APPLICATION OF ULTRASONIC GUIDED WAVES FOR INVESTIGATION OF COMPOSITE CONSTRUCTIONAL COMPONENTS OF TIDAL POWER PLANTS

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1 The 12 th International Conference of the Slovenian Society for Non-Destructive Testing»Application of Contemporary Non-Destructive Testing in Engineering«September 4-6, 2013, Portorož, Slovenia More info about this article: APPLICATION OF ULTRASONIC GUIDED WAVES FOR INVESTIGATION OF COMPOSITE CONSTRUCTIONAL COMPONENTS OF TIDAL POWER PLANTS R.Raišutis 1, L.Mažeika 1, V.Samaitis 1, A.Jankauskas 1, P.Mayorga 2, A.Garcia 3, M.Correa 3, B.Neal 4 1 Ultrasound Institute, Kaunas University of Technology, Kaunas, Lithuania, renaldas.raisutis@ktu.lt 2 EnerOcean S.L., Málaga, Spain, pedro.mayorga@enerocean.com 3 Innotecuk Ltd., Cambridge, UK, alvaro.garcia@innotecuk.com 4 TWI Ltd, Granta Park, Great Abington, Cambridge, UK, ben.neal@twi.co.uk ABSTRACT The tidal power is one of the promising sources of the renewable energy to be picked up by the tidal stream generators. However, due to continuous operation under the harsh marine conditions, such constructions should be periodically tested against the faults. The objective of this work is to select the modes of ultrasonic guided waves for inspection of the most critical regions of composite components of the hydrofoils used in tidal power plants and to determine the parameters of their excitation and propagation along the sample. The analysis of the multi-layered structures of the composite components to be inspected using ultrasonic guided waves was performed. The geometry, material type, properties and the critical regions that should be tested were identified. The dispersion curves of phase velocity of the guided waves propagating in the multi-layered composites structures have been determined using SAFE method. The propagating modes of guided waves in the multi-layered glass fibre (GFRP) structures of the skin and the main spar (CFRP) of the hydrofoils were identified using the modelling and the experiments. Key words: tidal power plant, multi-layered composite, ultrasonic guided waves, modelling 1. Introduction The tidal power is one of the promising sources of the renewable energy to be picked up by the tidal stream generators. During the last decade various types of the tidal power plant prototypes were developed, some of them contain special hydrofoils and hydro-buoys made of composites. However, due to continuous operation under the harsh marine conditions, such constructions should be periodically tested against the faults. One of the non-destructive testing techniques which enable to detect the defects both during manufacturing or in-service inspection is based on application of ultrasonic guided waves. The parameters of the Lamb wave modes in composite 277

2 materials depend on elastic properties of the laminate, plate thickness and thickness of different layers, fibre orientation, lay-up and on the presence of internal discontinuities, such as delaminations, porosity, ply gaps, foreign matter, and changes in fibre volume ratio [1,2]. The basic factors which determine particular mode of Lamb waves and operating frequency to be used are the following: dispersion, attenuation, sensitivity, excitability, detectability, selectivity and sensitivity to defects [3]. In the case of interaction of the Lamb waves with defects present on the propagation path, the wave are reflected, scattered and converted into other modes. Also, the attenuation of the guided waves in composites is relatively high comparing to the metal plates. Due to these facts, the analysis of the received multi-mode signal affected by the internal non-homogeneities becomes complicated. The identification of the various guided waves modes in the received multi-mode signal is complicated. Usually in order to avoid or at least to simplify these problems the measurements are performed in low frequency ranges where only two primary fundamental modes A 0 and S 0 propagate [4,5]. The group velocities of the A 0 and the S 0 modes are different, what enables to avoid overlapping of the signals in time domain. The objective of the presented work is to select the modes of ultrasonic guided waves to be used for inspection of the most critical regions of composite components of the hydrofoils used in tidal power plants and to determine the parameters of their excitation, propagation along the sample and to identify the regions of coverage. 2. Object under investigation and the expected types of possible defects The hydrofoil sample was used as the object for investigation. There are three regions of the sample possessing different arrangement of the layers. The first type is the leading (I). The second one is glass fibre skin, thick layer of epoxy glue and the carbon fibre body of the main spar (II). The last one is similar to the first one - trailing (III). The expected types of possible defects inside hydrofoil are the following (Fig. 1) [6]: Delamination between the skin and the adhesive layer; Delamination between the main spar and the adhesive layer; Adhesive joint failure between the skins along the leading and the trailing s; Internal multiple delaminations or splitting between the layers of the skin and the main spar. I II III Fig.1: The image of the hydrofoil sample with the regions I, II, III possessing different arrangement of the adjacent layers and the expected types of possible defects inside the hydrofoil 278

