Experiments and Models of Thermo-Induced Shape Memory Polymers

Recent advances in experiments and models of thermo-induced shape memory polymers (TSMPs) were reviewed. Some important visco-elastic and visco-plastic features, such as rate-dependent and temperature-dependent stress-strain curves and nonuniform temperature distribution were experimentally investigated, and the interaction between the mechanical deformation and the internal heat generation was discussed. The influences of loading rate and peak strain on the shape memory effect (SME) and shape memory degeneration of TSMPs were revealed under monotonic and cyclic thermo-mechanical loadings, respectively. Based on experimental observations, the capability of recent developed visco-elastic and visco-plastic models for predicting the SME was evaluated, and the thermo-mechanically coupled models were used to reasonably predict the thermo-mechanical responses of TSMPs

A viscoelastic-viscoplastic constitutive model and its finite element implementation of amorphous polymers

The amorphous polymers present remarkable temperature- and rate-dependent deformation behaviors. Based on a combination of the multiple relaxation viscoelastic-viscoplastic model and the three-element viscoelastic model, a constitutive model was constructed to describe the changes in mechanical properties of amorphous polymers from below to above glass transition temperature (θg). In this model, an exponential evolution equation of volume fraction was constructed to reflect the changes in glassy and rubbery phases at different temperatures. The proposed model was implemented into ABAQUS using the user-defined material subroutine (VUMAT). The strain-softening after yield and rate-dependent behaviors above and below θg were reasonably captured by the present model. Meanwhile, the creep and relaxation behaviors of the material were described. Finally, the processes of the tensile deformation of a dumbbell plate with a circular hole and the rate-dependent pressed film molding were simulated by the VUMAT. The results show that the implemented model can reasonably predict the structural responses of amorphous polymers

Finite Element Implementation of a Temperature-Dependent Cyclic Plastic Model for SA508-3 Steel

A new temperature-dependent cyclic plastic model, combining the nonlinear cyclic softening and kinematic hardening rules is established for a nuclear material of SA508-3 steel. A modified isotropic hardening rule is proposed to capture the temperature-dependent cyclic softening, and a modified kinematic hardening rule is established to improve the prediction of the ratcheting behavior by introducing an exponential function related to the accumulated plastic strain. The stress is updated by the radial return mapping algorithm based on the backward Euler integration. A new expression of consistent tangent modulus for the equilibrium iteration is derived, and then the proposed model is implemented into the finite element software ABAQUS by using the user material subroutine (UMAT) to simulate the temperature-dependent ratcheting behaviors of SA508-3 steel. Finally, the ratcheting evolutions of notched bars at elevated temperature are obtained by uniaxial stress-controlled cyclic tests, and the nonuniform strain fields on the surface of plates with a center hole is measured by using the digital image correlation (DIC) technology. Comparisons between experimental and simulated results of a material point and structural examples show that the implemented model can provide reasonable predictions for ratcheting behaviors and nonuniform strain fields of structures at different temperatures for SA508-3 steel

Synergetic-Deformation-Induced Strengthening in Gradient Nano-Grained Metals: A 3D Discrete Dislocation Dynamics Study

Gradient nano-grained (GNG) metals have shown high synergetic strength and good ductility due to their unique gradient microstructure. In this study, the mechanical behavior of gradient nano-grained metals was investigated by three-dimensional discrete dislocation dynamics. The simulation results show a trend that the successive yielding and gradual “transmission” of dislocations along the gradient direction result in a gradient distribution of stress and plastic strain. The distribution of geometrically necessary dislocations is more inhomogeneous in the gradient nano-grained (GNG) sample compared with those homogenous counterparts. The non-uniform deformation response of component layers induces the synergetic-deformation-induced (SDI) strengthening in the GNG sample. The back stress originates from geometrically necessary dislocations that pile up near the interface of gradient layers and leads to a significant hardening while there is a slight softening in different gradient layers in the GNG sample. This study provides a deeper insight into the SDI strengthening in gradient structure from the submicron scale

In Situ Observation on Rate-Dependent Strain Localization of Thermo-Induced Shape Memory Polyurethane

In situ monotonic tensile experiments of thermo-induced shape memory polyurethane (SMPU) at different loading rates were carried out by the digital image correlation (DIC) method and infrared camera FLIR®-A655sc in natural convection (NC) and forced convection (FC) conditions, respectively. The multiform strain localization of SMPU was observed by the DIC method, and the influence of thermo–mechanical coupling on the strain localization was analyzed by using the FLIR to measure the temperature field caused by the internal heat generation. The experimental results show that the strain localization mode strongly depends on the strain rate and convection condition, and the strain localization mode can be transformed by changing the convection condition from NC to FC. The competition mechanism between the strain hardening induced by the increasing loading rate and strain softening induced by the internal heat generation is indicated, the transition modes of strain localization are clarified, and the influences of thermo–mechanical coupling on shape memory effect are discussed

Atomistic study on the super-elasticity of nanocrystalline NiTi shape memory alloy subjected to a cyclic deformation

By establishing some atomistic simulation cells with the same size but different numbers of grains, molecular dynamics simulations are performed to investigate the super-elasticity of nanocrystalline NiTi shape memory alloy subjected to a cyclic tension-unloading and its dependence on the grain size. The effect of grain boundaries on the martensite transformation stress as well as the nucleation and growth of martensite phase is addressed. The degeneration of super-elasticity and the initiation and growth of defects in the nanocrystalline NiTi SMA during the cyclic tension-unloading are discussed. The results show that the super-elasticity degeneration occurs during the cyclic deformation of nanocrystalline NiTi SMA, and the residual strain accumulates progressively with the increasing number of cycles, which becomes more significant with the decrease of grain size. The grain boundaries can enhance the martensite transformation stress of nanocrystalline NiTi SMA and suppress the instability occurred during the martensite transformation. It is revealed that the interstitial atoms and plastic deformation are mainly concentrated within the grain boundaries

Atomistic study on the super-elasticity of single crystal bulk NiTi shape memory alloy under adiabatic condition

The temperature-induced phase transition and the super-elasticity (from the stress-induced phase transition) of equiatomic single crystal bulk NiTi shape memory alloys are investigated by the molecular dynamics method. By the simulation to the thermo-mechanical response of the single crystal NiTi alloy along the 〈0 0 1〉B2 under the compression/unloading and an adiabatic condition, the temperature change and the nucleation and growth of martensite transformation during the compression/unloading are discussed. The simulated results of molecular dynamics show that the single crystal bulk NiTi shape memory alloy exhibits a significant temperature change during the martensite transformation and its reverse under an adiabatic condition; moreover, a localized instability occurs apparently in the process of martensite transformation, which is closely related to the nucleation and growth rates of martensite phase; finally the effect of model size and strain rate on the thermo-mechanical response of the single crystal bulk NiTi alloy is also discussed, and no instability is observed in the simulated stress-strain curves if the model size is relatively larger, e.g., 8V0 and 13.824V0