Collapse of clamped and simply supported composite sandwich beams in three-point bending

Composite sandwich beams, comprising glass–vinylester face sheets and a PVC foam core, have been manufactured and tested quasi-statically. Clamped and simply supported beams were tested in three-point bending in order to investigate the initial collapse modes, the mechanisms that govern the post-yield deformation and parameters that set the ultimate strength of these beams. Initial collapse is by three competing mechanisms: face microbuckling, core shear and indentation. Simple formulae for the initial collapse loads of clamped and simply supported beams along with analytical expressions for the finite deflection behaviour of clamped beams are presented. The simply supported beams display a softening post-yield response, while the clamped beams exhibit hardening behaviour due to membrane stretching of the face sheets. Good agreement is found between the measured, analytical and finite element predictions of the load versus deflection response of the simply supported and clamped beams. Collapse mechanism maps with contours of initial collapse load and energy absorption are plotted. These maps are used to determine the minimum mass designs of sandwich beams comprising woven glass face sheets and a PVC foam core.     

Shear repair of RC beams using epoxy injection and hybrid external FRP

This experimental study aims at investigating the behavior of reinforced concrete (RC) beams strengthened by unidirectional and hybrid bidirectional fiber-reinforced polymer (FRP) sheets and subjected to cyclic loading. RC beams tested under cycled loading were subsequently repaired using both epoxy injection and external FRP sheets, and then re-tested under monotonic loading. Six RC beam specimens, two of which were control specimens and four were shear deficient, were upgraded with side-bonded FRP sheets in the first phase of the experimental program. In the second phase, three of the damaged beams were repaired using epoxy injection and unidirectional carbon fiber polymer (CFRP) sheets. The repairing method, FRP type, and FRP wrapping scheme were the test variables investigated. Test results show that the repair schemes imparted significant mechanical improvements in terms of ultimate shear capacity and ductility. The simultaneous application of epoxy injection and externally bonded FRP sheets was found to be a highly effective repair technique.     

Free vibration analysis of simply supported sandwich beams with lattice truss core

Free vibration of AISI 304 stainless steel sandwich beams with pyramidal truss core is investigated in the present paper. The lattice truss core is transformed to a continuous homogeneous material. Considering the deformation characteristics of the sandwich beam, the following assumptions are made: (1) the thickness of the sandwich beam remains constant during deformation; (2) for the thin face sheets, only bending deformation is considered, neglecting the effect of transverse shear deformation; (3) for the core, only shear deformation is considered as the core is too weak to provide a significant contribution to the bending stiffness of the sandwich beam. The shear stress is assumed to be constant along the thickness of the core. The governing equation of free vibration is derived from Hamilton's principle, and the natural frequencies are calculated under simply supported boundary conditions. Finally, numerical simulation is carried out to get the mode shapes and natural frequencies. Our results show that the theoretical solutions agree well with the numerical results. It indicates the present method would be useful for free vibration analysis of sandwich beams with lattice truss core.     

Two-dimensional elasticity solution for transient response of simply supported beams under moving loads

A semi-analytical analysis for the transient elastodynamic response of an arbitrarily thick simply supported beam due to the action of an arbitrary moving transverse load is presented, based on the linear theory of elasticity. The solution of the problem is derived by means of the powerful state space technique in conjunction with the Laplace transformation with respect to the time coordinate. The inversion of Laplace transform has been carried out numerically using Durbin??s approach based on Fourier series expansion. Special convergence enhancement techniques are invoked to completely eradicate spurious oscillations and obtain uniformly convergent solutions. Detailed numerical results for the transient vibratory responses of concrete beams of selected thickness parameters are obtained and compared for three types of harmonic moving concentrated loads: accelerated, decelerated and uniform. The effects of the load velocity, pulsation frequency and beam aspect ratio on the dynamic response are examined. Also, comparisons are made against solutions based on Euler?CBernoulli and Timoshenko beam models. Limiting cases are considered, and the validity of the model is established by comparison with the solutions available in the existing literature as well as with the aid of a commercial finite element package.     

点阵材料夹芯简支梁在冲击载荷下的动力响应

首先给出了两端可移点阵材料夹芯简支梁受到均布冲击载荷时的刚塑性动力响应分析,然后将理论预测的夹芯梁中点的最大挠度和结构响应时间与有限元结果进行了比较,理论预测结果与有限元计算结果一致性较好。通过与质量和材料相同的单层实心梁进行对比分析,证实了点阵材料夹芯简支梁具有很好的抗冲击性能。通过对四棱锥夹芯简支梁进行拓扑构型设计,发现两端可移夹芯简支梁的最大挠度和结构响应时间对芯层的相对密度和相对厚度、面板与杆元的夹角十分敏感。随着夹芯梁所承受的单位面积冲量增大 , 夹芯梁中点的挠度增大。通过对由应变率敏感的304不锈钢制成的四棱锥桁架夹芯梁进行精细有限元分析,发现当考虑应变率效应时,梁的最大挠度小于不考虑应变率时的最大挠度。     