3 3. The optimum arrangement of transducers At low frequencies (below 200kHz) the transducers for excitation and reception of guided waves usually possess diameter relatively small comparing to the wavelength of the excited waves. So, the shape or geometry of the transducer do not affects essentially the directivity pattern and as consequence the focusing cannot be achieved by such a means. The focusing of the beam can be obtained just by application of the set of transducers or transducer array. Usually, in order to achieve the effect of focusing the elements of the transducer array should be excited with pulses of appropriate amplitude and delay time or special spacing between transducers should be used. However, for efficient focusing the big number of the transducers and multichannel NDT instrument is needed what increase the complexity and price of inspection. Therefore, in order to achieve the directivity and the higher amplitude of the excited guided waves in structure of hydrofoil, it is proposed to use the small set of transmitters connected in parallel. Such arrangement is partially equivalent to the transmitter with wider lateral dimensions. However, in order to achieve the sufficient covering of the hydrofoil structure with guided waves and the reliable detection of defects it is proposed to use a set of the receivers and to perform commutation in appropriate order. The proposed configuration of transducers arrangement in the case of the adhesive joint inspection between the skin and the main spar is presented in Fig.2a and adhesive joint inspection between the internal layers of the skin along the sample is presented in Fig.2b and across the sample is presented in Fig.3. The very important fact is that the propagation of guided waves extremely depends on the all parameters of composite (the structure, the geometry, thickness of the layer, orientation of the fibres, the material and etc.). This means that some times even small changes in the structure or parameters of the composite component can lead to essential changes in the parameters of the propagating guided waves. This leads to the conclusion that the inspection or monitoring technique should be developed and tested on the samples which are as close as possible to the real structure of the component under investigation. Boundaries of the region Boundaries of the region Leading Transmitters Direct beam of multiple transmitters Main spar Boundaries of the region Beam divergence of single transmitter Upper shell Trailing Beam divergence of single transmitter Direct beam of multiple transmitters Transmitters Leading Transmitters Upper shell Main spar Beam divergence of single transmitter Direct beam of multiple transmitters Lower shell a) b) Lower shell Trailing Fig.2. The proposed configuration of transducers arrangement in the case of adhesive joint inspection along the hydrofoil: lack of cohesion between the skin and the main spar (a), lack of cohesion between the internal layers of the skin (b) 279

4 Boundaries of the region Leading Transmitters Boundaries of the region Direct beam of multiple transmitters Beam divergence of single transmitter Main spar Upper shell Trailing Lower shell Fig.3. The proposed configuration of transducers arrangement in the case of adhesive joint inspection across the hydrofoil (lack of cohesion between the internal layers of the skin) 4. Modelling of guided waves propagation in the multi-layered composite structure of the hydrofoil At first, the effects of guided waves propagating in the skin of the hydrofoil should be investigated and the modes most suitable for inspection should be determined. The overall structure of the hydrofoil skin (total thickness mm) consist of the three layers of differently oriented laminates (Table 1): GFRP (0 /-45 /90 /+45 ), thickness of 1 mm; GFRP (90 ), thickness of 2 mm; GFRP (0 /-45 /90 /+45 ), thickness of 1 mm. The overall multi-layered structure of the skin, the thickness of each layer and the appropriate orientation angle of the plies in each layer has been taken into account during simulations by SAFE method [7,8]. The stiffness matrix has been rotated according to the orientation angle of the plies for each particular layer (Table 1). The dispersion curves of the guided wave modes propagating along the laminate structure of the skin (direction 0 ) are presented in Fig.4a and propagating across the laminate structure of the skin (direction 90 ) are presented in Fig.4b. Table 1: Arrangement of the composite layers inside the skin of the hydrofoil NUMBER OF LAYER ORIENTATION 1 0º º º º º 2 6 0º º º º 0.25 LAYER THICKNESS, MM 280

