[Elenco soci]

Neri Paolo

Ricercatore TD(B)

Università degli Studi di Pisa

Sito istituzionale
SCOPUS ID: 56317034900
Orcid: 0000-0003-0730-0893

Pubblicazioni scientifiche

[1] Tamburrino F., Neri P., Paoli A., Razionale A.V., Effects of Coating Post-processing on Mechanical Properties of Strut-and-Node-Based SLA Lattice Structures, Lecture Notes in Mechanical Engineering, 937-945, (2023). Abstract

Abstract: Lattice structures play an increasingly crucial role in Additive Manufacturing (AM) to enhance the performance of parts for industrial and biomedical applications. Among AM technologies, VAT photopolymerization is one of the most suitable in producing shapes characterized by a good resolution and fine details as required for lattice structures. High stiffness and strength photoresins are commonly adopted when strut-and-node lattice structures, based on stretch-dominated unit cells, are printed. However, this choice can lead to brittle and sudden structural failures, undermining the use of these structures due to safety reasons. This work evaluates the effect of chemical post-processing on the deformation behavior and the tensile properties of SLA strut-and-node-based lattice structures. FCC (Face-Centered Cubic) lattice structures with two different layer heights were tested, and a highly deformable UV resin was used as a coating product. Results evidenced an increase in specimen elongation up to 64% for coated FCC lattice structures with respect to as printed samples. Chemical post-processing based on resin coating demonstrated to be an effective solution to get additively manufactured strut-and-node-based lattice structures characterized both by high strength and high strain.

Keywords: Coating post-processing | Lattice structures | Tensile properties | VAT photopolymerization

[2] Neri P., Aruanno B., Barone S., Paoli A., Razionale A.V., Semi-automatic Point Clouds Registration for Upper Limb Anatomy, Lecture Notes in Mechanical Engineering, 733-742, (2023). Abstract

Abstract: In this paper, a semi-automatic procedure to perform point clouds registration is presented. The method was developed for upper limb 3D scanning. During the acquisition, several frames are acquired from different points of view, to obtain a full 360° acquisition of the arm. Each frame stores both the point clouds coordinates and the corresponding RGB image. During post-processing, the RGB image is elaborated through a neural network, to detect relevant key points of the hand, which are then projected to the point clouds. The corresponding key points detected from different acquisitions are then used to automatically obtain a rough 3D rotation that aligns the point clouds corresponding to different perspectives in a common reference frame. Finally, the registration is refined through an iterative closest point algorithm. The method was tested on actual arm acquisitions, and the registration results are compared with the conventional fully manual 3-2-1 registration procedure, showing promising results of the proposed method.

Keywords: Neural network | Semi-automatic registration | Upper limb 3D scan

[3] Santus C., Romanelli L., Grossi T., Neri P., Romoli L., Lutey A.H.A., Pedranz M., Benedetti M., Torsional-loaded notched specimen fatigue strength prediction based on mode I and mode III critical distances and fracture surface investigations with a 3D optical profilometer, International Journal of Fatigue, 161, (2022). Abstract

Abstract: Torsional and axial fatigue tests were performed on steel 42CrMo4+QT and aluminium alloy 7075-T6, and a 3D optical profilometer was used for the investigations of the fracture surfaces. The mode I and mode III critical distances of the two materials were regarded as reference scales for evaluating the initial orientation of the cracks and then identifying the tests valid for the calibration of the normal and those for the shear-fatigue models. The torsional strength of blunt notched specimens was then predicted with the Smith-Watson-Topper and the Fatemi-Socie criteria, combined with the critical distances and the Chaboche model for the steel.

Keywords: 3D optical profile | Chaboche kinematic hardening model | Fatemi-Socie model | Smith-Watson-Topper model | Theory of critical distances

[4] Neri P., Frequency-band down-sampled stereo-DIC: Beyond the limitation of single frequency excitation, Mechanical Systems and Signal Processing, 172, (2022). Abstract

Abstract: A low-speed stereo-camera DIC setup was used in this paper to measure the down-sampled bandpass vibration signal of a plate in a given frequency range, which is much higher than the available frame rate. Down-sampled vibration measurements are well known in the literature. However, they are always subject to the strong hypothesis of having a single frequency component in the excitation source (e.g., pure sinusoidal excitation). In this scenario, several approaches can be found in the literature to reconstruct the actual response from the down-sampled data. In this paper, a data postprocessing algorithm is newly introduced to properly reconstruct the target response in the case of a frequency band excitation, thus relaxing the single frequency excitation constraint and allowing to explore a given frequency range with a single measurement. Additionally, a custom excitation signal is presented in this paper to achieve a constant intensity in the studied frequency range. The proposed approach is presented and experimentally validated by measuring a cantilever plate vibration in the kHz range using a 100 fps acquisition. A single acquisition of 98 frames allowed to describe the deformed shapes at 49 different frequency values in the range 1110–1160 Hz, highlighting two resonance peaks. The comparison with the results of multiple conventional single-frequency tests and LDV measurements confirmed the effectiveness of the proposed approach.

Keywords: Digital image correlation | Down-sampled bandpass measurement | Frequency band excitation | Low-speed camera | Vibration measurement

[5] Neri P., Barone S., Paoli A., Razionale A.V., Tamburrino F., A Depth-Camera Based System for the Real-Time Scanning of Upper Limb Anatomy, Lecture Notes in Mechanical Engineering, 245-255, (2022). Abstract

Abstract: The 3D reconstruction of upper limb anatomy plays a significant role in many biomedical fields as ergonomics, motion rehabilitation, prosthesis design. Conventional manual measurements have been progressively replaced by 3D optical scanning in collecting and storing 3D anatomical data, thus increasing reliability and data accuracy, shortening, at the same time, the overall acquisition process. However, the real-time scanning of human body parts still represents a complex task since it is challenging to keep the arm in a stable position and avoid artifacts in the collected data. Also, optical undercut geometries often impair the 3D reconstruction’s completeness. In this paper, a compact and low-cost 3D scanning system has been developed by integrating three D415 Intel RealSense cameras. The three depth cameras have been assembled in a circular rig to define a lightweight handheld scanner capable of carrying out 3D data acquisition in different scenarios. The optical system has been validated through anthropometric measurements on different subjects.

Keywords: 3D scanning | D415 Intel realSense | Depth-camera | Upper limb anatomy

[6] Barone S., Neri P., Paoli A., Razionale A.V., Tamburrino F., Effects of Coating Post-processing on the Compressive Properties of Strut-and-Node-Based FDM Lattice Structures, Lecture Notes in Mechanical Engineering, 442-450, (2022). Abstract

Abstract: Lattice structures have many outstanding properties, and their use in diversified industrial and biomedical fields is widely studied. The advent of additive manufacturing (AM) technologies has further pushed the design of these cellular structures allowing for the fabrication of complex trusses and tailored local geometries. However, geometrical defects introduced by the AM process into printed lattice structures significantly affect their mechanical properties. In this work, the effect of chemical post-processing on the compressive properties of FDM-PLA strut-and-node-based lattice structures is evaluated. A UV resin has been used as a coating film on samples fabricated using Simple Cubic (SC) and Face-Centered Cubic (FCC) unit cells. Results demonstrated a 65% increase in compressive strength for SC unit cells and a 12% increase for FCC unit cells with respect to as-printed samples. Resin coating demonstrated to represent an effective approach to minimize defects of strut-and-node-based lattice structures, thus enhancing mechanical properties.

