Stefan Dan Pastrama

Methodology for in situ stress intensity factor determination on cracked structures by digital image correlation

Purpose – The purpose of this paper is to develop a methodology for in situ stress intensity factor (SIF) determination that can be used for the analysis of cracked structures. The technique is based on digital image correlation (DIC) combined with an overdetermined algorithm.Design/methodology/approach – The linear overdeterministic algorithm for calculating the SIF based on stress values around the crack tip is applied to a strain field obtained by DIC.Findings – As long as the image quality is sufficiently high, a good accuracy can be obtained for the measured SIF. The crack tip can be automatically detected based on the same strain field. The use of the strain field instead of the displacement field, eliminates problems related to the rigid body motion of the analysed structure.Practical implications – In future works, based on the applied techniques, the SIF of complex cracked plane stress structures can be accurately determined in real engineering applications.Originality/value – The paper demonstra...

Contact Studies between Total Knee Replacement Components Developed Using Explicit Finite Elements Analysis

A pneumatic simulator of the knee joint with five DOF was developed to determine the correlation between the kinematics of the knee joint, and the wear of the polyethylene componenent of a TKR prosthesis. A physical model of the knee joint with total knee replacement (TKR) was built by rapid-prototyping based on CT images from a patient. A clinically-available prosthesis was mounted on the knee model. Using a video analysis system, and two force and contact pressure plates, the kinematics and kinetics data were recorded during normal walking of the patient. The quadriceps muscle force during movement was computed using the Anybody software. Joint loadings were generated by the simulator based on recorded and computed data. Using the video analysis system, the precise kinematics of the artificial joint from the simulator was recorded and used as input for an explicit dynamics FE analysis of the joint. The distribution of the contact stresses in the implant was computed during the walking cycle to analyze the prosthesis behavior. The results suggest that the combination of axial loading and anterior-posterior stress is responsible for the abrasive wear of the polyethylene component of the prosthesis.

Experimental simulation and computational modeling of the contact between femoral and tibial components in total knee arthroplasty

To study the biomechanical changes that appear in different pathological cases and to establish the efficiency of some types of osteothomies with different fixation systems and prostheses, a special five degrees of freedom simulator for the knee joint was developed. A model of the knee joint bones of a patient with total knee replacement (TKR) was reconstructed physically by rapid-prototyping from CT scans and a real prosthesis with metallic femoral component, metallic tibial tray and polyethylene plate was mounted on the artificial joint. Joint loadings was reconstructed on the simulator for normal walking based on data recorded from patient using force and pressure plates. Using a video analysis system, kinematics of the artificial joint from the simulator was recorded and used as input data for an explicit dynamics finite elements analysis of the tibial and femoral prosthetic components. Value and distribution of the contact stress in the polyethylene tibial component was computed during the walking cycle proving that this methodology of non-invasive investigation of TKR is helpful for prosthetic research and development.

Numerical Analysis for Determining the Displacements of a Lung Tumor

In this paper, a numerical analysis is undertaken in order to establish the three dimensional linear displacementsof tumors located inside the lung. In this way, the position of the tumor can be emphasized in various stages of the breathing process, helping thus the healthcare professionals both in surgical operations and in performing transbronchial biopsy. First, a geometrical model of the lung and airways was achieved with a 3D reconstruction program using CT scan images. The model was then imported in the ANSYS finite element software, which was used to perform the numerical simulations. The breathing process was simulated by applying external pressure on the surfaces of the lung. The values of this pressure were chosen as to correspond to the inspiration phase of the breathing process. Finally, conclusions are drawn regarding the values of the displacement of the nodules during breathing.