Applicability and Limitations of Ru’s Formulation for Vibration Modelling of Double-Walled Carbon Nanotubes

In this paper, a comparison is conducted between two different formulations of the van der Waals interaction coefficient between layers, as applied to the vibrations of double-walled carbon nanotubes (DWCNTs); specifically, the evaluation of the natural frequencies is achieved through Ru’s and He’s formulations. The actual discrete DWCNT is modelled by means of a couple of concentric equivalent continuous thin cylindrical shells, where Donnell shell theory is adopted to obtain strain-displacement relationships. In order to take into account the chirality effect of DWCNT, an anisotropic elastic shell model is considered. Simply supported boundary conditions are imposed and the Rayleigh–Ritz method is used to obtain approximate natural frequencies and mode shapes. A parametric analysis considering different values of diameters and numbers of waves along longitudinal and circumferential directions is performed by adopting Ru’s and He’s formulations. From the comparisons, it is evident that Ru’s formulation provides unsatisfactory results for relatively low values of diameters and relatively high numbers of circumferential waves with respect to the more accurate He’s formulation. This behaviour is observed for every number of longitudinal half-waves. Therefore, Ru’s formulation cannot be used for the vibration modelling of DWCNTs in a large range of diameters and wavenumbers

Nonlinear vibrations of functionally graded cylindrical shells: Effect of companion mode participation

In this paper, the nonlinear vibrations of functionally graded (FGM) circular cylindrical shells are analyzed. The Sanders-Koiter theory is applied to model the nonlinear dynamics of the system in the case of finite amplitude of vibration. The shell deformation is described in terms of longitudinal, circumferential and radial displacement fields. Simply supported boundary conditions are considered. The displacement fields are expanded by means of a double mixed series based on Chebyshev orthogonal polynomials for the longitudinal variable and harmonic functions for the circumferential variable. Both driven and companion modes are considered, allowing for the travelling-wave response of the shell. Numerical analyses are carried out in order to characterize the nonlinear response when the shell is subjected to an harmonic external load. A convergence analysis is carried out to obtain the correct number of axisymmetric and asymmetric modes describing the actual nonlinear behavior of the shells. The effect of the geometry on the nonlinear vibrations of the shells is analyzed, and a comparison of nonlinear amplitude-frequency curves of cylindrical shells with different geometries is carried out. The influence of the companion mode participation on the nonlinear response of the shells is analyzed; frequency-response curves with companion mode participation (i.e. the actual response of the shell) are obtained. The present model is validated in the linear field (natural frequencies) by means of data present in the literature

Multi-layer composite beam modelling and optimization for high speed mechanical applications

