[Truncated abstract] This PhD thesis aims to investigate the thermomechanical behaviour of functionally graded NiTi shape memory alloys (SMAs). Due to microstructural, compositional or geometrical gradient, a complex transformation field is created within the SMA structure. This provides gradient stress over stress-induced martensitic transformation, widened controlling window for stress- and thermally-induced martensitic transformations and better controllability of SMA element in actuation application. Analytical and numerical models are introduced to predict the deformation behaviour of such components under mechanical loads that are validated with actual experiments. The analytical models provide closed-form solutions for global stress-strain variation of such functionally graded alloys and can be used as effective engineering tools for mechanism design. The current thesis is presented in the form of research papers that are published or under consideration for publication in scholarly international journals. It is categorised in the following sections: (1) Microstructurally graded 1D and 2D SMA structures. By applying designed heat treatment gradient along the length of a shape memory alloy wire, transformation stress and strain gradients are created. Thus, the material exhibits distinctive inclined stress plateaus with positive slopes, corresponding to the property gradient within the sample. General polynomials are used to describe the transformation stress and strain variations with respect to the length variable. Closed-form solutions are derived for nominal stress-strain variations that are closely validated by experimental data for shape memory effect and pseudoelastic behaviour of NiTi wires. The average slope of the stress plateau is found to increase with increasing temperature range of the gradient heat treatment.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2013|