Eddy Current Metal Testing for Conservation

Innovations in Eddy Current Technology

by Curtis Desselles

Blythe McCarthy - Freer Gallery

Valuable information about cultural materials is not always visible to the naked eye.

Tattoodjj, “Statue of Prometheus”, June 8, 2008, online image, Flickr. Rockefeller Center, NY.

Opacity, “statue of Bastet, gilded bronze, Late Period”, March 8, 2009, online image, flickr.

Northwestern State University & NCPTT are continuing research into use of eddy current testing

on cultural heritage.

NCPTT, Photograph courtesy of Jason Church

Without nondestructive testing, cultural information will be lost.

Corroded Penny - www.wildbell.com/.../ 2008/05/img_9834.jpg

By making affordable eddy current instruments, preservationists can have access to the technology.

Eddy Current Testing, Freer Gallery

Affordable eddy current instruments can be used to analyze cultural materials.

Eddy Current Testing, Freer Gallery

Eddy Currents have been used in a variety of nondestructive applications.

Adam Hoke, an aviation maintenance technician with the U.S. Coast Guard, inspects the belly of a Boeing 737 using an eddy current

scanner. (Photo by Randy Montoya) Sandia Labs.

Eddy currents were discovered in 1851 by Foucault.

Léon Foucault (1819-1868), Wikipedia.

Applications of eddy currents include detection of cracks, discontinuities, flaws, and thicknesses.

Measuring paint thickness - NDT Resource Center

Testing for Cracks - NDT Resource Center

VoltageVoltage

is a measurement of the energy contained within an electric field, or an electric circuit, at a given point, measured in volts

is a measurement of the energy contained within an electric field, or an electric circuit, at a given point, measured in volts

CurrentCurrent

a flow of electricity through a conductor, measured in amperes

a flow of electricity through a conductor, measured in amperes

ResistanceResistance

a material's opposition to the flow of electric current, measured in ohms

a material's opposition to the flow of electric current, measured in ohms

CapacitanceCapacitance

a capacitor is an electronic component used for storing charge and energy, measured in farads

a capacitor is an electronic component used for storing charge and energy, measured in farads

InductanceInductance

a measure of the reaction of electrical components (especially coils) to changes in current flow by creating a magnetic field and inducing a voltage. Its unit is the henry

a measure of the reaction of electrical components (especially coils) to changes in current flow by creating a magnetic field and inducing a voltage. Its unit is the henry

ConductanceConductance

a measure of a material's ability to conduct electric charge; the reciprocal of the resistance, measures in Siemens

a measure of a material's ability to conduct electric charge; the reciprocal of the resistance, measures in Siemens

ImpedanceImpedance

the total effective resistance in an electric circuit to the flow of an alternating current

the total effective resistance in an electric circuit to the flow of an alternating current

Reactance Reactance

opposition to the flow of alternating current caused by the inductance and capacitance in a circuit rather than by resistance.

opposition to the flow of alternating current caused by the inductance and capacitance in a circuit rather than by resistance.

What are eddy currents?

Eddy Currents - NDT Resource Center

Eddy currents are analogous to a flowing stream.

EDDIES IN THE OYASHIO CURRENT, G. K. Vallis, Princeton University

Similarly, eddy currents can be created in metals.

Drawing Courtesy of Curtis Desselles

Here are the steps to construct an affordable eddy current analyzer.

Chichen Itza - Travelblog.org

Eddy currents can be created in the follow way in cultural materials.

• Create an alternating current (AC) in the form of a square or sine wave (excite circuit).• Excite a series LC circuit (coil and capacitor) who’s resonant frequency matches that of the excite circuit.• Bring the probe(s) (coil) near the metal to be tested.• Convert the voltage across the probe(s) to direct current (DC) using a diode.• Divide and store the voltage to a capacitor.• Measure the voltage in the capacitor using an analog to digital converter (ADC) ADC0831.

A Complete Eddy Current Analyzer Circuit

A Complete Eddy Current Analyzer Circuit

Magnet wire is used to constructed for use as probes.

Magnet Wire RadioShack® - $5.99

Coil Construction - 30 Gauge Magnet Wire

Photographs Courtesy of Curtis Desselles and Jason Church

Determine the inductance value with an LCR meter.

LCR Meter - Photograph Courtesy of Curtis Desselles

Use a square or sine wave to excite the LC circuit

LC Circuit - Drawing Courtesy of Curtis Desselles

Circuitry is built to excite the LC circuit to resonance

Propeller Demo Board - Parallax, Inc.

The frequency of the square or sine wave must match the resonant frequency of the LC circuit.

L = Inductance in Henries

C = Capacitance in Farads

Once at resonance, the voltage and current across the coil is at its maximum.

Drawing Courtesy of Curtis Desselles

The AC voltage is converted to DC using a diode and it is stored in a capacitor.

We then convert the analog signal to a digital signal using an ADC.

ADC PBasic code - Curtis Desselles

ADC0831 - Parallax Inc.

Using Microsoft Visual Basic an algorithm was created to convert the data to a bitmap.

Once the eddy current analyzer is built and calibrated, metal artifacts were tested.

Reference Samples - Freer Gallery

Four Techniques used in Eddy Current Analysis

• Identification of defects in metals.

