Bounded Elastic Moduli Multiplier Technique for Limit Loads: Part 1 - Theory

Statically admissible stress distributions are necessary to evaluate lower bound limit loads. Over the last three decades, several methods have been postulated to obtain these distributions using iterative elastic finite element analyses. Some of the pioneering techniques are the reduced modulus, r-node, elastic compensation, and linear matching methods, to mention a few. A new method, called the Bounded Elastic Moduli Multiplier Technique (BEMMT), is proposed and the theoretical underpinnings thereof are explained in this paper. BEMMT demonstrates greater robustness, more generality, and better stress distributions, consistently leading to lower-bound limit loads that are closer to elastoplastic finite element analysis estimates. BEMMT also questions the validity of the prevailing power law based stationary stress distributions. An accompanying research offers several case studies to validate this claim.

Numerical biomechanics modelling of indirect mitral annuloplasty treatments for functional mitral regurgitation

Mitral valve regurgitation (MR) is a common valvular heart disease where an improper closure leads to leakage from the left ventricle into the left atrium. There is a need for less-invasive treatments such as percutaneous repairs for a large inoperable patient population. The aim of this study is to compare several indirect mitral annuloplasty (IMA) percutaneous repair techniques by finite-element analyses. Two types of generic IMA devices were considered, based on coronary sinus vein shortening (IMA-CS) to reduce the annulus perimeter and based on shortening of the anterior–posterior diameter (IMA-AP). The disease, its treatments, and the heart function post-repair were modelled by modifying the living heart human model (Dassault Systèmes). A functional MR pathology that represents ischaemic MR was generated and the IMA treatments were simulated in it, followed by heart function simulations with the devices and leakage quantification from blood flow simulations. All treatments were able to reduce leakage, the IMA-AP device achieved better sealing, and there was a correlation between the IMA-CS device length and the reduction in leakage. The results of this study can help in bringing IMA-AP to market, expanding the use of IMA devices, and optimizing future designs of such devices.

REINFORCEMENT OF AGEING SHIP STRUCTURES

The objective of the present study is to investigate the possibility to recover the ultimate strength of a rectangular steel plate with a manhole shape opening subjected to a uniaxial compressive load and non-uniform corrosion degradation reinforced by additional stiffeners. Finite element analyses have been carried out to verify the possible design solutions. A total of four finite element models are generated, including 63 sub-structured models. The non-uniform corrosion has been generated by the Monte Carlo simulation. The reinforcement process covers three scenarios that include mounting of two longitudinal stiffeners, two longitudinal and two transverse stiffeners and the flange on the opening. The positioning of the stiffeners has also been studied. A total of 10 cases has been selected and tested for the numerical experiment. Three different assessments have been performed to evaluate the ultimate strength, weight and cost. Two additional studies on the effect of the plate thickness and slenderness have been also carried out.

Buckling Analysis of a Large Shelter with Composites

We present here linear and nonlinear finite element analyses of a newly designed deployable rapid assembly shelter (DRASH J) manufactured by DHS Systems. The structural analysis is carried out in three stages. Firstly, single composite tubes (struts) under three-point bending are modeled with five layers of orthotropic materials in three different orientations and the simulation results are compared with the actual test data for validation. Secondly, a comprehensive structural model for the entire shelter is constructed with the consideration of two types of strut scissor points, namely natural and forced scissor (crossing) points, as well as partial-fixed hub joints, which allow rotations along individual hub slots (grooves). Finally, a simplified structural model is created by introducing fixed joints for the scissor points as well as rigid links for the hubs. With sufficient verifications with experiments and different modeling methods, linear and nonlinear finite element analyses are then carried out for both the comprehensive and simplified shelter models. Based on the simulation results, we are able to identify a few critical issues pertaining to proper design and modifications of such shelter systems, such as various end wall supports pertaining to the overall structural stability.

