Micromechanics and Modelling of High Performance Adaptive Shape Memory Composites with Multifunctional Materials

Chetan S, Jarali (2010) Micromechanics and Modelling of High Performance Adaptive Shape Memory Composites with Multifunctional Materials. PhD thesis, CSIR NAL and Visvesvaraya Technological University, Belgaum.

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    Abstract

    The present work investigates the micromechanics approaches to model the thermomechanical behaviours of shape memory alloy composites. The research is primarily focussed on modelling the pseudoelastic and shape memory behaviours of smart composites, which are inherent in multifunctional materials like shape memory alloys and polymers. In the study, non- adaptive and adaptive matrix materials are used to address the adaptive fibernon-adaptive matrix, and adaptive fiber- adaptive matrix concepts, respectively. Nickel- Titanium shape memory alloy wire is used as an adaptive shape memory fiber. Similarly, epoxy matrix that does not exhibit shape memory behaviours is considered as non-adaptive, while matrix possessing such behaviours has been employed as an adaptive matrix. The importance of the present research is to develop the modelling procedures for shape memory composites useful in high performance applications. The first and the foremost requirements are to propose the constitutive relations, which should be simpler in computation and at the same time address the fundamental mechanics of constituent materials. Therefore, simple analytical approaches are streamlined and the existing shortcomings are overcome by proposing the suitable modifications. The significant contributions include the derivation of consistent forms of the phase transformation, and hygro-thermal inelastic composite relations, addressing the effect of moisture in matrix on the composite behaviour, the derivation of interface stress model to compute the interfacial axial and shear stress distributions, shape memory fiber and shape memory polymer composite model development, proposing the thermomechanical actuation cycle for the composite, and the study on the high performance multifunctional laminate behaviours. The methodology adopted includes the application of micromechanics approaches such as the method of mixture, method of inclusion, and the method of cells. Method of mixture and method of inclusion is derived using zeroth order displacement field, while the method of cells is based on the first order displacement field. Emphasis has been placed on the evaluation of effective elastic properties as well as the composite behaviours. The pseudoelastic behaviour of non-adaptive composite is analysed first. In addition, the fibermatrix interface effects are introduced to investigate the stress distribution in the nonadaptive composite based on the equilibrium equations of elasticity. Next, both pseudoelastic as well as the shape memory effects of adaptive composite with phase changes in fiber and matrix is studied using the method of mixture and method of inclusion. The effective properties and thermo-mechanical behaviour of adaptive shape memory composite laminate are also examined by adopting a two step homogenization scheme. In the first step, the effective properties of each layer are determined using method of mixtures and method of cells with iso-strain conditions. In the second step, the effective properties evaluation has been extended to the laminate through a thin plate theory assumption with transverse shear deformations. The possible elastic couplings are discussed. Extensive results are presented by using the constitutive relations with the proposed modifications and derivations. The thermo-elastic and hygro-thermo-elastic nature of multifunctional composite are computed. A comparison is also made with the strain energy approach for a simple uniaxial loading. The variation in the composite stiffness and the stresses are studied for different fiber volume fractions, fiber modulus, and fiber cross sections. The modifications in the modeling approaches are highlighted with analytical case studies involving hysteretic stress–strain behaviors. Further, the interfacial axial and shear stress distributions in the composite due to the phase transformations in the fiber in view of the applied boundary conditions on the matrix, is computed. Furthermore, adaptive fiber adaptive matrix composite model development is presented for the first time in this research work. The highlight of the study is the proposed thermomechanical cycle, which has been adopted for the actuation of the adaptive shape memory composite. Due to the multifunctional nature of adaptive materials, the effect of change in stiffness of the constituent phases on the overall stiffness of the composite is examined primarily. The variations in force and moment resultants are simulated for different laminate configurations with respect to fiber orientations and stacking sequences. The present work also takes into account the effects of phase transformations and the resulting change in the fiber-matrix modulus. Major conclusions can be drawn from the present research work. As compared to the passive composites (without smart materials), the active composites with adaptive fibers and nonadaptive matrix are able to carry large stress with high energy absorption capability due to the associated hysteresis. As such, it is observed that the effect of moisture in the matrix will also influence the high performance behaviours of composites. The results of the interface effects also suggest that a considerable shear stress is developed within the matrix accommodating the shape memory fiber. The shear stress increases more rapidly as the fiber radius is increased. Also, it is evident that the interface effect of shape memory composites is influenced by the fiber stiffness as compared to the geometric parameters. The modeling approach is further successfully validated extensively for different geometric and volumetric parameters under various loading conditions. Additionally, it has been brought out that adaptive composites with adaptive fiber and matrix are able to sustain large elastic deformations. Subsequently, it is noticed that the proposed modeling procedure for adaptive composites is able to reproduce consistently the overall composite response by taking into consideration not only the phase transformations, variable modulus and transformation stresses in the fiber but also the variable modulus, the evolution of stored strain and thermal strain in the adaptive matrix. Furthermore, the results prove that adaptive composite laminates can be developed, which provides shape controllability via tunable laminate stiffnesses, leading to have optimal structural response. Moreover, the work presents the necessary framework for a reliable and efficient analysis of high performance adaptive composites for practical structure and applications. Finally, the work concludes that efficient adaptive laminate development for high performance composite applications, exhibiting large shape adjustments, high stresses and increased stiffnesses, are feasible by incorporating shape memory fiber and shape memory polymer matrix as multifunctional materials.

    Item Type: Thesis (PhD)
    Uncontrolled Keywords: Smart Materials, Shape Memory Alloys, Shape Memory Polymers, Shape Memory Composites, Micromechanics.
    Subjects: ENGINEERING > Mechanical Engineering
    Division/Department: Structures Division
    Depositing User: Dr. Chetan S Jarali
    Date Deposited: 23 Aug 2011 10:35
    Last Modified: 26 Aug 2011 10:51
    URI: http://nal-ir.nal.res.in/id/eprint/9112

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