Alena Kruisová

Linearized forward and inverse problems of the resonant ultrasound spectroscopy for the evaluation of thin surface layers

In this paper, linearized approximations of both the forward and the inverse problems of resonant ultrasound spectroscopy for the determination of mechanical properties of thin surface layers are presented. The linear relations between the frequency shifts induced by the deposition of the layer and the in-plane elastic coefficients of the layer are derived and inverted, the applicability range of the obtained linear model is discussed by a comparison with nonlinear models and finite element method (FEM), and an algorithm for the estimation of experimental errors in the inversely determined elastic coefficients is described. In the final part of the paper, the linearized inverse procedure is applied to evaluate elastic coefficients of a 310 nm thick diamond-like carbon layer deposited on a silicon substrate.

Simulations of Self-Expanding Braided Stent Using Macroscopic Model of NiTi Shape Memory Alloys Covering R-Phase

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 minimal cost and with reduced hospital stays. However, since stent is an external mechanical healing tool implemented into human body for quite a long time, information on the mechanical performance of it is of fundamental importance with respect to patient’s safety and comfort. Advantageously, computational structural analysis can provide valuable information on the response of the product in an environment where in vivo experimentation is extremely expensive or impossible. With this motivation, a numerical model of a particular braided self-expanding stent was developed. As a reasonable approximation substantially reducing computational demands, the stent was considered to be composed of a set of helical springs with specific constrains reflecting geometry of the structure. An advanced constitutive model for NiTi-based shape memory alloys including R-phase transition was employed in analysis. Comparison to measurements shows a very good match between the numerical solution and experimental results. Relation between diameter of the stent and uniform radial pressure on its surface is estimated. Information about internal phase and stress state of the material during compression loading provided by the model is used to estimate fatigue properties of the stent during cyclic loading.

Sensitivity of the resonant ultrasound spectroscopy to weak gradients of elastic properties

The applicability of resonant ultrasound spectroscopy on materials with weak spatial gradients in elastic coefficients and density is analyzed. It is shown that such gradients do not affect measurably the resonant spectrum but have a significant impact on the modal shapes. A numerical inverse procedure is proposed to explore the possibility of reconstructing the gradients from experimentally obtained modal shapes. This procedure is tested on synthetic data and applied to determine the gradient of the shear modulus in a continuously graded silicon nitride ceramic material. The results are in a good agreement with the gradient calculated for the examined material theoretically as well as with the results of other experimental methods.

Acoustic metamaterial behavior of three-dimensional periodic architectures assembled by robocasting

Ultrasonic measurements combined with numerical modelling are used to analyze the elastic and acoustic properties of morphologically complex ceramic bodies assembled by the Robocasting technique. It is shown that the micromechanics of the robocast periodic scaffolds leads to several metamaterial-like wave propagation phenomena. Besides the expectable prominent elastic anisotropy and the frequency band structure resulting from the periodicity of the scaffold, a wave-mode mixing is observed that disables the conventional distinguishing between quasi-longitudinal, quasi-shear, and pure shear modes for propagation in the symmetry planes of the structure. This paper proves the capability of Robocasting, as a versatile three-dimensional (3D) printing method, to produce tailored acoustic metamaterials with very low damping and outstandingly strong acoustic anomalies.

Finite Elements Modeling of Mechanical and Acoustic Properties of a Ceramic Metamaterial Assembled by Robocasting

Finite element modeling (FEM) was used for numerical simulations of mechanical performance of aperiodic silicon-carbide scaffold manufactured by robocasting. The FEM approach enabled reliable calculation of theeffective anisotropic elastic properties of the scaffold at the macro-scale, as well as of the acoustic band structureindicating the metamaterial-like behavior of the material at the micro-scale. In addition, the micromechanics of thescaffold was discussed based on the outputs of the model: the mechanisms of the extremely soft shearing modes wereidentified and the corresponding stress concentrations arising at the contact points in the scaffold were analyzedwith respect to the possible failure modes of the robocast structure.

