Leonardo Pagnotta

A full-field approach for the elastic characterization of anisotropic materials

In the paper a procedure is described whereby the characterization of isotropic and anisotropic laminae is carried out by full-field measurement of the surface rotations under proper flexural loads. The analytic formulation does not necessarily imply a numerical calibration and the elastic constants are determined by integrating the whole experimental data on the surface of the specimen. Measurements were carried out by phase-stepped speckle interferometry using a shearometer based on Michelson design. The experimental instrumentation, including the loading device, and the procedures for the manipulation of experimental data are detailedly described; experimental results obtained on a steel specimen and a composite laminate are also reported.

A full-field procedure for evaluating the elastic properties of advanced ceramics

In the present paper, a procedure is described whereby the elastic properties of a ceramic material are evaluated during a biaxial flexure test. The disk specimen is supported on three points and loaded by a uniform pressure on the opposite face. The whole displacement field undergone by the upper face, measured by a digital speckle interferometer, is approximated by a set of polynomials whose weights depend on the elastic properties. This dependence, previously determined by finite element analysis, is exploited to derive the values of the elastic properties from the displacement field experimentally detected. The procedure proposed was applied to a silicon carbide specimen.

Measurement of the dynamic elastic properties of a thin coating

The use of thin coatings as protective layers for mechanical components has dramatically increased in the last two decades. The performance of a coated system depends mainly on a knowledge of the basic mechanical properties of coatings and, therefore, experimental techniques for determining Young’s modulus and Poisson’s ratio are needed. In this technical note a procedure is presented that allows the well known sonic resonance methods, for the elastic characterization of homogeneous and isotropic materials, to be extended to thin square plates. It requires the measurement of two of the first four natural frequencies of the sample in order to obtain the dynamic elastic properties of the material of which it is made. The procedure is based on suitable approximate relationships relating the resonant frequencies to size, density, and elastic constants of the material. These relations are derived from an extensive series of numerical analyses carried out by a finite element code. In addition, inexpensive exper...

Indentation fracture toughness of single-crystal Bi2Te3 topological insulators

Bismuth telluride (Bi2Te3) is one of the most important commercial thermoelectric materials. In recent years, the discovery of topologically protected surface states in Bi chalcogenides has paved the way for their application in nanoelectronics. Determination of the fracture toughness plays a crucial role for the potential application of topological insulators in flexible electronics and nanoelectromechanical devices. Using depth-sensing nanoindentation tests, we investigated for the first time the fracture toughness of bulk single crystals of Bi2Te3 topological insulators, grown using the Bridgman-Stockbarger method. Our results highlight one of the possible pitfalls of the technology based on topological insulators.

Determining elastic constants of materials with interferometric techniques

The article describes a method that combines finite element analysis and genetic algorithms in order to determine the elastic properties of materials from the full-field measurement of the surface displacements of plates under flexural loads. An optimizing process updates the elastic constants in a numerical model until the calculated displacements fit the experimental data. The unknown parameters are identified simultaneously by a single test and without damaging the structure. An original method is also proposed for optimizing the loading and constraining conditions of the specimen with a view to obtaining faster and more stable solutions. The procedures were applied to a steel specimen whose displacement field was detected by speckle interferometry.

Laser speckle decorrelation in NDT

Abstract The random noise of the laser speckle field which develops at the focusing plane of an imaging system, is, by now, efficiently used in several interferometric techniques as an information carrier of the macroscopic wavefront distortion induced by the surface displacement field of the object under investigation. The actual noise in this kind of techniques is represented by the speckle decorrelation at the image plane — i.e. the destruction of the carrier — which may be caused by the modification of the texture surface (e.g. by yielding under a severe stress state), but it is inherently produced by the same displacement field under measurement. In the paper the phenomenon of laser speckle decorrelation is numerically simulated and experimentally investigated with the aim of estimating its sensitivity to local deformation and assessing a possible field of application. Satisfactory results in the field of NDT of multilayer fiber-reinforced composites were obtained by reducing the diaphragm of the lens to increase the sensitivity of the imaging system to speckle decorrelation induced by local deformation; unfortunately this simple approach requires a considerable amount of laser power for illuminating the object. Different aperture shapes were therefore numerically simulated which provided improved efficiency and sensitivity and whereby a semi-quantitative analysis of the displacement field could be experimented.

Measurement of residual internal stresses in optical fiber preforms

A non-destructive experimental procedure is presented which enables the determination of residual thermal stresses in optical fiber preforms. The procedure is based on integrated photoelasticity. We carry out the measurement of the optical retardation using the traditional Sénarmont compensation method combined with a fringe shifting technique. The radial distribution of the axial stress is reconstructed using Abel transform. We have investigated two different refractive-index profiles, adopted in the realization of monomode and multimode optical fibers. It was shown that in both cases the results obtained experimentally and those analytically predicted by the Timoshenko elastic model were in good agreement. To obtain accurate experimental results, it was necessary to apply a correction procedure to take into account the fact that the trafectories of the light rays inside the preforms are not straight lines.

A numerical study of squeeze-film damping in MEMS-based structures including rarefaction effects

In a variety of MEMS applications, the thin film of fluid responsible of squeeze-film damping results to be rarefied and, thus, not suitable to be modeled though the classical Navier-Stokes equation. The simplest way to consider fluid rarefaction is the introduction of a slight modification into its ordinary formulation, by substituting the standard fluid viscosity with an effective viscosity term. In the present paper, some squeeze-film damping problems of both parallel and torsion plates at decreasing pressure are studied by numerical solving a full 3D Navier-Stokes equation, where the effective viscosity is computed according to proper expressions already included in the literature. Furthermore, the same expressions for the effective viscosity are implemented within known analytical models, still derived from the Navier-Stokes equation. In all the considered cases, the numerical results are shown to be very promising, providing comparable or even better agreement with the experimental data than the corresponding analytical results, even at low air pressure. Thus, unlike what is usually agreed in the literature, the effective viscosity approach can be efficiently applied at low pressure regimes, especially when this is combined with a finite element analysis (FEA

A review of patented works on the mechanical characterization of materials at micro- and nano-scale.

In recent years, the development of cost-effective processing techniques, novel design concepts and new materials paved the way to a widespread diffusion of micro- and nano-electro-mechanical systems (NEMS/MEMS). Obviously, the reliability as well as the performance of NEMS/MEMS depend on the corresponding materials properties, which in turn should be determined using ad-hoc small samples fabricated at the relevant size-scale. For this reason, in the last decade research efforts have been devoted to the development of experimental techniques suitable for the mechanical characterization of materials at micro- and nano-scale. There are many contributions stemming from this research area, the purpose of the present work is to give an overview of the most recent patented works. The focus will be directed to selected patents on the mechanical characterization of both micro- and nanosamples, like nanotubes and nanowires. Special emphasis will be given to the methods suited for the determination of elastic properties, fracture resistance and residual stresses of materials.

An Inverse Procedure for Determining the Material Constants of Isotropic Square Plates by Impulse Excitation of Vibration

The paper presents a procedure whereby the Poisson’s ratio and dynamic Young’s modulus of isotropic and homogeneous materials are determined using two of the first four frequencies of natural vibration in thin square plates. The procedure is based on suitable approximate relationships relating the resonant frequencies to the elastic constants of the material. These relations were derived from an extensive series of numerical analysis carried out by a finite element code. To measure the fundamental resonant frequencies, inexpensive computerized equipment is proposed. The procedure has been validated on Carbon Steel specimens.