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Shape-memory polymers (SMPs) represent a class of smart materials able to store a temporary shape and to recover the original shape upon an external stimulus, such as temperature. The present paper proposes a three-dimensional finite-strain phenomenological model for thermo-responsive SMPs, which distinguishes between two material phases presenting different properties and is based on a rule of mixtures. The proposed model is motivated by the earlier work of Reese et al. (2010) and it considers several significant material features that had not been addressed in previous phenomenological approaches. Specifically, the model reproduces both heating-stretching-cooling and cold drawing shape-fixing procedures and it takes into account the non-ideal behavior of realistic SMPs (i.e. imperfect shape-fixing and incomplete shape-recovery). Several numerical tests are reported to assess model performances, from simple uniaxial and biaxial tests to complex simulations of biomedical devices. Comparisons with experimental data taken from the literature are also provided to validate the model. The proposed improvements increase the model applicability over a wide range of polymer types and operating conditions. [ABSTRACT FROM AUTHOR]

The current work deals with periodic thermomechanical composite media, in which the material constituents are considered to obey the generalized standard materials laws. The aim is to provide a proper homogenization framework that takes into account both the equilibrium and the thermodynamics laws in microscale and macroscale levels. The study is based on the asymptotic expansion homogenization technique, which permits to deduce useful results about the general structure of microscale and macroscale energy potentials and constitutive laws. The paper also proposes an incremental, linearized formulation that allows to identify suitable thermomechanical tangent moduli for the macroscale problem. The capabilities of this framework are illustrated with numerical examples on multilayered composites. [ABSTRACT FROM AUTHOR]

Experimental results show that the martensite reorientation and reorientation-induced plasticity play critical roles on the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy (SMA) (Song et al., 2014b). In this paper, the multiaxial transformation ratchetting of the NiTi shape memory alloy is described by developing a new constitutive model in the framework of crystal plasticity. Besides the well-known inelastic mechanisms of NiTi shape memory alloys, such as the martensite transformation, martensite reorientation, transformation-induced plasticity and accumulated residual martensite, a new inelastic deformation mechanism, i.e., reorientation-induced plasticity is included in the proposed model to address its effect on multiaxial transformation ratchetting. The constitutive model is constructed in the single crystal scale and extended to a polycrystalline version by adopting an explicit scale-transition rule. By comparing the predicted results with the corresponding experimental ones, it is verified that the proposed model describes the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy more reasonably by considering the martensite reorientation and reorientation-induced plasticity simultaneously. [ABSTRACT FROM AUTHOR]

Abstract: A viable model for the phenomenon of transformation induced plasticity must be able to predict the strain response to arbitrary, also non-proportional loading paths. The constitutive model presented in this paper is interfacing between the macro- and the mesoscale by stress partitioning in the parent and product phase, using a nonlinear scale transition rule. As one of the key features a mean field tensor representing the orientation strain contribution is introduced taking into account backstress effects. A good agreement of the model results with experiments on a maraging steel is achieved. [Copyright &y& Elsevier]

Abstract: The loadings of variable combined stresses may cause different instabilities of materials such as the shear localization instability or non-localized thermal softening instability. The purpose of this paper is to present a theoretical analysis of instability behaviors of material under the loading conditions of combined stresses and apply the analytical results to interpret the removal mechanisms of chip materials in cutting process. First, the perturbation analysis of material instability under the loading conditions of combined stresses and relatively critical conditions are presented, followed by discussions on relations between the instability behaviors of materials and the loading conditions. Then, the removal mechanisms of chip materials in cutting process are studied by numerical simulation. The formation of continuous and discontinuous chips is shown closely related to the plastic flow and instability behaviors of the chip material, and the transformation between the two types of chips is found dependent on the ratio of plastic work dissipated by dilatation deformation and that dissipated by shear deformation. [Copyright &y& Elsevier]

Abstract: Plasticity in heterogeneous metallic materials of small volumes is governed by the interactions of the dislocations in the bulk and at the interfaces. The temperature distribution and accumulated heat in the medium, due to the high rate of loading, can also strongly affect the mechanical properties in micro and nanosystems. In this paper, the framework of higher-order strain gradient plasticity with interfacial energy effect is used to investigate the coupling of thermal and mechanical responses of materials in small scales and fast transient process. In order to accurately describe the size effect exhibited in micro/nano structured materials and to address the strengthening and hardening mechanisms, the theory is developed based on the decomposition of the mechanical state variables into energetic and dissipative counterparts. This, in turn, endowed the constitutive equations to have both energetic and dissipative gradient length scales ℓ en and ℓ dis respectively. The effect of the material microstructural interface between two materials is also incorporated into the proposed formulation. Hence four material length scales are introduced: two for the bulk and the other two for the interface. In addition, the temperature distribution in the system due to conversion of plastic work into heat and the partial dissipation of heat due to fast transient time, is included into the model using a generalized heat equation. The microstructural interactions (i.e. photon–electron coupling) and the effect of fast transients is considered by incorporating two time scales into the heat equation. The developed framework is solved numerically by using a thin film on an elastic substrate under biaxial straining. Furthermore, the obtained results are compared with experimental data. [Copyright &y& Elsevier]

This paper is concerned with the experimental behaviour of a 16MND5 steel (french vessel steel) under complex loading. A particular attention is paid to plasticity induced by phase transformation. We present an experimental set-up to apply thermo-mechanical loads under tension-torsion. This apparatus enables us to reach temperature of 1200 °C at a maximum heating rate of 60 °C s−1 and a high cooling rate of −30 °C s−1. A series of tests is performed in order to show the rule of loading on transformation plasticity. [Copyright &y& Elsevier]