FIBER-OPTIC ULTRASOUND SENSORS FOR PROCESS MONITORING P.A. Fomitchov, J.D. Achenbach & S. Krishnaswamy Why optical fibers? • can be configured to sense most parameters of interest in process monitoring { temperature, pressure, strain, ultrasound, flow rate, pH, composition,viscosity, voltage, current } • compatible with reinforced composites • very good measurement sensitivity • large bandwidth • small, light weight, and potentially rugged • potentially low cost with sensor multiplexing • can be used in hostile environments • can withstand high temperatures during processing • good SNR even in high EMI (electromagnetic interference) situations • pose less of a fire hazard near explosives than electrical sensors Center for Intelligent Processing of Composites

FIBER-OPTIC ULTRASOUND SENSOR S FOR PROCESS MONITORING How does ultrasound affect light propagating inside a fiber? ultrasound

optical fiber

• Impinging ultrasound causes strain in fiber • Strain in fiber leads to phase shift in light propagating in the fiber • Optical phase shift is demodulated using optical interferometry

Center for Intelligent Processing of Composites

FIBER-OPTIC

SENSORS

Ultrasonics in Cure Monitoring:

FOR

PROCESS

MONITORING

ultrasound generated using lasers or pzt-transducers

fiber sensor 1 fiber sensor 2

laminated composite

MEASURE at different times during cure: • ultrasonic wavespeed from time of flight measurements • ultrasonic attenuation from amplitude measurements INFER: • gel point • glass transition temperature • degree of cure • porosity of cured component ADVANTAGE: monitor cure at any selected layer rather than get a through-thickness average measurement Center for Intelligent Processing of Composites

INTRINSIC

FIBER-OPTIC

FABRY-PEROT

SENSORS

Ref: Lee & Taylor, Electronic Letters, vol. 24, (1988); Dorighi, Krishnaswamy, Achenbach, IEEE Ultrasonics, vol 42, No.5, (1995).

L

Iref Iin

cavity

Itr

PARTIAL MIRRORS

Advantages: • Local sensor • good sensitivity to ultrasound Disadvantages: • Difficult to fabricate • Needs active stabilization Center for Intelligent Processing of Composites

INTRINSIC

FIBER-OPTIC

SAGNAC

SENSOR

Ref: Fomitchov, Krishnaswamy, Achenbach, CQE/NU Inv. Disc. (1997).

ls2 D tructure Laser

1

intrinsic probe mirrorized fiber-tip

Advantages: • Common path - no stabilization needed • good sensitivity to ultrasound • dual-probe Disadvantages: • narraw bandwidth Center for Intelligent Processing of Composites

Cure

monitoring using the Sagnac Interferometer

Approach: to characterize the polymerization process by monitoring ultrasonic speed and attenuation using embedded sensors Current stage

embedded sensor - Sagnac interferometer external source - piezo-electric transducer (PZT)

Future work

embedded sensor - Sagnac interferometer embedded source - laser based ultrasonic source

Center for Intelligent Processing of Composites

CURE

MONITORING:

Set-up SENSOR SIGNAL

Mold

Epoxy

0.08

First fiber

0.06 0.04 0.02 0 -0.02 -0.04

Second fiber

-0.06

Sensing fibers

PZT

-0.08

2

4

6

time (µsec)

Center for Intelligent Processing of Composites

8

Results for cure monitoring DER 331 epoxy 25 2800

20 2600 15

2400

10

2200

5

2000

velocity

1800

0 0

50

100

150

200

250

300

350

Curing time, min Center for Intelligent Processing of Composites

Velocity, m/s

Attenuation, dB/cm

attenuation

Some practical considerations Problem: PZTs cannot be used at high temperatures.

DER 331 epoxy • can be cured at room temperature • typical curing time 2 - 5 hours • pzt generation of ultrasound acceptable

GY6010 epoxy • recommended curing temperature 100 - 200 degree C • typical curing time 2 - 5 hours at high temperature • typical curing time at room temperature - 7 days! • use of pzt generators not acceptable Solution: Use fiber-optic embedded Laser Ultrasonic Source Center for Intelligent Processing of Composites

Fiberized Embedded Laser Source

Ultrasonic

Optical Fiber

YAG Laser Mold Epoxy

Embedded Laser Ultrasonic Source

Center for Intelligent Processing of Composites

Characterization of Fiberized Laser Ultrasonic

Directivity of ultrasound generation

PZT detected signal

Ultrasound Amplitude

0

1

2

3

4

5

6

Time, us

Frequency 1.5 - 2 MHz

Source

7

8

1.2 1.0 0.80 0.60 0.40 0.20 0.0 0.20 0.40 0.60 0.80 1.0 1.2

Fiber

Theory

Center for Intelligent Processing of Composites

Experimental

Complete

Laser-Based Proce ss Monitoring System for High-Temperature Curing

YAG laser

Sagnac Interferometer

Sensing Fiber

Laser Ultrasonic Source

Mold Center for Intelligent Processing of Composites