5 vph, m/s vph, m/s f,khz a) f,khz b) Fig.4. The dispersion curves of the guided wave modes propagating: a - across (direction 0 ) the laminate structure of the skin; b - along (direction 90 ) the laminate structure of the skin The main spar is manufactured using unidirectional (direction of fibres is 0 ) CFRP composite. It is known that ultrasound velocities in the CFRP composites differs essentially from though in the GFRP. So, the dispersion curves of the guided wave modes propagating along (direction 0 ) the laminate structure of solidly glued skin and main spar are presented in Fig.5a. The dispersion curves of the guided wave modes propagating across (direction 90 ) the laminate structure of solidly glued skin and main spar are presented in Fig.5b. Analysis of the obtained dispersion curves shows that it is possible to use the fundamental modes of guided waves (A 0 and S 0 ) for investigation of the main spar segment of the hydrofoil in the frequency range below 20 khz. This is almost out of ultrasound ranges, however at higher frequencies, the multiple modes can propagate and waveforms of the received signals will be more complicated for analysis. v ph, m/s SH v ph, m/s A f, khz 2000 SH a) b) A 0 f, khz Fig.5. The dispersion curves of the guided wave modes propagating along (a) and across (b) the laminate structure of solidly glued skin and main spar 281

6 5. Experimental investigation of the mock-up sample of the hydrofoil In order to investigate experimentally the guided waves propagating in hydrofoil like structure the special flat sample was used. The geometry, structure and dimension of the sample are presented in Fig. 6. It consists of shell base (dimensions of 1 m x 1 m, having thickness of 8 mm) manufactured from glass fibre (GFRP, laminate orientation +/-45 fibres), beam with shear webs of glass fibre (+/-45, having thickness of 8 mm) and spar cap of unidirectional carbon (having thickness of 40 mm) fixed to shell base using the epoxy glue. The experimental set-up for investigation of guided waves propagation along the shell base (GFRP) only is presented in Fig.6a. The experimental set-up in the case of guided waves propagation along the overall structure of the hydrofoil is presented in Fig.6b. The parameters of the experiments for the case 1 (on shell base) and the case 2 (on spar cap) are summarized in Table mm MFC transmitter (active ceramic area mm) 100 mm 1000 mm 1m Scanning direction 300 mm Shear webs +/-45 0 fibres Approx 8mm thick (measured 7.98mm) Glass fibre 1000 mm 1m Scanning direction Moving receiver Shear webs +/-45 0 fibres Approx 8mm thick (measured 7.98mm) Glass fibre Moving receiver MFC transmitter (active ceramic area mm) 300 mm Epoxy structural paste adhesive 100 mm Epoxy structural paste adhesive 270 mm Spar cap Undirectional carbon 0 0 Approx 40mm thick 1000 mm 1m Shell base +/-45 0 fibres Approx 8mm thick (measured 8.01mm) Glass fibre Spar cap Undirectional carbon 0 0 Approx 40mm thick 1000 mm 1m Shell base +/-45 0 fibres Approx 8mm thick (measured 8.01mm) Glass fibre Fig.6. The experimental set-up for investigation of guided waves propagation along the shell base (GFRP) only (a), and for excitation-reception of the propagating guided waves on the spar cap (CFRP) (b) PARAMETER a) b) Table 2: Measurement parameters for the case 1 and case 2 Distance between transmitter and the of the object, (mm) VALUE MEASUREMENTS ON THE SHELL BASE Initial distance between transducers, (mm) Scanning distance, (mm) Duration of excitation pulse, (μs) Excitation voltage, (V) Scanning step of the receiver, (mm) 1 1 MEASUREMENTS ON THE SPAR CAP 282