Elastic constants of nanoporous III-V semiconductors

Resonant ultrasound spectroscopy is applied to determine elastic constants of nanoporous gallium arsenide and indium phosphide single crystals with various pore morphologies. Three samples with approximately the same level of porosity (30%) are studied; it is shown that in all cases this porosity leads to a decrease of Youngs moduli by more than 50% and to a significant increase of the Poissons ratios, while the strength of the resulting elastic anisotropy of the nanoporous material follows from the particular morphology of the pores. The experimentally obtained elastic constants are compared to those predicted for the given morphologies by finite elements modeling. It is observed that the numerical models give acceptably realistic predictions of the elastic constants, although they tend to underestimate the decrease of the elastic moduli due to the porosity as well as the corresponding increase of the Poissons ratio.

Ultrasonic bandgaps in 3D-printed periodic ceramic microlattices

HIGHLIGHTSTransmission of longitudinal ultrasonic waves through ceramic microlattices was studied.Microlattices with tetragonal and hexagonal spatial arrangements were used.All studied structures exhibit sharp bandgaps at frequencies above 3 MHz.The locations of the bandgaps are in agreement with FEM modeling predictions. ABSTRACT The transmission of longitudinal ultrasonic waves through periodic ceramic microlattices fabricated by Robocasting was measured in the 2–12 MHz frequency range. It was observed that these structures (scaffolds of tetragonal and hexagonal spatial arrangements with periodicity at length‐scales of Symbol100 Symbolm) exhibit well‐detectable acoustic band structures with bandgaps. The locations of these gaps at relatively high frequencies were shown to be in close agreement with the predictions of numerical models, especially for tetragonal scaffolds. For hexagonal scaffolds, a mixing between longitudinal and shear polarizations of the propagation modes was observed in the model, which blurred the matching of the calculated band structures with the experimentally measured bandgaps. Symbol. No caption available. Symbol. No caption available.

Ceramic phononic crystals with MHz-range frequency band gaps

Robocasting is an additive manufacturing method, which is capable of fabricating microarchitectured scaffolds, consisting of periodically repeating thin ceramic rods in various spatial arrangements. Fully sintered ceramic scaffolds are obtained by a combination of layer-by-layer 3D printing and subsequent presureless spark plasma sintering of the printed green ceramic bodies. Due to the complex structures with easily tunable geometric parameters, phononic crystals can be fabricated by the robocasting method. In this contribution, elastic and acoustic properties of the robocast silicon carbide scaffold are shown, utilizing a combination of resonant ultrasound spectroscopy measurement and finite element modeling. The scaffold is highly anisotropic in elastic properties, which leads to a strong acoustic energy focusing along the principal axes of the silicon carbide rods. Moreover, frequency band gaps in MHz range are detected by measuring longitudinal wave transmission, which is compared with a theoretical prediction by the finite element modeling.Robocasting is an additive manufacturing method, which is capable of fabricating microarchitectured scaffolds, consisting of periodically repeating thin ceramic rods in various spatial arrangements. Fully sintered ceramic scaffolds are obtained by a combination of layer-by-layer 3D printing and subsequent presureless spark plasma sintering of the printed green ceramic bodies. Due to the complex structures with easily tunable geometric parameters, phononic crystals can be fabricated by the robocasting method. In this contribution, elastic and acoustic properties of the robocast silicon carbide scaffold are shown, utilizing a combination of resonant ultrasound spectroscopy measurement and finite element modeling. The scaffold is highly anisotropic in elastic properties, which leads to a strong acoustic energy focusing along the principal axes of the silicon carbide rods. Moreover, frequency band gaps in MHz range are detected by measuring longitudinal wave transmission, which is compared with a theoretical ...

Simulation of Mechanical Behavior of NiTi Shape Memory Alloys Under Complex Loading: Model Formulation and its Performance in Applications

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 this work a constitutive model tailored for non-proportionally loaded shape memory alloys exhibiting R-phase transition, transformation strain anisotropy, tension-compression asymmetry is employed to reveal and interpret relation between macroscopic response of such an actuator and microscopic state within the shape memory material. Numerical simulations confirm good predictive capability of the model and demonstrate that because of naturally non-proportional loading mode, phase and stress distributions within cross-section of the wire may be rather complex and counterintuitive.Copyright