The Advantages of using Eddy Current Technology

for Weld Inspection

by John Hansen

The Theory Behind Eddy Current Inspection

Eddy currents are created by using a coil to induce an ac current into a conductive material.

The current is concentrated at the surface and in ferrous material there is little or no penetration.

Changes in the impedance with a simple coil are indicated with lift-off giving a distinctly different signal to a defect. This for weld inspection makes inspection very difficult. So there has to be a better way……….

History Of The WeldProbe

• 1966: Admiralty Materials Laboratory in the UK develops the Amlec for ferrous weld inspection (Hocking NDT License the technology) using a simple absolute probe.

• 1982: The Hocking WeldScan probe was developed by John Calvert and John Hansen and others for use with impedance plane eddy current instruments.

• 2000: Inspection procedure is standardised by BS EN1711 eddy current examination of welds by complex plane analysis.

Why Use Eddy Current Probes For Weld Inspection?

• MPI and Dye Penetrant inspection require the removal of the coating before inspection, costing both money and time.

• Eddy current probes allow welds to be efficiently inspected for near-surface cracks.

• Eddy current can inspect welds through paint or metallic coatings.

Why Use A Special Probe For Weld Inspection?

By using a differential coil configuration, eddy current weld probes:

• Allow inspection without the need to remove any coating / paint

• Are in-sensitive to lift-off and variations in material properties caused by the heat affected zone unlike a simple absolute probe.

Probe Anatomy

• A probe was designed with two orthogonal tangential interleaved coils.

• The coils positioning and accuracy of manufacture is critical to producing signals as required in the inspection procedure.

Scanning With An Eddy Current WeldProbe

• The active part of the probe is 3mm x 3mm.

• In order to scan the material, multiple scans need to performed. These are:

1. Zig zag scan of the toe in the HAZ (heat affected zone) along the length of the toe

2. Scan of the weld cap

3. A sweep along the toe

What Makes The Weld Probe So Special?

• Minimal spurious signals caused by lift-off because of differential connection

• Sensitive to linear or near linear indications (e.g. cracks), insensitive to symmetrical indications (e.g. holes and pits)

• Low lift-off sensitivity variation; 8 db per mm as compared with the pencil probe at 40 db per mm.

• Directionality means that the defects in line with scan give a positive indication and defects at right angle give a negative indication with a null at + and - 45o.

• The probe has been modeled and it shows an extremely linear field in the active area leading to good sizing capability and excellent lift off characteristics.

Probe Response

Applications:Offshore

• Widely used for weld inspection where it replaces MPI due to not needing to remove surface coating

• Readily applicable to rope access inspection. So further reducing the cost of inspection

Nuclear

• Widely used in the nuclear NDT industry.

• Used for conventional manual inspection and also in automated scanners.

Rail

Rail Head

Web

Foot

• Used for manual inspection of defects in rail and wheels.

• Axle inspection where it has been shown to be much more reliable than UT.

• Weld inspection on chassis and bogeys.

Other Applications• Bridges.• Ships/Submarines• Cranes• Steel framed buildings.• Modern Art• Overhead traffic lights.• Amusement rides.• Prison bars to detect saw cuts.

Equipment Used To Perform A WeldProbe Inspection.

• Impedance Plane Display Eddy Current Instrument (e.g. Vantage or Veritor).

• Weld probe.

• Paint probe to assess coating thickness and compensate calibration by means of shims.

• Test block with 2, 1 and 0.5mm Notches and four 0.5mm shims.

Standardisation• The technique has been widely accepted by various bodies including

BINDT-PCN, DNV and Lloyds Register.

• In 2000 BS EN1711 “Eddy current examination of welds by complex plane analysis” was issued. Topics covered by this standard include:

• Equipment and Settings.• Personnel Requirements.• Calibration and Calibration Blocks.• Acceptance Criteria and Weld Considerations.• Scanning Methods.• Coating Measurements.• Testing Plans.• Detectability of Flaws.• Method Flow Diagrams.• Non-acceptable Indications.• Recording and Reporting of Inspections.

Weld Probe Specification

• Straight, 90deg Inline, 90deg Right Angle• Diameters 11.0 (Small), 16 (Medium), 32mm (Large)• Dis-connectable and integral probe cables• Cable lengths from 1.5 to 50meters• Frequency range 100, 20, 100-600kHz (Fe NFE)• Minimal lift off signal, can find cracks though paint, oil

and conductive and nonconductive coatings• Made from hard wearing PET• Stainless steel and ceramic tips available on request

ETher NDE Weld Probes

Standard Bridge - Straight Standard Bridge – 90 Degree Inline

Standard Bridge – 90 Degree Transverse

4-Way Lemo Connector

Standard Disconnectable Weld Probes

Straight

Inline right angle

6mm dia. Weld Probes

Special Weld Probes

Waterproof

High Temp 200o C Ceramic Tipped

Straight 90deg Inline90deg Transverse

Flat Faced

Stainless Steel Steel Dome Faced

Advantages of EC NDT with Weld Probes

• Due to their size weld probes are much more suited to difficult access inspections plus custom versions can be used to further improve accessibility.

• The equipment is much more portable.• There is a Euro Norm standardising the procedure for

application (BS EN 1711:2000) • Weld probes are smaller• Can apply to a vast range of applications • Quick inspection time.• Can inspect through paint or metallic coatings.• Insensitive to Lift off in comparison with conventional absolute

probes• Huge range of standard probes available.

Negatives of EC NDT with Weld Probes

• Sensitivity reduces with increasing coating thickness 8dB/mm put procedure for compensating for this.

• Not good at discriminating or classifying defects beyond 5 mm depth in Ferrous material.

• Requires manual calibration on a test block.• Standard probes will operate to 100oC and there

are special probes for use at up to 200oC

ConclusionsThe WeldProbe provides a good alternative to MPI inspection as:

• It is not necessary to remove the surface coating.• Better suited for rope access inspectors.• The probe is relatively easy to use so finds application in

axle inspection.• National standards, training courses with operator

certification and acceptance by certifying authorities mean that the WeldProbe becomes a recognised solution.

• Mature technology with over 30 years of application success