Miroslav Frost

The Czech Academy of Sciences | AVCR · Institute of Thermomechanics

Nickel-titanium B19’ martensite is a strongly plastically anisotropic material with only one available slip system, which is the [100](001)M slip. Despite this, B19’ martensite polycrystals can be homogeneously plastically formed, reaching up to very high plastic strains. The absence of other slip systems is compensated by plastic twinning, in part...

Great attention has been recently paid to the investigation of plastic deformation in NiTi. Experimental investigations of the mechanical response of NiTi polycrystalline samples within a broad stress-strain-temperature state space have revealed a complex response involving martensitic transformation, reorientation, and plastic deformation processe...

Irreversible plastic forming of B19$^\prime$ martensite of the NiTi shape memory alloy is discussed within the framework of continuum mechanics. It is suggested that the main mechanism arises from coupling between martensite reorientation via twinning and a coordinated dislocation slip. A heuristic model is proposed, showing that the ${(20\bar{1})_...

The incremental energy minimization principle provides a compact variational formulation for evolutionary boundary problems based on constitutive models of rate-independent dissipative solids. In this work, we develop and implement a versatile computational tool for the resolution of these problems via the finite element method (FEM). The implement...

This paper presents an extension of a well-established constitutive model for NiTi covering both reversible (elastic, martensitic transformation, martensite reorientation) and irreversible (plastic) deformation mechanisms. Besides the inclusion of mechanisms of plastic deformation in both austenitic and martensitic phases in an independent manner,...

Specific heat treatment or addition of a ternary element may induce a two-stage transformation sequence in NiTi shape memory alloys (SMA). A typical example of intermediating phases is so-called R-phase, a rhombohedral distortion of the cubic austenitic phase, which exhibits much lower transformation strain and thermal hysteresis than the subsequen...

Macroscopic austenite-martensite transition zones in thin superelastic NiTi wires subjected to stretching, twisting and their combination were reconstructed via diffraction/scattering computed tomography. The method allows to visualize spatial distributions of both phases from a series of X-ray diffraction scans. A cone-shaped, radially symmetric t...

An important group of NiTi-based alloys employed in applications exhibits an intermediate R-phase transformation, which substantially modifies the material's thermomechanical response. Finite element analysis of products from such alloys requires a constitutive model covering the two-stage transformation sequence in its full thermodynamic nature. W...

A phenomenological model for polycrystalline NiTi shape-memory alloys with a refined dissipation function is here enhanced by a thermomechanical coupling and rigorously analyzed as far as existence of weak solutions and numerical stability and convergence of the numerical approximation performed by a staggered time discretization. Moreover, the mod...

The work presents a thermomechanical model for polycrystalline NiTi-based shape memory alloys developed within the framework of generalized standard solids, which is able to cover loading-mode dependent localization of the martensitic transformation. The key point is the introduction of a novel austenite-martensite interaction term responsible for...

We successfully reconstructed spatial phase distributions within a cross-section of a stretched NiTi helical spring via X-ray diffraction/scattering computed tomography (DSCT) and compared them with predictions obtained from a finite element simulation; the simulation employed an established phenomenological constitutive model tailored for NiTi sha...

The work presents a thermomechanical model for polycrystalline NiTi-based shape memory alloys developed within the framework of generalized standard solids, which is able to cover loading-mode dependent localization of the martensitic transformation. The key point is the introduction of a novel austenite-martensite interaction term responsible for...

A sharp-interface model describing static equilibrium configurations of shape memory alloys by means of interfacial polyconvex energy density introduced by Šilhavý and extended to a quasistatic situation by Knüpfer and Kružík is computationally tested. Elastic properties of variants of martensite and the austenite are described by polyconvex energy...

A sharp-interface model describing static equilibrium configurations of shape mory alloys by means of interfacial polyconvex energy density introduced by \v{S}ilhav\'y in 2010 and extended to a quasistatic situation by Kn\"upfer and Kru\v{z}\'ik in 2016 is computationally tested. Elastic properties of variants of martensite and the austenite are de...

Numerous constitutive models of shape memory alloys aiming to reliable predictions of their remarkable thermomechanical response are available in the literature. Often, experimentally documented specifics as tension-compression asymmetry or anisotropy of some physical properties of martensite are not considered, e.g. because it is difficult to dete...