Repair of heat-damaged RC shallow beams using advanced composites

Several repair techniques for restoring the structural capacity of heat-damaged high-strength reinforced concrete shallow beams using advanced composites are proposed. A series of 16 under-reinforced concrete hidden beams were cast, heated at 600°C for 3 h, repaired, and then tested under four point-loading. Tests were conducted to study the effectiveness of externally applied composite materials on increasing the flexural capacity of beams. The composites used include high strength fiber reinforced concrete jackets; ferrocement laminates; and high-strength fiber glass sheets. The beams repaired with steel and high performance polypropylene fiber reinforced concrete jackets regained up to 108 and 99% of the control beams’ ultimate load capacity, with a corresponding increase in stiffness of up to 104 and 98%, respectively. The beams repaired with fiber glass sheets and ferrocement meshes regained up to 126 and 99% of the control beams’ ultimate load capacity, with a corresponding increase in stiffness of up to 160 and 156%, respectively. Most of the beams repaired showed a typical flexural failure with very fine and well-distributed hairline cracks in the constant moment region.     

Vibration control of a simply supported cylindrical shell using a laminated piezoelectric actuator

Summary This paper develops a novel laminated piezoelectric actuator (LPA) to control the vibration of a cylindrical shell structure, which is fabricated through bonding multiple piezoelectric layers of the same property together. The electromechanically coupled equations of the system are derived based on the classic shell theory. A parametric study is then conducted to evaluate the effects of geometric and physical properties of the actuator on actuating forces. The results show that as the number of layers increases, the actuating forces per voltage produced by LPA in the axial, circumferential and radial directions of the shell all increase noticeably. The active vibration control of a simply supported cylindrical shell using LPA of different layer numbers is simulated as well under a velocity feedback scheme. It is indicated that with the same control voltage the LPA can obtain a better control performance than the conventional single layer piezoelectric actuator as expected and the targeted radial modal vibration of the shell is attenuated significantly.     

Production of Raman laser beams using injection-locking technique

We produced two Raman-laser beams with a frequency offset of 9.2 GHz by injection-locking of a master diode-laser to a slave diode-laser. The master laser was phase-modulated at 9.2 GHz while the laser beam passed through an electrooptic modulator. The phase-modulated beam was injected into the slave laser that was oscillating around one of the side-bands of the master laser. The relative linewidth of the two lasers was less than 10 Hz. Using these laser beams, coherent population trapping resonance of cesium atoms was observed for the purpose of testing the phase-coherence of the laser beams.     

Repair evaluation of concrete cracks using surface and through-transmission wave measurements

Surface opening cracks are common defects in large civil structures like bridges. They allow penetration of water or other agents that result in loss of durability earlier than expected. Their repair can be conducted by the injection of epoxy material that seals the crack sides keeping out any aggressive substances in addition to the recovery of strength. In order to evaluate crack parameters before impregnation as well as to determine the final repair effectiveness, a combination of Rayleigh and longitudinal waves is applied. Rayleigh waves demonstrate the filling condition of the material into the shallow layer near the surface while tomography using longitudinal waves through the thickness yields information about the area inside the structure. Wave propagation dispersion features are exploited by the proposed tomography at different frequencies, demonstrating that higher frequencies lead to more accurate characterization.     

Interfacial stresses in plated beams with cracks

In this paper, an analytical interfacial stress analysis is presented for cracked reinforced concrete beams bonded with thin plates and subjected to symmetrical tensile forces and/or end bending moments. A representative element between two contiguous cracks is considered under the assumption of plane stress. By means of the principle of minimum complementary energy, the analysis provides a closed-form solution that can be used easily to predict the stress distributions near the cracks in the beams. Numerical results from the present analysis are presented to study the effects of both material and geometric properties of the beams on the distributions of the interfacial stresses.     

简支矩形板声辐射的有源控制

文章以简支平板为例 ,通过声辐射模态研究结构声辐射的有源控制。首先分析了声辐射模态的数学和物理意义。由于在中、低频时 ,声辐射模态对应的辐射效率随着模态阶数的增加而迅速降低。在此基础上 ,文中提出了一种新的控制策略 ,即抵消前k阶声辐射模态的伴随系数 ,使得前k阶声辐射模态的声功率为零。文中以压电陶瓷作动器作为控制力源进行了数值计算研究。并与传统的控制策略———声功率最小化策略进行了比较。     

DSC analysis of a simply supported anisotropic rectangular plate

The discrete singular convolution (DSC) algorithm is used to analyze the deflection and free vibration behavior of a simply supported anisotropic rectangular plate. A novel approach is proposed to solve the difficulty in using DSC to handle the simply supported boundary conditions with bending–twisting coupling. DSC results are presented for bending under distributed load and a center concentrated load, and natural frequencies of flexural vibrations. It is shown that the DSC with proposed method to apply the simply supported boundary conditions yields very accurate results as compared to exact solutions or results obtained by methods of differential quadrature and finite element with fine meshes. It is also verified that neglecting the bending–twisting coupling in applying the simply supported boundary conditions may result incorrect solutions, especially for the bending analysis of anisotropic plates.     

