Thesis ANÁLISIS DEL COMPORTAMIENTO BAJO CARGAS DINÁMICAS DE UNA PRÓTESIS DE PIE FABRICADA MEDIANTE IMPRESIÓN 3D
Date
2019
Authors
PASTÉN CORTÉS, DANIEL IGNACIO
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Abstract
Dado el creciente número de pacientes con discapacidades motoras originadas ya sea por amputaciones o por pérdida de función muscular, se hace cada día más necesario implementar nuevos procesos de manufactura que permitan llegar a diseños de prótesis más eficientes en términos de costos y desempeño. En el presente trabajo se desarrolla un estudio de la vida útil esperada de una prótesis de pie fabricada mediante impresión 3D en la cual se utiliza ABS como material de base. Tomando tres diferentes patrones de marcha en un paciente promedio (caminata, caminata rápida, trote ligero), se desarrolló un análisis para cuantificar las cargas dinámicas más críticas, y con estas, hacer un análisis del estado de esfuerzos medios y fluctuantes en puntos críticos de la prótesis. Como una primera aproximación al análisis del estado de esfuerzos en este tipo de materiales celulares, se utilizaron las propiedades mecánicas de rigidez y resistencia para un material impreso y se asumió la prótesis como un sistema multi-cuerpo compuesto por piezas fabricadas con un material isotrópico y homogéneo. Las propiedades a fatiga del material fueron adaptadas de las propiedades del material inyectado y utilizadas para determinar la vida a fatiga de cada una de las piezas de acuerdo con el criterio de Goodman-modificado. Dada la complejidad de la estructura celular y los múltiples concentradores de tensiones la resistencia a fatiga estimada para el material impreso es muy baja (Se~7MPa para 30x103 ciclos). Los esfuerzos medios inducidos por el apriete en los tornillos que se utilizan para ajustar el ensamble son elevados en comparación con los esfuerzos alternantes generados por los tres patrones de marcha estudiados. Como resultado de esta condición la vida útil calculada para la prótesis es insensible al patrón de marcha llegando a tan solo 27 días (105 ciclos). Basado en los resultados obtenidos en el presente estudio se recomienda modificar el diseño de la prótesis evitando en lo posible tener concentradores de esfuerzos como lo ocasionados por el apriete de los tornillos en el proceso mismo de ensamble. De igual manera se observa que el efecto que tienen los resortes en la absorción de impacto en este diseño en particular no es significativo. Se recomienda remover los resortes y utilizar solamente polímeros densos entre partes móviles para atenuar el impacto de las cargas entre piezas y hacia el mismo paciente.
Given the increasing number of patients with mobility impairments caused either by amputation or by loss of muscle function, it is getting more necessary to implement new manufacturing procedures that allow more efficient prosthesis designs in terms costs and performance. In the present work a study of the expected lifespan of a foot prosthesis manufactured by 3D printing is developed in which ABS is used as a base material. Taking three different walking patterns in an average patient (walking, fast walking, light jogging), an analysis was developed to quantify the most critical dynamic loads, and with these, to make an analysis of the state of mean and alternating stresses at critical points of the prosthesis. As a first approach to the analysis of the state of stress in this type of cellular materials, the mechanical properties of stiffness and strength for a printed material were used and the prosthesis was assumed as a multi-body system composed of pieces made of an isotropic and homogeneous material. The fatigue properties of the material were adapted from the properties of the injected material and used to determine the fatigue life of each of the parts according to the Goodman-modified criteria. Given the complexity of the cell structure and the multiple stress concentrations, the fatigue resistance estimated for the printed material is very low (Se ~ 7MPa for 30x103 cycles). The mean stresses induced by the adjustment of the screws used to regulate the assembly are high compared to the alternating stresses generated by the three running patterns studied. As a result of this condition, the calculated life for the prosthesis is indifferent to the walking pattern, reaching only 27 days (105 cycles). Based on the results obtained in the present study it is recommended to modify the design of the prosthesis avoiding as much as possible stress concentrations as caused by the fit of the screws in the assembly process itself. Similarly, it is observed that the effect that springs have on the absorption of impact in this particular design is not significant. It is recommended to remove the springs and use only dense polymers between moving parts to attenuate the impact of the loads between pieces and towards the patient.
Given the increasing number of patients with mobility impairments caused either by amputation or by loss of muscle function, it is getting more necessary to implement new manufacturing procedures that allow more efficient prosthesis designs in terms costs and performance. In the present work a study of the expected lifespan of a foot prosthesis manufactured by 3D printing is developed in which ABS is used as a base material. Taking three different walking patterns in an average patient (walking, fast walking, light jogging), an analysis was developed to quantify the most critical dynamic loads, and with these, to make an analysis of the state of mean and alternating stresses at critical points of the prosthesis. As a first approach to the analysis of the state of stress in this type of cellular materials, the mechanical properties of stiffness and strength for a printed material were used and the prosthesis was assumed as a multi-body system composed of pieces made of an isotropic and homogeneous material. The fatigue properties of the material were adapted from the properties of the injected material and used to determine the fatigue life of each of the parts according to the Goodman-modified criteria. Given the complexity of the cell structure and the multiple stress concentrations, the fatigue resistance estimated for the printed material is very low (Se ~ 7MPa for 30x103 cycles). The mean stresses induced by the adjustment of the screws used to regulate the assembly are high compared to the alternating stresses generated by the three running patterns studied. As a result of this condition, the calculated life for the prosthesis is indifferent to the walking pattern, reaching only 27 days (105 cycles). Based on the results obtained in the present study it is recommended to modify the design of the prosthesis avoiding as much as possible stress concentrations as caused by the fit of the screws in the assembly process itself. Similarly, it is observed that the effect that springs have on the absorption of impact in this particular design is not significant. It is recommended to remove the springs and use only dense polymers between moving parts to attenuate the impact of the loads between pieces and towards the patient.
Description
Keywords
PROTESIS DE PIE , MANUFACTURA ADITIVA , IMPRESION 3D , FATIGA MULTI-AXIAL