Particle Testing) — Definition & NDT Use
A portable electromagnet device used for magnetic particle testing, shaped like an inverted U. The yoke magnetizes the test area between its two poles. Yokes are convenient for field inspection as they are portable and require only 110V power. Yoke lift-off capability determines magnetization quality. Different yoke designs accommodate various component geometries and space constraints.
On the job, Yoke sits between the procedure and the indication — its calibration record, serial number, and condition all flow into the inspection report and the audit trail. The magnetising current creates a field that runs continuous through the part; at a discontinuity the lines of flux squeeze around the gap and break the surface as a leakage field, where dry powder or wet-suspension particles cluster and outline the flaw to the inspector's eye. As the alternating coil approaches the conductive surface it drives circulating eddy currents; any change in the part — a crack, a thickness change, a permeability shift — perturbs those currents and registers as a phase-and-amplitude shift on the impedance plane. Calibration certificates, condition logs, and traceable serial numbers are what make the difference between an instrument that shows a number and an instrument whose number stands up in court or in front of an auditor.
The instrument's inspection scope is set by its OEM specification, its current calibration certificate, and any customer-specific qualifications that have been logged against it; a Yoke that is in calibration but unqualified for a customer's procedure is still off the job.
- Etymology / Origin
- Old English geoc (a wooden bar across draught animals); the magnetic-particle yoke is named for its U-shape.
- Formula
- Magnetising force on test surface: lift power test ≥ 4.5 kg (10 lb) for AC yoke, ≥ 18 kg (40 lb) for DC yoke per ASTM E709.
- Units
- Lift in kg or lb; magnetising current in A; pole spacing 75–200 mm typical.
- Typical Range
- AC yokes for surface cracks (skin depth ~0.5 mm in steel); DC/half-wave yokes for slightly subsurface flaws (~3 mm).
- Measured / Produced By
- Lift test on a known steel weight every shift or every 8 hours of use; pie-gauge field-direction check on the test piece.
- Code References
- ASTM E709 (MT guide); ASTM E1444 (MT practice); ISO 9934 (MT general principles)
- Worked Example
- Pole spacing set at 150 mm; magnetic field is longitudinal between the legs; rotating the yoke 90° between shots gives multidirectional coverage of the bead.
ASTM E709 / E1444
Standard guide and practice for magnetic-particle examination.
ISO 9934
Non-destructive testing — magnetic particle testing (general principles, media, equipment).
ASTM E215 / E376 / E2884
Eddy-current testing of tubes, conductivity, and array probes.
ISO 15548-1
Equipment characterization for eddy-current examination.
A frequent finding in audits is a yoke marked "in-cal" on the spreadsheet but with a current condition (damaged cable, missing cap) that would have invalidated the calibration if checked physically.
What does "Yoke" mean in NDT?
A portable electromagnet device used for magnetic particle testing, shaped like an inverted U. The yoke magnetizes the test area between its two poles
Which standards govern the use of Yoke?
Yoke is most often referenced under ASME Section V together with the relevant ASTM practice or the matching ISO standard for the method; the contract or purchase order will name the controlling document and edition for any specific job.
What other NDT concepts should I read alongside Yoke?
The most directly related entries in this glossary are "magnetic particle testing", "electromagnet", "magnetization"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.