Propagation Of Phase Transformation Front Research Articles

Establishing regularities in the propagation of phase transformation front during timber thermal modification

The creation of environmentally friendly protective materials for building structures made of wood could make it possible to influence the processes of stability and the physical-chemical properties at the thermal modification of hornbeam wood over a certain time. That necessitates studying the conditions for investigating phase transformations when the timber is exposed to high temperature, as well as establishing the mechanism of hornbeam wood thermal modification. Given this, a mathematical model of the phase transformation process during the transfer of heat flux to a sample was built. Based on the derived dependences, it was established that when hornbeam wood is exposed to temperature treatment, it undergoes endothermic phase transformations characterized by the heat absorption and change in the color of hornbeam wood. In particular, at a temperature of 200 °C, the temperature in the wood decreases by 5 % due to the chemical changes in the structure of cell wall components (lignin, cellulose, and hemicellulose). It was found that the process of thermal modification is accompanied by the decomposition of hemicellulose and the amorphous part of cellulose, a decrease in moisture absorption, as well as a decrease in the volume of substances that are a medium for the development of fungi. In addition, lignin and the resulting pseudo lignin undergo a process of polymerization and redistribution throughout the cell volume. At the same time, they give the cell walls higher density, hardness, increase hydrophobicity (water repellency), thereby reducing the ability to absorb moisture and swell. It was established that the most effective parameter of phase transformations is the temperature and aging duration. The results of moisture absorption have been given; it has been found that over 6 hours of modified timber exposure, its moisture absorption decreases by more than 10 times, which allows its application at facilities with high humidity

Multiscale analysis of the effect of interfacial energy on non-monotonic stress–strain response in shape memory alloys

The effect of formation and evolution of stress-induced martensitic microstructures on macroscopic mechanical properties of shape memory alloys in the pseudoelastic regime is investigated with account for size-dependent energy of interfaces. A quantitative relationship is established between the changes in free energy and dissipation on the interfaces at three microstructural scales and the overall mechanical characteristic of the material under tensile loading. The multiscale analysis carried out for a polycrystalline NiTi shape memory alloy has revealed that the interfacial energy storage and dissipation can strongly affect the shape and width of the stress–strain hysteresis loop. The predicted non-monotonic stress–strain response for the material of a selected grain size shows a remarkable similarity to the experimental one extracted from a tensile test of a laminate by Hallai and Kyriakides (2013). By the classical Maxwell construction, the non-monotonic response for a material element results in a commonly observed stress plateau for a tensile specimen, which is associated with the propagation of phase transformation fronts. This behaviour is confirmed with striking accuracy by 3D finite-element computations performed for a macroscopic tensile specimen, in which propagating instability bands are treated explicitly.

Modeling of propagation of phase transformation fronts in NiTi under uniaxial tension

Abstract Tension induced solid-state transformation in a nearly equiatomic NiTi results in inhomogeneous deformation with transformed and untransformed material coexisting. Strains of the order of 7% are recovered on unloading with the material reverting to its original phase. In a thin strip the two coexisting phases and deformation regimes are separated by inclined fronts monitored with digital image correlation. The fronts propagate along the strip until the transformation is completed with the stress remaining nearly unchanged. A recently developed constitutive model for the pseudoelastic behavior of NiTi is implemented in a finite element analysis used to simulate the strip experiment. The model is capable of addressing the tension/compression asymmetry by incorporating both the tensile and compressive responses of the material. Softening moduli are introduced over the extents of the plateaus of the tensile response at approximately the Maxwell stress levels. The numerical solution reproduced the closed hysteresis with the correct stress plateau levels and extents. Localized deformation initiates in a single narrow band of higher strain inclined to the axis of the strip. The high strain zone propagates initially with inclined fronts and subsequently with a crisscross pattern with the stress remaining essentially unchanged. The details of the front propagation are influenced by in-plane moment caused by secondary kinking of the strip. The strip unloads by tracing a lower stress plateau developing a similar propagation of localization patterns. Overall, the numerical simulation captures the main features of the response and the associated localization configurations. The sensitivity of the solution to the mesh, the boundary conditions, and the softening moduli are discussed.