Keywords: Additive manufacturing | Coating post-process | Compressive properties | Lattice structures

[7] Urquhart L.W.R., Tamburrino F., Wodehouse A., Neri P., Razionale A.V., Examination of Additively Manufactured Auxetic Components Using a Novel Testing Setup, Lecture Notes in Mechanical Engineering, 451-461, (2022). Abstract

Abstract: This paper reports upon the results of an initial test cycle using a bespoke testing rig designed expressly to examine additively manufactured auxetic components. Firstly, the key problems facing practical researchers in the field of auxetics is explored with the treatment of the boundary condition identified as a key issue. The testing setup that is then introduced utilises a novel method of part mounting and facilitates optical analysis and real-time force measurements. The study analyses three different auxetic structures (re-entrant, chiral, and semi-rigid), a set of samples of which were additively manufactured in TPU material. A range of parameters were varied across the three designs including interior geometry and wall thicknesses in order to demonstrate the effectiveness of the setup for the examination of different structures. Several key results were distilled from the tests that were then further analysed through numerical modelling and discussed with respect to future testing. Our investigation shows a close alignment between the physical testing results and the simulations, indicating that the testing configuration is rigorous and may be used to explore the mechanical behaviour of more complex auxetic componentry.

Keywords: Additive manufacturing | Auxetics | Mechanical analysis | Simulation

[8] Neri P., Paoli A., Razionale A.V., Santus C., Low-speed cameras system for 3D-DIC vibration measurements in the kHz range, Mechanical Systems and Signal Processing, 162, (2022). Abstract

Abstract: The digital image correlation (DIC) was used in this paper to obtain full-field measurements of a target vibrating at a frequency higher than the maximum cameras’ frame rate. The down-sampling technique was implemented to compensate for the cameras’ moderate frame rate, thus getting an accurate displacement acquisition even at 6.5 kHz. Two innovative methods to support the DIC application were introduced. The use of fringe projection (or structured light), initially applied on the sample at rest, reduced the effort and time required for the stereo matching task's solution and improved this setting's accuracy and reliability. Additionally, a new time-domain image filtering was proposed and tested to improve the quality of the obtained DIC maps. In combination with the down-sampling, the effect of this filtering technique was tested in this work at (approx.) 2500 and 6500 Hz by measuring the response of a bladed disk to sinusoidal excitation. Evidence of improved results was observed for both frequencies for amplitudes in the range of 10 µm.

Keywords: Bladed disk | Digital image correlation | Down-sampling approach | Low-speed camera | Reverse engineering | Vibration measurement

[9] Neri P., Paoli A., Razionale A.V., Santus C., Time-domain image filtering for DIC vibration measurements, Journal of Physics: Conference Series, 2041(1), (2021). Abstract

Abstract: A time-domain filter is presented in this paper for the noise reduction of the camera images, then elaborated for the full-field 3D-DIC vibration analysis of mechanical components. The basic idea behind this filtering is to initially decompose the light signal of each pixel of the raw images, both for the left and the right cameras, with the Fast Fourier Transform (FFT). The signal is then reconstructed with the Inverse FFT by keeping the zero-order harmonic and the first harmonic and just discarding all the others, which only introduce noise contributions. The down-sampling approach is also used in combination with this filtering to reach high loading frequencies, even without (expensive) high-speed cameras, provided that a quite short exposure time is available. A cantilever aluminum plate was tested at 591 Hz, and a similar experiment was repeated on a bladed disk (or blisk) excited at high frequency, 6458 Hz, obtaining clear and smooth displacement maps even in the range of 10 microns. In the blisk application, a single blade with a significantly larger displacement amplitude was clearly observed, which could be interpreted as mistuning evidence.

[10] Neri P., Paoli A., Santus C., Stereo-dic measurements of a vibrating bladed disk: In-depth analysis of full-field deformed shapes, Applied Sciences (Switzerland), 11(12), (2021). Abstract

Abstract: Vibration measurements of turbomachinery components are of utmost importance to char-acterize the dynamic behavior of rotating machines, thus preventing undesired operating conditions. Local techniques such as strain gauges or laser Doppler vibrometers are usually adopted to collect vibration data. However, these approaches provide single-point and generally 1D measurements. The present work proposes an optical technique, which uses two low-speed cameras, a multimedia projector, and three-dimensional digital image correlation (3D-DIC) to provide full-field measurements of a bladed disk undergoing harmonic response analysis (i.e., pure sinusoidal excitation) in the kHz range. The proposed approach exploits a downsampling strategy to overcome the limitations introduced by low-speed cameras. The developed experimental setup was used to measure the response of a bladed disk subjected to an excitation frequency above 6 kHz, providing a deep insight in the deformed shapes, in terms of amplitude and phase distributions, which could not be feasible with single-point sensors. Results demonstrated the system’s effectiveness in measuring amplitudes of few microns, also evidencing blade mistuning effects. A deeper insight into the deformed shape analysis was provided by considering the phase maps on the entire blisk geometry, and phase variation lines were observed on the blades for high excitation frequency.

Keywords: Bladed disk vibration | Digital image correlation | Downsampling | Low-speed cameras | Mistuning

[11] Santus C., Neri P., Romoli L., Lutey A., Raghavendra S., Benedetti M., Fatigue fracture surface investigations with a 3D optical profiler, Procedia Structural Integrity, 39(C), 450-459, (2021). Abstract

Abstract: In this paper a set of specimens, used for the critical distance determination, are investigated with a non-contact 3D optical profiler. The fatigue fracture surfaces of both plain and V-notched specimens, under axial (mode I) and torsional (mode III) loadings are observed, investigating steel 42CrMo4+QT and aluminium alloy 7075-T6. The fatigue fracture profiles are compared to be previously obtained critical distances, both for mode I and mode III. The stage I to stage II transition was found at a smaller size than the axial critical distance, for the steel, while for the torsional load a local plateau at the nucleation was observed. The fracture surface of the axial loading was instead much irregular at the scale of the mode I critical distance, for the aluminium alloy, resembling a not concluded stage I, while again a relatively flat surface was observed for the mode III loading.

Keywords: 3D optical profile | Fatigue fracture surface | mode I | mode III loadings | Theory of critical distances

[12] Macoretta G., Monelli B.D., Neri P., Bucciarelli F., Checcacci D., Giusti E., Full-scale vibration testing of nozzle guide vanes, Proceedings of the ASME Turbo Expo, 9B-2021, (2021). Abstract

Abstract: An increasing number of turboexpanders are equipped with Nozzle Guide Vane (NGV) as the first stator stage. By varying the throat area of the first stator vane the NGV enables an additional control methodology to the line-up power output allowing higher operational flexibility and higher efficiency at partial load and partial speed. The design of this component might become critical for enabling high expander availability considering its exposure to high temperature, thermal loading, and fluid induced vibrations. This is especially true also considering that the vibration frequencies of this sub-assembly are influenced by internal clearances and by the value of the friction coefficient, which leaves a relevant margin of error when using numerical methods (such as FEM) for predicting the actual structural behavior of this component. In this paper, the design of a full-scale test bench for the determination of both friction coefficients and modal behavior of a nozzle guide vane geometry is described. The bench enables us to simulate the pre-load due to aerodynamic forces on the NGV airfoil simulating the actual working conditions of bushes and bearings.

Keywords: Experimental Modal Analysis | Finite Element Method | Friction | Nozzle Guide Vane

[13] Barnaba A., Neri P., Bucchi F., Passarelli D., Parise M., Vibration of a cryomodule’s coupler during road transportation: Multibody analysis and experimental validation, JVC/Journal of Vibration and Control, (2021). Abstract

Abstract: The road transportation of critical components for cryogenic cryomodules is a challenging task. Indeed, the shipping and the handling of fundamental sub-assemblies can expose these components to harmful dynamic loads that can jeopardize their structural integrity. This article has the purpose of developing a finite element multibody model capable of assessing the acceleration on a test coupler for a cavity of the Single Spoke Resonator 1 subjected to a typical road transportation trip. A multibody model consisting of rigid bodies and finite element–imported flexible bodies has been created allowing a comparison of the simulated and experimental acceleration. Afterwards, an experimental on-road test, whose set-up is described in this article, has been carried out at Fermilab with instrumented components. Being able to effectively simulate and validate the dynamic effects on such sub-assemblies makes it possible, for the future, to simulate more complex structures subject to dynamic loadings, such as the entire cryomodules during shipping. Furthermore, the results from the simulations can be used to guide the design of new suspension systems suitable for the reduction mitigation of structural vibration during the road transportation of cryomodules.

