Browsing by Author "Baptista, Ricardo"
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- An algorithm for fatigue crack growth applied to mixed and biaxial mode loadingsPublication . Baptista, Ricardo; Infante, V.; Freitas, M.Fatigue is still one of the main concerns when dealing with mechanical components failure. While it is fundamental to experimentally determine the fatigue material behavior using standard specimens, testing large and complex component geometries can be complicated. In these cases, the Finite Element Method can be a cost-effective solution but developing fatigue crack growth models is still a complicated task. In order to solve this problem, an algorithm for automatic crack propagation was developed. Using three different modules, the algorithm can generate a complex Finite Element Method model including a fatigue crack; solve this model considering complex loading conditions, by applying the superposition method; and calculate the fatigue crack propagation rate, using it to update the original model. In order to benchmark this solution two different problems were analyzed, a modified compact tension specimen and a cruciform specimen. By modifying the compact tension specimen hole location and simulating an initial crack, it was possible to understand how mixed mode conditions influence the fatigue crack path. Different load ratios and initial crack directions on the cruciform specimen were analyzed. Increasing the load ratio will increase the crack deflecting angle. The obtain solutions were compared with experimental results, showing good agreement. Therefore the developed algorithm can be used to predict the fatigue crack growth behavior on complex geometries and when different types of loads are applied to the component.
- Automatic fatigue crack propagation calculation on welded high strength steelsPublication . Baptista, Ricardo
- Desenvolvimento de filmes para regeneração ósseaPublication . Baptista, Ricardo; Caria, Helena; Silva, SusanaA regeneração óssea constitui um campo de crescente importância tendo em contas as limitações inerentes às abordagens tradicionais, utilizadas no tratamento de lesões ósseas. Nesse contexto, surge a necessidade de desenvolver soluções inovadoras e eficazes para a restauração do tecido ósseo danificado. O potencial impacto deste estudo é de magnitude considerável, uma vez que se apresenta como uma possível via para melhorar a qualidade de vida dos doentes que enfrentam lesões ósseas, acelerando o processo de recuperação e mitigando as complicações associadas. O principal objetivo deste trabalho consiste no desenvolvimento de filmes compostos à base de biomateriais, com o propósito de servir como estrutura de suporte à regeneração do tecido ósseo. Para tal, foram desenvolvidas formulações e processos de fabrico destes filmes, seguindo- se a avaliação das suas propriedades físicas, químicas e biológicas, visando determinar a sua eficácia como estruturas que promovam a regeneração óssea. Foram produzidas três composições base dos filmes, fazendo variar as percentagens de alginato e ι-carragenina, possibilitando assim, analisar o impacto da composição base no resultado final. Adicionalmente, foram exploradas duas metodologias para a incorporação de hidroxiapatite (HA) nos filmes: a suspensão direta de HA nas soluções filmogénicas e a deposição da HA durante o processo de cura. O processo de cura compreendeu a imersão de uma ou ambas as faces do filme numa solução de cloreto de cálcio. Os resultados indicam que a composição mais eficaz foi a 75%/25%. Além disso, a cura em ambas as faces do filme proporcionou, em geral, melhores resultados, atingindo um módulo de elasticidade de 6794 MPa e uma tensão de carga máxima de 66 MPa. A inclusão de HA na composição dos filmes resultou numa redução de aproximadamente 65 % do módulo de elasticidade, no entanto, a incorporação de HA na solução de cura demonstrou melhorias nos ensaios de molhabilidade, com um ângulo de contacto mínimo de 9,0 º. Por fim, os ensaios biológicos validaram os filmes como uma alternativa viável para a regeneração óssea, evidenciando a adesão e proliferação celular. O filme que se destacou com melhores resultados foi o filme 75-25-C2L, apresentando uma viabilidade média de 84%, contribuindo assim para avanços práticos e inovadores no campo dos biomateriais para aplicações ortopédicas e traumáticas
- Design optimization of cruciform specimens for biaxial fatigue loadingPublication . Baptista, Ricardo; Cláudio, Ricardo; Reis, L.; Guelho, I.; Freitas, M.; Madeira, J. F. A.In order to correctly assess the biaxial fatigue material properties one must experimentally test different load conditions and stress levels. With the rise of new in-plane biaxial fatigue testing machines, using smaller and more efficient electrical motors, instead of the conventional hydraulic machines, it is necessary to reduce the specimen size and to ensure that the specimen geometry is appropriated for the load capacity installed. At the present time there are no standard specimen’s geometries and the indications on literature how to design an efficient test specimen are insufficient. The main goal of this paper is to present the methodology on how to obtain an optimal cruciform specimen geometry, with thickness reduction in the gauge area, appropriated for fatigue crack initiation, as a function of the base material sheet thickness used to build the specimen. The geometry is optimized for maximum stress using several parameters, ensuring that in the gauge area the stress is uniform and maximum with two limit phase shift loading conditions. Therefore the fatigue damage will always initiate on the center of the specimen, avoiding failure outside this region. Using the Renard Series of preferred numbers for the base material sheet thickness as a reference, the reaming geometry parameters are optimized using a derivative-free methodology, called direct multi search (DMS) method. The final optimal geometry as a function of the base material sheet thickness is proposed, as a guide line for cruciform specimens design, and as a possible contribution for a future standard on in-plane biaxial fatigue tests.
- Fatigue behavior of different geometry scaffolds for bone replacementPublication . Baptista, Ricardo; Guedes, M.When transplanting bone tissue is not a possibility, tissue engineering is responsible for developing solutions to substitute the functions of the missing bone structure or support the process of bone regeneration. Scaffolds can be used to fulfill this mission by supporting loads that were applied to the missing bone, supporting the cell regenerating process, allowing for the necessary nutrients and oxygen diffusion and delivering growth factors or drugs. Scaffold geometry design must support static and dynamic loads up to 20 MPa in order to replace human trabecular bone. Also, it should generate macro and micro pores to support cell growth and mineral precipitation, while all pores should be interconnected for nutrient and oxygen diffusion. Scaffolds were fabricated according to ASTM-695 standard, using two different layouts, 50% porosity and a theoretical distance of 0.8 mm between each filament. A 400 µm diameter nozzle was used, and scaffolds were produced at 215 ºC with deposition rate of 30 mm/s. Both designs were fatigue tested until 3600 cycles, using different load amplitudes and a frequency of 0.25 Hz. The orthogonal scaffold showed improved behavior, with compression modulus reaching 680 MPa, when a maximum stress of 14.5 MPa was applied.
- Modelling of a prosthetic hand featuring individual grip on each one of the five fingersPublication . Rosa, Joana; Sousa, Joana; Baptista, RicardoThe aim of this work is to develop a prosthetic hand with individual grip functionality on each finger, reproducing the natural movement of the hand. After the preparation of sketches and creating a virtual model, the final model shows dimensions: 124,04 mm × 246,57 mm × 38 mm (palm-covered) and an estimated weight of 1 kg, with all components. Afterwards was executed the functioning simulation of the prosthetic hand, elaborated prehension experiments were made using a metal cylinder, a rubber ball and a wooden pencil, with the tool Motion Study from Solidworks. However in the tests with the pencil was difficult to reach equilibrium, tells us that the hand is not functional with width smaller objects, because they require a bigger finger precision. Was made the study of forces applied in links thinner in different prehensions using the Finite Element Method. This study concluded that the aluminum alloy 2024 T3 has enough resistance to the stresses incurred for the grasping. Finally, was made an estimate cost of production for a single working prototype prosthetic hand. The result was 1325€.