NON- DESTRUCTIVE TESTING
B. Tech VI Semester
BY
Mr. Vishnu Pratap Singh, Assistant Professor
DEPARTMENT OF MECHANICAL ENGINEERING
BUDDHA INSTITUTE OF TECHNOLOGY
GIDA GORAKHPUR
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UNIT-V
EDDY CURRENT TESTING
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Introduction
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Outline
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Electromagnetic Induction
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electromagnetic induction.
Electromagnetic Induction (cont.)
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If another electrical conductor is brought into the proximity of
this changing magnetic field, the reverse effect will occur. Magnetic field cutting through the second conductor will cause an “induced” current to flow in this second conductor. Eddy currents are a form of induced currents!
Current Flow
Generation of Eddy Currents
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Eddy currents are induced electrical currents that flow in a circular path. They get their name from “eddies” that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right.
Test Probe
Eddy Currents
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Generation of Eddy Currents (cont.)
In order to generate eddy currents for an inspection a “probe” is used. Inside the probe is a length of electrical conductor which is formed into a coil.
Generation of Eddy Currents (cont.)
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Alternating current is allowed to flow in the coil at a frequency
chosen by the technician for the type of test involved.
Generation of Eddy Currents (cont.)
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A dynamic expanding and collapsing magnetic field forms in and around the coil as the alternating current flows through the coil.
Generation of Eddy Currents (cont.)
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When an electrically conductive material is placed in the coil’s dynamic magnetic field, electromagnetic induction will occur and eddy currents will be induced in the material.
Generation of Eddy Currents (cont.)
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Eddy currents flowing in the material will generate their own “secondary” magnetic field which will oppose the coil’s “primary” magnetic field.
Generation of Eddy Currents (cont.)
This entire electromagnetic induction process to produce eddy currents may occur from several hundred to several million times each second depending upon inspection frequency.
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Generation of Eddy Currents (cont.)
Depth
Depth
Eddy Current Density Low Frequency Low Conductivity Low Permeability
Standard Depth of
Penetration
(Skin Depth)
1/e or 37 %
of surface density
Eddy Current Density High Frequency High Conductivity
High Permeability
Eddy currents are strongest at the surface of the material and decrease in strength below the surface. The depth that the eddy currents are only 37% as strong as they are on the surface is known as the standard depth of penetration or skin depth. This depth changes with probe frequency, material conductivity and permeability.
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Inspection Data
the strength of the induced eddy currents.
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Inspection Data (cont.)
Impedance (Z) in an eddy current coil is the total opposition to current flow. In a coil, Z is made up of resistance (R) and inductive reactance (XL).
In an AC coil, induction from the magnetic field of one loop of the coil causes a secondary current in all other loops. The secondary current opposes the
primary current.
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Inspection Applications
One of the major advantages of eddy current as an NDT tool is the variety of inspections that can be performed. The following slides depict some of the these capabilities.
Material Thickness Measurement
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Material Thickness Measurement (cont.)
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Eddy current inspection is often used in the aviation industries to detect material loss due to corrosion and erosion.
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Material Thickness Measurement (cont.)
Eddy current inspection is used extensively to inspect tubing at power generation and petrochemical facilities for corrosion and erosion.
Crack Detection
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Crack detection is one of the primary uses of eddy current inspection. Cracks cause a disruption in the circular flow patterns of the eddy currents and weaken their strength. change in strength at the crack location can be detected.
This
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Crack Detection (cont.)
Eddy current inspection is exceptionally well suited for the detection of cracks, with an especially high sensitivity to detection of surface breaking cracks.
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Crack Detection (cont.)
Eddy current inspection of “bead seat” area on aircraft wheel for cracks using special probe that conforms to the shape of the rim.
Nonconductive Coating Measurement
Nonconductive coatings on electrically conductive substrates can be measured very accurately with eddy current inspection.
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Nonconductive Coating Measurement (cont.)
The photo to the left shows an aircraft panel paint thickness inspection. On the right, the display of a digital eddy current inspection instrument shows the different signals obtained by measuring eight different thicknesses of paint on aluminum.
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Monitoring Conductivity and Permeability
Variations
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Eddy current inspection is sensitive to changes in a material’s
electrical conductivity and magnetic permeability. This
“sensitivity” allows the inspection method to be used for such inspection procedures as:
Conductivity Measurements
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Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at a various points to evaluate its uniformity.
Equipment
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Instrumentation - Meters
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Meters are typically the simplest form of eddy current instrumentation.
The two general categories of meters are digital and analog.
Digital Meters
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Digital meters are typically designed to examine one specific attribute of a test component such as conductivity or nonconductive coating thickness. These meters tend to have slightly higher accuracy than analog devices.
Analog Meters
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Analog meters can be used for many different inspection applications such as crack detection, material thickness measurements, nonconductive coating measurements or conductive coating measurements.
Portable Eddy Scopes
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Eddy Current Probes
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Eddy Current Probes (cont.)
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data.
Eddy Current Probes (cont.)
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Eddy Current Probes (cont.)
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Surface probes are coils that are typically mounted close to one end of a plastic housing. As the name implies, the technician moves the coil end of the probe over the surface of the test component.
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Eddy Current Probes (cont.)
Some surface probes are specifically designed for crack detection of fastener holes. These include sliding probes, ring probes and hole probes.
Eddy Current Probes (cont.)
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Surface probes can be very small in size to allow accessibility to confined areas.
Finger Probe
Eddy Current Probes (cont.)
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Inside Diameter (I.D.) probes, also known as bobbin probes, are coils that are usually wound circumferentially around a plastic housing. These probes are primarily designed for inspection inside of tubular materials.
Eddy Current Probes (cont.)
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Outside Diameter (O.D.) probes are coils that are wound the circumference of a hollow fixture. The coil is designed such that the test part is ran through the middle of the coil. These probes can be used to inspect bars, rods as well as tubes.
Reference Standards
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Reference Standards (cont.)
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Reference Standards (cont.)
Material thickness standards used to help determine such things as material thinning caused by corrosion or erosion.
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Reference Standards (cont.)
Crack Standards:
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Reference Standards (cont.)
ASME Tubing Pit Standard:
Reference Standards (cont.)
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Nonconductive coating (paint) standard with various thickness
of paint on aluminum substrate.
Advantages of Eddy Current Inspection
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Limitations of Eddy Current Inspection
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Glossary of Terms
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direction.
Glossary of Terms
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one or more electron guns for generating an image.
flow through it.
Glossary of Terms
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Glossary of Terms
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Glossary of Terms
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magnetized.