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ALESSANDRO DE FELICE

Ricercatore Legge 240/10 - t.det.
Dipartimento di Ingegneria "Enzo Ferrari"

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Pubblicazioni

2024 - Dynamic analysis of parametrically excited stable rotors with unbalance [Relazione in Atti di Convegno]
De Felice, Alessandro; Sorrentino, Silvio
abstract

The dynamic analysis of parametrically excited rotors is a research field of great interest and practical importance, since instability and resonant behavior can cause issues ranging from anomalous noise and wear to catastrophic failures. This study is focused on the effects of unbalance on a parametrically excited rotor operating in the asymptotically stable domain, a research topic which in the scientific literature has not been investigated. Rotor unbalance causes an additional harmonic load acting on flexural deflection, influencing the frequency response together with the parametric excitation, yielding additional combination external resonances. As a first insight into this problem, to study the effects of angular speed independently of variations of the natural frequencies, and to facilitate decoupling of the equations of motion, a simplified model of a distributed-parameter slender rotor is considered, consisting of a homogeneous Euler-Bernoulli beam with circular section, rotating at constant angular speed about its longitudinal axis on isotropic supports. It is affected by unbalance and loaded by an axial end thrust, assumed to have a harmonic time-dependent component. The steady-state response is studied after decoupling the equations of motion, reducing the problem to the analysis of a non-homogeneous single-degree-of-freedom damped Mathieu equation.

2024 - Identification of lumped stiffness parameters for a motorcycle model in investigating weave and wobble [Articolo su rivista]
Passigato, Francesco; Schramm, Alexander; Diermeyer, Frank; Sorrentino, Silvio; Gordner, Achim; De Felice, Alessandro
abstract

In motorcycle dynamics, great importance is attributed to the study of the weave and wobble vibration modes and, in particular, to the effects of the flexibility of structural components on their stability. Therefore, appropriate motorcycle models for studying weave and wobble should include flexible elements for describing the flexural behavior of components such as the main frame, front assembly, and rear swingarm. Different approaches are possible formodeling flexibilities: the most common among them are the lumped stiffness and the flexible multibody approaches. While the latter certainly provides higher accuracy, the former has advantages in terms of computational load, but, above all, it makes it easier to understand in the design phase how technical parameters, such as torsional and bending stiffness of a given structural component, can influence the stability of weave and wobble. The accuracy of lumped stiffness models strongly depends on parameter identification. In this study, a general method is proposed to determine appropriate lumped stiffness parameters for any given motorcycle component. The proposed method is tested and validated by comparing the weave and wobble modal behavior with the results of flexible multibody analysis. The lumped stiffness model is then adopted to carry out a sensitivity analysis aimed at identifying the effects on the weave and wobble stability of the torsional and bending stiffness of specific structural components of the motorcycle to optimize their design.

2023 - A new approach to the study and prevention of the clutch judder [Relazione in Atti di Convegno]
Tentarelli, Manuel; Cantelli, Stefano; Sorrentino, Silvio; DE FELICE, Alessandro
abstract

The clutch judder observed in hydraulically actuated dual clutch transmissions cannot be fully explained by the current scientific literature. Using a new methodological approach for the analysis of clutch judder, this study shows that a fluctuating oil pressure makes the clutch a parametrically excited system and gives rise to regions of instability as a function of the operating conditions of the clutch. The system’s dynamic behaviour, characterized by both mechanical and structural high complexity, is here investigated through stability analysis of a mathematical model of the driveline. In particular, stability boundaries are identified by means of Floquet analysis, as well as the occurrence of both parametric resonance regions and linear resonance conditions. This opens a new perspective on NVH of automotive drivelines, showing that clutch judder can potentially occur in a much wider range of conditions than documented in the current scientific literature. The results of the experiments in progress on a test bench, designed in partnership with CNH Industrial, are validating the proposed model and will allow to calibrate the parameters of a predictive mathematical model to avoid the phenomenon of clutch judder in the design phase.

2023 - Friction-induced parametric oscillations in automotive drivelines: experimental analysis and modelling [Relazione in Atti di Convegno]
Tentarelli, Manuel; Cantelli, Stefano; Sorrentino, Silvio; De Felice, Alessandro
abstract

Clutch judder is a friction-induced self-excited vibration occurring in automotive drivelines, an NVH issue studied for more than forty years and attributed by the scientific community to three possible causes: stick-slip, negative gradient of the coefficient of friction and geometric disturbances. However, these explanations fail to describe the kind of judder studied in this contribution, arising in presence of an oscillating component (dither) in the clutch actuation pressure. The analysis of experimental data collected on a dual-clutch transmission mounted on a specific test bench suggested the presence of a parametric resonance, generated by the dither. A specific 4 degrees of freedom model was then developed, able to predict with good accuracy the unstable parametric region in which judder occurs and useful in the design stage.