Keywords: couplers | experimental data analysis | finite element analysis modelling | reduced order methods | road transportation | vibration measurement

[14] Bisogni L., Mollaiyan R., Pettinari M., Neri P., Gabiccini M., Automatic calibration of a two-axis rotary table for 3d scanning purposes, Sensors (Switzerland), 20(24), 1-21, (2020). Abstract

Abstract: Rotary tables are often used to speed up the acquisition time during the 3D scanning of complex geometries. In order to avoid manual registration of the point clouds acquired with different orientations, automatic algorithms to compensate the rotation were developed. Alternatively, a proper calibration of the rotary axis with respect to the camera system is needed. Several methods are available in the literature, but they only consider a single-axis calibration. In this paper, a method for the simultaneous calibration of both axes of the table is proposed. A checkerboard is attached to the table, and several images with different poses are acquired. An optimization algorithm is then setup to determine the orientation and the locations of the two axes. A metric to assess the calibration quality was also defined by computing the average mean reprojection error. This metric is used to investigate the optimal number and distribution of the calibration poses, demonstrating that the optimum calibration results are achieved when a wider dispersion of the calibration poses is adopted.

Keywords: 3D scanning | Optical system | Rotary table calibration

[15] Neri P., Bucchi F., Passarelli D., A multilevel finite element-multibody approach to design the suspension system for the road transportation of SSR1 cryomodule, Transportation Engineering, 2, (2020). Abstract

Abstract: Cryomodules (CM) represents edge-frontier assemblies in particle physics research field. The road transportation of CM is a critical phase during which the structures can be subjected to significant dynamic loads. It is therefore necessary to design a Transportation Tool (TT) equipped with an appropriate suspension system. This work describes the approach adopted for the design of the TT for the CM PIP-II SSR1 (Proton Improvement Plan-II – Single Spoke Resonators 1), which is firstly introduced in the CM research field. Initially a Finite Element (FE) model was developed, considering the main sub-assemblies of the CM. However, this model was not suitable for the design of the TT due to the high computational burden. For this reason the model was exported as a Modal Neutral File and imported into a MultiBody software (MB) where the remaining components were modeled as concentrated stiffnesses or rigid bodies. The MB model thus obtained has drastically reduced the calculation time, proving to be fundamental in the TT iterative design phase, which involves the use of Helical Isolators (HI) performing the function of mechanical filters. To validate the effectiveness of TT in reducing dynamic loads, a 3D acceleration profile measured during the transport of a similar cryomodule (Linear Coherent Light Source II, LCLS-II) was used. Furthermore, the results of the MB model were used to perform the structural verification of some critical components of the CM.

Keywords: Cryomodule transportation | Mechanical filter | Reduced order modeling | Suspension system | Vibration isolation

[16] Paoli A., Neri P., Razionale A.V., Tamburrino F., Barone S., Sensor architectures and technologies for upper limb 3d surface reconstruction: A review, Sensors (Switzerland), 20(22), 1-33, (2020). Abstract

Abstract: 3D digital models of the upper limb anatomy represent the starting point for the design process of bespoke devices, such as orthoses and prostheses, which can be modeled on the actual patient’s anatomy by using CAD (Computer Aided Design) tools. The ongoing research on optical scanning methodologies has allowed the development of technologies that allow the surface reconstruction of the upper limb anatomy through procedures characterized by minimum discomfort for the patient. However, the 3D optical scanning of upper limbs is a complex task that requires solving problematic aspects, such as the difficulty of keeping the hand in a stable position and the presence of artefacts due to involuntary movements. Scientific literature, indeed, investigated different approaches in this regard by either integrating commercial devices, to create customized sensor architectures, or by developing innovative 3D acquisition techniques. The present work is aimed at presenting an overview of the state of the art of optical technologies and sensor architectures for the surface acquisition of upper limb anatomies. The review analyzes the working principles at the basis of existing devices and proposes a categorization of the approaches based on handling, pre/post-processing effort, and potentialities in real-time scanning. An in-depth analysis of strengths and weaknesses of the approaches proposed by the research community is also provided to give valuable support in selecting the most appropriate solution for the specific application to be addressed.

Keywords: Body scanner | Depth cameras | Handheld scanner | Stationary scanner | Structured light scanning | Upper limb 3D scanning

[17] Barone S., Neri P., Paoli A., Razionale A.V., 3D acquisition and stereo-camera calibration by active devices: A unique structured light encoding framework, Optics and Lasers in Engineering, 127, (2020). Abstract

Abstract: The use of light encoding techniques is widespread in the field of 3D surface reconstruction. This paper presents a stereo-camera calibration methodology, which integrates structured light encoding with an active digital device. The structured light encoding approach is proposed to unambiguously solve the stereo matching issue for stereo-camera setups. A sequence of vertical and horizontal binary striped patterns, combined with a checkerboard pattern, is displayed by a high-resolution LCD screen, which is used as calibration board. A bundle adjustment technique is adopted to simultaneously adjust both camera parameters and screen geometry, as part of the stereo-camera calibration process, thus taking into account the possible inaccuracies of the digital display. The same structured light approach, with small variants, is projected by a multimedia digital projector to carry out 3D surface reconstruction. The proposed methodology defines a comprehensive framework for the development of a 3D optical scanner, from calibration to 3D acquisition, which has been validated by measuring primitive surfaces and reconstructing free-form shapes with different stereo-camera setups.

Keywords: Binary-code | LCD screen | Stereo-camera calibration | Stripe shifting | Structured light encoding

[18] Neri P., Fiaschi M., Menchini G., Semi-Automatic tool for vitiligo detection and analysis, Journal of Imaging, 6(3), (2020). Abstract

Abstract: Vitiligo vulgaris is an autoimmune disease which causes a strong reduction of the cells producing melanin, which is the main skin pigment. This results in the growth of white patches on patients' skin, which are more or less visible, depending on the skin phototype. Precise, objective and fast detection of vitiligo patches would be crucial to produce statistically relevant data on huge populations, thus giving an insight on the disease. However, few methods are available in literature. In the present paper, a semi-automatic tool based on image processing to detect facial vitiligo patches is described. The tool requires pictures to be captured under black light illumination, which enhances patches contrast with respect to healthy skin. The user is only required to roughly define the regions of interest and set a global threshold, thus, no specific image-processing skills are required. An adaptive algorithm then automatically discerns between vitiligo and healthy skin pixels. The tools also allow for a statistical data interpretation by overlapping the detected patches of all patients on a face template through an occurrence map. Preliminary results obtained on a small population of 15 patients allowed us to assess the tool's performance. Patch detection was checked by an experienced dermatologist, who confirmed the detection for all the studied patients, thus supporting the effectiveness of the proposed tool.

Keywords: Black light | Image processing | Semi-automatic vitiligo detection | Vitiligo

[19] Barone S., Neri P., Orsi S., Paoli A., Razionale A.V., Tamburrino F., Two coatings that enhance mechanical properties of fused filament-fabricated carbon-fiber reinforced composites, Additive Manufacturing, 32, (2020). Abstract

Abstract: Moisture absorption degrades the mechanical properties of polymeric parts that are 3D-printed by fused filament fabrication (FFF). This limitation is particularly significant for short fiber-reinforced polymers because the mechanical enhancement obtained by the fiber reinforcement can be compromised by the plasticizing effect introduced by water absorption. Therefore, the present work investigates the effects of two different coatings, a UV cured acrylate resin and an acrylic varnish, on the moisture absorption of FFF 3D-printed samples consisting of polyamide reinforced by short carbon fibers. Water content (CI) and open porosity (OP) were estimated through water absorption tests in distilled water for 2, 24, and 168 h, and after reconditioning. The coating effects were evaluated by conducting tensile tests to compare the Young's modulus, yield stress, and ultimate stress of the coated and uncoated specimens. The results demonstrated a significant reduction of CI and OP with both the acrylic and UV resin coatings, as well as considerable enhancements of these samples’ mechanical properties. Stress-strain curves evidenced a strain reduction after water immersion, which can be ascribed to a greater stability against different moisture conditions. These findings indicate the significant potential of the proposed coating processes to extend the use of FFF 3D-printed composite materials to a broader range of applications.

