Papers by Author: Elisabetta Ceretti

Abstract: Lattice metamaterials with adjustable auxetic behavior are characterized by periodic configurations of interconnecting struts and nodes, allowing for precise control over their macroscopic mechanical properties. Different lattice configurations were examined, two-unit cell variants with varying void fractions were assembled into crystalline-inspired designs, specifically simple cubic and body-centered cubic. Using vat photopolymerization, fabrication was carried out using a transparent biomedical elastomeric resin that was chosen for its exceptional ductility and strain tolerance. The curing, crosslinking and thermal mechanical stability of the resin were examined using Fourier Transform Infrared Spectroscopy and Differential Scanning Calorimetry, before and after polymerization. In order to determine specific stiffness, specific yield strength, mechanical characterization involved quasi-static uniaxial compression testing. The effect of different aspects of the macroscopic structures was also observed, exploiting diverse possible applications. The combination of geometry and the behavior of the elastomeric material allowed the creation of lightweight structures that could support large reversible deformations that could be used in soft robotics and healthcare devices.

Abstract: Additive manufacturing (AM) technologies have enabled the fabrication of customizable, low-cost capacitive sensors for a wide range of applications, including robotics, automation, and bioelectronics. Although various AM techniques have been explored, structural inconsistencies often remain a challenge, limiting the performance and reproducibility of printed dielectric layers. Stereolithography (SLA), offers higher resolution and denser prints, yet the use of commercial photopolymer resins as dielectric materials remains underexplored. This study investigates two commercial SLA-compatible resins, a flexible medical-grade elastic resin and a dental-grade resin, as potential dielectric layers for capacitive force sensors. Both resins are biocompatible for short-term use or skin contact, making them suitable also for medical applications. The elastic 50A-V1 resin exhibited a Young’s modulus of E = 5.0 ± 0.2 MPa up to approximately 60% strain, whereas the Dental Clear V2 resin showed a significantly higher modulus of E = 1020 ± 80 MPa under the same conditions. Therefore, the elastic resin was subsequently chosen as the dielectric material to fabricate a proof-of-concept capacitive force sensor, which exhibited a final capacitance of 1.13 ± 0.03 pF within a force range of 10 to 400 N. The findings serve as a preliminary step towards the development of fully 3D-printed capacitive force sensors for integration into soft robotic and smart biomedical systems.

Abstract: In this research the effect of the geometric features of an auxetic metamaterial structure was investigated by the authors. In particular, a re-entrant honeycomb geometry was selected as case study. Connectors inclination, width and length have been changed to study mechanical behavior and deformation under compression. The procedure adopted was both experimental and numerical. Solutions proposed highlight benefits in terms of compression load and controlled lateral displacement that is possible to achieve.

Abstract: Biomedical prostheses are artificial devices suitable for the replacement of missing or inefficient parts of the body, implanted to reduce the anatomical or functional deficiency, and sometimes also applied for aesthetic purposes. Despite this type of medical devices represents today a very innovative sector from the medical and engineering point of view, some issues, inherent to the interaction between human body and the external hosts must be considered. It is important that the weight and porosity of the prosthesis respect the patient’s physiological equilibrium which permit an appropriate osseointegration where needed. A typical solution is a lattice structure, which can be manufactured by Additive Manufacturing techniques which, as known, permit to build complex geometries in comparison with other processing routes. Lattice structure are typically characterized by both stiffness and strength significantly lower than the full part of the structure. Generally, for this reason, the lattices are applied to the low-stress areas, leaving a portion of solid sufficient to transmit the loads involved, or in such a way to guarantee the desired flexibility of the part-itself. During the design of lattices some limitations regarding their printability must be considered, such as the minimum printable dimension and the necessary support parts. A Design of Experiment analysis was conducted to identify the optimal parameters to manufacture a spinal cage with negligible porosity via laser powder bed fusion using Ti6Al4V alloy.

Abstract: Incremental Sheet Forming is a flexible process characterized by low costs and higher process times with respect to traditional forming technologies. It is therefore suitable for prototypes, small series or custom mass productions. Its flexibility derives from the use of a hemispherical punch that is moved by a CNC machine and gradually deforms the sheet in presence, or not, of a counter die. As a consequence, the sheet clamping is reduced and the part accuracy is lower than traditional sheet forming process as stamping. Therefore, the improvement of the part accuracy in Incremental Sheet Forming is a relevant research topic and solutions for error reduction are required for improving the process quality.The present paper describes the use of an Iterative Learning Control (ILC) algorithm for compensating the ISF part geometrical error. In particular, it iteratively corrects the part geometry on the basis of the error map obtained as the difference between formed and target part geometries. The ILC uses the target geometry to form a first trial part, it measures the obtained geometry and estimates the geometrical error map. Then the error map is used to modify the target geometry and another part is formed. This procedure gets iterated until the desired geometrical tolerance is achieved.The correction algorithm was experimentally tested in forming both axisymmetric and not axisymmetric parts using aluminum sheets. Results showed that in few iteration steps it was possible to significantly improve the part accuracy and to achieve geometrical tolerances comparable with the traditional sheet forming processes.

Abstract: Ring Rolling is a complex hot forming process where different rolls are involved in the production of seamless rings characterized by extreme dimensions (i.e. external diameter higher more than 1m). Because each roll can be independently controlled from the other ones different speed laws must be set; usually, in the industrial environment, a milling curve is introduced to monitor the shape of the workpiece during the deformation in order to ensure a correct ring production. In former works the authors focused their attention on the influence of different milling curves for an industrial case and the results underlined that a ring produced with a good quality and lower loads and energy could be obtained imposing a linearly descending trend to the Idle roll speed law. However different approaches could be used in order to evaluate the mentioned speed law.In this work the authors enhanced the knowledge about the optimization of the Idle roll speed law: different Idle roll speed laws were designed and simulated and the results were compared in order to identify the best speed law that guarantees a good quality ring with lower loads and energy required for manufacturing.

Abstract: Ring Rolling is a complex hot forming process used for the production of shaped rings, seamless and axis symmetrical workpieces. The main advantage of workpieces produced by ring rolling, compared to other technological processes, is given by the size and orientation of grains, especially on the worked surface which give to the final product excellent mechanical properties. In this process different rolls (Idle, Axial, Guide and Driver) are involved in generating the desired ring shape. Since each roll is characterized by a speed law that can be set independently by the speed law imposed to the other rolls, an optimization is more critical compared with other deformation processes. Usually, in industrial environment, a milling curve is introduced in order to correlate the Idle and Axial roll displacement, however it must be underlined that different milling curves lead to different loads and energy for ring realization. In this work an industrial case study was modeled by a numerical approach: different milling curves characterized by different Idle and Axial roll speed laws (linearly decreasing, constant, linearly increasing) were designed and simulated. The results were compared in order to identify the best milling curve that guarantees a good quality ring (higher diameter, lower fishtail) with lower loads and energy required for manufacturing.

Abstract: Friction Stir Welding (FSW) is a solid-state welding process introduced and developed in last decades. In this process a rotating tool is pressed on the two parts to be welded (mainly two plates), driven into the material and then translated along the parts interface. Academic and industrial interest is focused on the characteristics of the joined part in terms of mechanical resistance and fatigue resistance of the joints. These characteristics are heavily related to the process parameters chosen since the material stirring and the material temperature greatly depend on the pin rotating and translating speed. In fact, the stirring phenomena and the friction acting between the shoulder of the pin and the sheets, greatly increase the part temperature so that the material greatly changes its structural characteristics due to softening effect: grain dimensions, local hardness, grain orientation. Moreover, due to the physical material movement different types of defects (mainly voids) can be present in the welded zone (nugget). In particular three different areas can be identified: the heat affected zone (HAZ), the thermo-mechanical affect zone (TMAZ) and the nugget. The extension and the characteristics of these zone are very important in order to define the joint quality. These investigations are very important especially when FSW is applied in industrial fields such as aerospace, automotive and naval. To cut and to investigate an experimentally obtained joint is interesting for understanding the weld quality, but FEM simulation of the process can add very useful information in defining how the process parameter influence the joint behaviour and the three different zone extensions. As an example the heat flux, and consequently the temperature distribution inside the material, depend on the combination of rotation and welding speeds. For this reason, in the last years several efforts were oriented to the numerical simulation of the process, in order to investigate thermo-mechanical aspects, stress and strain distributions, thermal flow, residual stresses. The present paper deals with the set up of a FE model for the simulation of the FSW process whose results are correlated with the experimental observations carried out when joining AA6060-T6 aluminium alloy plates 5mm thick with a cylindrical tool with flat shoulder. The experimental campaign was performed under different welding conditions varying the tool rotational speed and the welding speed. A three-dimensional piezoelectric load cell was used to measure the welding forces in the main directions. The numerical model was developed and set up in DEFORM 3D environment. The information obtained from the model helped in the understanding of the welding phenomena.

Abstract: In extrusion operations, material solid state welding takes place thanks to the very high pressure and temperature at which the material undergoes between the several welding criteria developed, the attention was particularly focused on the Piwnik and Plata one. In this criterion a suitable parameter w, calculated as the interface pressure and the effective stress ratio integrated along the time, has been defined. According to this criterion the material should start to weld as this parameter exceed a limit value w_lim. In the present paper a new procedure for the w_lim identification as function of the temperature, based on coupled experimental-simulative strategy, is proposed. In particular, flat rolling experimental tests of sandwiches made of two rectangular specimens in AA6082 alloy were performed. The specimens were characterized by different heights in order to consider different compression ratio, that means different interface pressure and effective stress distributions. All the tests were repeated at different temperature. Once the experimental tests were performed, FEM simulations of the rolling process were run for the very same conditions. Thanks to a suitable user routine developed and implemented in the calculus code, it was possible to evaluate the steady state value of the w parameter for all the different conditions at the steady state conditions. By verifying if the actual experimental test demonstrated the presence or less of material bonding, it was possible to identify the w_lim values as function of the temperature. Particular attention must be paid to the study of the macrostructure of the welded material in order to identify the influence of the process parameter on the weld quality. This means that it is possible to identify not only if the weld will take place, but also if it will be qualitatively adequate.

Abstract: In cutting field, residual stress distribution analysis on the workpiece is a very interesting topic. Indeed, the residual stress distribution affects fatigue life, corrosion resistance and other functional aspects of the workpiece. Recent studies showed that the development of residual stresses is influenced by the cutting parameters, tool geometry and workpiece material. For reducing the costs of experimental tests and residual stress measurement, analytical and numerical models have been developed. The aim of these models is the possibility of forecasting the residual stress distribution into the workpiece as a function of the selected process parameters. In this work the residual stress distributions obtained simulating cutting operations using a 3D FEM software and the corresponding simulation procedure are reported. In particular, orthogonal cutting operations of AISI 1045 and AISI 316L steels were performed. The FEM results were compared with the experimental residual stress distribution in order to validate the model effectiveness.