2023 - NONLINEAR EFFECTS ON THE SELF-EXCITED CHATTER OSCILLATIONS IN MOTORCYCLE DYNAMICS, INCLUDING TYRE RELAXATION [Articolo su rivista]
Schramm, ALEXANDER EUGENE; Sorrentino, Silvio; DE FELICE, Alessandro
abstract

Nonlinear effects on the stability of the motorcycle rear ‘chatter’ phenomenon are investi-gated by means of a minimal two degrees of freedom model, including tyre relaxation since this aspect has not been investigated in-depth in previous studies. Motorcycle ‘chat-ter’ manifests itself as a self-excited oscillation, which arises during braking in the fre-quency range between 17 and 22 Hz, affecting safety and performance. The study of the linearised system gives indications on the initiation of self-excited vibrations and thus helps to prevent them using proper design techniques. Post-bifurcation behaviour is ana-lysed focusing on limit cycles and bifurcation diagrams, studied by means of specific ap-plications of the harmonic balance method and Floquet theory. This allows the detection of the validity of the linear results, and to identify the meaningful parameters in limit cycle generation, their amplitude, and their stability.

2023 - On the self-excited chatter vibration in motorcycles [Abstract in Atti di Convegno]
Schramm, ALEXANDER EUGENE; Sorrentino, Silvio; DE FELICE, Alessandro
abstract

This study is focused on the understanding of the nonlinear effects of tyre forces and chain geometry on motorcycle chatter, a self-excited oscillation arising at the rear wheel during heavy braking manoeuvres. In particular the post-bifurcation criticality is assessed, whether subcritical or supercritical, with respect to travelling speed.

2023 - Parametric excitation as a cause of clutch judder: theoretical study and experimental validation [Articolo su rivista]
Tentarelli, Manuel; Cantelli, Stefano; Sorrentino, Silvio; DE FELICE, Alessandro
abstract

Judder is a friction-induced torsional vibration generated during clutch engagement in automotive drivelines. To the best of the authors’ knowledge, three are the causes attributed by the scientific community to the onset of the clutch judder: stick-slip phenomena, negative gradient of the coefficient of friction and geometric disturbances. With the help of a methodological approach that integrates the analysis of a mathematical model with the experimental data obtained on a specially designed test bench, this paper shows that, in some cases, the clutch judder may also be due to the presence of parametric excitation. This new understanding overcomes the limits of the existing explanations because it does not preclude the occurrence of clutch judder at high slip speeds, with positive gradients of the friction coefficient and at excitation frequencies different from the eigenfrequencies of the system. The four degrees of freedom model developed for the study of the transmission provides maps that allow to identify the instability conditions of the system. The analysis described in this paper has been aimed at solving a judder problem in a transmission already in production, but the same approach can be used to avoid the conditions that cause the onset of clutch judder on other transmissions in the design phase.

2023 - Some recent advances in the dynamic analysis of parametrically excited continuous rotor systems [Abstract in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

Dynamic analysis of parametrically excited rotors is a research field of great interest and practical importance, since instability and resonant behavior can cause issues ranging from anomalous noise and wear to catastrophic failures. An overview is presented of some advances recently proposed by the authors on the dynamic analysis of parametrically excited continuous rotor systems, including results regarding stability analysis (parametric resonances) and preliminary insights into resonant behavior in the asymptotically stable domain, due to unbalance (external resonances). An axisymmetric shaft described by a spinning Timoshenko beam is studied, loaded by oscillating axial end thrust and twisting moment, carrying additional inertial elements like discs. Both isotropic and anisotropic supports are considered, as well as gyroscopic effects and different kinds of damping distributions (both external and internal), which represents a model including all the general features of slender rotors which are relevant for their dynamic analysis. Stability is studied after discretization of the equations of motion into a set of coupled ordinary differential Mathieu-Hill equations. Stability maps in the form of Ince-Strutt diagrams are discussed to highlight the occurrence of simple and combination critical solutions, as well as the influence of angular speed, damping, and anisotropy in the supports. Steady-state response is studied in the asymptotically stable domain under the effect of unbalance, yielding an additional external harmonic load, acting on flexural deflection. As a first insight into this problem, to study the effects of angular speed independently to variations of natural frequencies and to facilitate decoupling of the equations of motion, the Timoshenko model is simplified into the Euler-Bernoulli model, neglecting the gyroscopic effects, additional discs, anisotropy in the supports and twisting moment at the ends of the shaft.

2023 - Stability analysis of parametrically excited isotropic rotors on anisotropic supports [Relazione in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

The stability of slender rotors which are parametrically excited by external loads is studied under the effects of an-isotropic supports. An axisymmetric shaft is considered, describ- ed by scaling a spinning Timoshenko beam, carrying a disk and loaded by oscillating axial end thrust and twisting moment. The supports are modelled including ‘principal’ stiffness and damp- ing distributions, able to modify the closely separated modes generated by angular speed (with ‘splitting’ of eigenfrequencies in forward and backward pairs). The proposed model includes all the general features of slender rotors which are relevant for this kind of stability analysis, gyroscopic effects comprised. Stability is studied after discretization of the equations of motion into a set of coupled ordinary differential Mathieu-Hill equations. The influence of angular speed combined with anisotropy in the supports is analyzed with respect to frequency and amplitude of the external loads on stability charts in the form of Ince-Strutt diagrams. The occurrence of different kinds of critical solutions, simple and combination, is investigated, highlighting their dependency on both the degree of anisotropy in the supports and angular speed.

