Mechanical Behavior of PET-G Tooth Aligners Under Cyclic Loading

Invisible aligners are medical devices, which allow repositioning of teeth through a treatment designed by the orthodontists. During this orthodontic treatment, patients use several aligners each for a couple of weeks. The aligner will apply a system of forces on the teeth to shift them to desired position. Since aligners exert forces thanks to their particular shape, it is important that during lifetime's service they do not undergo significant deformations. This research aims to study the mechanical behavior of invisible aligners made by polyethylene terephthalate-glycol (PET-G), which is one of most used the plastic materials to produce such devices. In this study, cyclic compression tests in atmospheric environment (∼25°C) as well as in the presence of saliva (to simulate intraoral environment) were performed. The mechanical behavior of aligners with two different thicknesses (0.75 and 0.88 mm) was studied. In particular, each aligner was subjected to 22500 load cycles from 0 to 50 N. The chosen number of load cycles simulates the average load history to which an aligner is subjected during its lifetime. The tests were performed on a testing machine, using a hard resin dental cast properly fixed to the machine. Analysis of the results shows that the stiffness of the aligners increases during the cyclic test. In particular, a gradual reduction of the crosshead displacement was observed during the test, highlighting the occurrence of cyclic hardening phenomena. It was also found that the aligners show a residual strain recovery after removing the applied load. Moreover, in the analyzed range of load rate, the aligners show a low tendency to accumulate residual strains as loading cycles progress

Effect of ElectroSpark Process Parameters on the WE43 Magnesium Alloy Deposition Quality

This research aims to investigate the effects of process parameters on the quality of WE43 coatings deposited on homologue substrate by ElectroSpark Deposition (ESD) technology. ESD is new technology used to apply coatings or for the restoration and refurbishment of worn or damaged high valued parts. The depositions were processed using five different levels of Energy input (Es, Spark Energy). The microstructure of both the base material and deposits cross-section were characterized by optical and scanning electron microscopies. Also, X-ray diffraction technique was used. In addition, stereological studies of the through-thickness heterogeneities of the deposits (e.g., voids) were performed. The mechanical properties were evaluated by Vickers micro-hardness. The results show that the deposits exhibited a fine grained microstructure due to the rapid solidification. The average micro-hardness values of the deposits are lower than that of the substrate and distributed in a small range (49–60 HV). The lower hardness of the deposits respect to the base material is due to the presence of defectiveness such as spherical, laminar and random shaped voids. The defects area percentage inside the deposits remains well below than 11%. All the deposits were mainly affected by laminar morphology defects. The results indicate that the deposits defectiveness decreases as the energy input increases

Study of the Direct Metal Deposition of AA2024 by ElectroSpark for Coating and Reparation Scopes

ElectroSpark Deposition (ESD) is a pulsed micro-welding process that is capable of depositing wear and corrosion resistance deposit to repair, improve, and to extend the service life of the components and tools. Major new applications have taken place in gas turbine blades and steam turbine blade protection and repair, and in military, medical, metal-working, and recreational equipment applications. In this study, the ESD technique was exploited to fabricate 2024 aluminum alloy deposit on a similar substrate. The deposits were deposited using different process parameters. Heat input was varied on three levels. The outcoming microstructure was analyzed by optical and scanning electron microscopies. The deposit was characterized by the overlapping of layers with a mixed microstructure. The average hardness was independent from the process parameters. Both porosity inside the deposits and cracks at the deposit/substrate interface were detected. The porosity lowered with the heat input and increased the average length of cracks

Effect of ElectroSpark Process Parameters on the WE43 Magnesium Alloy Deposition Quality

