Vietnam Journal of Mechanics

Trong nghiên cứu này, phân tích dao động tự do của các vi mô Timoshenko làm từ Vật liệu Phân lớp Chức năng (FGM) trên nền móng đàn hồi Winkler-Pasternak được thực hiện dựa trên Lý thuyết Căng thẳng Kết hợp Đã Điều chỉnh (MCST). Đặc tính vật liệu của vi mô thay đổi xuyên suốt độ dày theo phân phối công suất và được ước lượng thông qua các kỹ thuật đồng hóa Mori–Tanaka, Hashin-Shtrikman và Voigt. Mô hình vi mô Timoshenko xem xét tham số độ dài được áp dụng. Các phương trình vi phân dao động tự do của các vi mô FGM được thiết lập dựa trên Phương pháp Phần tử Hữu hạn (FEM) và các hàm hình dạng của Kosmatka. Ảnh hưởng của hiệu ứng kích thước, nền móng, vật liệu và các tham số hình học đến tần số dao động sau đó được phân tích. Kết quả cho thấy rằng nghiên cứu này có thể áp dụng cho các FGM khác cũng như các cấu trúc vi mô phức tạp hơn.

This paper addresses the problem of repairing multiple cracked beams subjected to static load using piezoelectric patches. First, the problem is formulated and solved analytically for the case of two cracks that results in ratio of restoring moments produced by employed piezoelectric patches. Since the ratio is dependent only on crack positions but not their depth, the result obtained for case of two cracks has been extended for the case of multiple cracks. This proposition is then validated by finite element simulation where repairing piezoelectric patches are replaced by mechanical moment load equivalent to the restoring bending moments produced by the piezoelectric patches. The excellent agreement between analytical solution and numerical simulation results in case of single and double cracks allows making a conclusion that a piezoelectric patch could productively repair a cracked beam by producing a restoring moment due to its piezoelectricity. Thus, the problem of repairing multiple cracked beam using piezoelectric patches is solved.

The interaction between nonlinear oscillations is an interesting problem which has attracted many researches. The interaction between forced and parametric excitations and between two parametric excitations of first and second degrees and first and third degrees, is informed in [1]. In the present paper, we study interaction between two parametric excitations of the second and third degrees. The asymptotic method of nonlinear mechanics in combination with a computer is used.

In [2], a modified Poincare method has been presented and applied to evaluate the period (frequency) of free oscillation in an undamped Duffing oscillator with large cubic restoring nonlinearity. There, the system examined is supposed to be near certain linear one with unknown (to be evaluated) frequency.

Dynamic modeling and analysis of flexible manipulators play an essential role in optimizing mechanical design parameters and control law of real robot systems. In this paper, a nonlinear dynamic model of a manipulator is formulated based on the Finite Element Method. To analyze the dynamic behavior effectively, a numerical simulation scheme is proposed by taking full advantages of MATLAB and SIMULINK toolboxes. In this manner, the effect of varying payload and link length ratio of the manipulator to its elastic displacement is dynamically taken into account. The simulation results show that the payload and length link ratio have significant influences on the elastic displacements of the system. In particular, a proper spectrum of the link length ratio, in which the flexural displacement of the end point of the manipulator is smallest, is demonstrated. To this end, the proposed methodology could be used further to select optimal geometric parameters for the links of new robot designs.

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Comparing with reinforced concrete structure, member rigidity of steel structure is less, and displacement of steel structure is larger than that of reinforced concrete structure, so the secondary moment due to axial force has to be considered, i e., the problem of P-Delta effect. For most building structures, especially tall steel buildings, the P-Delta effect of most concern occurs in the columns due to gravity load, including dead and live load. The column axial forces are compressive, making the structure more flexible against lateral loads. If compressive P-forces are present and are large enough, the structure may buckle. Local buckling of individual members or global buckling of the whole structure is possible. This paper presents the method to analysis stability of steel frames with semi-rigid connections and rigid zones by using P-Delta effect.

The main aim of this paper is to investigate the nonlinear buckling and post-buckling of eccentrically stiffened sandwich functionally graded porous (FGP) cylindrical shells surrounded by elastic foundations in thermal environments and under torsional load by analytical approach in terms of the displacement components. The shells are reinforced by eccentric rings and stringers attached to the inside and material properties of face sheets and stiffeners are assumed to be continuously graded in the thickness direction. The sandwich cylindrical shell is composed of FG porous core and two FG layer coating. Based on the first order shear deformation theory (FSDT) with von Kármán geometrical nonlinearity and smeared stiffeners technique, the governing equations are derived. Using Galerkin method, the closed form to find critical torsional load and post-buckling load-deflection curves are obtained. The effects of porosity parameters, the thickness of the porous core, temperature, stiffener, foundation, material and dimensional parameters are analyzed.

In civil engineering, distinct mechanical properties and behaviors of structural steel rods necessitate a novel approach to material modeling. This study extends the application of recently proposed strain-hardening laws, originally developed for automotive sheet metals, to several structural steel rods (CB240-T and CB300-T). Standard uniaxial tensile tests are conducted for each examined material to obtain experimental stress-strain data. Various curve fitting methods are then employed to refine the parameters of the strain-hardening laws, enabling accurate representation of the steel rods mechanical behavior. Subsequently, these laws are implemented in Abaqus software for numerical simulation of uniaxial tensile tests, facilitating the analyses of material response under uniaxial tensile loading condition. Compared to the measured data, the predicted force-displacement curves are in good agreement with the measurements until the tail of the curves. The comparisons verify the ability and potential of the examined hardening law for studying the post-necking behavior of structural steels. The outcomes provide a framework for more precise characterization of structural steel materials.

A simplified model is proposed for modal analysis of cracked 3D-framed overhead crane of H-form frame using the well-known dynamic stiffness method. First, the dynamic stiffness of a cracked beam is derived as the inverse of the frequency response function and used for establishing a model of the cracked beam called dynamic stiffness one. Then, the dynamic stiffness model of cracked beams is employed for representing two supporting beams in the H-form frame crane. So that a simplified model of the overhead crane is conducted as a beam with artificial elastic supports of stiffness obtained above for supporting beam. Finally, the free vibration problem of the simplified overhead crane is formulated and solved for the crane fundamental frequency analysis in dependence upon crack parameters and supporting positions. Numerical computation is accomplished for illustrating the proposed theory and providing some conclusions on the simplified model of 3D overhead cranes.