Dislocation Density Based Flow Stress Model Applied to the PFEM Simulation of Orthogonal Cutting Processes of Ti-6Al-4V.

Machining of metals is an essential operation in the manufacturing industry. Chip formation in metal cutting is associated with large plastic strains, large deformations, high strain rates and high temperatures, mainly located in the primary and in the secondary shear zones. During the last decades, there has been significant progress in numerical methods and constitutive modeling for machining operations. In this work, the Particle Finite Element Method (PFEM) together with a dislocation density (DD) constitutive model are introduced to simulate the machining of Ti-6Al-4V. The work includes a study of two constitutive models for the titanium material, the physically based plasticity DD model and the phenomenology based Johnson-Cook model. Both constitutive models were implemented into an in-house PFEM software and setup to simulate deformation behaviour of titanium Ti6Al4V during an orthogonal cutting process. Validation show that numerical and experimental results are in agreement for different cutting speeds and feeds. The dislocation density model, although it needs more thorough calibration, shows an excellent match with the results. This paper shows that the combination of PFEM together with a dislocation density constitutive model is an excellent candidate for future numerical simulations of mechanical cutting.

Coronal advanced flap in combination with a connective tissue graft. Is the thickness of the flap a predictor for root coverage? A prospective clinical study.

AIM: Evaluate if there is any relationship between the flap thickness (FT) and the presence of complete root coverage (CRC) when performing coronally advanced flaps in combination with a connective tissue graft (CTG). MATERIALS AND METHODS: Prospective clinical study, in which multiple Miller class I and II recessions were treated with a coronally advanced flap and a CTG standardized at 1 mm of thickness. Individual stents permitted repeated measurements of conventional periodontal parameters at the same point. The primary outcome variable was CRC. Secondary outcomes were recession reduction, gingival thickness and width of keratinized tissue (KT) achieved at 6 months post-surgery. RESULTS: Forty-five recessions (2.4 ± 0.75 mm) were treated in 20 patients. Mean root coverage was 93.4 ± 10.98%; 65% achieved CRC. The mean FT was 1.01 mm ± 0.64 mm and 1.01 ± 0.61 mm at 2 and 5 mm from the gingival margin, respectively. No relationship could be found between FT and CRC (p > .05). Statistical significant changes (p < .05) were observed for recession depth, clinical attachment level, KT and soft tissue thickness at the end of the study. CONCLUSIONS: Flap thickness seems not to be a predictor for CRC when performing a coronally advanced flap plus a CTG. This technique may be of choice when treating thin biotypes.