• Identification and conductivity.

• Thickness determination on gilded or coated surfaces.

• Hallmark recovery on metal objects.

Photographs Courtesy of Jason Church

Four Techniques used in Eddy Current Analysis“Identification of defects in metals”

Drawing Courtesy of Curtis Desselles

• Eddy Current Parameter

• Phase lag is the shift in time between the eddy current response from a disruption on the surface and a disruption at some distance below the surface.

Four Techniques used in Eddy Current Analysis“Identification of defects in metals”

Drawing Courtesy of Curtis Desselles - 50 KHz

• Phase Lag

• Three defects at different depths and frequencies

R B G

Four Techniques used in Eddy Current Analysis“Identification of defects in metals”

• Phase Lag

• Three defects at different depths and frequencies

Drawing Courtesy of Curtis Desselles - 300 KHz

GBA

Four Techniques used in Eddy Current Analysis“Identification of defects in metals”

• Methodology

• Set frequency to 150 KHz

• Bring probe within 1 mm from test surface

• Scan object from left to right in a slow but steady manner

• Results appear on laptop screen

• High frequency

• Lower depth penetration and higher resolution

• Low frequency

• Greater depth penetration and lower resolution

Four Techniques used in Eddy Current Analysis“Identification of defects in metals”

Case Study

Screen Capture - 158.5 KHzAluminum (1mm) - 158.5 KHz

Four Techniques used in Eddy Current Analysis“Identification and Conductivity”

• Eddy Current Parameter

• Lift-off - This technique usually involves zeroing out a probe (coil) in air. Then placing the probe in contact with the sample surface. For nonmagnetic materials, the change in impedance of the coil can be correlated directly to the conductivity of the material.

• Resistance v. inductive reactance

Screen capture of id measurementsBlack is baseline conductivity

Screen capture of id measurementsBlack is baseline conductivity

XL

R

Four Techniques used in Eddy Current Analysis“Identification and Conductivity”

• Lift-Off

• Zero probe

•Touch probe to sample

• Lift off in a slow and steady manner

• Touch probe to sample

• Results will appear on screen

Photograph Courtesy of Jason Church

“Identification and Conductivity”“Identification and Conductivity”

Four Techniques used in Eddy Current Analysis“Thickness Measurements”

• Eddy Current Parameter

• Lift-Off

• Zero probe

•Touch probe to sample

• Lift off (1 mm) in a slow and steady manner

• Scan left to right

• Results will appear on screen

Four Techniques used in Eddy Current Analysis“Thickness Measurements”

Drawing Courtesy of Curtis Desselles

Lift-Off

Four Techniques used in Eddy Current Analysis“Recovery of Hallmarks”

• Eddy Current Parameter

• Phase lag is the shift in time between the eddy current response from a disruption on the surface and a disruption at some distance below the surface.

• Touch probe to sample

• Scan left to right

• Repeat scanning of entire object

• Results will appear on screen

Four Techniques used in Eddy Current Analysis“Recovery of Hallmarks”

Scan of handle area

Lead Bale Seal, Los Adaes, Williamson Museum, NSU

Building the InstrumentBuilding the Instrument

How do we get from this…How do we get from this…

…to this…to this

Pre-drill Holes to Later Insert Electrical Components

Pre-drill Holes to Later Insert Electrical Components

Copy Circuit Design to Copper Circuit Board

Use Sharpie Marker for Layout

Copy Circuit Design to Copper Circuit Board

Use Sharpie Marker for Layout

Completed Circuit DiagramCompleted Circuit Diagram

Etch Copper Board in Ferric Chloride Sharpie Ink Acts as a Resist

Etch Copper Board in Ferric Chloride Sharpie Ink Acts as a Resist

Periodic Agitation Speeds the ProcessPeriodic Agitation Speeds the Process

Solder on the ComponentsSolder on the Components

560 pf Capacitor

↓

↑RCA Coil Inlet

Rectifier Diode

↓

↑.1 µf Capacitor

100 mega ohm Resistor

↓

1 mega ohm Resistor

↑

741 IC 8pin Chip

↓×

ADC0831 Analog to Digital Converter

↓

Wires to Serial Port

↑

Coil Construction - 30 Gauge Magnet Wire

Coil Construction - 30 Gauge Magnet Wire

↓

Iron Ferrite Core

RCA Output

↓

Future DirectionsFuture Directions

iPhone Applications Tomographic Layers Database of Standard Parameters Better Resolution

iPhone Applications Tomographic Layers Database of Standard Parameters Better Resolution

iPhone ApplicationsiPhone Applications

Eddy Current ImagingEddy Current Imaging

Database of Standard Parameters

Database of Standard Parameters

Conclusion

• These case studies show the potential of the affordable eddy current instrument.

• As cultural resources age and decay, valuable information can be lost.

• This research allow for affordable eddy current instruments to be applied to cultural materials.

Cultural Material - Los Adaes - Photographs courtesy of Curtis Desselles

Acknowledgements

• Northwestern State University of Louisiana (NSU)

• National Center for Preservation Technology and Training

(NCPTT)

• Dr. Mary Striegel

• Dr. Hiram Gregory

• Dr. ElizaBeth Guin

• Jason Church

• Houren Zhu