Finite Element Analysis of Transhumeral and Transtibial Percutaneous Osseointegrated Endoprosthesis Implantation

Cadaveric mechanical testing of a percutaneous osseointegration docking system (PODS) for osseointegration (OI) prosthetic limb attachment revealed that translation of the exact system from the humerus to the tibia may not be suitable. The PODS, designed specifically for the humerus achieved 1.4–4.8 times greater mechanical stability in the humerus than in the tibia despite morphology that indicated translational feasibility. To better understand this discrepancy, finite element analyses (FEAs) modeled the implantation of the PODS into the bones. Models from cadaveric humeri (n = 3) and tibia (n = 3) were constructed from CT scans, and virtual implantation preparation of an array of endoprosthesis sizes that made contact with the endosteal surface but did not penetrate the outer cortex was performed. Final impaction of the endoprosthesis was simulated using a displacement ramp function to press the endoprosthesis model into the bone. Impaction force and maximum first principal (circumferential) stress were recorded to estimate stability and assess fracture risk of the system. We hypothesized that the humerus and tibia would have different optimal PODS sizing criteria that maximized impaction force and minimized first principal stress. The optimal sizing for the humerus corresponded to implantation instructions, whereas for the tibia optimal sizing was three times larger than the guidelines indicated. This FEA examination of impaction force and stress distribution lead us to believe that the same endoprosthesis strategy for the humerus is not suitable for the tibia because of thin medial and lateral cortices that compromise implantation.

Biomechanical Tests and Finite Element Analyses of Pelvic Stability using Bilateral Single Iliac Screw with Different Channels in Lumbo-iliac Fixation

Background: In lumbo-iliac fixation, the iliac screw can be placed in a number of locations and directions, and multiple screws can be placed to enhance the fixation effect. At present, there is no uniform standard for the placement of single iliac screw. Biomechanical tests and finite element analyses were used to compare the effect of bilateral single iliac screw with three channels on pelvic stability in lumbo-iliac fixation, so as to provide a basis for determining the best single iliac screw channel.Methods: Five adult embalmed cadaver pelvic specimens were selected. Unstable Tile C1 pelvic injury model (pubic symphysis separation and left sacral Denis II fracture) was established. The pubic symphysis was fixed with five-hole reconstruction plate. Lumbo-iliac fixation for the treatment of pelvic posterior ring injury: three channels of bilateral single iliac screw (channel A from PSIS to AIIS, channel B from 1 cm medial and 1 cm caudal of PSIS to AIIS, channel C from 2 cm below PSIS to AIIS). At the same time, the finite element model of unstable pelvic posterior ring injury treated with lumbo-iliac fixation was established, which were used to analyze and explore the effect of bilateral single iliac screw with three channels on the biomechanical stability of the pelvis, including the stress distribution and the maximum Von Mises stress of internal fixation, vertebral body and ilium.Results: Biomechanical tests revealed that under vertical compression load, the compressive stiffness of pelvic specimens fixed with three channels of bilateral single iliac screw was lower than that of complete pelvic specimens (P < 0.05). The vertical displacement fixed by channel B was smaller than that fixed by channel A and channel C; however, there was no significant difference between channel B and channel A (P > 0.05). The compressive stiffness fixed by channel B was better than that fixed by channel A and channel C. Under torsional load, the torsional stiffness fixed by channel B was stronger than that fixed by channel A and channel C. Finite element analyses conformed that the maximum Von Mises stress of the internal fixator fixed in channel B under the conditions of vertical, forward bending, backward extension, left bending, left rotating and right bending were significantly lower than that fixed in channel A and channel C. Under various working conditions, the maximum Von Mises stress of the internal fixture of channel B was less than that of channel A. In terms of the maximum Von Mises stress of the vertebral body and iliac, compared with the other two iliac screw channels, the overall stress distribution fixed by channel B was more reasonable.Conclusions: Bilateral single iliac screw with three channels in lumbo-iliac fixation could effectively restore pelvic stability. The construct stiffness of the channel from 1cm medial and 1cm caudal of PSIS to AIIS is better than that of the other two channels. This channel has the advantages of good biomechanical stability, reasonable stress distribution, small maximum Von Mises stress of internal fixation, strong fatigue resistance and not easy to break screws and robs.