Forces in mechanics最新文献

This present work uses the phase-field modelings to investigate the influence of interfacial effects on damage and mechanical behavior, as well as the optimal distribution of the inclusion shape within brittle inclusion-matrix structures in various typical cases. These two constituent phases in the structures are assumed to be either isotropic or anisotropic. To achieve these goals, this work will: (i) use the phase-field modelings either considering or neglecting interfacial debonding, and the anisotropic phase-field modeling; (ii) determine and incorporate the strain tensor orthogonal decompositions into each specific phase-field modeling to enhance the accuracy and effectiveness of the simulation methods; (iii) combine the phase-field modelings with the BESO topology optimization algorithm to analyze the influence of interfacial effects on relationship curves and the optimal distribution of the inclusion shape. Through proposed numerical examples, it is demonstrated that the interfacial effects strongly influence crack paths, behavior curves, and optimal material distribution in structures. When considering interfacial effects, cracks are almost unable to penetrate into the inclusion phase. However, when neglecting interfacial effects, cracks propagate into the inclusion phase. This reason makes the structure more difficult to damage than when considering the interfacial effects, as evidenced by greater peak load values in behavior curves and greater total fracture resistance of the material. Especially in the example of inclusion phase optimization, the total fracture resistance value of the case neglecting interfacial effects is more than 107.9% greater than that considering interfacial effects.

The laminated sandwich composites have wide structure-making applications in the automotive and aviation fields due to their lightweight and superior flexural rigidity properties. Making grooves or holes to assemble more than one structure induces crack discontinuities near the stress concentration region in these sandwich structures. The present work examines the effect of crack discontinuities on the mechanical performance and failure process of the sandwich structures under different loading conditions. Phase field method (PFM) has been presented and implemented using in-house developed MATLAB code. The effect of holes, multiple cracks, number of cores, and loading conditions are analyzed for the mechanical and fracture behavior of the structure. Load-carrying capacity, threshold displacement value for crack initiation, crack propagation trajectory, and energy absorption capacity are compared for various crack discontinuities under different loading conditions. Approximately 35% increase in load carrying capacity is observed in equivalent multiple core sandwich structures.

Ground mobile robots operating in outdoor environments face multiple challenges, being overcoming obstacles on uneven terrain a prominent one. This challenging task has been addressed by numerous researchers who have developed robots employing various strategies, all aimed at efficiently overcoming increasingly higher obstacles. This article describes 108 robots designed for this purpose, incorporating the principle of rolling for locomotion and obstacle overcoming. These robots have been categorized into six major groups based on their operating principle and strategy for overcoming obstacles. After conducting a meticulous review and comparison, it has been determined that both the definition of the strategy robot will use to overcome an obstacle and the optimized robot design from the early stages of its development through clearly established requirements are the elements that hold the greatest significance in enabling a mobile robot to efficiently overcome an obstacle. In this regard, specific requirements and parameters have been identified that must be considered in the design of the robot to fulfill its purpose. Among these, key considerations include dimensional optimization, robustness, adaptability, energy efficiency, sensory capability, and appropriate navigability.

In the past decade, the world has witnessed a new space race, driven by a growing commitment to reducing the environmental impact of space missions. This has led to the widespread adoption of liquid-propellant rocket engines, which offer several advantages over their solid-propellant counterparts. One key advantage is their reusability, which not only helps to reduce the generation of space debris but also makes space exploration cheaper. To further enhance the performance of liquid rocket engines, researchers have been exploring innovative cooling techniques and advanced materials. Among these materials, Ceramic Matrix Composites (CMCs) have shown great potential in reducing the overall engine weight when used instead of high-tech metal alloys, resulting in lower fuel consumption and emissions during launches. This paper focuses on the mass minimization of inner liners made of CMCs in rocket thrust chambers. At this aim, a computationally efficient preliminary design approach, based on an analytical one-dimensional thermo-mechanical model, is proposed. A case study of mass minimization of an inner liner of rocket thrust chamber is also presented and discussed, by considering five different CMC materials.

