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.

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).

Mechanical Characterization of the Plastic Material GF-PA6 Manufactured Using FDM Technology for a Compression Uniaxial Stress Field via an Experimental and Numerical Analysis.

This manuscript presents an experimental and numerical analysis of the mechanical structural behavior of Nylstrong GF-PA6, a plastic material manufactured using FDM (fused deposition modeling) technology for a compression uniaxial stress field. Firstly, an experimental test using several test specimens fabricated in the Z and X-axis allows characterizing the elastic behavior of the reinforced GF-PA6 according to the ISO 604 standard for uniaxial compression stress environments in both Z and X manufacturing orientations. In a second stage, an experimental test analyzes the structural behavior of an industrial part manufactured under the same conditions as the test specimens. The experimental results for the test specimens manufactured in the Z and X-axis present differences in the stress-strain curve. Z-axis printed elements present a purely linear elastic behavior and lower structural integrity, while X-axis printed elements present a nonlinear elastic behavior typical of plastic and foam materials. In order to validate the experimental results, numerical analysis for an industrial part is carried out, defining the material GF-PA6 as elastic and isotropic with constant Young's compression modulus according to ISO standard 604. Simulations and experimental tests show good accuracy, obtaining errors of 0.91% on the Z axis and 0.56% on the X-axis between virtual and physical models.

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.