2022 - Cold Judder in Tractor Drivelines: An Essential Model for Stability Analysis [Relazione in Atti di Convegno]
Tentarelli, M.; Cantelli, S.; De Felice, A.; Sorrentino, S.
abstract

Current scientific literature classifies the well-known clutch judder phenomenon in friction judder and pressure-induced judder, based on the mechanisms that generate it. Friction judder is associated to a negative gradient of the friction coefficient and to stick-slip phenomena, while pressure-induced judder is mainly caused by geometric disturbances. The peculiarities of the torsional oscillations experimentally observed in dual-clutch transmissions of tractors, incompatible with the explanations found in the current literature, led to consider parametric excitation as a possible cause of instability. Consequently, the clutch was modelled by means of a system of three coupled linear differential equations with time-periodic coefficients. Stability analysis, performed by adopting Floquet theory, allowed to draw stability maps as functions of the two excitation parameters, i.e. the frequency and the amplitude of the dither signal in the clutch actuation pressure. Stability maps proved to be particularly promising to explain the experimentally observed phenomenon. In addition, it was possible to show that clutch judder can also arise for positive gradients of the friction coefficient.

2022 - Effects of anisotropic supports on the stability of parametrically excited slender rotors [Articolo su rivista]
De Felice, A.; Sorrentino, S.
abstract

This study is aimed at investigating the effects of anisotropic supports on the stability of slender rotors parametrically excited by external loads. An axisymmetric shaft described by scaling a spinning Timoshenko beam on anisotropic supports is studied, loaded by oscillating axial end thrust and twisting moment, with the possibility of carrying additional inertial elements like discs, which represents a model including all the general features of slender rotors which are relevant for this kind of stability analysis, gyroscopic effects comprised. Stability is studied after discretization of the equations of motion into a set of coupled ordinary differential Mathieu-Hill equations. The influence on stability of angular speed combined with anisotropy in the supports (including principal stiffness, principal damping and cross-elements) is analysed with respect to frequency and amplitude of the external loads on stability charts in the form of Ince-Strutt diagrams. The occurrence of different kinds of critical solutions, simple and combination, is investigated, highlighting their dependency on both the degree of anisotropy in the supports and angular speed.

2022 - Experimental Identification of Viscoelastic Properties of Plates Made of Quiet Aluminum [Capitolo/Saggio]
Grosso, P.; De Felice, A.; Sorrentino, S.
abstract

The present study is aimed at the identification of equivalent viscoelastic models for layered thin-walled structures, obtained from vibration measurement only. Accurate modeling of modal properties is fundamental for describing metal fatigue caused by forced vibration on structural components. A new approach is proposed, based on a definition of an equivalent modal damping ratio applied to the circle-fit technique, to overcome the difficulties related to the identification of modal parameters when adopting non-conventional viscoelastic models. When the structural internal dissipative effects are dominant, this procedure identifies the parameters of an equivalent Young’s modulus in the frequency domain. The proposed procedure is applied to the analysis of plates made by Quiet Aluminum, adopting the linear fractional Kelvin-Voigt viscoelastic model and assessing the accuracy of the identified parameters by comparison of numerically simulated with experimentally measured frequency response functions.

2022 - Experimental stability analysis of tractor drivelines affected by cold judder [Relazione in Atti di Convegno]
Tentarelli, Manuel; Cantelli, Stefano; Sorrentino, Silvio; DE FELICE, Alessandro
abstract

Clutch judder is a friction-generated self-excited vibration affecting automotive drivelines. Stick-slip, negative gradient of the friction coefficient and geometric disturbances are the only causes currently identified in the scientific literature. Nevertheless, they are not sufficient to explain the vibration occurring in drivelines (typically of tractors) with dither (oscillating component) added to clutch hydraulic actuation signals. In this study, experimental data acquired on a specifically designed test bench are processed to show that this particular kind of judder is an instability due to parametrical excitation exerted by the actuation pressure signal. A minimal model with four degrees of freedom is proposed, able to fit the experimental data, to explain the onset of the parametrically excited vibration, and to identify which parameters have a major role in controlling this phenomenon. Stability charts obtained from the proposed model also represent a useful tool to prevent instability already in the design phase.