In this study, an external semi-elliptical crack was modeled in a 3D API 5 L X52 pipeline with stress corrosion cracking (SCC). To make the crack, the finite element method (FEM) was used to obtain the failure pressure (P_f) and its mechanical behavior. The longitudinal crack had a constant length (2c) and varied with depth (a). The properties of X52 steel subjected to SCC using the slow strain rate technique (SSRT) were considered. True stress-strain curves were obtained in air and in an NS4 solution with pHs of 3.5 and 9.5 at temperatures of 25 and 50 °C. According to the SCC index calculated from the mechanical properties of the SSRT, the X52 steel is susceptible to SCC at pH 3.5 and 50 °C. The mechanical properties of the tensile test using the stress-strain curves decreased as the pH and temperature changed, compared to those carried out at room temperature. This was due to the corrosion and hydrogen embrittlement produced by the solution on the X52 steel. The failure pressure is sensitive to the stress-strain curve; if the stress-strain curve decreases, the failure pressure also decreases. Corrosion defects, such as longitudinal cracks in X52 steel, which could be susceptible to SCC in an NS4 solution, decrease the failure pressure (P_f) and its capacity to withstand pressures of up to 75 % in pipe-grade steel that transports hydrocarbons.

In this analysis, the creep responses of a non-constant thickness annular plate was presented. The material of disc is assumed functionally graded magneto-electro-elastic (FGMEE) in which the material properties change through the radius. Also, the heat transfer coefficients for convection and conduction are functions of radius and temperature. At first, the equation of heat transfer accounting for thermal gradient, convection boundary conditions, internal heat generation, and solar radiation effects was derived. The differential transformation method (DTM) was used to solve the resulting nonlinear differential equation. The equilibrium equation for the annular plate including creep strain effects was then obtained. Ignoring creep strains, an analytical solution was obtained for the zero-time of this equation. Then, creep strains were introduced using Norton's law and the Prandtl-Reuss relations to find the stress and strain rates under fixed temperature boundary conditions. Next, the equation of strain rates including creep strains was solved analytically. Finally, an iterative approach was used to evaluate the time-dependent redistribution of creep stresses at any time point. Numerical examples highlighted the influences of key parameters like internal heat generation, convective heat transfer, grading index, solar radiation, thickness profile, and angular speed on the stresses, deformations and electric and magnetic potentials.

For the first time, this paper investigates the forced vibrations of a functionally graded (FG) nanobeam with an accelerating moving load in a thermal environment. There is no exact coupling solution for the vibrations of nanobeam with an accelerated moving load, so this paper aims to provide a method to obtain an accurate solution for nanoscale structures with an accelerated moving load. The equations of motion are derived using Timoshenko's beam theory and the nonlocal strain gradient theory (NSGT). The Laplace method has been used to solve the coupling and exact differential equations. Then, by inverting Laplace from the coupled equations, an exact solution of the temporal response for FG nanobeam with constant acceleration and initial velocity in a thermal environment was obtained. The natural frequency was compared with previous works for validity and had acceptable results. Finally, the effect of parameters such as changes in acceleration and velocity of moving force, negative acceleration, power law index of FG material, different temperatures, nonlocal parameter, and longitudinal scale parameter on the maximum dynamic displacement of nanobeam is investigated. These results can be used better to design FG nanostructures with accelerated moving loads. Considering the sensitivity of data analysis in nano dimensions, it is necessary to provide an analytical solution method in these dimensions to reduce the error percentage in nano dimensions to zero. which is presented in this work for the first time.

Pitting corrosion defect is a common defect in pipelines employed in the oil and gas industry. This paper explores the mechanical behavior and failure pressure (P_f) of the pipelines, and compares the traditional empirical methods used in oil and gas pipelines with pit arrangements including an isolated pit and pitting colony (three and five pits aligned in the longitudinal direction). This study is based on nonlinear finite element method (FEM) and 3-D pipeline models. P_f was predicted by FEM and traditional methods considering the interaction of pitting corrosion defects. The pipeline P_f values obtained by FEM and methods were compared to determine similarities in applications.

This document reports a numerical and experimental crashworthiness study of a vehicle structural component. Particularly, a windshield A-pillar made of a novel high-strength steel was tested on an apparatus for impact tests that was in-house designed so as to reproduce the effects of a vehicle side impact crash. In parallel, numerical simulations were performed using the explicit finite element code LS-DYNA, considering different elastic-plastic material constitutive laws with damage. A comparison between numerical and experimental outcomes demonstrated a satisfactory correlation, with the GISSMO model that provided a good correlation and needed a relatively low computational effort.

Vibration isolation is a crucial technology in precision manufacturing. With the rapid development of smart sensors, smart actuators, and microprocessors, active vibration isolation has emerged as a viable approach to improve the performance of manufacturing equipment. In integrated skyhook active vibration isolation, the cut-off frequency limits the controller gain. Base on the repeated poles technique, this paper reveals the simple analytical forms of the optimal cut-off frequency and controller gain of the integrated skyhook active isolation. The analytical formula is useful in the design of active isolation. Numerical calculations are performed to show the good performance of the optimal design. An experiment is carried out to show the existence of the optimal controller gain.