Abstract
The present doctoral thesis aims towards exploration of the response of shape adaptive structures controlled by Shape Memory Alloy (SMA) actuators, that operate under partial phase transformation conditions. Particular attention is paid to the active load alleviation on large wind turbine blades through adaptation of appropriate airfoil sections along the blade span. Consequently, the conducted research is bi-fold and focuses on: (I) the study, understanding and prediction of the constitutive behaviour of SMA materials under loading conditions that lead to the formation of partial phase transformation and (II) the design, analysis and realization of appropriate camber-adaptive mechanisms, controlled by SMA actuators, able to induce congruous target shapes as well as follow prescribed target trajectories that permit the actively controlled reduction of the both fatigue and extreme loads developed on the blade root.In order to experimentally investigate the response of a Ni51Ti49 wt.% al ...
The present doctoral thesis aims towards exploration of the response of shape adaptive structures controlled by Shape Memory Alloy (SMA) actuators, that operate under partial phase transformation conditions. Particular attention is paid to the active load alleviation on large wind turbine blades through adaptation of appropriate airfoil sections along the blade span. Consequently, the conducted research is bi-fold and focuses on: (I) the study, understanding and prediction of the constitutive behaviour of SMA materials under loading conditions that lead to the formation of partial phase transformation and (II) the design, analysis and realization of appropriate camber-adaptive mechanisms, controlled by SMA actuators, able to induce congruous target shapes as well as follow prescribed target trajectories that permit the actively controlled reduction of the both fatigue and extreme loads developed on the blade root.In order to experimentally investigate the response of a Ni51Ti49 wt.% alloy in thin wire form, a specially designed in-house apparatus has been manufactured for the realization of thermal cycles under constant mechanical load conditions. The study concentrated on achieving closed partial transformation cycles that were initiated and formed in various areas on the branches and inside the major hysteresis cycle obtained after a full transformation cycle takes place. For the simulation of the material behaviour the well-established constitutive model developed by Professor Lagoudas and his coworkers has been adopted. The original form of the hardening functions was properly modified to account for the formation of closed partial transformation cycle branches. The proposed model modification introduces a modified internal state variable, i.e. the Martensitic volume fraction, that is subjected to proper scaling based on the respective fraction obtained during the reversal of the transformation direction. The extended constitutive equations and their predictions were compared with two other approaches based on literature, while all the models were implemented in a commercial finite element analysis software through appropriate user material subroutines. The numerical predictions were correlated with experimental measurements to assess and validate the modified constitutive equations. Subsequently, the proposed modification of the hardening function expression was proved to predict the experimentally obtained response without requiring complex mathematical expressions, neither demanding additional calibration parameters nor high computational cost. The aforementioned characteristics render the proposed model modification ideal for the simulation of complex shape adaptive structures entailing SMA actuators.Following, the specifications for the design and development of a novel, biomimetic, articulated mechanism for the active shape adaptation of the camber-line for large wind turbine blade sections are presented. SMA wire actuators were considered to be placed in an antagonistic configuration in order to enable both clock-wise and counter-clock-wise rotation and achieve a wide range of shapes. A physical prototype was designed and manufactured for the experimental investigation of the resultant movement. Testing results were compared with finite element model predictions that either included or neglected partial transformation behaviour. Implementation of the modified constitutive equations suggested that the formation of incomplete transformation branches drastically affects the produced movement, especially after a direction reversal takes place. Omitting the particular behaviour leads in erroneous predictions and large deviations that underestimate the capabilities of the actuators.Consecutively, the movement of the camber-adaptive mechanism was simulated for a series of input target time trajectories. A proper three component PID controller was developed for actively controlling actuators heating phase through Joule heating by calculating the required power input, while a series of On/Off controllers were also developed for the consideration of free or forced heat convection conditions. Conducted simulations without consideration of partial transformation behaviour proved that the mechanism can follow adequately the prescribed trajectories for active load alleviation of the developed moments at root of the blade. Nevertheless, delay in motion during reversal in movement directionality and mismatch in target shape during abrupt movements suggest that the obtained response cannot be considered ideal. Ignoring partial phase transformation under extreme loading conditions results in erroneous axial stress saturation that are responsible for the gradual saturation of the available transformation strain and evidently immobilize the mechanism. On the contrary, the modified constitutive equations predict a faster response, without delays during direction reversal, reduced axial stresses, lower temperature variation demands and, consequently, reduced power requirements. Concluding, consideration of the unique behaviour presented when SMAs operate under partial transformation is of great importance for the design, analysis and reliable prediction of associated response of shape adaptive structures entailing these materials as actuators, while it broadens the available application fields for their use in engineering as well as other sciences.
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