Guided Wave Propagation in Composite Structures; Application to ice detection on wind turbine blades

Licentiate thesis, 2016

Guided waves are an efficient non-destructive tool in inspection and fault detection of elongated structures. Due to the special characteristics of composite materials, study of guided wave propagation in them has been an interest. In the current work, application of guided waves is investigated in ice detection on composite materials which is a well-known problem in wind turbine industry. The possibility of detecting a layer of ice on a composite plate is first investigated by a 2D isotropic-anisotropic multilayer model. The wave equation is solved and dispersion curves are obtained. Results show that adding a second isotropic layer on top of an anisotropic material causes reduction in phase and group velocity of the first symmetric mode. Effects of low temperature on the received signal is investigated using an experimental test setup. Measurements show that lowering the temperature causes drop in amplitude and temporal phase shift in the received signal. These effects were handled by a modification of the Baseline Signal Stretch method. The modification is based on decomposing the signal into symmetric and asymmetric modes and applying two different stretch factors on each of them. Computational modelling of the problem is performed by first developing a 2D model which shows that accretion of ice causes reduction in phase and group velocities of the incident wave and creates reflections. The model is developed further to a 3D shell model, in which ice is placed on the plate by changing the properties of specific elements in the icing region. The Baseline Signal Stretch with the mode decomposition method is applied to the model for temperature variations. Effects of ice accretion on a composite plate is studied in time, frequency and wavenumber domains. In each case post-processing approaches are introduced for this specific application. Moreover, icing index is introduced which is sensitive to accumulated ice on the plate. The experimental study is performed in a cold climate lab in three different steps. The first part to get general understanding about the effects of ice accretion on waves propagating in a composite plate. Next, to understand the effects of temperature on the received signal and calibrate the temperature model and finally a more accurate study by installing 24 accelerometers and manufacturing a layer of ice on the plate to validate the results obtained by the computational model. Using the model and introduced criteria both thickness and location of ice on the plate are identified. All the results show that application of guided waves is a promising and accurate tool in ice detection on composite plates.