Keywords: Fused filament fabrication | Material extrusion | Mechanical properties enhancement | Moisture absorption | Short fiber-reinforced polymers

[20] Barone S., Neri P., Paoli A., Razionale A.V., Bertini L., Santus C., Optical Stereo-System for Full-Field High-Frequency 3D Vibration Measurements Based on Low-Frame-Rate Cameras, Lecture Notes in Mechanical Engineering, 155-164, (2020). Abstract

Abstract: Dynamic characterization of vibrating targets represents a critical issue for many industrial fields. In this paper, a stereo-camera system integrated with a Digital Image Correlation (DIC) algorithm is proposed with the aim at performing 3D full-field vibration measurements in the range of kHz. The system exploits two industrial low-speed cameras, and the Nyquist-Shannon frequency limitation is overcome by a down-sampling approach under the hypothesis that the vibration signal is characterized by a single known frequency component. Experimental results obtained from the measurement of vibrational responses of a cantilever plate excited at three high-frequency resonance values (1121 Hz, 2956 Hz and 4010 Hz) are provided. A comparison with numerical analyses evidences the effectiveness of the proposed approach.

Keywords: Digital Image Correlation | Low-frame-rate cameras | Stereo-camera setup | Vibration measurement

[21] Barone S., Neri P., Orsi S., Paoli A., Razionale A.V., Tamburrino F., Properties Enhancement of Carbon PA 3D-Printed Parts by Post-processing Coating-Based Treatments, Lecture Notes in Mechanical Engineering, 837-847, (2020). Abstract

Abstract: In recent years, Fused Deposition Modelling (FDM) has become one of the most attractive Additive Manufacturing (AM) techniques, due to the advantages in the production of complex shapes with a wide range of materials and low investment costs. The thermoplastic polymers used for FDM technology are characterized by low mechanical properties if compared to those of composites and metals. This issue is usually overcome by reinforcing the thermoplastic polymer with chopped fibres or particles. Moreover, a second issue arises, which is represented by the water absorption with a relevant impact on mechanical properties and dimensional stability of printed models. In this paper, an experimental study is presented with the aim at evaluating the water absorption influence on mechanical properties of Carbon PA (Polyamide matrix reinforced with Carbon Fiber at 20%) specimens fabricated with the FDM technique. Two post-processing treatments, based on the use of acrylic spray and photosensitive resin, have been also proposed to improve the behaviour of Carbon PA printed parts. Results of water absorption tests and tensile tests demonstrated a significant improvement in terms of weight stability and mechanical properties by adopting the proposed post-processing treatments.

Keywords: Carbon fiber | Coating treatment | Fused Deposition Modeling | Mechanical characterization | Water absorption

[22] Neri P., Bertini L., Santus C., Guglielmo A., Generalized safe diagram for mistuned bladed disks, Journal of Engineering for Gas Turbines and Power, 141(11), (2019). Abstract

Abstract: The Singh's advanced frequency evaluation (SAFE) diagram is a design tool commonly used to find and thus prevent bladed disk resonances, allowing those modal combinations with no match in terms of angular harmonic shape to be neglected. In principle, the SAFE diagram is based on the assumption that the disk structure is perfectly cyclic; hence, any degree of mistuning would prevent its validity. However, each natural mode of a mistuned bladed disk can be expressed as a superimposition of angular harmonic components. The numbers of nodal diameters of these harmonics are extracted as the discrete Fourier transform of the eigenvector. In this paper, a new graphical representation is proposed to account for this complete harmonic content, instead of just a single (main or nominal) number of nodal diameters as in the conventional SAFE diagram. In addition, the relative intensities of the modal components are shown in this generalized SAFE diagram, for an indication of the induced vibrational magnitude of each possible resonance. Numerical and experimental cases are presented, obtaining significant examples of the proposed diagram. Analyzing the signals of an aeromechanical test, unexpected resonances were observed, which were not predicted by the conventional SAFE, but were detected with the proposed diagram.

Keywords: Aeromechanical test | Campbell diagram | Experimental modal analysis | Mistuned bladed disks | Safe diagram

[23] Barone S., Neri P., Paoli A., Razionale A.V., Low-frame-rate single camera system for 3D full-field high-frequency vibration measurements, Mechanical Systems and Signal Processing, 123, 143-152, (2019). Abstract

Abstract: A single-camera stereo-digital image correlation (stereo-DIC) system to obtain 3D full-field vibration measurements is proposed. The optical setup is composed of two planar mirrors and a single low frame rate camera, thus resulting in a compact and low-cost equipment. The two mirrors are used to create pseudo-stereo images of a target surface on the camera sensor, which are then correlated by using stereo-DIC. The image acquisition process is carried out at low frame rates and the Nyquist-Shannon frequency limitation is overcome by adopting a down-sampling approach under the hypothesis that the vibration signal is characterized by a single known frequency component. The developed pseudo-stereo DIC system allows to obtain 3D full-field vibration measurements in a frequency range up to 4 kHz even with an available frame rate (at full resolution) of 178 fps. The effectiveness of the described approach has been verified by performing vibration measurements on a cantilever plate and a turbine blade.

Keywords: Digital image correlation | Down-sampling approach | Single low-speed camera | Vibration measurement

[24] Neri P., Time-domain simulator for short-term ship manoeuvring prediction: development and applications, Ships and Offshore Structures, 14(3), 249-264, (2019). Abstract

Abstract: This paper presents a short-term time-domain algorithm for ship motion simulation. Plane motion was considered, and thus, a three degrees of freedom model was used. The main inputs of the model are: propellers speed, rudder angles and initial conditions (initial position/orientation and linear/rotational velocity). The outputs are final ship position and orientation after a given simulation time. The data available in the Voyage Data Recorder and the results of sea trials performed before the ship delivery were used to estimate the needed parameters, implementing a simple but reliable linear regression scheme. The proposed model was applied to Costa Concordia manoeuvring simulation in order to assess its reliability in terms of trajectory prediction. Finally, as an example of application, the simulator was implemented in a real-time manoeuvring predictor, which is capable of foreseeing the ship position up to 40 s ahead in time, supporting the Commander in emergency operations.

Keywords: Costa Concordia shipwreck | manoeuvring predictor | plane motion model | Ship motion simulator

[25] Neri P., Interaction force between magnetic field and ferromagnetic target: analytical, numerical and experimental study, Simulation, 95(3), 209-218, (2019). Abstract

Abstract: In this study, an analytical model and a finite element (FE) model were developed in order to study the force produced by a permanent magnet on a ferromagnetic target. The study was aimed at estimating the magnetic action in order to design an excitation device for vibration tests. The dynamic analysis of rotating structures as compressors’ bladed wheels requires a solicitation that reflects the operational conditions. If the component is made of ferromagnetic material, it is possible to use magnetic fields for the excitation. The present paper reports the interaction between planar parallel surfaces, first studied analytically and numerically, and the results were compared with experimental results. Then the interaction between sloping surfaces was analyzed, allowing an analytical boundary loss model to be developed. Finally, the FE model was improved to study the interaction between double curvature surfaces. A comparison with experimental results measured on an actual bladed wheel was performed.

Keywords: bladed wheels excitation | boundary loss model | magnetic force model | Permanent magnet model

[26] Barone S., Neri P., Paoli A., Razionale A.V., Tamburrino F., Development of a DLP 3D printer for orthodontic applications, Procedia Manufacturing, 38, 1017-1025, (2019). Abstract

Abstract: Recent advances in Additive Manufacturing (AM) technologies have allowed a widespread diffusion of their use in different fields. 3D printing is becoming commonplace for biomedical applications requiring the custom fabrication of prostheses and appliances fitting patient-specific anatomies. In this work, the feasibility of a vat photopolymerization technology, based on Digital Light Processing (DLP), has been investigated for the manufacturing of polymeric orthodontic appliances. A custom DLP 3D printer has been developed by exploiting an off-the-shelf digital projector, with the aim at studying the influence of printing parameters on the surface roughness. The feasibility of using Dental LT Clear resin, a biocompatible photopolymer specifically designed for SLA technology, has been finally verified.

