An inclusion theory for the propagation of martensite band in NiTi shape memory alloy wires under tension

In this paper, we present analytical modelling for the pure mechanical response of uniaxial tensioned NiTi wire that experiences stress-induced martensitic transformation via a propagating martensite band at the superelastic temperature regime. The model aims to predict the overall behavior of the SMA wire as a structural response containing propagating instabilities. Based on the systematic experimental investigation of Shaw and Kyriakides (Shaw, J.A., Kyriakides, S., 1995. Thermomechemical aspects of NiTi. J. Mech. Phys. Solids 43, T243-1281 and Shaw, J.A., Kyriakides, S., 1997. On the nucleation and propogation of phase transformation fronts in a NiTi alloy. Acta Mater. 45(2), 638-700), the wire is modeled as an elastic rod containing a single cylindrical transformation inclusion with a uniform axisymmetric eigenstrain. The analytical expression of the free energy of this special matrix-inclusion system is formulated and the length of the martensite band is identified as the key variable describing the transformation process of the system. Theoretical predictions on the peak stress and the subsequent steady-state propagation stress of the wire during forward and reverse transformations are provided and compared with the available experimental data. Specimen size effect on the nominal stress-strain curves and general deformation features of the wire an discussed. (C) 2000 Elsevier Science Ltd. All rights reserved

Elastic solutions of a cylindrical rod containing periodically distributed inclusions with axisymmetric eigenstrains

In this paper, we give the elastic solution for a special type of microstructure - a circular cylindrical rod containing periodically distributed inclusions along its axial direction. Each inclusion has the same uniform axisymmetric transformation strain (eigenstrain). Analytical elastic solutions are obtained for the displacements, stresses and elastic strain energy of the rod. The effects of microstructure and its evolution (growth of inclusions) on the elastic stress and strain fields as well as the strain energy of the rod are quantitatively demonstrated. As a result of such microstructure evolution nominal stress-strain relation with strain softening is derived for a rod under uniaxial tension

Effect of elastic matrix constraint on the tensile deformation of NiTi superelastic fiber

In this paper we study the effect of elastic matrix constraint on the tensile deformation of an active NiTi shape memory alloy fiber, which, when no matrix constraint is present, will experience stress-induced phase transformation by nucleation and growth of a macroscopic martensite band. The effect of the constraint is measured by two factors: the relative Young's modulus (by dimensionless parameter E-(2)/E-(1)) and the relative dimension (by dimensionless parameter h/a) of the fiber and the matrix. The transformation process of the fiber through the martensite band growth under tension is modeled as an embedded elastic fiber containing growing cylindrical transformation inclusions. By Love's stress function, the elastic solutions of the inclusion-fiber-matrix system as well as the internal elastic energy during the transformation are obtained. Analytical expressions of the free energy of the system during the transformation are also formulated for the case of uniaxial tension. After introducing the band nucleation and growth criteria, the growth capability of a martensite band is examined. The results demonstrate that, depending on the magnitude of the matrix constraint, three distinct deformation patterns of the fiber exist: (1) with weak matrix constraint, single band growth dominates the transformation process of the fiber; (2) with intermediate matrix constraint, sequential bands nucleation and growth prevails in the fiber; and (3) with strong constraint, numerous bands form and grow, and macroscopically the fiber tends to deform homogeneously. Parametric studies on the macroscopic stress-strain response of the fiber-matrix system are performed and the obtained results are discussed. (C) 2003 Elsevier Ltd. All rights reserved

Deformation behavior of metallic glasses with shear band like atomic structure: a molecular dynamics study

Molecular dynamics simulations were employed to investigate the plastic deformation within the shear bands in three different metallic glasses (MGs). To mimic shear bands, MG specimens were first deformed until flow localization occurs, and then the volume of the material within the localized regions was extracted and replicated. Homogeneous deformation that is independent of the size of the specimen was observed in specimens with shear band like structure, even at a temperature that is far below the glass transition temperature. Structural relaxation and rapid cooling were employed to examine the effect of free volume content on the deformation behavior. This was followed by detailed atomic structure analyses, employing the concepts of Voronoi polyhedra and ``liquid-like'' regions that contain high fraction of sub-atomic size open volumes. Results suggest that the total fraction of atoms in liquid-like regions is a key parameter that controls the plastic deformation in MGs. These are discussed in the context of reported experimental results and possible strategies for synthesizing monolithic amorphous materials that can accommodate large tensile plasticity are suggested

On the critical thickness for non-localized to localized plastic flow transition in metallic glasses: A molecular dynamics study

Molecular dynamics simulations were employed to investigate the specimen thickness-dependent tensile behavior of a series of Cu(x)Z(100-x) (x = 20, 40, 50, 64 and 80 at%) metallic glass (MG) films, with a particular focus on the critical thickness, tc, below which non-localized plastic flow takes place. The simulation results reveal that while the transition occurs in all the alloys examined, t(c) is sensitive to the composition. We rationalize t(c) by postulating that the strain energy stored in the sample at the onset of plastic deformation has to be sufficient for the formation of shear bands. The composition-dependence of t(c) was found to correlate with the average activation energy of the atomic level plastic deformation events. (C) 2015 Elsevier Ltd. All rights reserved

Prefabricated construction enabled by the Internet-of-Things

Prefabricated construction has been used for public rental housing in Hong Kong. In order to speed up housing delivery, Hong Kong Housing Authority (HKHA) have employed advanced technologies, including Building Information Modelling (BIM) and Radio Frequency Identification (RFID), in some of their pilot prefabrication-based construction projects. However, the information obtained from BIM and RFID is not well connected and shared among relevant stakeholders. This paper introduces a multi-dimensional Internet of Things (IoT)-enabled BIM platform (MITBIMP) to achieve real-time visibility and traceability in prefabricated construction. Design considerations of a RFID Gateway Operating System, visibility and traceability tools, Data Source Interoperability Services, and decision support services are specified for developing the MITBIMP. A case study from a real-life construction project in Hong Kong is used as a pilot project to demonstrate advanced decision-making by using cutting-edge concepts and technologies within the MITBIMP to providing a basis for real-time visibility and traceability of the whole processes of prefabrication-based construction.Department of Building and Real Estat