Meccanica最新文献

A scissors-type bridge that applies a deployable scissors mechanism to its structural form has been researched and developed to support emergency restoration activities after disasters. The vibration characteristics of this bridge have been evaluated by eigenvalue analysis and walking experiments in past studies. However, the basic knowledge, including the design method for this bridge, was still limited. Hence, this study conducts a hammering test to obtain basic vibration characteristics and explore the change in vibration characteristics under different boundary conditions through eigenvalue analysis. Although the total weight is increased by installing the decks, it can be seen that installing the decks on the top or bottom of the scissors-type bridge significantly improves rigidity in the horizontal direction. Still, the influence in the vertical direction is less than that in the horizontal direction. Furthermore, based on the experimental and numerical results, an estimation formula for the natural frequency approximating a scissors-type bridge as a simple beam structure was proposed. It was possible to estimate the natural frequencies in the vertical primary mode with errors of less than 3(%) compared with the experimental results and errors of less than 10(%) compared with the numerical results. This finding has important implications for the developing a simple and reliable design of emergency structures that use the scissor mechanism.

The capillary imbibition phenomenon has been widely studied by establishing governing equations, some of which might be too complex to obtain accurate solutions. Moreover, the capillary rise can be influenced by the constraint of capillary edge (i.e., pinning phenomenon), which is difficult to describe by governing equations. Therefore, a new analysis method is proposed in this paper, which can predict the capillary rise with/without contact line movement considering the dynamic contact angle and entrance viscosity dissipation. Firstly, the capillary length is discretized into micro elements. Secondly, every micro fluid level rise is analyzed based on conservation of mechanical energy. Finally, the whole rise process can be predicted by the end-to-end linking of all micro-elements. The accuracy of the proposed method is verified by multiple sets of experimental data. Moreover, we discuss the pinning and depinning phenomena in detail and propose a dimensionless number to judge the occurrence of depinning phenomena.

In the present study, a novel type of nontraditional tuned mass damper (TMD), termed as a tuned mass rate-independent damper (TMRD), was proposed for structural vibration suppression. The proposed TMRD consisted of a spring element, a mass element, and a rate-independent linear damping (RILD) element, which generated damping forces independent of the excitation frequencies. Based on the fixed-point theory, analytical formula of optimally designing the characteristic parameters of the TMRD were derived. The advantages of the TMRD over TMDs in suppressing the structural vibration and damper stroke were demonstrated, when they were separately used in flexible structures subjected to harmonic and earthquake-induced ground motions. For practical applications, an approximate method was presented to passively realize the proposed TMRD. It was suggested that the proposed method can be used to physically realize the TMRD and provide a superior solution than the conventional TMD to suppressing the structural vibration and damper stroke.

To investigate the effects of axial misalignment, lead crown relief, and temperature on the meshing stiffness of gears, this study aims to derive high-precision gear stiffness values that are more aligned with practical engineering applications. Based on the thermal expansion theory, slice coupling effect, and involute profile theory, a thermal stiffness model for meshing spur gear pairs under multiple influencing factors has been established, integrating the improved potential energy method and the nonlinear contact stiffness calculation approach. A calculation method for the thermal stiffness of meshing spur gear pairs, considering the impacts of axial misalignment and lead crown relief, has been developed. Furthermore, the mechanisms by which varying temperature, modification amounts, and misalignment affect the meshing stiffness of spur gear pairs have been explored. The results indicate that both lead crown relief and axial misalignment alter the load distribution across the tooth width, leading to a relative load concentration and consequently affecting the deformation of the gears under external loading, thereby influencing their meshing stiffness. It was found that the meshing stiffness decreases with an increase in axial misalignment and lead crown relief amounts. Additionally, with the introduction of temperature effects, an increase in temperature further reduces the meshing stiffness of the gear pair. The thermal deformation induced by temperature variations results in profile errors, affecting the actual meshing positions of the gears and altering the dimensions of single and double tooth intervals along the meshing line. This research establishes a theoretical foundation for the design of gear systems and the study of gear transmission system dynamics.

In this study we evaluate the performance of different constitutive biomechanical models, focusing on their ability to reproduce the mechanical behavior of myocardial tissue under various deformation modes. Three constitutive models were analyzed assuming incompressible formulations: the invariant-based formulation of the Costa model, the Holzapfel–Ogden (HO) model, and its extended version (HOE). The study aimed to identify which model provides the best fit for different experimental data, including equibiaxial (EBx), true biaxial (TBx), simple triaxial shear (STS), and combined data sets (Equibiaxial + Shear, True biaxial + Shear). The results showed that the Costa model generally performed better when considering combined datasets, providing a good balance between fitting accuracy and parameter stability, while using the least number of parameters among the contrasted models. The HO model demonstrated reasonable fitting abilities but struggled with non-equibiaxial conditions and clearly orthotropic simple shear datasets. The extended HOE model improved the fitting performance of the standard HO formulation for more complex data, particularly in shear tests, but introduced additional complexity and a higher number of parameters. Therefore, our study highlights the importance of analyzing which validated constitutive formulation is able to adapt to the available experimental data, especially when mixed deformation modes are involved. While all the three models tested performed adequately, the Costa model proved to be the most versatile, especially when dealing with various experimental conditions, providing insights for future research on biomechanical modeling of cardiac tissue.

