Catálogo de publicaciones - libros

This paper reports briefly the observed deformation instability and domain morphology evolution during stress-induced austenite → martensite (A→M) phase transformation in a superelastic NiTi polycrystalline shape memory alloy microtube. High-speed data and image acquisition techniques were used to investigate the dynamic and quasistatic events which happened in displacement controlled quasi-static tensile loading/unloading process of the tube. These events include dynamic formation, self-merging, topology transition and front instability of a macroscopic deformation band. The reported phenomena brought up several important issues in the fundamental understanding of the instability and pattern evolution in polycrystals under mechanical force. These issues are believed to be essential in the theoretical modeling and worth further investigation in the future.

The application of shape memory alloys (SMA), as an advanced material, has accelerated in recent years, especially in biomechanical engineering. However, there is a lack of understanding of the fracture behavior of SMA devices. This paper presents our consideration on the theoretical study of the fracture properties of SMA. Owing to the existence of a new transformed phase near the crack-tip region, the transformation strain, including the transformation volume strain and the shear strain, the plastic deformation, and the mismatch of elastic property will alter the crack-tip stress field and hence govern the fracture behavior of the SMA material. Therefore, it is vital to clarify the influence of these factors on the fracture toughness. Following this consideration, the paper reports our recent research progress in this direction. First, a simple study is carried out to show the influence of transformation consisting of pure volume contraction. These results reveal that the phase transformation with volume contraction in SMA tends to reduce their fracture resistance and increase the brittleness. Second, a constitutive model is established to quantify the effect of stabilization of plasticity on the stress-induced martensitic trans-formation. Third, the effect of transformation strain with shear and volume components on the fracture toughness of a superelastic SMA is studied.

In this paper, a methodology based on three-dimensional (3D) finite element procedures is developed and used to simulate the superelastic behavior and the shape memory effect (SME) of shape memory alloys (SMAs). A 3D constitutive model for SMAs is implemented as a user defined subroutine for the finite element code ABAQUS. The kinetic law based on an experimentally defined stress-temperature phase diagram provides the phase fraction history for a given loading path and specified initial value of the martensite fraction. With a simple 3D model, we demonstrate the capability of the proposed methodology to simulate the shape memory effect, pseudoelasticity and hysteresis behavior. This methodology provides a computational tool for the design of 3D actuators using SMAs.

A new general nonlinear constitutive model is proposed for giant magnetostrictive materials. For the convenience of engineering applications, the expressions of the 3-D (bulk), 2-D (film) and 1-D (rod) models are respectively given for an isotropic material and their applicable regions are also identified. It is found that the new model can accurately predict the magnetostrictive strain curves in low, moderate and high magnetic field regions for various compressive pre-stress levels. The numerical simulation illustrates that, the new model, whether for magnetostrictive rods or for thin films, can effectively describe the effects of the pre-stress or residual stress on the magnetization and the magnetostrictive strain curves, which can not be captured by previous models.

In this paper, the magnetostrictive actuator was analyzed with the nonlinear constitutive equation of magnetostriction and, for comparison, the linear piezomagnetic equation. Theoretical model shows that, for the case of relatively large excitation magnetic field, the nonlinear constitutive equation should be adopted in order to give an accurate prediction in the design of actuators. The actuator resonates not only at the natural frequencies of the vibration system, but also arises when the frequency of the excitation current in the coil equals half of any one of the natural frequencies of the system. This conclusion cannot be reached by the formulation with the linear piezomagnetic equation. It is verified that the results obtained from the nonlinear equation reduce to be that of linear piezomagnetic equation when the amplitude of the excitation magnetic field provided by the coil is very small compared to the bias magnetic field and its frequency does not induce resonance of the system.

This paper introduces the 1:12 model test of the feed support system of a 500-meter aperture radio telescope. The main purpose of this research is to study the controllability and performance of a cable-driven Gough-Stewart platform used as a positioning platform for the feed of a large radio telescope. The Gough-Stewart platform is used for vibration control, the upper platform of which is driven by long cables and the lower one is to be stabilized in tracing the observation trajectories. The similarity for the scaled model and the implementation are illustrated. The test study has been carried out in two steps. The first step involves the uncoupled cable-car tracking control tests and the Gough-Stewart platform vibration suppression tests, while the second step carries out the coupled test of the system. The general aspects of the control system and control law are introduced and the problems arising in the tests are also addressed. Results on the 50m×50m×12m are also presented.

The motion of biofilm in aqueous environment is modeled as two-phase mixture flow which is governed by a generalized Novier-Stokes equation. Noticed that the propagation of bacteria colonies obeys the diffusion law with the similar instability mechanism in the coarsening of metallic grains during solidification, a thermodynamic framework has been derived which reveals the two competing mechanisms in biofilm growth: the absorption of nutrition from water phase that tends to maximize the contact area to environment and a loose-surface layer formation that tends to minimizes biofilm surface. Therefore an additional condition has been obtained which governs the formation of finger-shaped biofilm. 2D numerical simulation has been performed.

Heart is the pump to make blood circulate in the vessels. The actuating heart muscles can generate intrinsic force making the heart chambers contract and diastole rhythmically under the stimulation of electronic signals. We carried out in-vivo experiments that screwed the lead tip of a heart pacer into the designated sites of heart and applied on controlled external excitation on the lead-myocardium interface. A special design of the helix tip using strain gauges and fiber optic sensors is instrumental for the measurement of the push-pull forces of lead-myocardium interactions during the tests. We observed that the interaction was mainly caused by the heart beating and influenced by the lungs breath. The in vivo heart had an ability of self-adjusting to protect itself from outside excitations. The security of the pacer attachment to the heart muscle is estimated for the fatigue and damage tests. In-vitro heart muscle specimen tests are also carried out for the parameters that characterize the constitutive relation of heart muscles.