2022 - Multibody Efficiency Analysis of Chain Drives in Racing Motorcycles [Articolo su rivista]
De Rossi, L.; Schramm, A.; De Felice, A.
abstract

In racing motorcycles, the maximization of power transmission from the engine to the rear wheel is one of the critical aspects for improving the performance. Therefore, it is important to improve as much as possible the efficiency of the chain drive, consisting of a front sprocket on the output shaft of the transmission and a rear sprocket connected to the rear wheel, linked by a roller chain. In this study, a multibody model of a chain drive of a racing motorcycle involving high rotational speeds is developed and validated. The energy losses are analyzed, highlighting their dependency on working conditions, and the efficiency is studied as a function of number of teeth on the sprockets, mounting of the chain and sprockets, selected speed ratio, and chain pitch. As a result, it is found that the efficiency is improved by a larger number of teeth with an equal speed ratio, by reducing the chain pitch (while keeping the sprocket diameters constant), and by larger diameter sprockets (in every working condition, including those of high rotational speeds, in contrast to findings in previous literature). Variations in clearances in the chain influence the efficiency, while variations of center distance between sprockets is not influential if the clearances are kept constant.

2022 - Nonlinear effects on the self-excited oscillations known as motorcycle chatter [Relazione in Atti di Convegno]
Schramm, Alexander; Sorrentino, Silvio; DE FELICE, Alessandro
abstract

Nonlinear effects on the stability of the motorcycle rear ‘chatter’ phenomenon are investigated by means of a minimal two degrees of freedom model. Motorcycle ‘chatter’ consists of a self-excited oscillation which occurs during braking manoeuvres in the frequency range between 17 and 20 Hz. It causes loss of rider confidence and leads to decreased performance. The study of the linearised system gives indications on the initiation of self-excited vibrations and thus helps to prevent them using proper design techniques, while nonlinear analysis can lead to completely new insights. Post-bifurcation behaviour is analysed focusing on limit cycles, studied by means of an application of the harmonic balance method. This allows the detection of the validity of the linear results, and to identify the most influential parameters in limit cycle generation, their amplitude, and their stability.

2021 - A method for the experimental identification of equivalent viscoelastic models from vibration of thin plates [Articolo su rivista]
Grosso, Pasquale; DE FELICE, Alessandro; Sorrentino, Silvio
abstract

Aim of the present study is the identification of equivalent viscoelastic models for layered thin walled structures, obtained from vibration measurement only, able to fit the experimental data on a relatively wide frequency range by means of a minimum number of parameters. A novel approach is proposed, based on a definition of an equivalent modal damping ratio applied to the circle-fit technique, to overcome the difficulties related to the identification of modal parameters when adopting non-conventional viscoelastic models. When the structural internal dissipative effects are dominant, this procedure identifies the parameters of an equivalent Young’s modulus in the frequency domain, representing the viscoelastic properties of a homogenized structure as a scalar function with frequency-dependent real and imaginary parts. The proposed procedure is applied to the analysis of Aluminum plates coated by damping pads and of plates made by Quiet Aluminum. To fit the experimentally found equivalent modal damping ratios, several viscoelastic models are adopted and compared (viscous, hysteretic, generalized Maxwell, fractional derivative damping, and in particular the Fractional Kelvin-Voigt model), assessing the accuracy of the identified parameters by comparison of numerically simulated with experimentally measured frequency response functions

2021 - Damping and gyroscopic effects on the stability of parametrically excited continuous rotor systems. [Articolo su rivista]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

This study is aimed at analysing damping and gyroscopic effects on the stability of parametrically excited continuous rotor systems, taking into account both external (non-rotating) and internal (rotating) damping distributions. As case-study giving rise to a set of coupled differential Mathieu-Hill equations with both damping and gyroscopic terms, a balanced shaft is considered, modelled as a spinning Timoshenko beam loaded by oscillating axial end thrust and twisting moment, with the possibility of carrying additional inertial elements like discs or flywheels. After discretization of the equations of motion into a set of coupled ordinary differential Mathieu-Hill equations, stability is studied via eigenproblem formulation, obtained by applying the harmonic balance method. The occurrence of simple and combination parametric resonances is analysed introducing the notion of characteristic circle on the complex plane and deriving analytical expressions for critical solutions, including combination parametric resonances, valid for a large class of rotors. A numerical algorithm is then developed for computing global stability thresholds in presence of both damping and gyroscopic terms, also valid when closed-form expressions of critical solutions do not exist. The influence on stability of damping distributions and gyroscopic actions is then analysed with respect to frequency and amplitude of the external loads on stability charts in the form of Ince-Strutt diagrams.

2021 - Experimental identification of equivalent viscoelastic models for vibrating plates. [Relazione in Atti di Convegno]
Grosso, Pasquale; De Felice, Alessandro; Sorrentino, Silvio
abstract

Aim of the present study is the identification of equivalent viscoelastic models for layered plates, obtained from vibration measurements only, able to fit the experimental data on relatively broad frequency ranges by means of a minimum number of parameters. A novel approach is proposed, based on a definition of an equivalent modal damping ratio applied to the circle-fit technique, to overcome the difficulties related to the identification of modal parameters when adopting non-conventional viscoelastic models. If the structural internal dissipative effects are dominant, this procedure identifies the parameters of an equivalent Young’s modulus in the frequency domain, representing the viscoelastic properties of a homogenized structure as a scalar function with frequency-dependent real and imaginary parts. The proposed procedure is applied to the analysis of Aluminum plates coated by damping pads and of plates made by Quiet Aluminum. To fit the experimentally found equivalent modal damping ratios, the linear Fractional Kelvin-Voigt viscoelastic model is adopted, assessing the accuracy of the identified parameters by comparison of numerically simulated with experimentally measured frequency response functions.