This paper presents a novel method for spur gear tooth profile optimization, addressing the challenge of designing gears with improved performance. Traditional gear designs often compromise between contact stress, wear, and noise. This research explores a wider design space to identify gear profiles offering a better balance. The proposed approach leverages tensor-based kinematics combined with the Reuleaux method for conjugate profile generation, creating a robust framework for exploring potential designs. This framework defines an objective function considering multiple performance criteria. Differential evolution is employed to search for novel tooth profiles minimizing this function. The performance of optimized profiles is compared against existing designs, including involute, S-gears, and cosine gears. Key performance indicators include Hertz contact and subsurface shear stresses, normal force, sliding factor, specific sliding, contact ratio, and gear mesh stiffness. Results demonstrate the method’s effectiveness in generating improved tooth profiles. Optimized solutions exhibited contact and shear stress reductions comparable to 30-degree involute and S-gears, suggesting improved pitting resistance and wear. Some designs showed substantial specific sliding reductions, indicating the potential for reduced heat generation and surface wear. While cosine gears showed reduced contact stress, they also exhibited lower contact ratios, potentially increasing dynamic loads. These optimized solutions offer a promising path towards designing high-performance gears tailored to specific applications. The method effectively explores the vast solution space and generates tooth profiles fulfilling desired optimization trade-offs, paving the way for future research incorporating additional performance criteria and exploring more complex gear geometries.

A concept of an ichthyoid propulsor mimicking the undulating motion of a swimming fish is proposed and verified. The propulsor consists of an articulated fluid-conveying pipe with a triangular fin attached to its free end. A sufficiently high flow velocity in the propulsor leads to the instability of the system and the possible appearance of snake-like flutter vibrations. A dynamical model of the system is proposed. It is based on classical Benjamin’s model of the dynamics of an articulated fluid-conveying pipe and Lighthill’s elongated body theory, which quantifies hydrodynamic forces generated by the swimming fish. Parameters of the system for which the propulsor is subject to dynamic loss of stability, leading to the appearance of periodic flutter vibrations, are identified. Methods of bifurcation analysis, supported by numerical simulations, prove that the system can undergo a supercritical Hopf bifurcation. This soft self-excitation yields a stable limit cycle of the system, for which the thrust and lateral forces generated by the propulsor are calculated. It is shown that the mean value of the thrust is positive for a range of swimming speeds. The performance of the propulsor is assessed in relation to the swimming speed. The research may broaden knowledge about articulated pipes conveying fluid and support possible applications of the proposed propulsor.

The parallel robots workspace plays an important role in their design and construction. In this paper, a new algorithm is presented based on the concepts of interval analysis to determine the parallel robots workspace by considering joints ranges of motion. The proposed algorithm is based on the kinematics equations, interval arithmetic computations and refinement method. In this algorithm, the system of interval nonlinear equations obtained from kinematic analysis is solved simultaneously and the refinement operation is carried out to accurately calculate the intervals of the equations. A type of refinement operation, namely the slope form, is used in this method to eliminate the excess width of intervals of equations. The proposed algorithm is implemented on 3 and 4-DOF Delta parallel robots of Human and Robot Interaction Laboratory of Tehran university. The workspace of 3-DOF Delta parallel robot and the constant-orientation workspace of 4-DOF Delta parallel obtained from the proposed method for the different active joints ranges of motion are compared with the results of the method based on interval analysis without refinement operation and the geometric method. The results show that the proposed algorithm calculates the parallel robot workspace with appropriate accuracy.

The current study aims to analyze the load transfer between bone and three different TPMS-stem implants designed for tibia-total knee replacement (TKR) application through an initial stage of recovery and after healing time under static and dynamic loading conditions. The TPMS-based scaffolds, Schwarz, diamond, and gyroid, are used for tibia-stem designs. The mechanical performance of TPMS-stem implants was investigated based on von Mises stress for various loading conditions using ANSYS 2021R1. The results showed that TPMS-stem implants increase the maximum von Mises stress on the bone surface under the tibia tray by 14–24% under static loading and 15–36% under dynamic loading compared to solid-stem implants. Also, TPMS-stem implants reduced the maximum von Mises stress in the stem tip area when static and dynamic loading were considered. Stress reductions of 21.3, 21, and 17% were obtained under static loading for diamond, Schwarz, and gyroid stem implants, respectively.

In recent decades, there has been an increasing number of researches and applications involving hyperelastic structures, integrating different areas of engineering structures and materials, driven by technological advances in the manufacturing process, many involving multistability and the practical use of snap-through buckling. However, there is little information on the stability of hyperelastic multistable structural elements. The objective of this work is, therefore, to study experimentally and numerically the stability of hyperelastic arches, a structural form found in many applications. The arches are made of rubber-like material (polyvinyl siloxane), an elastomer that closely conforms to the incompressible hyperelastic ideal, which is described by the constitutive polynomial model. Uniaxial tests are used to determine the material constants. The aid of a digital image record during the tests allows an in-depth analysis of the deformation field. Several specimens are tested, covering a large range of rise-to-span ratios and two cross-section geometries, thus allowing for an in-depth understanding of the multistable behavior of pre-compressed hyperelastic arches. The tests are conducted under displacement control, allowing the determination of load and displacement limit points. Excellent correlation is obtained between the experiments and the nonlinear equilibrium paths obtained using three-dimensional finite element models. The results obtained show that the arches, due to the flexibility of hyperelastic materials, can undergo large displacements and rotations without damage, giving them great potential for energy absorption and storage. Density is a crucial property of rubber, significantly influencing its structural response. The important role of self-weight on bifurcation loads and nonlinear equilibrium paths is demonstrated here. Understanding the non-linear behavior and stability of these structures is important in practical applications such as vibration control, energy absorption and harvesting, metamaterials development, bioengineering, medicine, and flexible robots, among others.