Keywords: Additive manufacturing | Custom DLP 3D printer | Orthodontic appliances

[27] Barone S., Neri P., Paoli A., Razionale A.V., Flexible calibration of a stereo vision system by active display, Procedia Manufacturing, 38, 564-572, (2019). Abstract

Abstract: Camera calibration plays a fundamental role for 3D computer vision since it is the first step to recover reliable metric information from 2D images. The calibration of a stereo-vision system is a two-step process: firstly, the calibration of the individual cameras must be carried out, then the two individual calibrations are combined to retrieve the relative placement between the two cameras, and to refine intrinsic and extrinsic parameters. The most commonly adopted calibration methodology uses multiple images of a physical checkerboard pattern. However, the process is time-consuming since the operator must move the calibration target into different positions, typically from 15 to 20. Moreover, the calibration of different optical setups requires the use of calibration boards, which differ for size and number of target points depending on the desired working volume. This paper proposes an innovative approach to the calibration, which is based on the use of a conventional computer screen to actively display the calibration checkerboard. The potential non-planarity of the screen is compensated by an iterative approach, which also estimate the actual screen shape during the calibration process. The use of an active display greatly enhances the flexibility of the stereo-camera calibration process since the same device can be used to calibrate different optical setups by simply varying number and size of the displayed squared patterns.

Keywords: Active display | Reverse Engineering | Stereo camera calibration | Structured light scanner

[28] Cucinotta F., Sfravara F., Neri P., Razionale A., Composite sandwich impact response: Experimental and numerical analysis, Frattura ed Integrita Strutturale, 13(47), 367-382, (2019). Abstract

Abstract: The use of composite materials allows to have a great flexibility in terms of mechanical and physical characteristics. One of the most used composite structure in naval field, is the sandwich, which is composed by a stacking sequence of different plies. The designer, in preliminary phase, must handle a great quantity of degree of freedom (types of materials, orientation of the fibres, position along the stack, thickness, etc.) in order to reach the best compromise between mechanical behaviour, environmental impacts and production costs. Finite Element analysis represents a useful tool in order to optimize all these parameters and to estimate the outcome of experimental tests at design stage. The main goal of this work is to develop and to validate a FE model for the simulation of a particular family of composites, widely used in naval field and, in particular, in High Speed Crafts and powerboats. The first part of the paper concerns the experimental tests on two different types of sandwich specimens. Two families of tests were conducted: four-point bending tests and impact drop tests. The second part of the paper focuses on the validation of a FE model for both experimental setups.

Keywords: Finite Element | Impact tests | Lightweight composites | Offshore powerboats | Sandwich composites

[29] Barone S., Neri P., Paoli A., Razionale A.V., Automatic technical documentation of lithic artefacts by digital techniques, Digital Applications in Archaeology and Cultural Heritage, 11, (2018). Abstract

Abstract: Despite the existence of a wide variety of standards to create hand-made illustrations of lithic artefacts, the conventional process is laborious, time-consuming and the quality of the drawings is highly variable. In this paper, a novel computer-based methodology to create automatic technical documentation of lithic artefacts, in the form of manual-like drawings, is presented. The method exploits the artefact digital model obtained by a 3D optical scanner. An optimization process is proposed to orient the digital model reproducing the conventional positioning. A lighting model is used to introduce an illumination source having different directions, to highlight surface details. A set of images is then created and segmented to retrieve the artefact outline and the internal ridges between flake scars. Potentialities of the proposed methodology are illustrated by analyzing three different stone artefacts acquired by a structured light scanner. 2D technological drawings are automatically created and compared to those obtained by an experienced lithic illustrator.

Keywords: 3D scanning | Automatic technical drawing | Lithic artefact | Reverse engineering

[30] Neri P., Excitation device for high frequency vibration analysis: Design and test results, JVC/Journal of Vibration and Control, 24(19), 4620-4629, (2018). Abstract

Abstract: In the present paper, the design and optimization of a high frequency excitation source is presented. The device was developed for a harmonic response analysis test bench, aimed at dynamic characterization and resonance prediction of mechanical structures. A wide frequency range must be covered, depending on the analyzed structure: the range 1–10 kHz was considered in the present work. The device was designed for a test bench aimed at investigating the vibrational response of centrifugal compressor bladed wheels. A really compact solution was needed since the final test bench provides one exciter for each blade (up to 20 devices on the circumference hoop). Both contact and contactless solutions were considered, but only the contact solution was found to fulfill all the specifications. Finally, different stinger solutions were proposed and compared in the paper. The investigated solutions were: a beam stinger (diameter 1 mm); a wire stinger (diameter 0.2 mm); and a ball stinger (diameter 3 mm) with two different support solutions. Experimental tests performed on a device prototype allowed to verify the specifications fulfillment and to choose the best stinger solution for the application.

Keywords: bladed wheels dynamic characterization | custom electromagnetic shaker | electromagnet excitation | harmonic response analysis | High frequency excitation source | modal analysis

[31] Barone S., Neri P., Paoli A., Razionale A.V., Structured light stereo catadioptric scanner based on a spherical mirror, Optics and Lasers in Engineering, 107, 1-12, (2018). Abstract

Abstract: The present paper describes the development and characterization of a structured light stereo catadioptric scanner for the omnidirectional reconstruction of internal surfaces. The proposed approach integrates two digital cameras, a multimedia projector and a spherical mirror, which is used to project the structured light patterns generated by the light emitter and, at the same time, to reflect into the cameras the modulated fringe patterns diffused from the target surface. The adopted optical setup defines a non-central catadioptric system, thus relaxing any geometrical constraint in the relative placement between optical devices. An analytical solution for the reflection on a spherical surface is proposed with the aim at modelling forward and backward projection tasks for a non-central catadioptric setup. The feasibility of the proposed active catadioptric scanner has been verified by reconstructing various target surfaces. Results demonstrated a great influence of the target surface distance from the mirror's centre on the measurement accuracy. The adopted optical configuration allows the definition of a metrological 3D scanner for surfaces disposed within 120 mm from the mirror centre.

Keywords: Catadioptric system | Reverse engineering | Spherical mirror | Structured light stereo system

[32] Barone S., Paoli A., Neri P., Razionale A.V., De Santis M.L., Mailland F., Automatic illustration of lithics from 3d scanned models, IOP Conference Series: Materials Science and Engineering, 364(1), (2018). Abstract

Abstract: The manual drafting of lithics artefacts could be a very time-consuming work, and it could be cumbersome on the archaeological site. In this case, a 3D digital model of the object could be very useful. Nowadays, several digitizing technologies are available to easily acquire information about the shape of an object. Virtual models could be used to create a digital museum or to share information between researchers. On the other hand, the manual drafting of a lithic object contains information about the technologies used to realize it. Information about the core setup, types of chipping surfaces, detach sequence of supports, and much more. In this work a method to easily obtain a hand-made-like draft of lithic artefacts is proposed. The method is based on the 3D acquisition of the object with a structured-light based scanner and a sequence of digital processing of the acquired data.

[33] Barone S., Carulli M., Neri P., Paoli A., Razionale A.V., An omnidirectional vision sensor based on a spherical mirror catadioptric system, Sensors (Switzerland), 18(2), (2018). Abstract

Abstract: The combination of mirrors and lenses, which defines a catadioptric sensor, is widely used in the computer vision field. The definition of a catadioptric sensors is based on three main features: hardware setup, projection modelling and calibration process. In this paper, a complete description of these aspects is given for an omnidirectional sensor based on a spherical mirror. The projection model of a catadioptric system can be described by the forward projection task (FP, from 3D scene point to 2D pixel coordinates) and backward projection task (BP, from 2D coordinates to 3D direction of the incident light). The forward projection of non-central catadioptric vision systems, typically obtained by using curved mirrors, is usually modelled by using a central approximation and/or by adopting iterative approaches. In this paper, an analytical closed-form solution to compute both forward and backward projection for a non-central catadioptric system with a spherical mirror is presented. In particular, the forward projection is reduced to a 4th order polynomial by determining the reflection point on the mirror surface through the intersection between a sphere and an ellipse. A matrix format of the implemented models, suitable for fast point clouds handling, is also described. A robust calibration procedure is also proposed and applied to calibrate a catadioptric sensor by determining the mirror radius and center with respect to the camera.

