SHM of Composite in tidal energy converters

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Contents

Introduction

Tidal current is being recognized as a resource to be exploited for the sustainable generation of electrical power. The high load factors resulting from the fact that water is 800 times denser than air and the predictable and reliable nature of tides compared with the wind makes tidal energy particularly attractive for Electric power generation. Condition monitoring is the key for exploiting it cost- efficiently.

Composite materials in Tidal Energy Converters

Composite materials are becoming ever more common place in engineering as the limits of more traditional materials increasingly become a limiting design factor and the cost and complexity of manufacturing composite material components reduces. The wind turbine industry very quickly learned that the high-strength, lightweight properties of glass and carbon fibre reinforced thermoset plastic (GFRP and CFRP respectively) were well suited to shape and load characteristic required for turbine blades. The tidal energy conversion (TEC) industry benefits from the learning gained in the development of composite blades for the wind industry and as a result, most TEC devices incorporate composite turbine blades. TEC device turbine blades are typically lower aspect ratio (short and wider) and have a denser structure incorporating greater quantities of carbon fibre than their wind turbine equivalents, power for power.

Structural health of composites in Tidal Energy Converters

This materials during exploitation under dynamic loads, continuous vibrations, fatigue and harsh environmental conditions (for example, marine environment) must be periodically tested during the maintenance in order to avoid the dangerous defects, like internal delamination, breakage of fibres, cracks and etc.

On other hand, during manufacturing, the appropriate quality control of such materials should be performed, also. In the case of non-destructive testing (NDT) of objects having large dimensions and complex geometry structure the access to their surfaces is often possible only from one side. During the time, the constructional components of the structure may degrade and failure of the construction may occur if the strength reduces below the designed value. Therefore, the periodical structure health monitoring should be performed.

Use of Guided waves to monitor defects in composite tidal energy converter elements

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 using only single-side access. Such waves propagate in different elongated objects such as plates, rods, pipes, rails and spar profiles. The guided waves propagating in plates are called Lamb waves. The parameters of the Lamb wave modes in composite 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 .

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 .

Defects detection capability

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 difficult. Usually in order to avoid or at least to simplify these problems the measurements are performed in low frequency ranges where two primary fundamental modes A0 and S0 propagate . The group velocities of A0 and S0 modes are different, what enables to avoid overlapping of the signals in time domain. The S0 mode possesses higher group velocity and propagates faster then the A0 mode. The signals used in the measurements usually are not narrow band. Due to dispersion, the higher frequency components propagate with different velocity comparing to the lower frequency components. As the result, waveform of the received waves becomes elongated [8]. During the long range inspection, the length of the structure which can be inspected from a single location will be determined by the degree of attenuation of the chosen mode. Therefore, it is necessary to choose a mode with the lowest attenuation. The Lamb waves are attenuated due to several phenomenons: signal losses due to dispersion and beam divergence, scattering and leakage into surrounding medium. The reduction of the signal amplitude according the beam divergence is given by the inverse square root dependency on the distance. The more complex scattering phenomena occurs in the case of interaction of Lamb waves with internal structural non-homogeneities and defects of the object being investigated. Such effects complicate the interpretation of received signals, and distinguishing of the appropriate mode remains problematic .

The types of damage expected in the composite structure of the hydrofoil of a tidal power plant are assumed to be: 1. Delamination between skin and adhesive layer, 2. Delamination between main spar and adhesive layer; 3. Adhesive joint failure between skins along leading and trailing edges; 4. Internal multiple delaminations or splitting between layers of the skin and the main spar.

Validation of the detection on real tidal devices components

The guided waves technology to detect defects in tidal devices has been tested in two devices components by a consortium of SMEs and supporting research centres working in the project TidalsenseDEMO:

- Hydrobuoy mooring system developed by Nautricity Limited. A buoyancy and lift tension creating hydrofoil made mainly of composite (GFRP) materials. - Aqua Energy Solutions sails. This device tries to cover a large swept area by securing a high number of composite made sails attached to two wires moving on a elongated loop and that transfer power to a generator at one of the ends of this loops.

Two implementation strategies were applied in two demonstration over those real elements: Automatic periodic Reading/Measurements Strategy: A permanent installation of transducers and interfacing electronics in the tidal device that allows an automatic periodic reading of the sensors. This strategy allows to perform remote condition monitoring of the critical or more expensive composite material elements. Spot Measurements (SM) strategy: Permanent installation of the transducer in the critical components that allows the user to perform measurements during maintenance interventions/visits to the tidal device. Allow the user to forecast the replacement of the element in a future intervention, or immediately in the current visit to the machine, if significant damage is detected. The maintainability of the structural health monitoring system was demonstrated by the study of correlation after a transducer replacement on a working element.

TidalSense Demo is a Seventh Framework Programme project, funded under Research for the Benefits of SMEs | Cordis Reference to the Program framework. The TidalSense Demo project is a Demonstration Action under the FP7 "Capacities" programme with Grant Agreement no. 286989. This EU programme is managed by the Research Executive Agency REA in Brussels, on behalf of the European Commission, http://ec.europa.eu/research/rea/FP7/2007_2013


Related Pages

Bibliography

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