Interpreting finite element results for brittle materials in endodontic restorations

Background: Finite element simulation has been used in last years for analysing the biomechanical performance of post-core restorations in endodontics, but results of these simulations have been interpreted in most of the works using von Mises stress criterion. However, the validity of this failure criterion for brittle materials, which are present in these restorations, is questionable. The objective of the paper is to analyse how finite element results for brittle materials of endodontic restorations should be interpreted to obtain correct conclusions about the possible failure in the restoration. Methods: Different failure criteria (Von Mises, Rankine, Coulomb-Mohr, Modified Mohr and Christensen) and material strength data (diametral tensile strength and flexural strength) were considered in the study. Three finite element models (FEM) were developed to simulate an endodontic restoration and two typical material tests: diametral tensile test and flexural test. Results: Results showed that the Christensen criterion predicts similar results as the Von Mises criterion for ductile components, while it predicts similar results to all other criteria for brittle components. The different criteria predict different failure points for the diametral tensile test, all of them under multi-axial stress states. All criteria except Von Mises predict failure for flexural test at the same point of the specimen, with this point under uniaxial tensile stress. Conclusions: From the results it is concluded that the Christensen criterion is recommended for FEM result interpretation in endodontic restorations and that the flexural test is recommended to estimate tensile strength instead of the diametral tensile tes

Numerical tooth contact analysis of gear transmissions through the discretization and adaptive refinement of the contact surfaces

The tooth contact analysis (TCA) is an important resource for the design of gear drives. This widely used analysis provides the contact pattern, contact path and the function of transmission errors that are directly related to the performance of the gear set. In this work, a new geometric approach for the TCA is proposed. This approach is general, deterministic and independent from the type or alignment status of the gears. It is based on the discretization of the contact surfaces of the reference teeth pair and on a geometrically adaptive refinement to solve the contact problem and to compute the instantaneous contact area for each position of the gear set along the gearing cycle. The new algorithm demonstrated to be versatile, robust and efficient through different test cases, obtaining accurate results with a relatively low computational cost.The authors express their deep gratitude to the Spanish Ministry of Science and Innovation (MICINN) for the financial support of research project ref. DPI2013-47702-C2-2-P

Estudio del efecto de abducción del pulgar en una prótesis de mano de bajo coste: una propuesta de Benchmark

Comunicació presentada a la IX Reunión del Capítulo Español de la Sociedad Europea de Biomecánica (ESB 2019)La irrupción en el ámbito doméstico de la tecnología de impresión en 3D por deposición fundida (FDM) ha permitido mejorar los diseños de prótesis de mano bajo coste (BC) que intentan cubrir las necesidades más básicas con el mínimo tiempo y coste (normalmente menos de 450€1,2), bajo la premisa de "hágalo usted mismo" (DIY). Hoy en día, muchos diseños de manos de BC pueden descargarse libremente como un archivo de estereolitografía (.stl) desde distintos repositorios de diseño (instructables.com, thingiverse.com) o, más explícitamente, desde iniciativas sin ánimo de lucro como e-NABLE3, Open Hand Project1 y Openbionics4. La experiencia de uso con estos diseños sigue poniendo de manifiesto la necesidad de estandarizar las pruebas para caracterizar y comparar la mecánica de todas las tipologías de prótesis de mano de BC. El presente trabajo compara la funcionalidad y la actuación mecánica de dos variantes del modelo de una prótesis de mano de bajo coste. En el proceso, se proponen diferentes técnicas y se evalúa su interés para formar parte de un benchmark

Affordable 3D-printed tendon prosthetic hands: Expectations and benchmarking questioned

The popularization of 3D-printing has allowed enhancing affordable prostheses for persons with amputations in developing countries, yet manufacturers are not subjected to any control from any medical regulatory authority. Adopted evaluation protocols seem to cherish optimistic expectations. A reduced performance test, derived from the Southampton Hand Assessment Procedure and two bench tests, to evaluate the mechanical advantage in the fingers and the slip resistance, are proposed to assess affordable tendon-driven devices. Ultimately, five models amongst those most commonly found in the scientific literature and the Internet have been evaluated. Three subjects participated with the aid of an able-bodied adaptor. The reduced test of performance provides consistent results but with a more direct interpretation of the failed patterns of prehension. All these models create far more expectations than the results deliver. With the supplementary material provided, an affordable benchmarking can be established with this reduced performance test and the two bench tests. They can lead to improved designs, prescriptions and regulations