Keywords: Backward projection model | Catadioptric sensor | Computer vision | Forward projection model | Spherical mirror

[34] Bertini L., Bucchi F., Monelli B.D., Neri P., Development of a simplified model for the vibration analysis of lawn mowers, Procedia Structural Integrity, 8, 509-516, (2018). Abstract

Abstract: The vibrational behavior of vehicles is a crucial issue for the comfort, especially for the professional vehicles. This paper presents a simplified modelling approach for studying the vibrational behavior of a lawn tractor. The vibrational response of a real vehicle is analyzed by an extensive experimental modal analysis and Finite Element model (FE) simulating the modal behavior of the whole tractor. The FEM was then validated by the comparison with the experimental results and then used for identifying the components and connections effectively driving the modal response. Based on these results, a simplified Multi-Body (MB) model, able to reproduce the vibrational response of the studied lawn mower, was then setup, showing good correspondences with experimental results. General guidelines for defining effective vehicles Multi-Body modal models were also derived.

Keywords: Experimental modal analysis | Finite Element modal model | lawn mower | Multi-Body modal model

[35] Barone S., Braglia M., Gabbrielli R., Miceli S., Neri P., Paoli A., Razionale A.V., Fabrication of fluidic reactors by a customized 3D printing process, Procedia Structural Integrity, 12, 113-121, (2018). Abstract

Abstract: Microfluidic systems demonstrated to improve the analysis of biological and chemical processes by providing a more controlled fluid-handling environment. Typically, microfluidic systems are created in monolithic form by means of microfabrication techniques that constrain designers to work in a two-dimensional space. In this regard, Additive Manufacturing (AM) is a powerful set of technologies that can deal with the complexity of 3D structures producing flow paths with sections differing in size and direction. In this work, the use of a commercial laser-based stereolithography 3D printer has been firstly explored to fabricate transparent channels for flow reactors. A custom 3D printer, based on Digital Light Processing Stereolithography (DLP-SLA), has then been developed with the aim at gaining flexibility and overcoming typical limitations raised from standard commercial solutions. The effectiveness of the developed DLP-SLA 3D printer has been experienced by printing transparent fluidic devices with embedded channels with a specifically designed three-step printing process.

Keywords: Additive manufacturing | DLP 3D printing | fluidic reactor | laser-based stereolithograpy

[36] Barone S., Neri P., Paoli A., Razionale A.V., 3D vibration measurements by a virtual-stereo-camera system based on a single low frame rate camera, Procedia Structural Integrity, 12, 122-129, (2018). Abstract

Abstract: A 3D full-field optical system for high frequency vibration measurement is proposed. The system is composed of a single low-frame-rate camera and two planar mirrors. This compact optical setup overcomes the typical drawback of capturing synchronous acquisitions in the case of a camera pair. Moreover, planar mirrors allow for the use of the classical pinhole model and, thus, conventional stereo-calibration techniques. The use of a low-frame-rate camera provides on the one hand a high-resolution sensor with a relatively low-cost hardware but imposes, on the other, the adoption of a down-sampling approach, which is applicable only when a single (known) sinusoidal load is applied to the structure. The effectiveness of the proposed setup has been verified by the 3D vibration measurement of two different targets up to a frequency of 1 kHz, corresponding to a displacement amplitude of 0.01 mm.

Keywords: digital image correlation | down-sampling approach | Reverse engineering | single low-speed camera

[37] Barone S., Neri P., Paoli A., Razionale A.V., Catadioptric stereo-vision system using a spherical mirror, Procedia Structural Integrity, 8, 83-91, (2018). Abstract

Abstract: In the computer vision field, the reconstruction of target surfaces is usually achieved by using 3D optical scanners assembled integrating digital cameras and light emitters. However, these solutions are limited by the low field of view, which requires multiple acquisition from different views to reconstruct complex free-form geometries. The combination of mirrors and lenses (catadioptric systems) can be adopted to overcome this issue. In this work, a stereo catadioptric optical scanner has been developed by assembling two digital cameras, a spherical mirror and a multimedia white light projector. The adopted configuration defines a non-single viewpoint system, thus a non-central catadioptric camera model has been developed. An analytical solution to compute the projection of a scene point onto the image plane (forward projection) and vice-versa (backward projection) is presented. The proposed optical setup allows omnidirectional stereo vision thus allowing the reconstruction of target surfaces with a single acquisition. Preliminary results, obtained measuring a hollow specimen, demonstrated the effectiveness of the described approach.

Keywords: 3D acquisition | catadioptric stereo vision system | internal geometries acquisition | spherical mirror projection model | structured light scanning

[38] Barone S., Neri P., Paoli A., Razionale A.V., Design and manufacturing of patient-specific orthodontic appliances by computer-aided engineering techniques, Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, 232(1), 54-66, (2018). Abstract

Abstract: Orthodontic treatments are usually performed using fixed brackets or removable oral appliances, which are traditionally made from alginate impressions and wax registrations. Among removable devices, eruption guidance appliances are used for early orthodontic treatments in order to intercept and prevent malocclusion problems. Commercially available eruption guidance appliances, however, are symmetric devices produced using a few standard sizes. For this reason, they are not able to meet all the specific patient’s needs since the actual dental anatomies present various geometries and asymmetric conditions. In this article, a computer-aided design-based methodology for the design and manufacturing of a patient-specific eruption guidance appliances is presented. The proposed approach is based on the digitalization of several steps of the overall process: from the digital reconstruction of patients’ anatomies to the manufacturing of customized appliances. A finite element model has been developed to evaluate the temporomandibular joint disks stress level caused by using symmetric eruption guidance appliances with different teeth misalignment conditions. The developed model can then be used to guide the design of a patient-specific appliance with the aim at reducing the patient discomfort. At this purpose, two different customization levels are proposed in order to face both arches and single tooth misalignment issues. A low-cost manufacturing process, based on an additive manufacturing technique, is finally presented and discussed.

Keywords: additive manufacturing | eruption guidance appliance | finite element model | Orthodontics | temporomandibular joint

[39] Neri P., Bladed wheels damage detection through Non-Harmonic Fourier Analysis improved algorithm, Mechanical Systems and Signal Processing, 88, 1-8, (2017). Abstract

Abstract: Recent papers introduced the Non-Harmonic Fourier Analysis for bladed wheels damage detection. This technique showed its potential in estimating the frequency of sinusoidal signals even when the acquisition time is short with respect to the vibration period, provided that some hypothesis are fulfilled. Anyway, previously proposed algorithms showed severe limitations in cracks detection at their early stage. The present paper proposes an improved algorithm which allows to detect a blade vibration frequency shift due to a crack whose size is really small compared to the blade width. Such a technique could be implemented for condition-based maintenance, allowing to use non-contact methods for vibration measurements. A stator-fixed laser sensor could monitor all the blades as they pass in front of the spot, giving precious information about the wheel health. This configuration determines an acquisition time for each blade which become shorter as the machine rotational speed increases. In this situation, traditional Discrete Fourier Transform analysis results in poor frequency resolution, being not suitable for small frequency shift detection. Non-Harmonic Fourier Analysis instead showed high reliability in vibration frequency estimation even with data samples collected in a short time range. A description of the improved algorithm is provided in the paper, along with a comparison with the previous one. Finally, a validation of the method is presented, based on finite element simulations results.