We briefly review the recent experimental and theoretical research on the modeling of localized deformation in NiTi shape memory alloys. These alloys exhibit a remarkable response to mechanical loading with a relatively large reversible strains called superelasticity. We point up factors affecting formation, topology and propagation of localization...

It has been known for a long time that martensitic phase transformation in NiTi shape memory alloys loaded in tension develops inhomogeneously via formation and propagation of macroscopic deformation bands resembling well-known Lüders bands. Growing literature evidence supports the view that NiTi, in fact, develops a variety of localized deformatio...

One of the major drawbacks of mechanically loaded superelastic NiTi structures is their short fatigue life, which is believed to be related to cyclic variation of relatively large strains due to martensitic phase transformation. Thus, determination of spatial and temporal evolution of martensite within such structures may help to avoid critical loa...

Reversible and volume preserving martensitic transformation /MT/ proceeding in shape memory alloys such as NiTi differs from that in steels. Especially in superelastic NiTi largely used in medical devices, the effect of MT on the material response at stress risers and the role of MT in crack nucleation and propagation remain unclear and widely unex...

Elastic constants of polycrystalline NiTi and NiTiCu shape memory alloys in the martensite phase were determined by resonant ultrasound spectroscopy; the evolution of these constants was studied with subsequent applications of compressive loads inducing reorientation of martensitic variants. While the initial thermally-induced martensite exhibited...

Martensitic phase transformation in NiTi shape memory alloys (SMA) can spread either homogenously or in localized martensitic transformation bands. Transformation band propagation is usually observed in particular loading modes (tension) and geometries of specimen (wires, thin ribbons). In this work, a well established NiTi SMA constitutive model i...

See details on the topic and download the 3D-XRD dataset from the Special Topics website of the Department of Functional Materials http://ofm.fzu.cz/

See details on the topic and download the 3D-XRD dataset from the Special Topics website of the Department of Functional Materials http://ofm.fzu.cz/

Bend it, shape it, remember it Shape-memory alloys have the useful property of returning to their original shape after being greatly deformed. This process depends on the collective behavior of many small mineral grains in the metal. Using three-dimensional x-ray diffraction, Sedmák et al. tracked over 15,000 grains in a nickel-titanium shape-memor...

We study the formation of modulated martensites in ferromagnetic shape memory alloys by a mathematical model originating from the nanotwinning concept. The results show that the incommensurateness, systematically observed in experiments for the modulated phases, may be understood as a precursor effect of the intermartensitic transitions, and its ap...

Textured polycrystals of NiTi-based shape memory alloys (SMA) exhibit pronounced anisotropic properties which significantly influence their response to mechanical and thermal loading. In this work, a constitutive model tailored for non-proportional multi-axial loading of NiTi SMA exhibiting two-stage phase transformation via R-phase is enhanced so...

Shape memory alloys (SMA) are metallic materials exhibiting unusual properties of being able to sustain and recover large strains (superelasticity) and to "remember" the initial configuration and return to it with temperature change (one-way shape memory effect). These properties make them very suitable for utilization as actuators. One of the simp...

Actuators in the form of a helical spring made from shape memory alloy are attractive due to light weight, large recoverable deformation, high energy density and manufacturing simplicity. For their optimal design and control detailed information on evolution of phase and stress distribution within the material during operation is advantageous. In t...

Self-expanding stents or stentgrafts made from Nitinol superelastic alloy are widely used for a less invasive treatment of disease-induced localized flow constriction in the cardiovascular system. The therapy is based on insertion of a stent into a blood vessel to maintain the inner diameter of the vessel; it provides highly effective results at mi...

We present a three-dimensional constitutive model for NiTi polycrystalline shape memory alloys exhibiting transformations between three solid phases (austenite, R-phase, martensite). The ‘full modelling sequence’ comprised of formulation of modelling assumptions, construction of the model, mathematical analysis and numerical implementation and vali...

Behavior of NiTi shape memory alloys under complex loading is still a subject of both experimental and theoretical investigations. One of the simplest geometries, in which the material is loaded in combined mode and which has also several practical applications, is a simple helical spring. In this contribution, mechanical response of NiTi superelas...

