Papers by Author: Cristian Sorin Nes

Abstract: This work presents an experimental program to determine the mechanical properties of cancellous bone in the femoral head as a function of location. To achieve this several specimens of cancellous bone of approximately 10 mm height and 10 mm diameter were obtained from one human femoral head, starting the sampling from its main loading compressive direction. All specimens underwent compression testing in order to determine the mechanical properties of each specimen and thus a properties map of the cancellous bone in the femoral head was obtained. Based on the results a parametric file with material properties was created in order to be used by professionals in finite element analysis programs.

Abstract: Aluminium clad steel reinforced cables are widely used in long distance electrical energy transport. Under the influence of weather conditions, the conductors are subjected to additional stress, which may lead to accelerated deterioration and premature breakage. This phenomenon depends much on contact mechanics. Due to the rough geometry of real mechanical surfaces, the elastic-plastic contact between bodies occurs at several points simultaneously. The contact between wires or between the wires and the suspension clamp, which is regarded as a critical location, changes the mechanical properties of the conductor with the emergence and development of contact indents. In order to understand the development of the indents ant their influence in the properties of the wires, indentation, tensile and fatigue tests were performed on wires taken from a steel-aluminium conductor. The problem of normal contact when plastic deformations are much higher than the elastic ones is extremely complex. The convex profile of the wires brings additional complications. Specific strain intensity is proportional to the depth of penetration. Experimental researches have shown that the shape and size of the indents occurred at the contact points have a significant influence on the tensile strength and lifetime of the conductors and local deformations determine certain features of fatigue crack initiation and propagation.

Abstract: Objectives: The mechanical characteristics of bone cement depend on the chemical composition, mixing technique, polymerization period and temperature. Additionally, the shape of the cement mantle can also lead to variations in the mechanical properties of the material, as thicker walls are significantly more non-homogeneous and less permeable for the body fluids. This paper presents experimental tensile tests on bone cement focused on the study of the influence of the test specimen shape on the obtained results.Methods: High viscosity bone cement, produced and supplied as polymer powder and monomer liquid portions was used. The mixed cement was inserted during the working time into two types of silicon molds: one with the shape and size according to ISO 527 (“dogbone” shape with rectangular cross section area) and other with the shape and size according to ASTM F 2118 (“dogbone” shape with circular cross section area). Both types of test specimens have approximately the same value of the cross sectional gauge area. All samples were polished with grit abrasive paper in the longitudinal direction and soaked in phosphate buffered saline (PBS) solution. The specimens were subjected to a tensile test on a hydraulic machine. To avoid the occurrence of critical stress areas on the specimen, special fixing devices were manufactured.Results and conclusions: The stress–strain curves of all specimens tested exhibited similar linear elastic regime followed by brittle fracture. However, different values of the ultimate tensile strength and elastic modulus of tested samples were recorded.

Abstract: Objectives: This paper presents a numerical fatigue life assessment of a self-expandable Nitinol stent. The analysis was performed using the ANSYS 11 software. Methods: Stent durability is an issue which must be addressed during the design of implants. Given the corrosive properties of blood and the cyclic loads that are applied on the stent (the cyclic variation of blood pressure), the determination of fracture parameters and fatigue characteristics of the implant is highly recommended. Breaking of the stent’s wire is particularly dangerous because it can cause the dislocation of a piece of stenotic plaque, which in turn can block a smaller artery, causing a heart attack. On the other hand, any discontinuity in stent structure acts as an accumulating place for stenosis particles, significantly shortening the life of the implant. The stent consists of a cylindrical tube 22.42 mm long, with a diameter of 8.3 millimeters. The wire section is square, 0.2x0.2 millimeters. The stent is only subjected to the pressure generated by the stenoted arterial wall. This evenly distributed pressure is defined at the outer surface of the stent and has a value of 2.5 MPa, corresponding to a 56% blood vessel stenosis. This way, the most severe loading conditions for the stent could be simulated. The stress distribution was then used to asses the fatigue life of the stent. Results and conclusions: The results showed that, in normal conditions (with the maximal internal pressure of 139 mm Hg = 18533 Pa), no damage appears on the stent after 10⁷ cycles.

Abstract: This paper presents a finite element analysis in order to determinate the stress distribution in an proposed model of the artificial cruciate ligament of the knee joint during the gait cycle.