Determination of the ISO face load factor in spur gear drives by the finite element modeling of gears and shafts

The face load factor is a common coefficient used in gear design standards that takes into account the uneven distribution of load across the face width of the gears caused by the mesh misalignment. In this paper, a finite element model that includes the gears and the corresponding shafts is proposed. The results obtained from the application of finite element analysis to this model are compared with those obtained from application of the ISO Standard 6336 coefficient-based method (Method C). The influence of the length of gear shafts, the face width of the gears, the relative position of the gears over their shafts, the ratio between the pitch radii of the gears and the radii of their shafts, and the relation between the mesh misalignment and the face load factor, have been investigated.The authors express their deep gratitude to the Spanish Ministry of Science and Innovation for the financial support of research project refs. DPI2010-20388-C02-01 and DPI2010-20388-C02-02

Grasp modelling with a biomechanical model of the hand

The use of a biomechanical model for human grasp modelling is presented. A previously validated biomechanical model of the hand has been used. The equilibrium of the grasped object was added to the model through the consideration of a soft contact model. A grasping posture generation algorithm was also incorporated into the model. All the geometry was represented using a spherical extension of polytopes (s-topes) for efficient collision detection. The model was used to simulate an experiment in which a subject was asked to grasp two cylinders of different diameters and weights. Different objective functions were checked to solve the indeterminate problem. The normal finger forces estimated by the model were compared to those experimentally measured. The popular objective function sum of the squared muscle stresses was shown not suitable for the grasping simulation, requiring at least being complemented by task-dependent grasp quality measures

Implementation of Hertz theory and validation of a finite element model for stress analysis of gear drives with localized bearing contact

An analytical approach for stress analysis of gear drives with localized bearing contact based on the Hertz theory is proposed. The proposed approach provides a complete and effective solution of the contact problem but satisfaction of the hypotheses for application of the Hertz theory is its main drawback. On the other hand, a finite element model has been developed and validated in terms of the contact area, maximum contact pressure, pressure distribution, maximum Tresca stress, and Tresca stress distribution underneath the contacting surfaces. Validation of the finite element model is provided for those cases wherein the Hertz theory can be applied. The obtained results confirm the applicability of the proposed approach for gear drives with localized bearing contact wherein edge contact is avoided by surface modifications and whole crowning of tooth surfaces is provided

Interpreting finite element results for brittle materials in endodontic restorations

Abstract Background Finite element simulation has been used in last years for analysing the biomechanical performance of post-core restorations in endodontics, but results of these simulations have been interpreted in most of the works using von Mises stress criterion. However, the validity of this failure criterion for brittle materials, which are present in these restorations, is questionable. The objective of the paper is to analyse how finite element results for brittle materials of endodontic restorations should be interpreted to obtain correct conclusions about the possible failure in the restoration. Methods Different failure criteria (Von Mises, Rankine, Coulomb-Mohr, Modified Mohr and Christensen) and material strength data (diametral tensile strength and flexural strength) were considered in the study. Three finite element models (FEM) were developed to simulate an endodontic restoration and two typical material tests: diametral tensile test and flexural test. Results Results showed that the Christensen criterion predicts similar results as the Von Mises criterion for ductile components, while it predicts similar results to all other criteria for brittle components. The different criteria predict different failure points for the diametral tensile test, all of them under multi-axial stress states. All criteria except Von Mises predict failure for flexural test at the same point of the specimen, with this point under uniaxial tensile stress. Conclusions From the results it is concluded that the Christensen criterion is recommended for FEM result interpretation in endodontic restorations and that the flexural test is recommended to estimate tensile strength instead of the diametral tensile test.</p