2021 - Nonlinear investigation into the motorcycle chatter phenomenon using numerical simulation. [Relazione in Atti di Convegno]
Schramm, Alexander; Sorrentino, Silvio; DE FELICE, Alessandro
abstract

A minimal, two degrees of freedom model is used to investigate the nonlinear effects on the stability of the motorcycle rear ‘chatter’ phenomenon, which occurs during braking manoeuvres and has a frequency of approximately 20 Hz. Numerical simulations using the full non-linear equations of motion are performed at stable, neutral, and unstable equilibria, and with different initial conditions to investigate the nonlinear effects when compared to the linearised response. Attention is focused on the existence, stability, and amplitude of limit cycles in order to understand which are the most influential nonlinear terms and parameters, and in which conditions the results of linear analysis are still predictable.

2021 - Patter instability of racing motorcycles in straight braking manoeuvre [Articolo su rivista]
Cattabriga, Stefano; De Felice, Alessandro; Sorrentino, Silvio
abstract

The front wheel of a motorcycle during heavy braking in straight motion can lock before reaching the maximum longitudinal braking force, a problem affecting especially performances of road racing motorcycles. This is preceded by a self–excited vibration, referred to as front wheel patter, in the frequency range between 7 and 10 Hz. Aim of the present study is the identification of the actual switching mechanism to instability together with its governing parameters. A minimal model of the front assembly of a motorcycle is proposed, and its stability in equilibrium configurations is studied via eigenvalue analysis. The sensitivity with respect to all its governing parameters is analyzed by means of stability maps and the self–excitation mechanism is explained with the aid of energy balance analysis and phase–diagrams. The results are compared to those existing in the literature and to numerical simulations obtained from a full motorcycle multibody model.

2021 - Stability analysis of articulated bus in straight-ahead running manoeuvre [Articolo su rivista]
DE FELICE, Alessandro; Mercantini, Matteo; Sorrentino, Silvio
abstract

A comprehensive study on the stability of a planar linearized single-track model of a two-section pusher articulated bus is presented with the aid of a complete set of stability maps. The two sections of the vehicle model are connected at the hitch point by a revolute joint; an equivalent visco-elastic characteristic function describes its rotational visco-elastic properties, playing a major role in stability control and therefore in passive safety. The equations of motion are derived in analytical form, allowing easy implementation of the non-linear model (eventually including a non-linear viscoelastic characteristic functions of the joint). Stability of the linearized model is then studied in equilibrium configurations by means of sensitivity analysis with respect to the model’s governing parameters. Stability maps are drawn on the basis of sets of parameter values related to straight-ahead running, steady-state manoeuvres. The most important parameters controlling the onset of unstable motions are identified, paying attention to the role played by the equivalent rotational damping coefficient and the equivalent torsional stiffness characterizing the connection joint, with the aim of finding criteria for its design.

2020 - A new application of the Extended Kalman filter to the estimation of roll angles of a motorcycle with Inertial Measurement Unit [Articolo su rivista]
Romualdi, Lorenzo; Mancinelli, Nicolò; DE FELICE, Alessandro; Sorrentino, Silvio
abstract

The capability of providing a real–time reliable measure of the actual roll angle is of great importance for the racing motorcycle dynamics. This study presents a method based on an Extended Kalman Filter (EKF) for the estimation of the actual roll angle of a motorcycle equipped with an Inertial Measurement Unit (IMU), with the advantage of not needing the development and implementation of a complete motorcycle model. Measured data which, depending on the additional instrumentation available on the motorcycle, do not form a complete set of input data for the estimation algorithm, induce the introduction of approximations affecting the accuracy of the results. A thorough error analysis is carried out by means of numerical simulations along with experimental validations. As a result, a novel approximate form for the kinematical model of the IMU is developed, yielding an overall good accuracy in the roll angle estimates.

2020 - Dynamic characterization of polymeric vibration dampers [Relazione in Atti di Convegno]
Barbieri, Marco; DE FELICE, Alessandro; Pellicano, Francesco; Sorrentino, Silvio; Zippo, Antonio
abstract

Polymeric dampers are used in many fields for vibration isolation. These devices are based upon a low stiffness and high damping connection between a vibrating support and the item to be fastened. Whenever the connection is assured by a polymeric element, it is mandatory to be aware that stiffness and damping change with the excitation frequency. The characterization of stiffness and damping of the polymeric element in terms of storage modulus and loss modulus is commonly carried out at low frequency by means of a Dynamic Mechanical Analysis (DMA), nonetheless this approach cannot be applied at higher frequency. In the present study, a novel experimental approach for estimating the frequency dependent storage modulus and loss modulus in a polymeric vibration damper is presented. The proposed method is based on a direct measurement of the energy loss in hysteretic cycles and it is suitable for simple implementation using common instruments for vibration measurement.