Abstract: This paper presents a finite element analysis regarding the stress distribution in a cemented Austin Moore type hip prosthesis. The 3-D model was obtained using a Roland PICZA 3-D laser scanner. The applied loads simulate the normal gait cycle. The prosthesis is made from stainless steel with a femoral head of 45mm diameter. The numerical analysis was performed using the ABAQUS code. The results showed that the stress level in the cement is sensitive to the femoral neck angle. Starting with a standard, 125° angle, and increasing the angle with up to 5°, the resulting stress can be reduced with more than 10%. The proposed angle increase produces a more uniform stress distribution in the cemented section, increasing the durability of the arthoplasty.

Abstract: This paper presents a comparative bibliographic study of different materials with elevated biomechanical biocompatibility regarding the stent-blood vessel interaction. Only the materials used in coronary stents’ manufacturing are considered: stainless-steel (316L), Cobalt-Chromium alloys (CoCrMo, CoNiCrMo), Nickel-Titanium alloys (Nitinol), Tantalum. The main characteristics that result from the stress-strain curve of each material are presented, as well as the biocompatibility and durability. The stainless-steel has good mechanical properties, excellent biocompatibility and low price. Cobalt-Chromium alloys have excellent mechanical properties, excellent biocompatibility, acceptable shape memory properties, but high density and low flexibility. The Nitinol represents the best choice, with excellent mechanical properties, excellent biocompatibility, good corrosion resistance, high flexibility (super-elastic behavior), low density, but high price. Tantalum alloys present the best biocompatibility and high flexibility, but the mechanical properties are relative modest.

Abstract: This paper presents the durability analysis of a machine shaft subjected to bending with torsion, based upon the stress analysis at a notch which joins segments of different cross-sections (circular and square respectively) of the shaft. During the specific operating conditions of the equipment which’s part the shaft is, this notch has proven to be critical, causing premature failure. The durability analysis is carried out based on the real loading conditions, allowing the stress state evaluation at the base of the notch for different radii, using ABAQUS 6.9-3 finite element analysis program. The experimental studies carried out on the shaft’s material, 41Cr4 steel, resulted in the determination of material properties and of torsion fatigue curves in case of two different heat treatments. The cycles composing the equivalent stress spectrum are counted using the rainflow algorithm. The number of loading blocks to failure (number of load spectrum repetitions), considering the real operating conditions, is obtained using Miner’s rule, based on the rotating bending fatigue curve, corrected corresponding to the shaft’s specific characteristics. The proposed calculation method, based on the variation in time of the stress tensor during operating conditions, leads to the determination of the optimal notch radius for which the shaft’s durability falls between prescribed limits.

Abstract: Objectives: Numerical determinations of Stress Intensity Factors (SIFs) for a shaft under mixed-mode load with a crack of different sizes in the connection zone between square and circular cross-sections. Methods: Linear-elastic Fracture Mechanics principles are used. The numerical analysis program used was ABAQUS CAE version 6.9-3. The shaft consists in a circular section and a square section, with a fillet connecting zone. Cracks of different lengths and different depths are modeled. The shaft is subjected to torsion and bending. The SIFs were determined using the contour integral method. Results: The stress distribution was determined and plotted, and the stress concentration effect of the notches was highlighted. Crack propagation was also performed, using the XFEM module of ABAQUS code. The computed SIFs were plotted along the crack front. Conclusions: Crack initiation and propagation matched the pattern obtained in experimental tests, thus validating the model. The results confirmed that the fillet zone between the two sections acts as a stress concentrator. The fillet radius determines the magnitude of stress concentration. Crack geometry has a significant influence on SIFs, as well as on the global stress distribution.

Abstract: Objectives: Numerical stress intensity factors (SIFs) computation for several fabrication defect geometries in coronary stents. XFEM crack initiation and propagation was also performed. Methods: The model represents a self-expandable coronary stent, made from a shape memory alloy (L-605). Several flaw shapes are considered. The analysis was performed using the ABAQUS code. The loads and boundary conditions simulate the interaction between the blood vessels and stents, immediately after the angioplasty was performed. The mesh contains 3d stress hexahedral elements. For global stress and strain distributions, the model of a complete stent was used. For crack propagation analysis and SIF determination, the model represented a single segment of the stent. The stress intensity factors were computed using the contour integral method. Results and conclusions: The stress and strain fields highlight the negative effects of crack initiation and propagation on the residual life of the stent. Furthermore, by compromising the structural integrity of the stent, large strains may occur, thus increasing the risk of restenosis and further stenosis-related complications. The stress intensity factors indicate the most dangerous locations for the flaws (cracks), as well as the most dangerous geometries.