Multilayer composite beam applications are known and used to obtain functionally graded mechanical components whose properties can be tuned to obtain high performances, such as strength, stiffness, light inertia and damping behaviour. Nevertheless, the technical literature lacks in studies concerning multilayer dynamical beam modelling making it possible to identify the model parameters of known solutions and to design new solutions being able to overcome the limits associated to such solutions. A current research topic concerns high damping FGM applications in the aerospace field, where standard high stiffness, high strength slender shell components, such as turbine blades, must show a limited vibrational behaviour in operating conditions, i.e. the material employed should exhibit normally conflicting characteristics, such as high internal hysteresis and high stiffness as well. In this specific field, some recent applications were explored, mainly based on a experimental approach, and while some of them showed to be effective, the lack of a theoretical model inhibited the study of any possible improvement. In this paper beam-based mechanical components, to be used in high speed mechanical applications such as in the automotive and in the automation industry field, are considered and investigated from a mainly theoretical point of view. A basic dissipative mechanism is proposed to justify the damping behaviour associated to some known multi-layer material composite solutions experimentally shown by some researchers and by our research group in the past. A multi-layer beam modeling, based on the known zig-zag approach, was studied and here proposed, with the specific aim to simulate the dynamical behaviour of known solutions, to make it possible to identify the unknown model parameters associated to the manufacturing technology and to then optimize the multi-layer architecture to obtain the best possible design solution. A new piecewise cubic function describing the axial displacement of each layer of laminated composite beams, being able to correctly model the shear stress at the different layer interfaces, is proposed. The contribution of this function is added to the classic Timoshenko beam theory component to provide a realistic representation of the deformation states of transverse shear flexible laminated composite beams in the case of plane strain hypothesis. The function is defined in terms of the axial displacements at the layer interfaces, presenting cubic variation across the thickness of every layer of the composite beam and continuity at the layer interfaces, where its amplitude is supposed to vanish at the top and bottom surfaces of the beam. In addition, the continuity of the transverse shear stress at each layer interface can be imposed in order to obtain a well-conditioned system of equations of motion, but it can also be relaxed to model fricional actions between layers. A procedure making it possible to obtain the value of the axial displacement at each layer interface starting from the axial displacement of the previous layer interface is developed. The profile of the axial displacements along the beam thickness is obtained in the most general form, being not dependent in shape by the boundary conditions and applied external forces. The effect of the thickness and shear modulus of each layer on the profile of the axial displacements along the beam thickness by considering different numbers of layers is studied. Several geometric and material configurations are compared. From the analyses, some optimal configurations concerning heterogeneous materials and layer thicknesses, giving the most relevant deviation of the axial displacement distribution along the depth of the cross-section of the beam of the piecewise cubic zigzag function with respect to the linear axial displacement distribution given by the Timoshenko beam theory, are outlined. The influence of the heterogeneous axial displacement distribution along the beam thickness on the dynamical behaviour of laminated composite beams is investigated. The free and forced vibrational behaviour of composite beams under different boundary conditions and time-varying transverse loading is obtained and the results critically discussed

Nonlinear vibrations of functionally graded cylindrical shells: Effect of the geometry

In this paper, the effect of the geometry on the nonlinear vibrations of functionally graded (FGM) cylindrical shells is analyzed. The Sanders-Koiter theory is applied to model the nonlinear dynamics of the system in the case of finite amplitude of vibration. The shell deformation is described in terms of longitudinal, circumferential and radial displacement fields. Simply supported boundary conditions are considered. The displacement fields are expanded by means of a double mixed series based on harmonic functions for the circumferential variable and Chebyshev polynomials for the longitudinal variable. In the linear analysis, after spatial discretization, mass and stiff matrices are computed, natural frequencies and mode shapes of the shell are obtained. In the nonlinear analysis, the three displacement fields are re-expanded by using approximate eigenfunctions obtained by the linear analysis; specific modes are selected. The Lagrange equations reduce nonlinear partial differential equations to a set of ordinary differential equations. Numerical analyses are carried out in order to characterize the nonlinear response of the shell. A convergence analysis is carried out to determine the correct number of the modes to be used. The analysis is focused on determining the nonlinear character of the response as the geometry of the shell varies

Effect of the geometry on the nonlinear vibrations of functionally graded cylindrical shells

In this paper, the effect of the geometry on the nonlinear vibrations of functionally graded (FGM) cylindrical shells is analyzed. The Sanders-Koiter theory is applied to model nonlinear dynamics of the system in the case of finite amplitude of vibration. Shell deformation is described in terms of longitudinal, circumferential and radial displacement fields; the theory considers geometric nonlinearities due to the large amplitude of vibration. Simply supported boundary conditions are considered. The displacement fields are expanded by means of a double mixed series based on harmonic functions for the circumferential variable and Chebyshev polynomials for the longitudinal variable. Both driven and companion modes are considered, allowing for the travelling-wave response of the shell. The functionally graded material is made of a uniform distribution of stainless steel and nickel, the material properties are graded in the thickness direction, according to a volume fraction power-law distribution.The first step of the procedure is the linear analysis, i.e. after spatial discretization mass and stiff matrices are computed and natural frequencies and mode shapes of the shell are obtained, the latter are represented by analytical continuous functions defined over all the shell domain. In the nonlinear model, the shell is subjected to an external harmonic radial excitation, close to the resonance of a shell mode, it induces nonlinear behaviors due to large amplitude of vibration. The three displacement fields are re-expanded by using approximate eigenfunctions, which were obtained by the linear analysis; specific modes are selected. An energy approach based on the Lagrange equations is considered, in order to reduce the nonlinear partial differential equations to a set of ordinary differential equations.Numerical analyses are carried out in order characterize the nonlinear response, considering different geometries and material distribution. A convergence analysis is carried out in order to determine the correct number of the modes to be used; the role of the axisymmetric and asymmetric modes carefully analyzed. The analysis is focused on determining the nonlinear character of the response as the geometry (thickness, radius, length) and material properties (power-law exponent and configurations of the constituent materials) vary; in particular, the effect of the constituent volume fractions and the configurations of the constituent materials on the natural frequencies and nonlinear response are studied.Results are validated using data available in literature, i.e. linear natural frequencies

