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 February 2008 Impact delamination in cross-ply laminate

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

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

4 Tomographic imaging enables visualization of internal damage 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

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

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

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

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

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

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

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

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

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

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 = s t = 0.76 s

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

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