Browsing by Author "Freitas, Manuel"
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- Characterisation and evaluation of the mechanical behaviour of endodontic-grade NiTi wiresPublication . Pereira, Samuel; Carvalho, André; Reis, Luis; Freitas, Manuel; Montalvao, DiogoWith the introduction of new materials and advances in medical science, the endodontic files have changed since the early days of root canal treatments. In the late days, we have seen an increasing use of Nickel-Titanium (NiTi) alloys. At body temperature, NiTi alloys present a superelastic behaviour, which allows to be more effective in the removal of the tooth pulp tissue, and in the protection of the tooth structure. Anyhow, these NiTi instruments will eventually fracture, usually without any visual signal of degradation. Thus, there is a need of studying these alloys, as they present a high hysteresis cycle and non-linearities in the Elastic domain. Currently, there is no international standard to test NiTi endodontic files, so various authors have attempted to design systems that can test them under fatigue loads, usually based on empirical setups. Following a systematic approach, this work presents the results of rotary fatigue tests for two Alfa Aesar (R) Nitinol wires with different diameters (0.58mm and 0.25mm).The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions.
- Characterization and evaluation of the mechanical behaviour of endodontic-grade NiTi wiresPublication . Pereira, Samuel; Carvalho, André; Reis, Luís; Freitas, Manuel; Montalvão, DiogoThe orthodontic files have been used in dentistry since the middle ages and, as so, the shape, material and operation mode have changed since those days. In the late days, we have seen an increasing use of Nickel-Titanium (NiTi) alloys, to the detriment of more conventional alloys. At body temperature, these NiTi alloys present a superelastic behaviour, which allow the file to follow the teeth root in an easier way comparing to conventional alloys and have been reported to be more effective in the removal of the tooth pulp tissue, and in the protection of the tooth structure. Not withs tanding, these NiTi instruments, as all the others being subjected to bending loading, they fracture without any visual signal of degradation. Thereby, there is a need of studying these alloys, as they present a high hysteresis cycle and a high non-linearity in the Elastic Domain. Currently, there is not an international standard for these alloys, so various authors have attempted to design systems that can test NiTi endodontic files under fatigue loads, usually based on empirical setups. Following a systematic approach, this work presents the results of rotary fatigue tests for two Alfa Aesar® Nitinol wires with different diameters (0,58mm and 0,25mm).
- Determination of the rotary fatigue life of NiTi alloy wiresPublication . Carvalho, André; Montalvao, Diogo; Freitas, Manuel; Reis, Luís; Fonte, M.Nickel-Titanium (NiTi) alloys with superelastic properties have been increasingly introduced as a substitute to more conventional alloys, such as stainless steel, in a variety of applications. In Dentistry, NiTi alloys are used in tools such as Endodontic rotary files, allowing the file to follow teeth root canals more easily than their stainless steel counterparts. Nevertheless, during surgery, the file is subjected to cyclic bending loading, since it is rotating while being deformed inside the curved canals and is prone to fracture due to fatigue, without showing any visible signals of degradation. Following a systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (Memry and Euroflex). The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure. Experimental tests as well as numerical Finite Element Analysis (FEA) simulations are presented to better understand the fatigue fracture mechanisms present in NiTi alloys, showing that there is good agreement between the predicted strains (difficult to measure in such small wires) and the cycles to failure.
- Fatigue damage map of AZ31B-F magnesium alloys under multiaxial loading conditionsPublication . Anes, Vitor; Reis, Luís; Freitas, ManuelIn this work, the mechanical behavior of the AZ31B-F magnesium alloy under cyclic loading is analyzed with the goal of contributing to the advancement of its use in the design of AZ31B-F components and structures. To achieve this goal, an experimental program was implemented to evaluate the cyclic response of the AZ31B-F under specific proportional loads with different stress amplitude ratios. Afterwards, regression methods were applied to extend the experimental data to a wide range of proportional loads. As a result, the AZ31B-F damage map, a material property that stablishes the damage scale between normal and shear stresses for finite life loading regimes, was obtained. In addition, a safety factor was developed for the AZ31B-F material when subjected to proportional loading. The achieved results have a direct application in mechanical design of components/structures made of AZ31B-F contributing to its reliability.
- Magnesium alloy elastoplastic behaviour under multiaxial loading conditionsPublication . Anes, Vitor; Moreira, Rogério; Freitas, Manuel; Reis, LuísThe reduction of pollution and fuel consumption is an important goal to the transportation industry. The weight reduction of vehicles has a strong effect on the reduction of greenhouse gas emissions and on the fuel consumption. Nowadays magnesium alloys tend to replace steels and aluminium alloys in order to go further in the structural weight reduction. Magnesium alloys are greatly appreciated due to their high strength-to-weight ratio, stiffness, and low density. However, magnesium alloys can exhibit complex types of cyclic plastic deformation, like twinning and de-twinning effect. Recent researches indicate that these type of plastic deformation cannot be fully characterized using the typical tools used in steels, thus it is required a new approach to fully capture their fatigue cyclic deformation and plasticity. This research aims to obtain a phenomenological cyclic elastic-plastic model that captures the cyclic deformation of magnesium alloys under multiaxial loading conditions. In order to validate the achieved model, the numeric estimates were correlated with the experimental data and with the estimates of the Jiang & Sehitoglu plasticity model. The results show that the developed model estimates are in agreement with the experimental data.
- Rotary fatigue life of NiTi alloy wires and FEA modelling of fatigue damagePublication . Montalvão, Diogo; Carvalho, André; Freitas, Manuel; Reis, LuísNickel-Titanium (NiTi) alloys with superelastic properties have been increasingly introduced as a substitute to more conventional alloys. For example, NiTi alloys used in Dentistry, such as in Endodontic rotary files, possess superelastic properties that allow for the file to follow teeth root canals more easily than their stainless-steel counterparts. Nevertheless, during surgery, the file is subjected to cyclic bending loading, since it is spinning while being deformed inside the curved canal. Therefore, these instruments are prone to fracture due to fatigue, without showing any visible signals of degradation. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear. However, most existing test setups ignore the fracture mechanics involved in the fatigue phenomenon. In this work, the results of rotary fatigue tests for NiTi wires from different manufacturers is presented. The formulation is described, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure. Experimental tests as well as numerical Finite Element Analysis (FEA) simulations are presented to better understand the fatigue fracture mechanisms present in NiTi alloys, showing that there is good agreement between the predicted strains (difficult to measure in such small wires) and the cycles to failure. One characteristic is that these alloys exhibit a large hysteresis in the elastic domain if loaded up to the mixture of austenitic and martensitic phases (also known as B19’ martensite) and then unloaded. Rotating bending fatigue tests of NiTi wires show that, when loaded up to the B19’ martensite, the number of cycles to failure decrease with the applied strain.
- Rotary fatigue testing machine to determine the fatigue life of NiTi alloy Wwres and endondontic filesPublication . Carvalho, André; Freitas, Manuel; Reis, Luís; Montalvao, Diogo; Fonte, M.Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (MemryTM and EuroflexTM). The tests were done on a versatile fully automatic rotary bending testing machine. The formulation is also presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure.
- Rotary fatigue testing to determine the fatigue life of NiTi alloy wires: an experimental and numerical analisysPublication . Carvalho, André; Freitas, Manuel; Reis, Luís; Montalvao, Diogo; Fonte, M.Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (Memry™ and Euroflex™). The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions.