Numerical study of the influence of the specimen geometry on split Hopkinson bar tensile test results

Finite element simulations of high strain rate tensile experiments oil sheet materials using different specimen geometries are presented. The simulations component ail experimental study, using a split Hopkinson tensile bar set-up, Coupled with a. full-field deformation measurement, device. The simulations give detailed information on the stress state. Due to the small size of the specimens and the way they are connected to the test device, non-axial stresses develop during loading. These stress components, are commonly neglected, but, as will be shown, have a distinct influence on the specimen behaviour and the stress-strain curve extracted from the experiment. The validity; of the basic assumptions of Hopkinson experiments is investigated: the uniaxiality of the stress state, the homogencity of the strain and the negligibleness of the deformation of the transition zones. The influence, of deviations from these assumptions on the material behaviour from a Hopkinson experiment is discussed

Numerical simulation of Nitinol peripheral stents: from laser-cutting to deployment in a patient specific anatomy

The current clinical trend is to use percutaneous techniques, exploiting Nitinol self-expanding stents, to treat peripheral occluded vessels such as carotid or superficial femoral arteries. Although this class of stents addresses the biomechanical requirements (i.e. flexibility, kink resistance, etc.), it has been observed that many of these stents implanted in peripheral vessels are fractured. Numerical simulations have shown to be very useful in the investigation and optimization of stents and also to provide novel insights into fatigue/fracture mechanics. To date most finite element based stent simulations are performed in a straight simplified anatomy and neglect the actual deployment process; consequently there is a need for more realistic simulations taking into account the different stages of the stent design process and the insertion in the target anatomy into account. This study proposes a virtual framework to analyze numerically Nitinol stents from the laser-cutting stage to the deployment in a (patient specific) tortuous anatomy

Inverse modelling of image-based patient-specific blood vessels : zero-pressure geometry and in vivo stress incorporation

In vivo visualization of cardiovascular structures is possible using medical images. However, one has to realize that the resulting 3D geometries correspond to in vivo conditions. This entails an internal stress state to be present in the in vivo measured geometry of e.g. a blood vessel due to the presence of the blood pressure. In order to correct for this in vivo stress, this paper presents an inverse method to restore the original zero-pressure geometry of a structure, and to recover the in vivo stress field of the final, loaded structure. The proposed backward displacement method is able to solve the inverse problem iteratively using fixed point iterations, but can be significantly accelerated by a quasi-Newton technique in which a least-squares model is used to approximate the inverse of the Jacobian. The here proposed backward displacement method allows for a straightforward implementation of the algorithm in combination with existing structural solvers, even if the structural solver is a black box, as only an update of the coordinates of the mesh needs to be performed

A new method for improved standardisation in three-dimensional computed tomography cephalometry

Interest for three-dimensional computed tomography cephalometry has risen over the last two decades. Current methods commonly rely on the examiner to manually point-pick the landmarks and/or orientate the skull. In this study, a new approach is presented, in which landmarks are calculated after selection of the landmark region on a triangular model and in which the skull is automatically orientated in a standardised way. Two examiners each performed five analyses on three skull models. Landmark reproducibility was tested by calculating the standard deviation for each observer and the difference between the mean values of both observers. The variation can be limited to 0.1 mm for most landmarks. However, some landmarks perform less well and require further investigation. With the proposed reference system, a symmetrical orientation of the skulls is obtained. The presented methods contribute to standardisation in cephalometry and could therefore allow improved comparison of patient data