Behavior of NiTi wire under proportional and non-proportional loading was recently subject of intensive experimental research. Based on analysis of experimental results, a new thermomechanical model for simulation of NiTi under non-proportional loading was proposed. The model is formulated within the framework of generalized standard materials and...

Due to unique mechanical properties (e.g. superelasticity) and excellent biocompatibility, NiTi shape memory alloys are utilized in medical applications as surgical tools or reinforcements for arteries and veins (stents). The components are usually made from polycrystals, which are frequently strongly textured, thus anisotropic, and in which R-phas...

Detachable fastening of two distinct objects using two interlocked NiTi hook arrays represents a novel NiTi functionality. Recently developed superelastic Velcro-like fasteners [1] may find engineering applications in e.g. medical devices, specialized fasteners for space or firefighter applications. In this presentation, in order to introduce this...

This work presents a thermomechanical model for polycrystalline NiTi-based shape memory alloys developed within the framework of continuum thermodynamics of irreversible processes. The model is capable of realistic simulations of several physical phenomena, involving transformation between austenite. R-phase and martensite and martensite reorientat...

A recently developed Velcro-like fastener utilizes superelastic deformation of two interlocked NiTi hooks when pulled apart. This work focuses on experimental analysis (evaluation of normal detachment force at different temperatures) and modeling (simulation by a finite element implemented SMA model) of the unhooking process. It is claimed that non...

A simple one-dimensional rate-independent model is proposed. It is able to capture responses of a NiTi shape memory alloy wire element to mechanical and thermal loadings. Since the model takes into account martensitic phase transformation as well as deformation processes in the martensite, both shape memory effects and pseudoelasticity can be simul...

Optimal design of SMA structures must be based on a deep knowledge of the material properties. As the SMA structures increase in complexity (stents, textiles, microdevices), modeling becomes increasingly involved. Macroscopic models of shape memory alloys are being developed with the ultimate aim to effectively simulate thermomechanical response of...

Shape memory (SM) fasteners based on the SM effect are already well-known applications. However, it has just been recently that NiTi hook arrays working as probabilistic fasteners were fabricated. Similarly to Velcrotype fasteners our fasteners are releasable with hooks returning back to their original shape after releasing. NiTi hooks may be incor...

We study shape memory hooks employed in releasable fasteners. The designed fastener in question consists of two identical parts, each one containing an array of hooks made of the shape memory alloy with the pseudoelastic behavior at room temperature. The fastening function consists in interlocking of the hooks of the either part together. The firmn...

A polycrystalline Ni 53.6 Mn 27.1 Ga 19.3 shape memory alloy was deformed in compression in four steps up to a final deformation of 3.0 % at room temperature. After each deformation step the relative elongation was measured in two thermal cycles in the temperature range 300 K to 640 K, where a thermoelastic martensitic phase transformation occurs....

Thin NiTi shape memory wires with unique thermomechanical properties are considered to be promising material for a wide range of new engineering applications. Based on broad experimental research on behavior of NiTi wires, which reveal a significant anisotropy in the transformation behaviors between tension and torsion, a new two-dimensional NiTi S...

Shape memory (SM) hooks belong to the category of SM fasteners utilizing the effects of the martensitic phase change transformation. The SM hooks may be incorporated into textiles and function in the same manner as Velcro fasteners do. The main objective of our research was to find the influence of the material properties and geometry parameters of...

Optimal design of NiTi wire structures (NiTi knitted braided self-expanding stents, NiTi plain or hybrid textiles, complex NiTi wire supported actuators) requires reliable knowledge and modelling of thermomechanical behaviour of NiTi wires which is still quite difficult task, particularly due to the multiple deformation mechanisms involved. This wo...

As cast ingot of the polycrystalline shape memory Ni53·6Mn27·1Ga19·3 alloy was composed of columnar and equiaxed grains. Dilatation characteristics of the samples with columnar grains were studied in the temperature range of 25–370°C and compared with the dilatation characteristics of the samples with equiaxed grains. The results show that shape me...

Nad názvem: Univerzita Karlova v Praze, Matematicko-fyzikální fakulta