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Damage State Awareness in Composite Laminates Via Ultrasonic Guided Waves

Cliff LissendenJoseph Rose Engineering Science & MechanicsThe Pennsylvania State University

Workshop on Prognosis of Aircraft and Space Devices, Components, and Systems

Sponsored by AFOSR

Cincinnati, OH

19-20 February 2008

Impact delamination in cross-ply laminate

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We focus on understanding guided wave prop-agation in order to improve penetration power and sensitivity to damage.

Analysis of guided waves in composite laminates

Mode excitation and tuning

Synthetic reconstruction

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Current issues and capabilities

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Ray distributions for 16 and 8 element arrays

Methods for tomographic imaging of internal damage are available.

Enable reconstruction based on signal difference, group velocity, etc.

Array of PWAS Air-coupled transducer test bed

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Tomographic imaging enables visualization of internal damage.

4.23 J Impact delamination in ([0/90]s)6 CFRP with a 178 mm diameter circular array of PWAS

C-scan 16 element array 8 element array

Needs: penetration distance, number of sensors, damage sensitivity, critical size

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Material anisotropy results in skew angles for guided wave propagation that must be accounted for in monitoring.

Unidirectional CFRP with fibers oriented at 22.5°. Wave activation is in the 0° direction.

[0/45/90/-45]s CFRP laminate 200 mm x 200 mm x 1.6 mm excited by 3 cycle toneburst from 10 mm diameter PWAS

300 kHz (movie)

100 mm1

00 m

m

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Focusing of energy from a phased array can sweep through a plate.

Beam control of a linear phased array for an aluminum plate, as pioneered by V. Giurgiutiu

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Guided wave ultrasonics can monitor structures. To be used effectively, the underlying wave mechanics must be well understood.

Dispersion curves and wave structure are the foundation

Mode & frequency selection encompasses

many application specific aspects

Sensor design

DAQ & analysis

FrequencyGroup velocityDispersivityExcitabilityAttenuationDamage sensitivity

Signal processingDamage mechanicsEtc.Angle beam

CombNatural tuningPhased array tuning

Prognostics

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Lamb-like and SH waves are not decoupled in anisotropic materials.

Phase velocity dispersion curves for guided waves propagating in the 0o direction of a [(0/45/90/-45)s]2 carbon/epoxy laminate.

Simply number modes sequentially

SAFE (lines) & GMM (symbols) compare well

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Group velocity and attenuation dispersion curves provide valuable information for health monitoring.

Group velocity dispersion curves Attenuation dispersion curves for guided waves propagating in the 0o direction of a [(0/45/90/-45)s]2 carbon/epoxy laminate.

Kelvin-Voigt model1 neper = 8.69 dB

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Multiple modes are often excited simultaneously.

5 cycle Hanning windowed tone burst excitation with 200 kHz central frequency – normal loading w/ 1 mm wide transducer.

Mode 1 dominates low frequency region

Natural tuning

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Mode 1Mode 2Mode 3Mode 4Mode 5Mode 6

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Frequency (MHz)

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The source influence can be shown clearly in phase velocity-frequency space.

10 mm transducer, 1 MHz central freq. 10 cycle excitation, variable incidence angle, 2 mm aluminum plate

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30 degree

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60 degree 1D model, F(x,t)

2D Fourier Transform, F(k,)

Source Influence spectrum, F(cp,f)

cp = /f, f = /2

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The source influence is determined using the normal mode expansion (NME).

Modes are orthogonal; v*(H) = complex conjugate of normalized velocity at top surface, T = traction vector, P = Poynting vector, x1 = prop. dir., x3 = thickness dir.

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*3

1

131131

3131

expexp4

expˆˆ,,

exp,,

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1

xikdikTP

Hvxa

tixxxaxtxx

tixxatxx

n

L

L nnn

n

nn

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nn

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σσ

vv

Velocity field

Stress field

Transducer length

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A linear phased array provides mode tuning by using time delays.

Elements uniformly spaced at distance d

time delays

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pc

dt

Guided wave

Li & Rose, 2001, IEEE Trans. 48(3):761

d

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Frequency tuning and time delays provide tremendous flexibility for mode tuning.

200 kHz central frequency 1 MHz central frequency

d = 2 mm

Mode 1, cp = 1.5 km/s, t = 1.33 s t = 1.09 s

Mode 3, cp = 6.6 km/s, t = 0.303 s t = 0.76 s

Frequency (MHz)

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Tuned mode 1 Tuned mode 3

Synthetic phased array tuning provides flexibility through reconstruction.

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In summary, phased array transducers can be used for synthetic focusing in composite laminates.

Mode selection and tuning can improve sensitivity and penetration power

Long range guided wave monitoring capabilities

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