This research aims to investigate the effects of process parameters on the quality of WE43 coatings deposited on homologue substrate by ElectroSpark Deposition (ESD) technology. ESD is new technology used to apply coatings or for the restoration and refurbishment of worn or damaged high valued parts. The depositions were processed using five different levels of Energy input (Es, Spark Energy). The microstructure of both the base material and deposits cross-section were characterized by optical and scanning electron microscopies. Also, X-ray diffraction technique was used. In addition, stereological studies of the through-thickness heterogeneities of the deposits (e.g., voids) were performed. The mechanical properties were evaluated by Vickers micro-hardness. The results show that the deposits exhibited a fine grained microstructure due to the rapid solidification. The average micro-hardness values of the deposits are lower than that of the substrate and distributed in a small range (49-60 HV). The lower hardness of the deposits respect to the base material is due to the presence of defectiveness such as spherical, laminar and random shaped voids. The defects area percentage inside the deposits remains well below than 11%. All the deposits were mainly affected by laminar morphology defects. The results indicate that the deposits defectiveness decreases as the energy input increases

Mechanical Behaviour of ABS-Fused Filament Fabrication Compounds under Impact Tensile Loadings

In the Fused Filament Fabrication (FFF) process, the part is built as a layer-by-layer deposition of a feedstock filament material. The continuous improvements of the FFF have changed the main purpose of this technique from rapid prototyping to a rapid manufacturing method. Then, it is fundamental to determine the material properties of FFF parts as a function of the service load. The impact loads and, in particular, a high strain rates tensile impact can be a critical issue in FFF part and, in general, for plastic materials. The aim of the present work is to characterise the mechanical behaviour of FFF parts under tensile impact loads. To this purpose, three different orientations (i.e., 0°, 45° and 90°) both single- and multilayer specimens, have been printed. Finally, the influence of the impact speed on the mechanical behaviour has also been tested under three different values of speed (3.78 m/s, 3.02 m/s and 2.67 m/s). The results show that the FFF parts are influenced by the raster orientation, confirming the orthotropic behaviour also under dynamic loads, while the variation of impact speed, on peak force and absorbed energy, is limited

Advancements in Electrospark Deposition (ESD) Technique: A Short Review

The need to use components with improved surface characteristics in relation to severe operating conditions, together with the aim of cost reduction associated with the replacement of damaged components, has led to an increasing use of coatings and repairing processes. The most common deposition processes are generally characterized by high equipment costs and, sometimes, by long deposition time. Furthermore, some repair technologies, especially those characterized by high heat input, are not suitable for alloys used in aerospace applications due to the degradation of their mechanical characteristics. In the last decades, a novel eco-friendly method capable of overcoming the limits set out above emerged: the electrospark deposition (ESD) technology. Thanks to its efficiency, simplicity, cost-effectiveness, and low heat input, this technology has proved to be suitable both for improving surface properties, such as thermo and wear resistance, higher hardness and corrosion resistance, and for the repair of high-value components. The aim of this review is to describe in detail some aspects of the ESD technique to understand the ESD processing preparation of alloys normally considered difficult to weld by traditional processes and to give some important clues to the readers to contribute to the defect-free repair of damaged areas and coatings deposition

Friction Stir Processing at High Rotation Rates of a Magnesium Alloy: Mechanical Properties at High Temperatures and Microstructure

A high-pressure die-cast magnesium alloy plate was friction stir processed at high rotation rates with different advancing speeds. The stirred zone was very narrow around the tool and this made the friction stir process difficult to occur in the whole thickness of the plate. Intermetallicphase network at grain boundaries was refined due to partial dissolution and fragmentation of Mg17Al12 β-phase during the friction stir process; the likely increment of solute content in solid solution was exploited for aging to improve hardness. The ductility of friction stir processed samples deformed at 300° and 350°C substantially increased compared to the base material and to room temperature strained samples

Caratterizzazione microstrutturale e meccanica di rivestimenti in lega AA 2024 ottenuti mediante Electrospark Deposition

