AbstractStructural health monitoring (SHM) systems and non-destructive evaluation (NDE) methods are commonly used to assess the integrity of aerospace structural components. These techniques allow a continuous and in-situ screening of both composite and metallic structures, thus saving operative costs and maintenance time. In order to collect large quantity of sensing data and due to the complexity of aerospace structures, surface bonded transducers need to be placed in a number of positions of an aircraft. However, transducer information on commercial airliners is still communicated to the user by costly and heavy cables. In the last decade, active wireless technology has been considered, even though all the energy available on aircrafts is already used for other purposes. Thus, energy harvesting systems are necessary to make the communication self-powered.
This work is organised in two main parts. In the first part of this work, three SHM systems for damage evaluation are introduced and validated. A novel nonlinear ultrasonic time domain approach, relying on the time of arrival estimation of the second harmonic nonlinear response, is presented and used to detect and localise material damage. Second order phase symmetry analysis filtering and burst excitation are used so that arrival times of the second order nonlinear elastic waves, measured by a number of receiver sensors, can be estimated through the Akaike Information Criterion approach. Then, Newton’s method and unconstrained optimisation are used, in combination, to solve a system of nonlinear equations so that material damage coordinates can be obtained. The methodology was validated through a number of experiments on an impact damaged composite plate and the results showed that defect position can be detected through this method with high accuracy as the maximum error in damage localisation was approximately 5 mm. Furthermore, a novel nonlinear ultrasonic frequency approach, relying on the evaluation of the second order nonlinear response amplitude via two different non-dimensional parameters, is proposed in order to achieve both material damage detection and localisation. Surface bonded transmitting-receiving sensors, placed at opposite sides of the structure, are coupled and the damage position projection on each path between the coupled sensors is calculated through a reciprocal relationship of the nonlinear elastic parameters. All the results, from each sensor pair, are averaged so that damage position is found. Experimental tests were performed on a damaged composite panel and the results showed that this technique provided high localisation accuracy as the maximum error between the real and the estimated damage location was approximately 13 mm. An evolution of this nonlinear ultrasonic method is also presented, allowing imaging of damage in composite structures. This novel technique considers either the second harmonic or the modulated elastic response, from pairs of surface bonded transducers, as input of a reciprocal relationship in order to find the damage position projection on the path between coupled sensors. A statistical approach is then used to select a cloud of points in order to picture the damaged area. The technique was validated on a complex damaged composite panel and the results showed accurate damage localisation and imaging as the maximum error between the real and the calculated damage area centres was only 1.3 mm. All the proposed SHM methods, unlike traditional linear ultrasonic techniques, allow detection and localisation of damage on composite materials without a priori knowledge of the ultrasonic wave velocity nor a baseline with the undamaged structure.
The second part of this work is focused on the development of thermo-electric energy harvesting in order to feed low-power wireless SHM systems. Temperature gradients, representing the heat waste produced by aircraft engines, are used as input of thermo-electric generators (TEGs) which transform thermal power into an electrical current. Considering that TEG power output can be enhanced by thermal diffusion systems, called heatsinks, a novel air cooling heatsink is here introduced and developed to further improve TEG performance. Numerical finite element thermal simulations were performed to assess the cooling performance of the proposed heatsink and experimental tests were carried out to validate simulation results. The results showed that the novel heatsink allowed an enhancement of TEG temperature difference and power output, in comparison with traditional cylindrical pin heat diffusion systems. In order to allow TEG achieving wireless SHM operative energy requirements of tens of mW, a heat dissipation system, composed by two novel heatsinks combined through a copper sheet and highly oriented pyrolytic graphite layers, was also here introduced and tested. Experimental results revealed that the considered heatsink-TEG arrangement was able to produce an output power over 25 mW, confirming the feasibility of a self-powered wireless SHM system for aerospace applications.
|Date of Award||19 Jun 2019|
|Supervisor||Michele Meo (Supervisor) & Fulvio Pinto (Supervisor)|