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1 Damage State Awareness in Composite Laminates Via Ultrasonic Guided Waves Cliff Lissenden Joseph Rose Engineering Science & Mechanics The 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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2 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 Current issues and capabilities

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3 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 PWASAir-coupled transducer test bed

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4 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-scan16 element array8 element array Needs: penetration distance, number of sensors, damage sensitivity, critical size

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5 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 mm

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6 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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7 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 Frequency Group velocity Dispersivity Excitability Attenuation Damage sensitivity Signal processing Damage mechanics Etc. Angle beam Comb Natural tuning Phased array tuning Prognostics

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8 Lamb-like and SH waves are not decoupled in anisotropic materials. Phase velocity dispersion curves for guided waves propagating in the 0 o 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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9 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 0 o direction of a [(0/45/90/-45) s ] 2 carbon/epoxy laminate. Kelvin-Voigt model 1 neper = 8.69 dB

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10 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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11 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 1D model, F(x,t) 2D Fourier Transform, F(k, ) Source Influence spectrum, F(c p,f) c p = /f, f = /2

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12 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, x 1 = prop. dir., x 3 = thickness dir. Logo Velocity field Stress field Transducer length

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13 A linear phased array provides mode tuning by using time delays. Elements uniformly spaced at distance d time delays Logo Guided wave Li & Rose, 2001, IEEE Trans. 48(3):761 d

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14 Frequency tuning and time delays provide tremendous flexibility for mode tuning. 200 kHz central frequency 1 MHz central frequency d = 2 mm Mode 1, c p = 1.5 km/s, t = 1.33 s t = 1.09 s Mode 3, c p = 6.6 km/s, t = 0.303 s t = 0.76 s

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15 Tuned mode 1Tuned mode 3 Synthetic phased array tuning provides flexibility through reconstruction.

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16 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 Questions? Logo

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