2020 - Experimental identification of viscoelastic properties of vibrating plates [Relazione in Atti di Convegno]
Grosso, Pasquale; DE FELICE, Alessandro; Sorrentino, Silvio
abstract

Aim of the present study is the identification of equivalent viscoelastic models for layered thin walled structures, obtained from vibration measurement only, able to fit the experimental data on a wide frequency range by means of a minimum number of parameters. An identification method of general validity is proposed, based on the concept of equivalent modal damping ratio and on the classical circle-fit technique. It is applied to the analysis of both Aluminum plates coated by damping pads and plates made by Quiet Aluminum. To fit the experimentally found equivalent modal damping ratios, the linear Fractional Kelvin viscoelastic model is adopted, assessing the accuracy of the identified parameters by comparison of numerically simulated with experimentally measured frequency response functions.

2020 - Front wheel patter instability of motorcycles in straight braking manoeuvre [Relazione in Atti di Convegno]
Cattabriga, Stefano; DE FELICE, Alessandro; Segatori, Davide; Sorrentino, Silvio
abstract

The front wheel of a motorcycle during heavy braking in straight motion can lock before reaching the maximum braking force. Before front locking, a self-excited vibration occurs, referred to as front wheel patter, in the frequency range between 7 and 10 Hz. In the present study the actual switching mechanism to instability is identified, together with its governing parameters. A minimal model of the front assembly of a motorcycle is defined, and its stability in equilibrium configurations is studied via eigenvalue analysis. The sensitivity with respect to all its governing parameters is analyzed by means of stability maps and the self-excitation mechanism is explained with the aid of energy balance analysis and phase-diagrams. The results are compared to those existing in the literature and to numerical simulations obtained from a full motorcycle multibody model.

2020 - Stability analysis of parametrically excited gyroscopic systems [Relazione in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

This study is aimed at analyzing and clarifying gyroscopic effects on the stability of parametrically excited rotor systems, a topic which in the literature is not sufficiently investigated. As case–study giving rise to a set of coupled differential Mathieu–Hill equations with both gyroscopic and damping terms, a balanced shaft is considered, modelled as a spinning Timoshenko beam loaded by oscillating axial end thrust and twisting moment, with possibility of carrying additional inertial elements. After discretization of the equations of motion into a set of coupled ordinary differential Mathieu-Hill equations, stability of Floquet solutions is studied via eigenproblem formulation, obtained by applying the harmonic balance method. A numerical algorithm is then developed for computing global stability thresholds in presence of both gyroscopic and damping terms, aimed at reducing the computational load. Finally, the influence on stability of the main characteristic parameters of the shaft is analyzed with respect to frequency and amplitude of the external loads on stability charts in the form of Ince-Strutt diagrams. As a result, it has been demonstrated that gyroscopic terms produce substantial differences in both critical solutions and stability thresholds: the former are generally non-periodic limited-amplitude functions, and modifications induced on stability thresholds consist of shifts and merging of unstable regions, depending on the separation of natural frequencies into pairs of forward and backward values.

2019 - First assessment on suspension parameter optimization for a solar–powered vehicle [Relazione in Atti di Convegno]
Sorrentino, Silvio; DE FELICE, Alessandro; Grosso, Pasquale; Minak, Giangiacomo
abstract

Optimization of suspension parameters with respect to comfort and road holding is a challenging issue for solar–powered cars, due to in–wheel electric engines on very light vehicles, carrying payloads which can exceed their total mass. The solar–powered car considered in this study was designed and manufactured for racing by the University of Bologna; with a mass of 300 kg and a payload of 320 kg due to four occupants, using 5 m2 of monocrystalline silicon photovoltaic panel on the roof, 64 kg of lithium–ion batteries and two electric engines coupled directly to the rear wheels, it can achieve either a range of 600 km at cruising speed, or velocitiy peaks of 120 km/h. In this cantribution, equivalent vertical stiffness and equivalent damping coefficients are optimized for both axles, achieving results that in terms of comfort and road holding are comparable to those of standard passenger cars.

2019 - In–plane vibration analysis of plates with periodic skeletal truss micro–structures [Articolo su rivista]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

The present contribution analyzes the dynamic in–plane behaviour of two–dimen- sional finite lattices consisting of periodic micro–skeletal truss structures made either by bar or beam elements. An homogenization asymptotic technique, employing the Fourier transform of the dynamic equilibrium equations of the discretized medium, is applied to derive the partial differential equations describing the in–plane vibration of an equivalent homogeneous plate. The resulting set of partial differential equations is then analyzed for deriving explicit relationships between reference–cell properties and homogenized continuum properties, i.e. parent material properties. Specific selection criteria are found for geometric and material parameters of the reference–cell in view of getting orthotropic as well as isotropic homogenized plates. Finally, the effects of micro–structural properties on the vibrating behaviour of the homogenized plate are investigated in terms of modal analysis, studying their influence on natural frequencies.

