EDDY CURRENT

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EDDY CURRENT

• Ara Abnous
• San Diego State University
• Mechanical Engineering Department
• ME 495
• 12/08/05

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Outline

• Introduction
• Basic Principal
• History
• Properties of Electricity
• Mutual Inductance
• Instruments
• Probes
• Applications
• Price and Manufacturing
• Summery

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Basic Principles of Eddy Current Inspection

• Eddy current inspection is one of several NDT
  methods that use the principal of
  electromagnetism as the basis for conducting
  examinations.
• Electromagnetic induction

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Eddy current can perform variety of inspections
and measurements.

• Crack Detection
• Material Thickness Measurements
• Thin materials such as Tube, sheet stock
• Coating Thickness Measurements
• Conductivity Measurements
• Heat Treatment Monitoring

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Some of the advantages of eddy current inspection

• Sensitive to small cracks and other defects
• Detects surface and near surface defects
• Immediate results
• Equipment is very portable
• Test probe does not need to contact the part
• Inspects complex shapes and sizes of conductive
  materials

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Eddy current inspection has some disadvantages or
limitation

• Only conductive materials can be inspected
• Skill and training required is more extensive
  than other techniques
• Surface finish and and roughness may interfere
• Reference standards needed for setup
• Depth of penetration is limited

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History of Eddy Current Testing

• Michael Faraday's discovery of electromagnetic
  induction in 1831
• practical use at Second World War

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Properties of Electricity

• Induction
• Generating electrical current in a conductor
  by changing magnetic field
• rate of magnetic field changed current induced
• Lenz's Law
• The induced current working
• against the primary current.

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Properties of Electricity

• Faraday's Law
• L value of inductance
• in henries
• Mutual-inductance
• Inductance is used to
• generate eddy currents
• in the test piece

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The Basis for Eddy Current Inspection

• The test probe is basically a coil of wire
  through which alternating current is passed.
• When the probe is brought in close to a
  conductive material, the probes changing magnetic
  field generates current flow in the material.
• The eddy currents produce their own magnetic
  fields that interact with the primary magnetic
  field of the coil.
• By measuring changes in the resistance and
  inductive reactance of the coil, information can
  be gathered about the test material

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Eddy current inspection system

• The solid line in the graph below shows the
  circuit's total current, which is affected by the
  total impedance of the circuit.
• Total impedance resistance inductive reactance

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The impedance plane diagram

• The strength of the eddy currents and the
  magnetic permeability of the test material cause
  the eddy current signal to react in a variety of
  different ways.
• Resistance component will increase
• Reactance of the coil decreases
• Crack present
• Resistance component will decreases
• Reactance of the coil go back up

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Eddy Current Instruments

• Large variety of configurations
• Digital
• Analog (simple bridge circuit )
• compares a balancing load to that
  measured on the test specimen.
• Crack detection
• Corrosion inspection
• Conductivity testing

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Probes - Mode of Operation

• probes can be custom designed for a wide variety
  of applications
• Surface probes
• Bolt hole probes
• ID probes
• OD probes

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Applications

• Surface Breaking Cracks
• Cracks are detected when
• they disrupt the path of
• eddy currents and
• weaken their strength
• SCD Using Sliding Probes
• Commercial aircraft, fatigue

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Applications

• Tube Inspection

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Applications

• Conductivity and heat treatment
  measurements
• Thickness Measurements of Thin material
• Thickness of thin metal sheet
• Metallic coatings
• Cross-sectional dimensions of cylindrical
• tubes

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Price and Manufacture

• Inspecting heat exchanger
• Surface crack detection
• Laptop computer easy Operator interface
• Multi-frequency eddy current instrument
• From 3000 up to 30000 and more US
• Eddy Current Technology (ECT) Incorporated was
  established 25 years ago in Canada
• UniWest eddy current products

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Summary

• NDT methods
• Electromagnetic induction
• Crack Detection
• Equipment is very portable
• Large variety of configurations
• Custom designed Probes
• Applications
• Tube Inspection
• Conductivity

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References

• NDT Resource Center
• http//www.eddycurrent.com/mainframesys.html
• http//www.uniwest.com/instrument.html

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Thank you Enduring my language
Have a nice winter break