Keywords: Blade damage detection | Bladed wheel resonance | Bladed wheels monitoring | Non-Harmonic Fourier Analysis

[40] Bertini L., Neri P., Santus C., Guglielmo A., Automated Experimental Modal Analysis of Bladed Wheels with an Anthropomorphic Robotic Station, Experimental Mechanics, 57(2), 273-285, (2017). Abstract

Abstract: Experimental modal analysis is challenging when the component has a highly three-dimensional shape, since a great number of measurement points are needed with accurate positioning. An anthropomorphic robotic station is proposed to automate this analysis, specifically on bladed wheels. This provides a reliable control of the spot location and of the beam orientation of a Laser Doppler Vibrometer. The modal frequencies were obtained along with the vibrational shapes and their spatial resolution was managed by exploiting the programming flexibility of the robotic station. The SAFE diagram was easily obtained by measuring a single point for each sector, and an extension of this diagram was demonstrated for the splitter blade wheels. The use of multiple measurement points, for each wheel sector, significantly improved the characterization of the modes having the same number of nodal diameters, hence the same shape coordinate on the SAFE diagram.

Keywords: Anthropomorphic robotic station | Bladed wheel dynamics | Experimental modal analysis | Laser doppler vibrometer | SAFE diagram

[41] Barone S., Neri P., Paoli A., Razionale A., Digital Image Correlation Based on Projected Pattern for High Frequency Vibration Measurements, Procedia Manufacturing, 11, 1592-1599, (2017). Abstract

Abstract: The dynamic characterization of mechanical components is a crucial issue in industry, especially in the field of rotating machinery. High frequency loads are typical in this field and experimental tools need to fulfill severe specifications to be able to analyze these high-speed phenomena. In this work, an experimental setup, based on a Digital Image Correlation (DIC) technique with a projected speckle pattern, is presented. The proposed approach allows the measurement of vibrational response characterized by a single sinusoidal component having a frequency up to 500 Hz and an amplitude lower than 10 μm.

Keywords: Geometrical inspection | Optiocal touch probe | Reverse Engineering

[42] Barone S., Neri P., Paoli A., Razionale A., Optical Tactile Probe for the Inspection of Mechanical Components, Procedia Manufacturing, 11, 1582-1591, (2017). Abstract

Abstract: Mechanical components are often subjected to tolerances and geometrical specification. This paper describes an automatic 3D measurement system based on the integration of a stereo structured light scanner and a tactile probe. The tactile probe is optically tracked by the optical scanner by means of 3D measurements of a prismatic flag, rigidly connected to the probe and equipped with multiple chessboard patterns. Both the stereo vision system and the tactile probe can be easily configured enabling complete reconstructions of components having complex shapes. For instance, structured light scanning can be used to acquire external and visible geometries while tactile probing can be limited to the acquisition of internal and hidden surfaces.

Keywords: Geometrical inspection | Optical touch probe | Reverse Engineering

[43] Bertini L., Neri P., Santus C., Guglielmo A., One exciter per sector test bench for bladed wheels harmonic response analysis, Proceedings of the ASME Turbo Expo, 7B-2017, (2017). Abstract

Abstract: Bladed wheel dynamic characterization is a crucial issue to avoid resonance excitations. The test bench presented in this paper was designed to independently excite the wheel sectors with one electromagnetic shaker each blade. Since a wide frequency range (1-10 kHz) is usually considered for bladed wheels, custom electromagnetic devices were designed, and then a closedloop control software was also implemented. The global mode shapes of the wheel were then reconstructed through subsequent accelerometer measurements on all sectors to evaluate the harmonic response. The main target of the test rig is the reproduction of any operational condition by experimentally simulating an arbitrary number of stator vanes. In this way the response levels of the differently excited modes are measured and the modal damping is optimally quantified by providing a selective excitation of any number of nodal diameters. Preliminary results showed how the test setup actually allows to excite those modes with a specific number of nodal diameters, however, also exposed some difficulties to avoid small load components with different numbers of nodal diameters.

[44] Barone S., Paoli A., Neri P., Razionale A.V., Giannese M., Mechanical and geometrical properties assessment of thermoplastic materials for biomedical application, Lecture Notes in Mechanical Engineering, 0, 437-446, (2017). Abstract

Abstract: Clear thermoplastic aligners are nowadays widely used in orthodontics for the correction of malocclusion or teeth misalignment defects. The treatment is virtually designed with a planning software that allows for a definition of a sequence of little movement steps from the initial tooth position to the final desired one. Every single step is transformed into a physical device, the aligner, by the use of a 3D printed model on which a thin foil of plastic material is thermoformed. Manufactured aligners could have inherent limitations such as dimensional instability, low strength, and poor wear resistance. These issues could be associated with material characteristics and/or with the manufacturing processes. The present work aims at the characterization of the manufactured orthodontic devices. Firstly, mechanical properties of different materials have been assessed through a set of tensile tests under different experimental conditions. The tests have the purpose of analyzing the effect that the forming process a d the normal use of the aligner may have on mechanical properties of the material. The manufacturing process could also introduce unexpected limitations in the resulting aligners. This would be a critical element to control in order to establish resulting forces on teeth. Several studies show that resulting forces could be greatly influenced by the aligner thickness. A method to easily measure the actual thickness of the manufactured aligner is proposed. The analysis of a number of real cases shows as the thickness is far to be uniform and could vary strongly along the surface of the tooth.

Keywords: 3D Human Modeling | Clear Aligner | Mechanical Properties Assessment | Optical 3D Scanner | Thermoforming Process | Virtual Design

[45] Neri P., Barone S., Paoli A., Razionale A., Finite element analysis of TMJ disks stress level due to orthodontic eruption guidance appliances, Lecture Notes in Mechanical Engineering, 0, 415-423, (2017). Abstract

Abstract: In the present work, the effect of Eruption Guidance Appliances (EGAs) on TemporoMandibular Joint (TMJ) disks stress level is studied. EGAs are orthodontic appliances used for early orthodontic treatments in order to prevent malocclusion problems. Commercially available EGAs are usually produced by using standard sizes. For this reason, they are not able to meet all the specific needs of each patient. In particular, EGAs are symmetric devices, while patient arches generally present asymmetric conditions. Thus, uneven stress levels may occur in TMJ disks, causing comfort reduction and potential damage to the most solicited disk. On the other hand, a customized EGA could overcome these issues, improving the treatment effectiveness. In this preliminary study, a Finite Element (FE) model was developed to investigate the effects of a symmetric EGA when applied to an asymmetric mouth. Different misalignment conditions were studied to compare the TMJ disks stress levels and to analyze the limitations of a symmetric EGA. The developed FE model can be used to design patient-specific EGAs, which could be manufactured by exploiting non-conventional techniques such as 3D printing.

Keywords: Eruption guidance appliance (EGA) | FE model | Patient-specific orthodontic appliance | TMJ disks stress | TMJ disorders

[46] Neri P., Neri B., A short term simulator for vessel manoeuvres prediction, Lecture Notes in Electrical Engineering, 409, 59-65, (2017). Abstract

Abstract: This paper presents a methodology for vessels manoeuvring prediction during navigation. The proposed algorithm allows to estimate future position and heading of the vessel on the basis of present position, velocity, propeller speed and rudder angle. The prediction algorithm, which can be very useful in emergency manoeuvres, uses a short term simulator which has been tuned up and validated in the framework of the Costa Concordia cruise ship trial. However, the methodology could be applied to any kind of ship, since it just requires a set of data recorded during a set of mandatory sea trials performed before the delivery of the ship. The prediction algorithm shows, on the electronic navigation chart, the foreseen position of the ship in the next 30–40 s, provided that helm and power settings remain unchanged, allowing the helmsman to check in advance the effect of the commands and to eventually modify the trajectory.

