Key Engineering Materials Vols. 340-341

Abstract: Superelasticity is one of the most important properties of shape memory alloy. In this paper, the superelastic deformation behavior of NiTi shape memory alloy subjected to cyclic loading with stable superelasticity is experimentally investigated. According to test data, a constitutive model for the superelasticity of shape memory alloy is presented based on the artificial neural network (ANN). Numerical results agree well with experimental observations that verified the constitutive model being of high accuracy. This model can avoid the difficulties of other models on the determination of the parameters and is suitable for practical engineering application. Thus, a new method is provided for building the constitutive model of shape memory alloy.

Abstract: Recent uniaxial tension tests have shown that stress-induced phase transformation in NiTi SMAs tubes can lead to helical-type localized deformation and propagation phenomena. Based on detailed experimental observation and possible deformation mechanism, a trilinear stress-strain relationship with intrinsic strain softening is employed to represent the material constitutive behavior in this paper, and a 3-D finite deformation simulation is performed to model the tube under tension by using nonlinear FEM. The simulations successfully reproduce the nucleation and evolution of the helical-type martensite band during stress-induced transformation observed in the experiments.

Abstract: The shape-memory composite belt with a TiNi-SMA wire fiber and a polyurethane-SMP sheet matrix was fabricated. The bending actuation characteristics of the belt were investigated by the thermomechanical tests. The results obtained can be summarized as follows. (1) Residual deflection close to the maximum deflection is obtained by cooling under constant maximum deflection. The residual deflection disappears by heating under no load. Both the rate of shape fixity and the rate of shape recovery are close to 100%. (2) Recovery force appears by heating under constant residual deflection. The recovery force is 93-94% of the maximum force. The development of high functionality of shape-memory composite elements is expected by various combinations of SMAs and SMPs.

Abstract: In this study, we performed the bending fatigue test and investigated the influence of strain ratio on fatigue life in TiNi shape memory thin wire. The pulsating plane bending, alternating plane bending and rotating bending fatigue tests were carried. Additionally, we carried out the observation of the fatigue fracture surface by a scanning electron microscope. The behavior of fatigue crack was investigated. The results obtained are summarized as follows. (1) The martensitic transformation (MT) stress of the superelastic thin wire (SE-NT) is higher than that of the SMA thin wire (SME-NT) and the fatigue life of SE-NT is shorter than that of SME-NT. Maximum bending strain at the fatigue limit is the MT starting strain. (2) The low-cycle fatigue life curve in plane bending for SE-NT is expressed by a power function of maximum strain εmax and the number of cycles to failure Nf. The smaller the strain ratio for the same εmax, the shorter the fatigue life. (3) In both the rotating bending and the plane bending, fatigue cracks nucleate on the surface of the wire and one fatigue crack grows preferentially. The region in which fatigue crack propagated is fan-shaped.

Abstract: In this study, the simulation of the motion of an interface during the stress-induced martensitic transformation of a shape memory alloy is performed using the level-set method. The kinetics of the phase transformation is defined as an anisotropic kinetic relation between the rate at which the weak discontinuity moves, given by its normal velocity, and the thermodynamics driving force. The latter is derived from a dissipation function, which obeys the 1st and 2nd law of thermodynamics and accounts for large strains. Furthermore, a hyperelastic constitutive framework is used to describe the constitutive behavior of the material. The model is implemented into the finite element method and is then used to solve a 2D phase transformation problem in a shape memory alloy.

Abstract: This study deals with deformation behavior of a shape-control plate which consists of an aluminum alloy plate and a pre-strained NiTi shape memory alloy (SMA) wire. The shape-control plate exhibits reciprocating bending deformation by heating and cooling. Deformation behavior of the plate is examined by electric heating and natural cooling of the SMA wire. Experimental results exhibit that the bending deformation of the plate is considerably stable over more than two thousand heating-cooling cycles and can be well controlled by electric current. Furthermore, the deformation behavior of the plate is analyzed by a simple beam theory for the aluminum alloy plate and Brinson’s one-dimensional constitutive model for the SMA wire taking account of not only martensitic transformation but also rhombohedral-phase transformation. Numerical results describe well the deformation behavior of the shape-control plate observed in the experiments.

Abstract: The characteristics of energy storage and dissipation in TiNi shape memory alloys were investigated experimentally based on the superelastic properties under various thermomechanical loading conditions. The results obtained can be summarized as follows. (1) The recoverable strain energy increases in proportion to the rise in temperature, but the dissipated work per unit volume depends slightly on temperature. In the case of low strain rates, the recoverable strain energy and dissipated work do not depend on both the strain rate and the temperature-controlled condition. (2) In the case of high strain rates, while the recoverable strain energy decreases and the dissipated work increases in proportion to the rise in strain rate under the temperature-controlled condition, the recoverable strain energy increases and the dissipated work decreases under the temperatureuncontrolled condition.

Abstract: The influence of the strain-holding conditions on shape recovery and secondary-shape forming was investigated. The results obtained are summarized as follows. (1) If strain is held at holding temperature Th = Tg+20K in a short time, irrecoverable strain starts to appear at the holding time th= 0.5h and strain is not recovered at all at th=8h. In the case of Th = Tg+10K, irrecoverable strain appears in a short holding time if holding strain is large and the rate of secondary-shape forming S is 42% at th=8h. (2) If strain of 50% is held at Th = Tg+10K in a long time, strain becomes not to be recovered and S is 93% at th =12h. In the case of Th = Tg, the increasing rate of S increases if th is longer than 40h. If Th is lower than Tg-10K for th= 72h, strain is recovered perfectly by heating and secondary-shape forming does not appear.

Abstract: In this paper, a soil-water coupled elasto-plastic finite element analysis is applied to the problem of seepage flow by incorporating unsaturated seepage characteristics and assuming the pore air pressure in the unsaturated soil region to be atmospheric pressure. It is shown that the proposed soil deformation–seepage flow coupled analysis method is applicable to safety investigations of river embankments and that the existing evaluation criterion for the seepage failure of river embankments is not always on the safe side.

Abstract: A tangent modulus of soil mass which allows for a piece-wise linear approximation of the hyperbolic response curve is particularly suited for incremental construction simulation. The parameter identification of nonlinear constitutive model of soil mass is based on an inverse analysis procedure, which consists of minimizing the objective function representing the difference between the experimental data and the calculated data of the mechanical model. The artificial neural network is applied to estimate the model parameters of soil mass. The weights of neural network are trained by using the Levenberg-Marquardt approximation which has a fast convergent ability. The parameter identification results illustrate that the proposed neural network has not only higher computing efficiency but also better identification accuracy. The numerically computational results with finite element method show that the forecasted displacements at observing points according to identified model parameters can precisely agree with the observed displacements.