Large thermoelastic deflections and stability of simply supported polygonal panels

Summary A Berger-type approximation for large deflections of thermally loaded initially curved elastic plates is derived. In case of simply supported boundaries and arbitrary polygonal planforms, dimensionless solution is found in terms of Dirichlet's Helmholtz-eigenvalues of the given domain. Stability of this equilibrium solution is exemplarily discussed, showing bifurcation and snapping behaviour in the sense of a cusp catastrophe. Additionally, possibility of mode-jumping is indicated.With 3 Figures     

简支边界条件下超椭圆蜂窝夹芯板的自由振动

基于Reddy三阶剪切板理论,将铝基蜂窝芯层等效为一正交异性层,等效弹性参数由修正后的Gibson公式得出,对简支边界条件下超椭圆蜂窝夹芯板弯曲振动的固有频率进行了理论推导,应用具体算例与实验值和有限元计算结果进行了验证,结果表明本文方法具有较高的计算精度。研究了板宽、板厚和芯层胞元结构参数对简支边界条件下超椭圆蜂窝夹芯板固有频率的影响,并绘出相应的曲线图,该曲线图对超椭圆蜂窝夹芯板的工程应用具有指导意义。     

On the elastic stability of simply supported anisotropic sandwich panels

Here, the elastic stability behavior of simply supported anisotropic sandwich flat panels subjected to mechanical in-plane loads is investigated using an analytical approach. The formulation is based on first-order shear deformation theory and the shear correction factors employed are based on energy consideration that depends on the lay-up as well as material properties. The governing equations are obtained using the Raleigh–Ritz method assuming a combination of sine and cosine functions in the form of double Fourier series for the displacement fields. The effectiveness of the integrated formulation is tested for global characteristics considering examples related to multi-layered laminates and sandwich panels for which solutions are available.     

Random vibrations of orthotropic plates clamped or simply supported all round

Acta Mechanica - An approximate method is presented for determining the probabilistic response of rectangular orthotropic plates clamped all round. For solving the stochastic boundary value...     

Vibration assessment of a simply supported footbridge under discrete pedestrian loading

Abstract

This paper presents an analytical approach to analyze the vertical vibration of a simply supported beam subjected to discrete pedestrian loading. Such loading is characterized as each identical footstep load acting at different locations along the beam depending on a step length, which differs from the moving load model used in current guidelines. It allows us to investigate how pacing parameters such as a pacing period, footstep contact duration, and step length affect the beam response induced by a pedestrian who walks or runs. Also, vibration serviceability estimations based on the proposed running and walking load models and the Sétra guideline are compared. It can be concluded that the proposed loading models provide more accurate vibration estimation of a simply supported footbridge under vertical pedestrian loading than other models.     

两端简支输流管道共振可靠度分析

采用基于Galerkin的加权残数法分析输流管道,利用 阶Galerkin截断建立试函数,推导出消除残数方程,得到输流管道的前 阶固有频率,并分析流速对固有频率的影响。建立输流管道共振可靠度的功能函数,利用点估计法计算功能函数的前四阶矩,采用修正Edgeworth级数法求得输流管道的共振可靠度,并讨论了流速对管道共振可靠度的影响。研究结果对于输流管道的防共振设计和共振可靠性评估具有参考价值。     

Bending vibration of simply supported rectangular plates with internal rigid support

A previously derived invariant expression for the amplitude of the displacement of homogeneous, isotropic, harmonically vibrating plates with internal rigid supports or cracks is supplemented here by terms representing possible point discontinuities at the tips of the support or of the crack. This expression being given in tensor notation can be easily adapted to curvilinear plates with regular boundaries (separable coordinate systems) and arbitrary discontinuities and, in particular, to rectangular plates with curvilinear discontinuities. Vibration of a rectangular simply supported plate with arbitrarily located rectilinear rigid support is discussed as an example. The unknown discontinuity of the shear force across the support is described by a coupled system of integro-algebraic equations valid for any position and inclination of the rigid support. This system is subsequently reduced to an infinite system of linear-algebraic equations. Fundamental frequencies of natural vibration are obtained for a square plate with centrally located but arbitrarily inclined rigid support of any length. In the special cases—support parallel to one edge of the plate or diagonal support reaching the corners—the results show very good agreement with the ones known from previous analyses.