Electrical Conductivity Testing Using Eddy Current Method

Electrical Conductivity Testing Using Eddy Current Method 

by Ravi Kumar Thammana

Introduction 

Electrical conductivity measurement can be done using Eddy current testing. This method gives good results for non magnetic materials. Works well with digital readout equipment or general purpose eddy current equipment. Among all NDT testing methods eddy current has more wider applications. 

Apart from flaw detection, eddy current testing can be used for material sorting, hardness, heat treatment, pain thickness, conductive coating thickness on non conductive materials.

Eddy current test method for detecting conductivity is applicable for non-ferromagnetic materials such as aluminium, titanium, magnesium, copper and other metals. Measurement can be performed on the materials that contains non conductive coating.

Material sorting using Eddy current testing is an effective method to segregate materials based on alloy type, ageing, heat treatment micro structures hot or cold working. This method effectively works on the materials which has drastic effect on the parameter that is being measured with respect to the conductivity of the material.

Conductivity Measurement Using Eddy Current 

For non destructive testing purposes, in Eddy current testing the unit of measurement for conductivity is %IACS. Whereas for general engineering applications electrical conductivity is measured in Siemens per metre. The conductivity of pure annealed copper is 0.58 X108 S/m which is equivalent to 100%IACS at 20 Deg C.

Examination results are purely dependent on the conductivity changed due to the change in variable. To get accurate results the user has to establish appropriate correlation curve for the property desired to be measured before commencement of testing.

 

Test Variables

 

Temperature is an important variable while performing conductivity test using Eddy current testing. Components under inspection shall be stabilised before performing conductivity testing. As per the international standards in general the testing has to be performed at 20 degree centigrade.

Second factor that has major influence on the probe and test specimen will be liftoff. It is the distance between front surface of the probe to the top surface of the object under examination. Liftoff has major influence on the test results smaller the lift off better will be the test results however, largely liftoff may reduce the life of the probe by accidental damage to the front surface.

Third factor is 'edge effect'. It is the distance from the probe to the edge of the material where there is a signal that starts coming from the edge even though there is no defect. Before starting the test contact the manufacturer to know about the edge effect for the specific probe.

Surface Preparation

The component surface shall be clean and free of oil dust dirt grease or any other extraneous matter that may have an impact on the conductivity results. Though light non conductive coatings may not have much influence on the examination results, we recommend to remove any surface coatings for better accuracy.

Limitation

Eddy current method of Conductivity Measurement for conducting materials has reasonably good accuracy. However two different alloys can have same conductivity though their chemistry remains same. Prior to confirming the conductivity values it is advised for the user to cross verify the results using various conventional chemical analysis techniques. This assures the results to be confident and reliable.

Minimum Thickness Limitation

Eddie currents varies across the thickness of the material. Measurement accuracy depends on the thickness of the specimen. Skin depth is a measurement that gives idea about the thickness limitation. Skinduct is nothing but the depth at which the Eddy current density will be 37% when compared to the surface eddy current density. This may vary from material to material and alloy to avoid and also depends upon the frequency employed. Roughly Eddy currents conductivity measurement will have reasonable accuracy provided the mental thickness is at least 2.6 times the skin depth for that material.

Skin Depth Calculation Formula for Eddy Current testing

δ = 503.3/ Square root of (fσ) (in Units of 'meter) where σ is in S⁄m

or

 δ = 50.3/ Square root of (µ.f.1/ρ) (in Units of 'millimeter') where ρ is in µΩ•cm and µr =1

or

δ = 660/Square root of  fσ (in mm) , σ in %IACS

µ = magnetic permeability of the specimen material,

µo = magnetic permeability of free space (air), µr = relative magnetic permeability of the specimen material, σ = electrical conductivity of the specimen material, ρ = electrical resistivity of the specimen material, and f = examination frequency in Hz

Reference Conductivity Blocks

 

Reference conductivity blocks are required to calibrate the eddy current equipment for conductivity measurements. Various manufacturers supply these conductivity blocks in calibration mode. Otherwise you can buy the blocks from the suppliers and get them calibrated from national physical laboratory npl.

Probe Selection

Equipment manufacturers supply probes exclusively made for measuring conductivity. For example olympus has a product range of probes that has capability for conductivity measurement. This probe can measure conductivity and pain thickness simultaneously on any test object if appropriately calibrated follow the manufacturer's instructions to perform the testing. This probe comes with integrated cable for obtaining better accurate results.

Eddy Current Equipment

 

Olympus make Nortec 500 or 600 Series or any other equivalent equipment and probe combination. 

 

Calibration for Electrical Conductivity Testing

For standardisation of adequate equipment turn the instrument on and allow sufficient time to stabilise according to the manufacturer's recommendations. I just balance and standardise the conductivity meter against the standards and compensate for conductivity meter for surface roughness and lift off in accordance with the instructions. If the lift of adjustment is not available determine the acceptable range of lift off that will meet the requirement. Standardisation or calibration of equipment has to be performed at periodic intervals.  

The eddy current equipment probe and different standards should be standardised while performing the temperature near the ambient temperature of the material it is recommended to maintain a temperature of 20 degree centigrade but it is not essentially required to maintain it. Temperature stability of a material under examination is more important than environmental ambient temperature. If the temperature changes substantially for the instrument recalibration shall be performed again. It is recommended to maintain the calibration at least before commencement of the testing and at the conclusion of the testing and in between at specific intervals as determined by the NDT level 2 eddy current testing operator. 

 

Procedure for Electrical Conductivity Testing

 

1. Connect the probe to the instrument which is already calibrated 

2. Switch on the equipment and alloy to stabilise for the duration recommended by the manufacturer

3. check the temperature of the components

4. Adjust the instrument settings to calibrate for measuring electrical conductivity using Eddy currents

5. Place the Eddy current proof on the test object read the results and record for preparing the test report

6. Post completion of testing verify the instrument calibration to ascertain the results are accurate. If there is any discrepancy in calibration recalibrate the equipment and redo the inspection for all the components since last valid calibration.

Interpretation of Examination Results

 

Conductivity measurement using Eddy current testing is based on the comparison of unknown component with the known conductivity reference standards. Therefore the accuracy of results depends upon the accuracy of calibration prior to testing we recommend to have at most care while calibrating the equipment.

Eddy Current test Report

A written report of Electrical conductivity measurement shall contain information required to be specified as per the standard being followed. There are ASTM, ISO, BS, EN standards available. The user can use any of the standards as per his or her requirement.

You can download Eddy Current Test report format from Trinity NDT website. 

About Trinity NDT - NABL accredited Labs in Bangalore, India

 

Trinity NDT is India's leading NDT and welding services company headquartered at Bangalore in India. The labs are accreted to iso 17025 international standard. Aerospace specialised NDT services are available for eddy current testing of conducting materials.

The labs are fully equipped with latest advanced Eddy current equipment for performing electrical conductivity measurements . NDT level 2 experience professionals are available at the centre to perform both in lab and on site services . 

An An ASNT level 3 in a Eddy Current testing available at the facility to look after consulting services for establishing entity techniques procedures performing the tests witnessing and training of the entity personal for level 1, level 2 and level 3. 

Contact us today for more information on Ultrasonic testing, magnetic particle testing, penetrant testing, ASNT Level III, Radiography testing, welding inspection.