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244 Victor Cotoros and Eugen Merticaru the computer through equations that can be solved. The partitioning of the entire structure in many small parts is named discretization. Through this discretization, a difficult problem (practicly impossible to solve) is replaced with a simplier one. Mechanical stresses obtained with finite element method analysis are known as "von Mises stresses", these being equivalent mechanical stresses, that are icluding both normal and shear stresses. These stresses are giving an overall image regading the stress distribution into the analyzed assembly, they being able to be used for dimensioning some technical elements. For the proposed purpose, there was started from the virtual model of the osteosynthesis assembly bone-implant, designed in CAD software SolidWorks and ProEngineering. The model was then imported in the software ANSYS for finite element analysis. ANSYS is a software for numerical simulation with finite element. It is used to analyse the complex problems of mechanical structures, thermal proceses, fluid mechanics, magnetism, electric field etc. ANSYS has many graphical abilities that may be used to assess and present the anaysis results. There were analyzed 3 surgical orthopedical procedures: osteosynthesis with malleolar screw; osteosynthesis with Kirschner broaches and link with metalic wire; osteosynthesis with Kirschner broach, link with metalic bracing wire and cortical screw. 2. Finite Element Analysis of the Osteosynthesis with Malleolar Screw There was started from the virtual model of the osteosynthesis assembly bone-implant screw, designed in the CAD software SolidWorks and ProEngineering. The model was then imported in the software ANSYS for finite element analysis. The geometrical complexity of the implanted functional unit leads to a complex static structural behavior which is impossible to be assessed by classic strength calculus. For this reason, there was used the finite element analysis with the software ANSYS. The assessment of the state of stress, strain and contact of the designed elements was done by structural static analysis of the assembly, in the same conditions of loading and support. Thus, there were assessed the effects of loading at the level of interface bone-screw. In order to assess the state of stress and strain that occurs in bone, due to malleolar screw, the osteosynthesis assembly was analyzed with the finite element method for traction loading, as can be seen in figure 1. This mechanical loading was considered to be the most disadvantageous for fracture osteosynthesis, that is, the type of loading which shows if such a type of assembly resists or not to maintain the contact between the fracture fragments. At the same time, the analytical data can be compared with the experimental data, because the experimental testing to traction is an usual loading for special testing machines.
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 245 The mechanical properties of the elements of the analyzed assembly, required for finite element analysis in ANSYS, are presented in Table 1. separati on F F Fig. 1 – Traction loading of the assembly bone-implant. Table 1 Mechanical properties required for FEA Materials Elascicity modulus E [MPa] Poisson coefficient, ν Austenitic stainless steel 1.93⋅10 5 0.31 7.86⋅10 -6 Porous bone 100 0.27 1⋅10 -6 Periosteum 15.3⋅10 3 0.28 1.8⋅10 -6 Compact (cortical) bone 17.5⋅10 3 0.28 -6 2⋅10 Density 3 ρ [kg/mm ] The first stage of the numerical analysis with finite element was to import the assembly CAD model and to declare all the materials involved in analysis. Thus, the uased materials were: austenitic stainless steel for the screw, porous bone for the internal layer of the lower epiphysis of tibia, periosteum for surface layer of tibia and compact (cortical) bone for lower layer of tibia diaphysis. The next stage was to declare the contact between elements. This operation was done automatically, the software ANSYS identifiyng the surfaces in contact and declaring links. Numerical analysis with finite element continued by dicretizing the assembly into finite elements. Discretization was done automatically, using tetrahedrical elements, respectively “10-Node Tetrahedral Structural Solid”. Initially, was tried manual discretization of the assembbly, but certain regions of the threaded zone could not be discretized due to assigning too small dimensions for discretized elements. Thus, the automatic discretization was chosen. Another reason for chosing automatic discretization is that the calculus requirements are increasing exponentially at the same time with decreasing the
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BULETINUL INSTITUTULUI POLITEHNIC D
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