A strain rate dependent model with decreasing Young's Modulus for cortical human bone.

In the existing literature, some studies have observed an increase in the elastic modulus of human cortical bone with strain rate, which has been described as a consequence of the viscoelastic properties of the bone. However, these results contradict the findings of other studies, in which an independence or decrease of the elastic modulus with strain rate is observed, which could be explained by other non-viscoelastic mechanisms. This research studies the dynamic behavior of human cortical bone specimens and investigates their mechanical properties . A full and objective strain rate dependent model is proposed and used to describe the experimental results obtained from uniaxial tensile tests of twenty-one human rib cortical bone specimens from twelve male post mortem human subjects (average age of 68.5 ± 12.3 years). In addition, a general discussion of some families of viscoelastic models is given and the caution with which they should be used when dealing with complex materials such as bone. The main experimental finding is that in the range of strain rate analyzed (ε̇=0.10-0.60), there is a significant decrease in Young's modulus (E≈ 18 GPa forε̇=0.10s-1andE≈ 8 GPa forε̇=0.50s-1), which is not of viscoelastic origin. Moreover, the most frequently used viscoelastic models analyzed in this study predict how the elastic modulus should not vary markedly with strain rate for small strains. In fact, the observed behavior seems related to the findings of other researchers who observed that the microcraking damage depends on the strain rate in the same sense found in our work. This allows us to interpret the qualitative results as a consequence of the microcracking that takes place within the cortical bone, and not related to viscoelastic effects.

Simulación Mediante Elemento Finito de Sistemas Intramedulares Telescópicos para Rehabilitación de Pacientes con Osteogénesis Imperfecta / Simulation by Finite Element Method of Intramedullary Telescopic Systems for Rehabilitation of Patients with Osteogenesis Imperfecta

Resumen En el trabajo se plantea el análisis de diferentes geometrías para un dispositivo intramedular, las cuales ayudan a reducir y evitar la migración, deformación y rotura del implante en tejido óseo afectado con Osteogénesis Imperfecta (OI). Se realizaron diseños en CAD de diferentes prototipos, donde se analizan las propiedades mecánicas en el alma del dispositivo, así como en las roscas distal y proximal de los implantes macho y hembra. Asimismo, se obtuvieron modelos 3D de huesos de un infante afectado con OI para realizar simulaciones mediante elemento finito de la interacción entre el hueso y el dispositivo intramedular. Los resultados muestran que los prototipos propuestos disminuyen la deformación del dispositivo, así como el aumento en la rigidez de la relación hueso-prótesis. Asimismo, las roscas generaron un menor esfuerzo en la unión con el hueso, lo que prevé un menor daño al tejido óseo. El trabajo se limitó al análisis numérico del rediseño de implantes telescópicos intramedulares para afectados con OI. Concluyendo que la geometría semicircular 3/4 de caña, otorga un óptimo resultado en las pruebas realizadas, al tiempo que las roscas ACME proveen una mejor sujeción en las epífisis distal y proximal de los huesos largos.

Abstract In this work we propose the analysis of different geometries for an intramedullary device, which help to reduce and avoid the migration, deformation and rupture of the implant in bone tissue affected with Osteogenesis Imperfecta (OI). Designs of different prototypes were made in CAD, where the mechanical properties in the device's soul are analyzed, as well as in the distal and proximal threads of the male and female implants. Likewise, 3D bone models of an affected infant with OI were obtained to perform finite element simulations of the interaction between the bone and the intramedullary device. The results show that the prototypes proposed decrease the strain of the device, as well as the increase in the stiffnes of the bone-prosthesis relationship. Also, the threads generated less stress in the union with the bone, which provides less damage to the bone tissue. The work was limited to the numerical analysis of the redesign of intramedullary telescopic implants for patients with OI. Concluding that the semicircular geometry 3/4 of cane, gives an optimal result in the tests carried out, while the ACME threads provide a better subjection in the distal and proximal epiphyses of the long bones.

Numerical Low-Back Booster Analysis on a 6-Year-Old Infant during a Frontal Crash Test.

This work studies descriptively the Head Injury Criterion (HIC) and Chest Severity Index (CSI), with a finite element model of the Hybrid III dummy type, for six-year-old subjects in a frontal vehicular collision, using the low-back booster (LBB) passive safety system. The vehicle seats and the passive safety systems were modelled in CAD (computer aided design) software. Then, the elements were analysed by the finite element method (FEM) in LS-DYNA® software. The boundary conditions were established for each study, according to the regulations established by the Federal Motor Vehicle Safety Standard (FMVSS), following the FMVSS 213 standard. The numerical simulations were performed during an interval of 120 ms and recording results every 1 ms. In order to analyse the efficiency of the system, the restraint performance of the LBB system is compared with the restraint configuration of the vehicle safety belt (VSB) only. The obtained injury criteria with the LBB system shows its ability to protect children in a frontal collision. The analyses allow obtaining the deceleration values to which the dummy head and chest was subjected. Of the studies herein performed, Study I: VSB obtained a HIC36 of 730.4 and CSI of 315.5, while Study II: LBB obtained a HIC36 of 554.3 and CSI of 281.9. The outcome shows that the restraint efficiency of each studied case differs. Used materials, the attachment system of the LBB, and the belt restraint system properly placed over the infant trunk are the main factors reducing the injury criteria rate.

Frontal crashworthiness characterisation of a vehicle segment using curve comparison metrics.

The objective of this work is to propose a methodology for the characterization of the collision behaviour and crashworthiness of a segment of vehicles, by selecting the vehicle that best represents that group. It would be useful in the development of deformable barriers, to be used in crash tests intended to study vehicle compatibility, as well as for the definition of the representative standard pulses used in numerical simulations or component testing. The characterisation and selection of representative vehicles is based on the objective comparison of the occupant compartment acceleration and barrier force pulses, obtained during crash tests, by using appropriate comparison metrics. This method is complemented with another one, based exclusively on the comparison of a few characteristic parameters of crash behaviour obtained from the previous curves. The method has been applied to different vehicle groups, using test data from a sample of vehicles. During this application, the performance of several metrics usually employed in the validation of simulation models have been analysed, and the most efficient ones have been selected for the task. The methodology finally defined is useful for vehicle segment characterization, taken into account aspects of crash behaviour related to the shape of the curves, difficult to represent by simple numerical parameters, and it may be tuned in future works when applied to larger and different samples.