Directed Acoustic Shearography Or What You Cant See Can Hurt You

DirectedAcousticShearography

-or-

What youcan not see

CAN hurt youRussell Kurtz, PHD

Chief Scientist, RANscitechApril 2010

During the past decade, a large percentage of the

aircraft accidents reported were caused by corrosion or other buried damage

LAYERS OF METAL

When these defects occur in modern materials made up of several layers, detection can be difficult and expensive.

Defect Disassembly itself causes damage and some composites can’t be disassembled at all.

Aloha Airlines Flight 243, April 28, 1988

It is , therefore, critical to develop a Nondestructive Evaluation (NDE) Technology that can find

these buried defects, corrosion, or damage spots rapidly, accurately, and efficiently.

HolographyEddy CurrentThermography

Existing NDE Technologies

Better at finding surface cracks than buried cracks

X-Ray ImagingTHz TransmissionComputed Tomography

Require access to both sides of a piece

Shearography Capable of detecting buried defects (with thermal source), but very slow

FAST

Directed Acoustic Shearography

Shearography using an acoustic stress source is the basis of

Directed Acoustic Shearography

THEORY

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Presentation to SPIEApril 2010

Orlando, Floridaby

Russell Kurtz, PHDChief Scientist

RAN Science & Technology

Differential Surface Metrology

Reference scan Stress scan Shear scan

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Applied stress interacts with any voids, cracks,

or defects

Stress scan subtracted from reference scan

Shearography Stress Methods

Apply Heat0.3-in. auto part

Acoustic Signal

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Takes between 3 and 60 seconds

for thermal equilibrium to be

reached

Stress propagates through depth

virtually instantly

However, a typical spherical wave is not a good choice

as its intensity decreases as fast as the depth squared

Directed Acoustic Shearography

An acoustic signal could be a good source of stress

IFit could be focused.

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This can be accomplished with a

Phased Array Acoustic Transducer (PA)

The PA

A 2D array of transducers whose relative phase and intensity can be controlled

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Able to focus the signal down to a narrow spot, or create a collimated beam of acoustic intensity

Advantages of DAS

0.3-in. auto part

Spherical Wave Acoustic Signal

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Acoustic Intensity:

0.274 W/cm²

Collimated Acoustic Beam(diameter 0.1 in)

Acoustic Intensity:

19.7 W/cm²

DAS Also Improves Resolution

For accurate determination of defect depth, a minimum of four scans must be performed at

different angles. The exact number depends on the thickness of the material.

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MODELING & EXPERIMENT

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Test Coupons0.1-in. thick aluminum, 2x4 inches, drilled on 1-inch centers.

One side (a) polished, the other (b) rough. Rough side with small defect made by a 1/32-inch drill.

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(a) (b)

Finite Element ModelingWe found that a 0.1-in. diameter collimated beam,

with an acoustic power of 125mW, would induce a surface deflectionof >7.5 µm if it encountered the 1/32-in. defect.

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Experiment

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• Run at low frequency (20 kHz) due to limitations of the equipment (ideal closer to 500 kHz)

• Tested at two voltages (0-9 V and 0-30 V) corresponding to 450mW and 1.5 W acoustic power

• Acoustic intensities equivalent to 590 mW/cm² at lower voltage and 2.0 W/cm² at higher

Results

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2.0

1.5

1.0

0.5

0.0

–0.5-0.4 -0.2 0.0 0.2 0.4

x-Position (in.)

Dif

fere

nce

(μ

m)

y-Po

siti

on

(in

.)

0.4

0.2

0.0

-0.2

-0.4

-0.4 -0.2 0.0 0.2 0.4x-Position (in.)

y-Po

siti

on

(in

.)

0.4

0.2

0.0

-0.2

-0.4

6

4

2

0

-2

Dif

fere

nce

(μ

m)

Net deflection increase 2.9 µm with 9 V

Net deflection increase 10 µm with 30 V

Summary

We found it!

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The concept of Directed

Acoustic Shearography

has been proven

Still keep your eye out for alligators

Conclusion