2019 - On the dynamic behaviour of rotating shafts under combined axial and torsional loads [Articolo su rivista]
De Felice, Alessandro; Sorrentino, Silvio
abstract

A comprehensive dynamic study on a distributed parameter model of a straight uniform rotating shaft is developed, aimed at presenting some clarifications and corrections to published results, together with novel contributions. The model includes the effects of transverse shear, rotatory inertia and gyroscopic moments with additional combined end thrust and twisting moment. The equations of motion are derived in both Newtonian and Lagrangian formulations according to the Timoshenko beam theory. A novel contribution is given in the development of complete modal analysis of the model under study, highlighting the properties of the operators involved and the relations among eigenfunctions represented in complex and real variables. The influence of the main governing parameters (slenderness ratio, angular velocity, external axial end thrust and twisting moment) is studied on natural frequencies, modal shapes and critical speeds of the rotor. New evidence of existence of a second frequency spectrum in the Timoshenko beam theory is presented, together with a novel definition for its identification, only possible if considering gyroscopic effects.

2019 - On the stability of parametrically excited continuous gyroscopic systems [Relazione in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

2019 - Passenger car steering pull and drift reduction considering suspension tolerances [Relazione in Atti di Convegno]
De Rosa, Mariano; DE FELICE, Alessandro; Fragassa, Cristiano; Sorrentino, Silvio
abstract

Pull to side, Steering wheel misalignment and Drift leeward are handling anomalies affecting passenger cars travelling on straight paths. They represent a cost factor in the automotive industry, occurring in small but not negligible percentage of the overall car production. The present contribution is aimed at understanding which are their causes in terms of suspension tolerances, and at reducing their extent, in the specific case of vehicles with front Double wishbone suspension and rear Five arms suspension. The most influential parameters on handling irregularities have been identified through a design of experiment analysis. Then, after performing a sensitivity analysis by multibody virtual modelling, a correlation with experimental data from presetting and wheel aligner benches has provided sufficient information for setting tolerance thresholds, able to keep the handling anomalies under study within acceptable bounds. Pull to side and Steering wheel misalignment have been found to be mainly related to the set up phase of wheel angles (front camber angles influencing Pull to side, front and rear toe angles influencing Steering wheel misalignment), while the main cause of Drift leeward has been identified with (rear) ride steer.

2019 - Straight path handling anomalies of passenger cars induced by suspension component and assembly tolerances [Articolo su rivista]
De Rosa, Mariano; DE FELICE, Alessandro; Grosso, Pasquale; Sorrentino, Silvio
abstract

The effects on handling due to suspension component and assembly tolerances are analysed focusing on three anomalies affecting passenger cars travelling on straight paths, namely ‘pull to side’, ‘steering wheel misalignment’ and ‘drift leeward’. These anomalies do not constitute safety problems but represent a cost factor in the automotive industry since they occur in small but not negligible percentage of the overall car production. The present contribution is therefore focused on understanding which are their causes, and on reducing their extent, in the specific case of vehicles with front Double wishbone suspension and rear Five arms suspension. To this purpose, the vehicle assembly process is analysed in terms of errors and tolerances for identify the most influential parameters on handling irregularities. Sensitivity analysis by multibody virtual modelling, and subsequent correlation with experimental data from pre-setting and wheel aligner benches, have provide sufficient information for setting tolerance thresholds, able to keep the handling anomalies under study within acceptable bounds. As a result, ‘pull to side’ and ‘steering wheel misalignment’ has been found to be mainly related to the set up phase of wheel angles (front camber angles influencing pull to side, front and rear toe angles influencing steering wheel misalignment), while the main cause of ‘drift leeward’ has been identified with (rear) ride steer. Application of the procedure to a production line led to a consistent reduction (from 3% down to about 1% of the overall production) of anomalous vehicles.

2018 - Experimental identification of static and dynamic stiffness of polymeric vibration dampers [Relazione in Atti di Convegno]
Barbieri, Marco; DE FELICE, Alessandro; Pellicano, Francesco; Sorrentino, Silvio; Zippo, Antonio
abstract

Polymeric dampers are frequently used in the automotive field in order to filter engine produced vibrations and thus protecting electronic devices. An accurate knowledge of damper stiffness is therefore important for a proper design of such mechanical filters. Since viscoelastic materials exhibit a significant stiffness–frequency dependency, characterization of dynamic stiffness is required for automotive vibration dampers. In the present paper, a method for broadband identification of the dynamic complex stiffness is described; since the proposed method is reliable at medium and high frequency only (500–1500 Hz), an additional static measurement has been performed. Therefore, the resulting method is a combination of static and dynamic measurements and it does not require specific DMA equipment: all experiments have been carried out using typical vibration testing equipment.

