Aleksandar Grbović

Application of Numerical Methods in Design and Analysis of Orthopedic Implant Integrity

In this paper a numerical analysis of hip implant model and hip implant model with a crack in a biomaterial is presented. Hip implants still exhibit problem of premature failure, promoting their integrity and life at the top of the list of problems to be solved in near future. Any damage due to wear or corrosion is ideal location for crack initiation and further fatigue growth. Therefore, this paper is focused on integrity of hip implants with an aim to improve their performance and reliability. Numerical models are based on the finite element method (FEM), including the extended FEM (X-FEM). FEM became a powerful and reliable numerical tool for analysis of structures subjected to different types of load in cases where solving of these problems was too complex for exclusively analytical methods. FEM is a method based on discretization of complex geometrical domains into much smaller and simpler ones, wherein field variables can be interpolated using shape functions. Numerical analysis was performed on three- dimensional models, to investigate mechanical behaviour of a hip implant at acting forces from 3.5 to 6.0 kN. Short theoretical background on the stress intensity factors computation is presented. Results presented in this paper indicate that acting forces can lead to implant failure due to stress field changes. For the simulation of crack propagation extended finite element method (XFEM) was used as one of the most advanced modelling techniques for this type of problem.

Measurement of the Stress State in the Lower Link of the Three-Point Hitch Mechanism

Agricultural machines and their implements are subjected to dynamic loads during farm operations. Depending on the type of operation (e.g. lifting or plowing), lower links of the three-point hitch mechanism are exposed to stresses caused by combination of bending moments and axial forces. In this paper we analyzed influence of the soil resistance during plowing in the lower link and the possibility of its failure. The stresses were measured using strain gauges at locations with uniform stress distribution in order to enable more reliable comparison with finite element analysis (FEA). Recorded stresses vs. time were used for identifying mean stresses and amplitudes for different plowing depth and different tractor speeds. Due to the geometry of the lower links and their joints in the three-point hitch mechanism, during plowing and transferring soil resistance, links are loaded not only by axial forces but also by bending moment in the horizontal plane. Under some assumptions, FEA provided us to make relations between the measured stresses and the loads that caused them. Measured stresses show that links have significant safety margin relative to tractor installed power and soil resistance, which enables the possibility of their design optimization. Obtained results may also serve for further analyses of fatigue life prediction, measurement of the draft forces etc.

Determination of the Region of Stabilization of Low-Cycle Fatigue HSLE Steel from Test Data

Steel, NIONIKRAL 70 (NN-70), selected in this study to investigate the experimental behaviour affected by fatigue loading, among other things, is used in shipbuilding and for manufacture of pressure vessels as well. The experiment was conducted using smooth round specimens made of steel NN-70 as parent material (PM). When selecting stabilized hysteresis as a representative of all of stabilized hysteresis for one strain level, and for the further processing of low-cycle fatigue test results, the recommendations of standards have been used as well as the methodology based on which linearity of the stabilization regions of low-cycle fatigue was numerically determined. In present paper, the behaviour of high-strength low-alloy (HSLA) steel under conditions of low-cycle fatigue (LCF) has been experimentally tested and analyzed. Based on the experimental results obtained in the programme EXCEL, characteristic regions of low-cycle fatigue of steel NN-70 have been determined, the most important being the region of stable behaviour of materials, so-called “the region of stabilization”. From this region, on the basis of pre-defined requirements, characteristic stabilized hysteresis have been isolated for each strain level, based on which the indicators of low-cycle fatigue of steel NN-70 have been identified. For selecting typical stabilized hysteresis and for further processing of the low-cycle fatigue test results, new methodology with two new methods were used: “Method of the middle stabilization (ms)” and “threshold NDT method (tNDT)”. The comparisons of those two methods are shown in this paper.

Evaluation of Stress Intensity Factors (SIFs) Using Extended Finite Element Method (XFEM)

Finite element method has been used for decades for calculating Stress Intensity Factors (SIFs), but it has some restrictions in crack propagation simulations mainly because the Finite Element mesh needs to be updated after each propagation step to track the crack path. Extended Finite Element Method (XFEM) suppresses the need to mesh and remesh the crack surfaces and can be used for modelling different discontinuities in 3D domains. Discontinuities can be represented independently of the FE mesh by exploiting-applying the partition of unity (PoU) method. In this method enrichment functions are added to the displacement approximation as long as the PoU condition is satisfied. Thanks to XFEM, problems involving static cracks in structures, evolving cracks, cracks emanating from voids etc., can be numerically studied and then the results can be compared against the analytical and experimental values. Cracks are propagated and after each step of the propagation the SIFs can be computed from the numerical solution at several points along the crack fronts. Interaction integrals are used to extract the mixed-mode SIFs with the help of-using the auxiliary fields. XFEM is still not fully recognized and needs to prove its practical value in order to be generally acknowledged. Results obtained by using XFEM for a complex 3D geometry are still not regarded as reliable without experimental checks. The purpose of this paper is to demonstrate that reliable values of SIFs can be obtained for three-dimensional structures with cracks. Results and conclusions given here should contribute to making a more objective judgment about XFEM usefulness in solving wide range of problems, especially those related to the fatigue design.