IN VITRO MECHANICAL EVALUATION OF SPINAL FIXATION ROD CONNECTORS

ABSTRACT Objective Evaluate and compare the mechanical resistance and the fatigue behavior associated with the use of three different modalities of vertebral fixation system rod connectors through in vitro pre-clinical mechanical tests developed specifically for this application (linear, lateral with square connector and lateral with oblique connector). Methods Cobalt chromium rods 5.5 mm in diameter were used and coupled with three types of connectors: a) side rod with oblique connector, b) side rod with square connector, and c) rod and linear connectors. Quasi-static mechanical four-point bending and fatigue tests were performed. The variables measured were (I) the bending moment at the yield limit, (II) the displacement at the yield limit, (III) the rigidity of the system in flexion and (IV) the number of cycles until system failure. Results The linear system presented the greatest force and the greatest moment at the yield limit, as well as the greatest stiffness equivalent to bending. All specimens with square and oblique connectors endured 2.5 million cycles in the minimum and maximum conditions of applied moment. The specimens with linear connector endured 2.5 million cycles with fractions of 40.14% of the bending moment at the yield limit, but failed with levels of 60.17% and 80.27%. Conclusions Systems with linear connectors showed greater mechanical resistance when compared to systems with square and oblique connectors. All systems supported cyclic loads that mimic in vivo demands. Level of evidence V; In vitro research.

Study for Control of Dimensional Distortion in Tempered Industrial Parts

The presence of residual stress generated in the manufacturing process defines one of the biggest problems found in the mechanical metal industry. In the search of mechanical properties to a specific application, it is mandatory to impose the parts at very critical mechanical, chemical or thermal requests. The study analyzes all the steps that apply considerable stresses to the component during the manufacturing process and, therefore, discover which ones add more stresses to the yield limit of the material. It was also intended to contribute with relevant information regarding the surface integrity of the material, with bigger coverage in the residual stresses generated where, due to their nature and expressiveness, they can be beneficial or harmful to the component’s useful life. Accordingly, the objective was to analyze the raw material of SAE 4140 steel through its microstructure and verification of its chemical and mechanical characterization. In order to reduce the occurrence of dimensional distortions in excess of tolerance, we sought to identify the most critical step and, therefore, act with viable possibilities and without relevant costs for the prevention of the problem encountered. In order to measure its stress and define the process in which the highest inclusion of residual tractive stresses is characterized, these being the deleterious ones for the process and for the product, we used the method of measuring residual stresses by ray diffraction in X. In your measurement, the stresses included on the surface of the specimens were measured at specific stages of the process. X-ray diffraction analysis analyzes the diffraction planes and their respective interplanar distances from a specific material, as well as the densities of atoms along the crystalline planes. Using mathematical models, it is possible to measure the residual stress existing in the investigated parts. In view of the analysis by X-ray diffraction, it was verified the existence of disordered variations and modifications of the crystalline phases on the material surface, at the end of the finish machining process. These crystalline phases which, together with a less aggressive fabrication, favor plastic deformation due to the presence of residual stresses which surpass those of the yield limit of the analyzed material. In this sense, it was possible to determine which the most critical operation related to the component request is the machining in which it is applied. Since the subsequent processes only aggravates this condition, resulting in an unusable component for the proposed application without adding a higher cost to the product, either through the use of some rework or scrap procedure.

Study on the stress of micro-S-shaped folding cantilever

Micro-cantilever has shown wide application prospect in the field of micro-sensors, actuators, gyroscope, and so on. There are abundant research studies on simple cantilever beam models, but there are few on S-shaped folding cantilever with complex structure, although it is widely used. In order to study the deformation failure of S-shaped folding cantilever, the force analysis of S-shaped folding cantilever was carried out in this article, and the stress values of different positions under the external load of the cantilever were deduced. The finite element model about S-shaped folding cantilever was built based on software ANSYS. The theoretical calculation was compared with the finite element calculation, and the results showed that the max stress is 681 MPa based on the derived theoretical formula, the max stress is 673 MPa based on the ANSYS, the error is 1.18%, which can prove formula is accurate. To further validate the stress predicted by the mathematical modeling, a micro-force testing platform was built to test the cantilever. Since the stress value cannot be measured directly in the test, the force corresponding to the stress was taken as standard and compared it with the simulation. The tested external force was corresponding the yield limit. The results showed that the experimental force was 0.06462 N before the plastic deformation occurred, the theoretical outcome was 0.065231 N corresponding the yield limit, the error was 0.94%. Both simulation and experimental results depict that the theoretical model is effective for predicting the stress of the S-shaped folded cantilever. The theoretical model helps to enhance the efficiency, and improve the performance, predictability, and control of the S-shaped folding cantilever.

Numerical Simulation of Buckling and Post-Buckling Behavior of a Central Notched Thin Aluminum Foil with Nonlinearity in Consideration

In thin notched sheets under tensile loading, wrinkling appears on the sheet surface, specifically around the cracked area. This is due to local buckling and compression stresses near the crack surfaces. This study aims to numerically study the buckling behavior of a thin sheet with a central crack under tension. A numerical model of a notched sheet under tensile loading is developed using the finite element method, which considers both material and geometrical nonlinearity. To overcome the convergence problem caused by the small thickness-to-length/width ratio and to stimulate the buckling, an imperfection is defined as a small perturbation in the numerical model. Both elastic and elasto-plastic behavior are applied, and the influence of them is studied on the critical buckling stress and the post-buckling behavior of the notched sheet. Numerical results for both elastic and elasto-plastic behavior reflect that very small perturbations need more energy for the activation of buckling mode, and a higher buckling mode is predominant. The influences of different parameters, including Poisson’s ratio, yield limit, crack length-to-sheet-width ratio, and the sheet aspect ratio are also evaluated with a focus on the critical buckling stress and the buckling mode shape. With increase in Poisson’s ratio. First, the critical buckling stress reduces and then remains constant. A higher yield limit results in increases in the critical buckling stress, and no change in the buckling mode shape while adopting various crack length-to-sheet-width ratios, and the sheet aspect ratio changes the buckling mode shape.

On forward and inverse uncertainty quantification for models involving hysteresis operators

Parameters within hysteresis operators modeling real world objects have to be identified from measurements and are therefore subject to corresponding errors. To investigate the influence of these errors, the methods of Uncertainty Quantification (UQ) are applied. Results of forward UQ for a play operator with a stochastic yield limit are presented. Moreover, inverse UQ is performed to identify the parameters in the weight function in a Prandtl-Ishlinskiĭ operator and the uncertainties of these parameters.

An Approach of Vibration Control Based on the Design of Three DOFs Active Vibration Damping Platform

In this paper, an active vibration control platform is developed for milling processes. In this system, the workpiece is driven by a specially designed active platform to control the relative vibration between the tool and workpiece during milling processes. Numerical simulations are carried out to validate the effectiveness of the control platform. Results indicate that maximum stress of the hinge mechanism of the platform is far less than the yield limit of the material, and the designed platform can meet the use requirements in terms of the maximum displacement and natural frequency.