2018 - Stability analysis of rotating shafts under axial and torsional periodic loads [Relazione in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

A contribution is given to clarify gyroscopic effects on stability of parametrically excited rotor systems, highlighting the role played by stabilizing damping distributions. As case–study of general interest, giving rise to a set of coupled differential Mathieu–Hill equations with both gyroscopic and damping terms, a continuous perfectly balanced shaft is considered, modelled as a spinning Timoshenko beam loaded by oscillating axial end thrust and twisting moment. After discretization of the equations of motion into a set of coupled ordinary differential Mathieu–Hill equations, stability of Floquet solutions is studied via eigenproblem formulation, obtained by applying the harmonic balance method. A numerical algorithm is then developed for computing global stability thresholds in presence of both gyroscopic and damping terms, aimed at reducing the computational load. Finally, the influence on stability of the main characteristic parameters of the shaft is analyzed with respect to frequency and amplitude of the external loads on stability charts in the form of Ince–Strutt diagrams.

2018 - The second spectrum in Timoshenko beam theory: a new approach for its identification [Relazione in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

The problem of existence and identification of a second frequency spectrum in the Timoshenko beam theory is reconsidered from a novel perspective, studying a Timoshenko beam rotating with constant angular speed about its longitudinal axis. The existence of a second spectrum in the case of non–rotating beams and general boundary conditions has been much debated in the literature, since it is possible to easily identify the companion natural frequencies constituting the second spectrum only in particular cases. Its existence in a non–rotating finite–length beam has been recently demonstrated on the basis of accurate experimental results, at least for free–free boundary conditions, and also by considering free waves in beams of infinite length. In this study new evidence of existence of a second spectrum together with a novel definition for its identification are presented, possible when considering gyroscopic effects. As a secondary result, it can be stated that the whole second spectrum gives no contribution to the forward critical speeds of the rotating beam.

2017 - A novel method for dynamic characterization of polymeric vibration dampers [Relazione in Atti di Convegno]
Barbieri, Marco; DE FELICE, Alessandro; Pellicano, Francesco; Sorrentino, Silvio; Zippo, Antonio
abstract

Polymeric vibration dampers are very important devices used in many fields for vibration isolation and damping. These devices are based upon a low stiffness and high damping connection between a vibrating support and the item to be fastened. Whenever the connection is assured by a polymeric element, it is mandatory to be aware that stiffness and damping change with the excitation frequency as well as with temperature. Characterization of stiffness and damping of the polymeric element in terms of storage modulus and loss modulus is commonly carried out at low frequency by means of a Dynamic Mechanical Analysis (DMA), nonetheless this approach cannot be applied at higher frequency. In the present paper, a novel experimental approach for estimating the frequency dependent storage modulus and loss modulus in a polymeric vibration damper is presented. The proposed method is based on a direct measurement of the energy loss in hysteretic cycles, and it is suitable for simple implementation using common instruments for vibration measurement.

2017 - Insights into the gyroscopic behaviour of axially and torsionally loaded rotating shafts [Relazione in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

A distributed parameter model of a straight uniform shaft rotating at constant angular velocity is analytically investigated including the effects of transverse shear, rotatory inertia, gyroscopic moments and considering the additional contribution of combined end thrust and twisting moment. The equations of motion are derived by applying Hamilton’s principle according to the Timoshenko beam theory, and cast in dimensionless form to highlight the influence of the main governing parameters (slenderness ratio, angular velocity, applied external end thrust and twisting moment) on natural frequencies and critical speeds of the rotor. The results of this study constitute the basis for further developments, including comparison with finite element models and rotor stability analysis under combined loads.

2017 - In–plane vibration analysis of plates with periodic skeletal truss micro–structures [Relazione in Atti di Convegno]
DE FELICE, Alessandro; Sorrentino, Silvio
abstract

The present contribution focuses on the dynamic in–plane behaviour of two–dimen- sional lattices consisting of periodic micro–skeletal truss structures. An established homogenization asymptotic technique is applied to derive the partial differential equations describing the in–plane vibration of an equivalent homogeneous plate. A discretized formulation is adopted for studying a planar reticulated medium, whose periodic pattern in this specific case is described by a rectangular reference–cell composed by truss elements. The size of the reference–cell is assumed to be much smaller than the length–scale of the whole structure. The adopted homogenization technique employs the Fourier transform of the dynamic equilibrium equations in both the time and space domains, leading to the definition of the so–called symbol of the periodic structure, which embodies all its elastodynamic properties. Upon Taylor series expansions and inverse transformation, the asymptotic value of the symbol yields continuous dynamic equilibrium equations, whose solution describes the system’s response as the reference–cell characteristic dimension tend to zero. The resulting set of partial differential equations is then analyzed for deriving explicit relationships between reference–cell properties and homogenized continuum properties, i.e. parent material properties. Specific selection criteria are found for geometric and material parameters of the reference–cell in view of getting orthotropic as well as isotropic homogenized plates. Finally, the effects of micro–structural properties on the vibrating behaviour of the homogenized plate are investigated in terms of modal analysis, studying their influence on natural frequencies.