Nonlinear vibrations of functionally graded circular cylindrical shells

In this paper, the effect of the geometry on the nonlinear vibrations of functionally graded cy- lindrical shells is analyzed. The Sanders-Koiter theory is applied to model nonlinear dynamics of the system in the case of finite amplitude of vibration. Shell deformation is described in terms of longitudinal, circumferential and radial displacement fields. Simply supported boundary conditions are considered. Numerical analyses are carried out in order to characterize the nonlinear response when the shell is subjected to an harmonic external load; different geometries and material distribu- tions are considered. A convergence analysis is carried out in order to determine the correct number of the modes to be used; the role of the axisymmetric and asymmetric modes is carefully analyzed. The analysis is focused on determining the nonlinear character of the response as the geometry (thickness, radius, length) and material properties (power-law exponent N and configurations of the constituent materials) vary. The effect of the constituent volume fractions and the configurations of the constituent materials on the natural frequencies and nonlinear response are studied

Condition monitoring and reliability of a resistance spot welding process

The reliability of a resistance spot welding (RSW) process is studied monitoring the quality of the corresponding welding points. Each welding point is uniquely represented by a specific resistance characteristic curve over time. Five learning resistance characteristic curves, the good quality of the related welding points was experimentally verified by means of a non-destructive technique, are selected as a reference to check the quality of welding points related to different process resistance characteristic curves. A first estimate of the quality of the welding point is made comparing the corresponding process resistance characteristic curve with the learning maximum, minimum and average resistance characteristic curves. Both good quality and defective (glued or squeezed) welding points are observed. In order to more correctly identify the quality level of each welding point, two different parameters comparing the related process resistance characteristic curve with the learning average resistance characteristic curve are applied. First, the residual resistance, as the difference at each instant of time between the two resistance characteristic curves, is considered. Then, the Euclidean distance, as the geometric distance at each instant of time between the two resistance characteristic curves, is adopted. Finally, the trend of the quality of the welding points as their number increases for welding electrodes with a fixed number of dressings is investigated

A Comparison of Signal Analysis Techniques for the Diagnostics of the IMS Rolling Element Bearing Dataset

In this paper, a comparison of signal analysis techniques for the diagnostics of rolling element bearings is carried out. Specifically, the comparison is performed in terms of fault detection, diagnosis and prognosis techniques with regards to the first rolling element bearing dataset released by NASA IMS Center in 2014. As for fault detection, it is obtained that RMS value, Kurtosis and Detectivity, as statistical parameters, are able to properly detect the arising of the fault on the defective bearings. Then, several signal processing techniques, such as deterministic/random signal separation, time-frequency and cyclostationary analyses are applied to perform fault diagnosis. Among these techniques, it is found that the combination of Cepstrum Pre-Whitening and Squared Envelope Spectrum, and Improved Envelope Spectrum, allow the faults to be correctly identified on specific bearing components. Finally, the Correlation, Monotonicity and Robustness of the previous statistical parameters are computed to identify the most accurate tools for bearing fault prognosis