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Title: Interfacial behaviours of smart composites
Authors: Poon, Chi-kin
Degree: Ph.D.
Issue Date: 2004
Abstract: The success of conventional fiber reinforced composites (FRC) relies on the quality of bonding between fibers and matrix. The excellent interfacial bond strength between shape memory alloy (SMA) inclusions and the host materials is also critically important for the success of SMA-composites. A review of literatures shows that there is a lack of theoretical models and experimental findings on the interfacial behaviours of the SMA-composites. Therefore, in the past, the operation limit as well as the ideal actuation condition of SMA inclusions could not be predicted accurately during the design stage. Some of the fabricated SMA-composite structures may therefore suffer a potential risk of sudden failure due to over-loading or over-actuation. The theoretical models developed in this research provide a study basis for the prediction of internal stresses and interfacial strength of the SMA-composites. Martensite volume fraction is considered as a critical parameter which determines the material properties and shape memory effect (SME) of the SMA inclusions. The proposed model reproduce the SMA behaviour inside a substrate, evolutions of martensite volume fraction and elastic modulus of SMA, and the internal stresses along the embedded length in different loading and actuation scenarios. The concepts of 'constant martensite volume fraction region (CMR)' and 'constant axial stress region (CASR)' are proposed to justify the desired SMA actuation which ensures the uniform material properties and mechanical performance within a target region. The minimum required geometric factors for satisfying the CMR and CASR are estimated from the associated numerical analysis. Three types of typical loading conditions and two possible actuation modes are identified to study the influence of loading-actuation scenarios on internal stress distributions and interfacial debond. The critical wire embedded length and matrix-to-wire radius ratio for achieving 'almost the same' mechanical response of different loading conditions are estimated from the corresponding parametric studies. The results suggest that the critical embedded length and the matrix-to-wire ratio are required to validate the loading condition employed in pullout test for determining the interfacial properties. In addition, mode-b actuation approach is found to be more effective for the initiation of the SME. The energy approach is applied in the formulation of the interfacial debonding criterion. The total elastic strain energy on both the SMA wire and matrix cylinder is taken into account for the prediction of interfacial debond. Solutions of stress distributions in the partially debonded and the remaining bonded regions are derived with the considerations of externally applied stress, axial position and actuation temperature. These solutions are then substituted into the debonding criterion that results in the numerical solutions of normalised strain energy release rate against the applied load. The substantial improvement of the initial debond stress is predicted with the increase of the actuation temperature. A pre-load or initial applied stress is found to be necessary to compensate the additional recovery stress arising from SMA actuation before the further development of the strain energy release rate. Single fibre pullout test is employed to validate the model developed in this study. Based on the knowledge obtained from the CMR and CASR analysis, the SMA wire/epoxy matrix cylinders with appropriate dimensions are fabricated for the test. In addition to the pullout test data measured by the MTS micro-force testing machine, the debonding processes are also closely monitored by using the high resolution digital video capturing system. Results are then compared with theoretical predictions. Both the critical stress at the initial debond and the debond stress development with the debonded length are found matching fairly well with the theoretical predictions. The 'Optimum Actuation Condition (OAC)' that ensures the reinforcement of SMA composite but avoids the failure of composite interface due to over-actuation is also defined to optimize the application of SME in the composite structure within a safety actuation limit. Feasibility of OAC is studied with the consideration of different prestrain values. It is found that the higher the prestrain value, the more difficult the OAC control. A simplified OAC (SOAC) is also developed to provide an analytical solution of OAC and thus the ideal actuation temperature for achieving such specific actuation condition can be estimated more easily. It is found that the results of OAC and SOAC are compromised for the sufficient wire embedded length. The stress distributions in the perfectly bonded and partially debonded SMA wire/matrix cylinder are simulated by using finite element analysis (FEA). The frictional contact surfaces at the debonded region are carefully defined and the nonlinear static analysis is employed to analyze the stress distributions inside the partially debonded two-cylinder models. The results indicate that both FEA and theoretical results agree well with each other especially in the perfectly bonded condition. Buckling control of composites with embedded SMA wires is employed as application examples. The buckling resistance of SMA-reinforced laminated beam is measured from the buckling test. The results agree fairly well with the theoretical predictions and hence convince the applications of such theoretical model for the estimation of buckling resistance in the design stage of the SMA-composites. Based on the overall developments, it appears that the proposed theoretical models and the approach of experimental study are the viable basis for the development of the SMA-composites as a new class of reliable materials for satisfying wide variety of engineering requirements. The concept of evolution of martensite volume fraction along the embedded SMA materials is particularly important for the predictions of mechanical response as well as material status of the SMA-composites.
Subjects: Hong Kong Polytechnic University -- Dissertations
Smart materials
Smart structures
Shape memory alloys
Pages: xiv, 266 leaves : ill. (some col.) ; 30 cm
Appears in Collections:Thesis

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