[47] Gubian P., Piccinelli M., Neri B., Neri P., Giurlanda F., The disaster of Costa Concordia cruise ship: An accurate reconstruction based on Black Box and automation system data, Proceedings of the 2015 2nd International Conference on Information and Communication Technologies for Disaster Management, ICT-DM 2015, 178-185, (2016). Abstract

Abstract: In this paper, an accurate reconstruction of the events preceding the January 13th, 2012 impact of the Costa Concordia cruise ship with the rocks of Isola del Giglio is presented, along with the emergency countermeasures activated by the ship automation system after the impact. The reconstruction is entirely based on data recorded by the information systems of the ship and demonstrates the importance of this kind of data from a scientific and forensics point of view. First the authors, three of whom have served as consultants in the trial in Grosseto, Italy, show how information stored in the Voyage Data Recorder, the so called Black Box, has been used to calculate the exact time and coordinates of the impact point. An accurate evaluation of these data represents a conditio sine qua non to ascertain the effects of the helmsman's error, who allegedly misunderstood the Master's orders just 20 seconds before the impact. Then, these effects were studied by means of a purposely developed short term manoeuvring simulator, whose results are also briefly summarized in the paper. Moreover, information stored in the hard disk of the automation system of the ship, concerning the activation of emergency source of energy, have been examined and some results are briefly reported in the paper. The most important data are those concerning the watertight doors, the passenger elevators and the Emergency Diesel Generator. According to the results of our investigations, i) the helmsman's error appears to have been determinant in the accident; ii) the emergency procedure, which started automatically after blackout of the main power source, does not appear to have performed correctly. In our opinion, both these facts could have influenced the outcome of the last phase of the ship evacuation, during which most of the victims lost their lives.

Keywords: black box | Costa Concordia | manoeuvres simulator | shipwreck | VDR

[48] Neri P., Peeters B., Non-Harmonic Fourier Analysis for bladed wheels damage detection, Journal of Sound and Vibration, 356, 181-194, (2015). Abstract

Abstract: Abstract The interaction between bladed wheels and the fluid distributed by the stator vanes results in cyclic loading of the rotating components. Compressors and turbines wheels are subject to vibration and fatigue issues, especially when resonance conditions are excited. Even if resonance conditions can be often predicted and avoided, high cycle fatigue failures can occur, causing safety issues and economic loss. Rigorous maintenance programs are then needed, forcing the system to expensive shut-down. Blade crack detection methods are beneficial for condition-based maintenance. While contact measurement systems are not always usable in exercise conditions (e.g. high temperature), non-contact methods can be more suitable. One (or more) stator-fixed sensor can measure all the blades as they pass by, in order to detect the damaged ones. The main drawback in this situation is the short acquisition time available for each blade, which is shortened by the high rotational speed of the components. A traditional Discrete Fourier Transform (DFT) analysis would result in a poor frequency resolution. A Non-Harmonic Fourier Analysis (NHFA) can be executed with an arbitrary frequency resolution instead, allowing to obtain frequency information even with short-time data samples. This paper shows an analytical investigation of the NHFA method. A data processing algorithm is then proposed to obtain frequency shift information from short time samples. The performances of this algorithm are then studied by experimental and numerical tests.

[49] Bertini L., Neri P., Santus C., Guglielmo A., Mariotti G., Analytical investigation of the SAFE diagram for bladed wheels, numerical and experimental validation, Journal of Sound and Vibration, 333(19), 4771-4788, (2014). Abstract

Abstract: Compressor and turbine bladed wheels interact with the fluid distributed by the stator vanes. They are subject to vibration and fatigue loading, especially when resonance conditions are excited. Avoiding resonance is fundamental when designing bladed wheels. The Campbell diagram approach is too conservative since bladed wheels show many close frequency natural modes, thus it is almost impossible to avoid frequency matching. Singh's Advanced Frequency Evaluation (SAFE) diagram, or interference diagram, also introduces shape matching in addition to the frequency, for resonance prediction, therefore many frequency matching cases can be identified as non-critical and thus tolerated. The present paper explains and demonstrates the SAFE diagram by introducing an analytical expression to identify bladed wheel resonance conditions. The mode shape matching with a harmonic component is investigated by means of specific examples. Symmetry properties of the matching distribution of harmonic indices and mode shapes are also introduced and demonstrated. Finally, a bladed wheel analysis is used for validation, both in FE simulations and experiments.

[50] Bertini L., Monelli B., Neri P., Santus C., Guglielmo A., Robot assisted modal analysis on a stationary bladed wheel, ASME 2014 12th Biennial Conference on Engineering Systems Design and Analysis, ESDA 2014, 2, (2014). Abstract

Abstract: This paper shows an automated procedure to experimentally find the eigenmodes of a bladed wheel with highly three-dimensional geometry. The stationary wheel is supported in free-free conditions, neglecting stress-stiffening effects. The single input / multiple output approach was followed. The vibration speed was measured by means of a laser-Doppler vibrometer, and an anthropomorphic robot was used for accurate orientation and positioning of the measuring laser beam, allowing multiple measurements during a limited testing time. The vibration at corresponding points on each blade was measured and the data elaborated in order to find the initial (lower frequency) modes. These modal shapes were then compared to finite element simulations and accurate frequency matching and exact number of nodal diameters obtained. Being the modes cyclically harmonic, the complex formulation could be attractive, being not affected by the angular phase of the mode representation. Nevertheless, stationary modes were experimentally detected, rather than rotating, and then the real representation was necessary. The discrete Fourier transform of the blade displacements easily allowed to find both the angular phase and the correct number of nodal diameters. Successful MAC experimental to analytical comparison was finally obtained with the real representation after introducing the proper angular phase for each mode.

[51] Neri P., Piccinelli M., Gubian P., Neri B., A ship motion short term time domain simulator and its application to costa concordia emergency manoeuvres just before the january 2012 accident, Proceedings - 28th European Conference on Modelling and Simulation, ECMS 2014, 64-70, (2014). Abstract

Abstract: In this paper we will present a simple but reliable methodology for short term prediction of a cruise ship behaviour during manoeuvres. The methodology is quite general and could be applied to any kind of ship, because it does not require the prior knowledge of any structural or mechanical parameter of the ship. It is based only on the results of manoeuvrability data contained in the Manoeuvring Booklet, which in turn is filled out after sea trials of the ship performed before his delivery to the owner. We developed this method to support the investigations around the Costa Concordia shipwreck, which happened near the shores of Italy in January 2012. It was then validated against the data recorded in the "black box" of the ship, from which we have been able to extract an entire week of voyage data before the shipwreck. The aim was investigating the possibility of avoiding the impact by performing an evasive manoeuvre (as ordered by the Captain some seconds before the impact, but allegedly misunderstood by the helmsman). The preliminary validation step showed a good matching between simulated and real values (course and heading of the ship) for a time interval of a few minutes. The fact that the method requires only the results registered in the VDR (Voyage Data Recorder) during sea trial tests, makes it very useful for several applications. Among them, we can cite forensic investigation, the development of components for autopilots, the prediction of the effects of a given manoeuvre in shallow water, the "a posteriori" verification of the correctness of a given manoeuvre and the use in training simulators for ship pilots and masters. Proceedings 28th European Conference on Modelling and Simulation © ECMS Flaminio Squazzoni, Fabio Baronio, Claudia Archetti, Marco Castellani (Editors).

Keywords: Costa Concordia | Ship manoeuvre | Simulation model

[52] Barzi E., Gallo G., Neri P., FEM analysis of Nb-Sn rutherford-type cables, IEEE Transactions on Applied Superconductivity, 22(3), (2012). Abstract

Abstract: Rutherford-type cables are used in superconducting accelerator magnets. To produce magnetic fields larger than 10 T, brittle superconductors like Nb 3Sn are typically used. The original round wire, in the form of a composite of Copper, Niobium and Tin, is assembled into a so-called Rutherford-type cable, which is used to wind the magnet. The magnet is then subjected to a high temperature heat treatment to produce the chemical reactions that make the material superconducting. At this stage the superconductor is brittle and its superconducting properties sensitive to strain. This work describes a 2D finite element model, which simulates the mechanical behavior of Rutherford-type cable before heat treatment. The model was applied to a number of different cable architectures. To validate a critical criterion adopted into the single Nb-Sn wire analysis, the results of the model were compared with those measured experimentally on cable cross sections. © 2002-2011 IEEE.

Keywords: Finite element model analysis | Nb Sn wires 3 | plastic work | principal strain | Rutherford-type cables