7 The B-scan images obtained in both scanned areas are presented in Fig.7a and Fig.7c. The experimental dispersion curves of phase velocities reconstructed from the B-scan data using 2D FFT are presented in Fig.7b and Fig.7d. For comparison on the same images the theoretical dispersion curves obtained by the semi analytical finite element (SAFE) method are presented also. In general the good coincidence can be observed. S 0 A 0 S 0 A 0 Selected region for 2D FFT calculation a) b) Selected region for 2D FFT calculation c) d) Fig.7. Investigation of guided waves propagation: B-scan along the shell base (a), dispersion curves of phase velocities along the shell base (b), B-scan on the spar cap (c), dispersion curves of phase velocities on the spar cap (d). By circles the theoretical dispersion curves are presented 6. Conclusions The dispersion curves of phase velocity of the guided waves propagating in the multilayered structure of hydrofoils have been determined using SAFE method. The propagating modes of guided waves in the multi-layered structure of the hydrofoil skin and in the multi-layered structure of the main spar were identified using the modelling and the experiments. The frequency range of operation below 200 khz should be used for inspection of the skin and even lower for inspection of the main spar. In these frequency ranges mainly two fundamental modes the A 0 and the S 0 are propagating; The testing of main spar should be performed in longitudinal direction and other parts in hydrofoil along perpendicular direction due to special orientation of fibres in the composite. The propagating modes in the complicated structure of the hydrofoil (multi-layered, CFRP, GFRP) were investigated by numerical modelling (SAFE) and experiments. It was 283

8 estimated, that in order to use the fundamental modes, the frequency of operation below 100 khz should be used for inspection of the skin and even lower for inspection of the main spar. 7. References [1] Vishnuvardhan J., Krishnamurthy C. V., Balasubramaniam K.: Genetic algorithm based reconstruction of the elastic moduli of orthotropic plates using an ultrasonic guided wave single-transmitter-multiple-receiver SHM array, Smart Mater. Struct., Vol.16, 2007, [2] Chimenti D. E., Martin R. W.: Nondestructive evaluation of composite laminates by leaky Lamb waves, Ultrasonics, Vol. 29, No. 1, 1991, [3] Wilcox P.D., Lowe M.J.S., Cawley P.: Mode and transducer selection for long range lamb wave inspection, Journal of intelligent material systems and structures, Vol. 12, 2001, [4] Ramadas C., Balasubramaniam K., Joshi M., Krishnamurthy C. V.: Interaction of the primary anti-symmetric Lamb mode (A0) with symmetric delaminations: numerical and experimental studies, Smart Materials and Structures, Vol. 18, No. 8, 2009, 1-7. [5] Su Z., Ye L., Lu Y.: Guided Lamb waves for identification of damage in composite structures: A review, Journal of Sound and Vibration, Vol. 295, 2006, [6] Raišutis R., Jasiūnienė E., Šliteris R., Vladišauskas A.: The review of non-destructive testing techniques suitable for inspection of the wind turbine blades, Ultragarsas/Ultrasound, Vol. 63, No. 2, 2008, [7] Hayashi T., Kawashima K., Rose J. L.: Calculation for guided waves in pipes and rails, Key engineering Materials, Vol , 2004, [8] Hayashi T., Song W. J., Rose J. L.: Guided wave dispersion curves for a bar with an arbitrary cross-section, a rod and rail example, Ultrasonics, Vol. 41, No. 3, 2003, Acknowlment The part of this work was sponsored by the European Union under the Framework-7 project TidalsenseDemo Demonstration of a condition monitoring system for tidal stream generators No