An Experimental Analysis of the High-Cycle Fatigue Fracture of H13 Hot Forging Tool Steels.

In this study, the axial fatigue behaviour of hot forging tool steels at room temperature was investigated. Fatigue tests were performed on two steels within the same H13 specification. The fatigue tests were carried out in the high-cycle fatigue domain under normal conditions. These tests were also performed on specimens in contact with a corrosive medium, applying stress values that led to the high-cycle fatigue domain under normal conditions for the sake of comparison. Both materials showed similar fatigue strengths when they were tested under normal conditions. In contrast, corrosion fatigue lives were much lower than in normal tests and differed significantly between the two steels. Crack initiation was triggered by microstructural and surface defects in the normal tests, whereas the formation of corrosion pits caused crack initiation in the corrosion fatigue tests. Moreover, a fracture surface analysis revealed dissimilar crack propagation areas between both steels, which suggested that both steels had different fracture toughness. These results were in line with the differences observed between the carbide and grain sizes of both of the material microstructures.

Characterization of Co-Cr-W Dental Alloys with Veneering Materials Manufactured via Subtractive Milling and Additive Manufacturing LDED Methods.

Laser-directed energy deposition (LDED) is an additive manufacturing (AM) technology which can be an alternative to the traditional subtractive milling process for the obtention of porcelain-fused-to-metal (PFM) prosthesis. Still, the adhesion performance of the veneering ceramics for this material has been not studied yet. The main objective of this study is to perform a systematic comparison of the adhesion performance of Co-Cr-W metal frameworks obtained through LDED and conventional milling techniques. Comparison includes microstructural, superficial, and adhesion analysis. Co-Cr manufactured via LDED technique presents similar behavior (p < 0.05) in comparison to the material obtained via milling techniques, and its performance was validated with the veneering ceramics and veneering composites currently employed in the dental industry.

Hyaluronic acid hydrogels reinforced with laser spun bioactive glass micro- and nanofibres doped with lithium.

The repair of articular cartilage lesions in weight-bearing joints remains as a significant challenge due to the low regenerative capacity of this tissue. Hydrogels are candidates to repair lesions as they have similar properties to cartilage extracellular matrix but they are unable to meet the mechanical and biological requirements for a successful outcome. Here, we reinforce hyaluronic acid (HA) hydrogels with 13-93-lithium bioactive glass micro- and nanofibres produced by laser spinning. The glass fibres are a reinforcement filler and a platform for the delivery of therapeutic lithium-ions. The elastic modulus of the composites is more than three times higher than in HA hydrogels. Modelling of the reinforcement corroborates the experimental results. ATDC5 chondrogenic cells seeded on the composites are viable and more proliferation occurs on the hydrogels containing fibres than in HA hydrogels alone. Furthermore, the chondrogenic behavior on HA constructs with fibres containing lithium is more marked than in hydrogels with no-lithium fibres.

Determination of forces on a split palatal screw after rapid maxillary expansion.

OBJECTIVES: Aim of this study was to develop a finite element model of the forces that patients with rapid maxillary expansion bear and to validate it by a mechanical test. METHODS: Computer-aided design models of the metallic screw and polymeric splint were modelled and discretized. Two forces were generated and considered independently: F1 at the temporary molar (2.5 N) and F2 at the permanent molar (2.5 N). The results of the finite element analysis were used to define the strain values which the anterior and posterior arms of the rapid maxillary expansion appliance bore as a linear function of F1-F2 by calculating the strain-force coefficient δ ij . Two strain gauge rosettes were attached to an appliance which was placed in an XY motorized stage to reproduce the same forces used in the finite element analysis. Once the system was validated, the matrix was inverted to determine forces F1 and F2 that a group of 40 patients underwent (median age 8.33 years, standard deviation 1.86 years) for 75 days, using their strain values. The parents of the patients activated a quarter turn (0.20 mm) twice a day until 50% transversal overcorrection was achieved. RESULTS: Finite element analysis showed that the effects of the forces on stress at the location of the arms were notably different. There was a satisfactory correlation between finite element analysis predictions and in vitro values. Dissipation of F1 and F2 in patients was predicted to be 62.5 and 80%, respectively, after 75 days of retention. CONCLUSION: These results back the finite element analysis model for force prediction.

Short-term effects of strain produced on a split palatal screw-type hyrax appliance after rapid maxillary expansion: A clinical trial.

INTRODUCTION: The aim of this study was to establish an accumulated strain pattern in different parts of rapid maxillary expansion appliances and relate them to different vertical growth patterns. A clinical study was conducted of 40 patients with posterior crossbite who required rapid palatal expansion. METHODS: Patients (mean age, 8.48 years) were recruited and treated at the Dental Hospital of Bellvitge, Hospitalet de Llobregat, Barcelona, Spain. Strain gauges were placed on the arms of the RME hyrax screw appliance to record deformation (strain) during the expansion and the retention phases for 75 days. A finite element model was used to place the gauge at the point where the strain was most expressed. The vertical coefficient of variation was used to classify the patients by their vertical growth pattern. P = 0.05 was considered to be statistically significant. RESULTS: During the expansion phase, the highest values of accumulated strain were measured in the posterior part of the appliance for all facial biotypes, but these values passed to the anterior area at the end of the retention phase of the mesocephalic and brachycephalic patients. There was statistically significant difference in the strain of the posterior arms in accordance with the vertical growth pattern (P = 0.05) during the retention phase. At 75 days of retention, 61.25% of the arms had already begun to have strain dissipation. CONCLUSIONS: The accumulated strain pattern in the rapid maxillary expansion appliance can vary depending on the facial biotype. In the future, orthodontists should try to tailor the activation and retention protocol based on each patient's characteristics.