TIPS AND TRICKS FOR CHARACTERIZING SHAPE MEMORY ALLOY WIRE: PART 3-LOCALIZATION AND PROPAGATION PHENOMENA

This is the third paper in our series to identify unusual phenomena and to provide recommendations for the thermo-mechanical characterization of shape memory alloy (SMA) wire. Part 1 provided some basic background of the martensitic transformations responsible for the shape memory effect and superelasticity 1 . The characterization of two typical NiTi SMA alloys began with differential scanning calorimetry (DSC) thermograms to measure their respective transformation temperatures, specific heats, and latent heats of transformation. It included an experiment for each alloy showing both shape memory and superelasticity, but in different temperature regimes. Part 2 reviewed the various methods for obtaining a fundamental set of isothermal mechanical responses and provided data on the same two SMA wire alloys over their relevant temperature windows 2 . In the process, it showed stress-induced transformations, which lead to an introduction of strain localization and propagation of phase transformation fronts. In this paper(Part 3), we narrow ourfocus on certain unusual phenomena occurring during superelastic tension responses, namely localization of strain and temperature fields. These are often overlooked by the novice, yet they play an important role in the kinetics of stress-induced phase transformation, and in turn, exacerbate the material response’s sensitivities to loading rate and ambient media (which will be the subject of the next paper in this series). Here, we introduce special experimental techniques for (otherwise) difficult to measure features in the underlying material response. We focus on the superelastic response of one of the two NiTi alloys that was used in our previous articles, that is, the alloy with an Austenite finish temperature (Af) below room temperature (designated as superelastic wire). Figure 1 shows four isothermal superelastic experiments, magnified from the set of fundamental mechanical responses of Fig. 8 in 2 . Each was done on a virgin specimen under elongation control at a slow rate, ˙L =± 1 × 10 −4 s −1 , in a temperature-controlled air chamber at different Editor’s Note: This ET feature series is intended as an introduction to this exciting area of experimental mechanics. It aims to increase awareness of active materials and to promote their consistent characterization by disseminating best practices from leading researchers in the field. Each article in the series will address the characterization of one commercially significant active material. Series editors: Nilesh D. Mankame and Paul W. Alexander.

Modelling He I–He II phase transformation in long channels containing superconductors

The problem of heat transport during sub-cooling below λ point in long, narrow channels containing superconductors immersed in liquid helium is discussed. In order to describe the propagation of phase transformation front the travelling plane wave description with suitable change of variables has been adopted. Nonlinear temperature profile corresponding to the conductivity function in He II has been assumed. The heat diffusion equation is solved in the volume of He I with the assumption that the main contribution to the specific heat is due to helium and the main contribution to longitudinal conduction is due to superconductor. For one dimensional problem with the temperatures clamped at both extremities of the channel the closed form analytical solutions for temperature profile are obtained. Channels with discontinuity in diameter are also analysed. Experimental validation of the model is based on 54 m long superconducting lines applied in the Large Hadron Collider (LHC).

Finite-strain micromechanical model of stress-induced martensitic transformations in shape memory alloys

A micromechanical model of stress-induced martensitic transformation in single crystals of shape memory alloys is developed. This model is a finite-strain counterpart to the approach presented recently in the small-strain setting [S. Stupkiewicz, H. Petryk, J. Mech. Phys. Solids 50 (2002) 2303–2331]. The stress-induced transformation is assumed to proceed by the formation and growth of parallel martensite plates within the austenite matrix. Propagation of phase transformation fronts is governed by a rate-independent thermodynamic criterion with a threshold value for the thermodynamic driving force, including in this way the intrinsic dissipation due to phase transition. This criterion selects the initial microstructure at the onset of transformation and governs the evolution of the laminated microstructure at the macroscopic level. A multiplicative decomposition of the deformation gradient into elastic and transformation parts is assumed, with full account for the elastic anisotropy of the phases. The pseudoelastic behavior of Cu–Zn–Al single crystal in tension and compression is studied as an application of the model.

On the longitudinal impact of two phase transforming bars. Elastic versus a rate-type approach. Part II: The rate-type case

We study the propagation of phase transformation fronts induced by the longitudinal impact of two shape memory alloy bars modeled by a general form of a rate-type approach to non-monotone elasticity. We illustrate that such a rate-type law should be seen like a kinetic law for phase transformation. This investigation continues in a comparative way the analysis of the dynamic theory of elastic bar considered in Part I in relation with a viscosity criterion. We focus here on mathematical, thermodynamical and experimental aspects related with the wave structure which accompanies both the forward and reverse transformation. We analyze the propagation of disturbances in a pure phase near and far from their sources, that is the instantaneous waves and the delayed waves as well as the traveling wave solutions and the accompanying dissipation. In the numerical experiments one focuses on the influence of the impact velocity on the way the phase boundary propagates and on the results which can indicate indirectly the existence of a phase transformation like the time of separation, the velocity–time profile at the rear end of the target and the stress history at the impact face.

On the nucleation and propagation of phase transformation fronts in a NiTi alloy

In this paper we present an experimental methodology for simultaneous full field monitoring of the deformation and thermal changes in NiTi during mechanically unstable regimes associated with the pseudoelastic material response. The deformation history is established by photographically recording surface changes of a brittle coating as austenite-martensite phase transition fronts traverse the specimen. Temperature changes are monitored by infrared thermal imaging. Synchronized sets of optical and thermal images and the corresponding stress histories are presented for uniaxial experiments conducted at two relatively slow end-displacement rates. The results help clarify the interactions among the prevailing heat transfer conditions, the loading rate and the fundamental sensitivity of transformation stress to temperature. It is shown that, during loading, nucleation of martensite in an austenitic region is a distinct event requiring a higher stress than the stress required subsequently to continue the transformation. By contrast, the nucleation stress of austenite in a martensitic region during unloading is lower than the stress required to continue the transformation. This distinction between the nucleation and propagation stresses, coupled with the local temperature change caused by the latent heats of the two transformations, govern the number of nucleations of a new phase. It is also shown that coexisting transition fronts tend to propagate at the same speed which is inversely proportional to the number of fronts in the specimen.