In questo studio sono stati caratterizzati rivestimenti in lega d’alluminio AA2024 realizzati su substrato omologo mediante ElectroSpark Deposition (ESD). L’ESD è un processo di microsaldatura ad arco pulsato che utilizza impulsi elettrici di breve durata, generati dalla scarica di un condensatore, per trasferire il materiale di un elettrodo mobile ad un substrato metallico. I rivestimenti prodotti, analizzati con indagini in microscopia ottica ed elettronica a scansione, sono di tipo strato su strato, componendosi di diversi strati sovrapposti. All’interno di ciascuno strato deposto la microstruttura è risultata abbastanza fine a causa delle elevate velocità di raffreddamento coinvolte nel processo. La durezza media dei rivestimenti non cambia in maniera significativa al variare dei parametri di processo considerati. I rivestimenti presentano difettosità quali vuoti e micro-cricche perpendicolari o parallele all’interfaccia con il substrato. Anche il substrato è caratterizzato dalla presenza di micro-cricche all’interfaccia. All’aumentare dell’energia di scarica si è osservata una riduzione delle difettosità all’interno del coating e un aumento della lunghezza media delle micro-cricche nel substrato

Investigation of Interlaminar Shear Properties of CFRP Composites at Elevated Temperatures Using the Lempel-Ziv Complexity of Acoustic Emission Signals

Three-point bending tests on Short Beam Shear (SBS) specimens are performed to investigate the interlaminar shear properties of plain weave fabric CFRP composites. The tests are performed in a controlled environmental chamber at two different elevated temperatures. The interlaminar shear properties of the specimens remain largely unaffected by the testing temperature. However, the SEM micrographs show different damage progressions between the specimens tested at 100 °C and 120 °C. Fibre ruptures and longer delamination between the plies, as a result of a high temperature, are observed in the specimens tested at 120 °C, which are not observed in the specimens tested at 100 °C. In addition, the acoustic emission activities during the tests are investigated by using piezoelectric sensors. The information-theoretic parameter, the Lempel-Ziv (LZ) complexity, is calculated for the recorded acoustic signals. The LZ Complexities are used for identifying the occurrence of the first delamination failure in the specimens. Additionally, the two features of the acoustic signals, LZ complexity and Weighted Peak Frequency (W.P-Freq), are used for distinguishing the different damage sources in the CFRP specimens. The results are well-supported by the time-frequency analysis of the acoustic signals using a Continuous Wavelet Transform (CWT)

A full-field DIC analysis of the mechanical-deformation behavior of polyethylene terephthalate glycol (PET-G) aligners

Objectives: The aim was to investigate the full-field mechanical-deformation behavior of clear aligners made by polyethylene terephthalate glycol (PET-G) subjected to cyclic compression tests.Methods: Digital Image Correlation (DIC) (Chu et al., 1985; Schreier et al., 2009), a contactless full-field measurement technique, and Optical Microscope (OM) analysis were applied to study two PET-G aligners thermoformed from discs of a thickness of 0.75 mm and 0.88 mm. The clear aligners were placed on dental shape resin casts and were subjected to cyclic compression up to 13000 load cycles from 0 to 50 N at room temperature. The chosen number of load cycles simulates the average load history to which an aligner is subjected for one week. Local displacements and strains were measured for each test at 2, 6, 10, 20, 1000, 5000 and 13000 loading cycles.Results: Both aligners showed greater displacements in the early stages of loading, more pronounced for the 0.88 mm one. Local displacement and strain maps are derived both along the load direction and to the transverse one (never done from other researchers). Load-displacement cyclic curves allowed to evaluate the evolution of the stored energy and the stiffness during the test. The OM analyses showed significant morphological variations on the aligners' surface, such as wear and tear, high depressions and cracks, especially for the 0.75 mm specimen.Significance: Full-field analysis allowed to understand the mechanical behavior of device with complex geometry and complex load distribution, like invisible aligners. The knowledge of the stiffness and the direction of the total displacement helps the orthodontist to implement the best strategy to improve the patient's comfort and the treatment time