Calculation of flutter and dynamic behavior of advanced composite swept wings with tapered cross section in unsteady incompressible flow

In this study, the aeroelastic stability and response of a swept composite wing in subsonic incompressible flow are investigated. The wing is modeled as an anisotropic tapered thin-walled beam with the circumferentially asymmetric stiffness structural configuration to establish proper coupling between bending and torsion. The structural model considers a number of nonclassical effects, such as transverse shear, material anisotropy, warping inhibition, nonuniform torsion, rotary inertia and three-dimensional strain effects. The aerodynamic strip method based on two-dimensional Wagner function in unsteady incompressible flow is used. Following the analysis, the mass, stiffness and the damping matrices of the nonconservative aeroelastic system are formed such that the extended Galerkin method and the separation of variables method can be employed. As a result, the coupled and linear governing system of dynamic equations is obtained. Then, by transforming matrices into the state-space and state-vector forms, the problem under study is finally converted into an eigenvalue problem. The flutter and the divergence speeds for various layer configurations with different geometric and material properties and fiber orientations are obtained. By solving the aforementioned equations of motion in the time domain, the aeroelastic responses of the tapered swept composite wing are computed. The obtained results are compared with the available literature.     

Non-linear buckling and postbuckling of shear deformable anisotropic laminated cylindrical shell subjected to varying external pressure loads

Non-linear buckling and postbuckling of a moderately thick anisotropic laminated cylindrical shell of finite length subjected to lateral pressure, hydrostatic pressure and external liquid pressure has been presented in the paper. The material of each layer of the shell is assumed to be linearly elastic, anisotropic and fiber-reinforced. The governing equations are based on a higher order shear deformation shell theory with von Kármán–Donnell-type of kinematic non-linearity and including the extension/twist, extension/flexural and flexural/twist couplings. The non-linear prebuckling deformations and initial geometric imperfections of the shell are both taken into account. A singular perturbation technique is employed to determine the buckling pressure and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling response of perfect and imperfect, moderately thick, anisotropic laminated cylindrical shells with different values of shell parameters and stacking sequence. The results confirm that there exists a circumferential stress along with an associate shear stress when the shell is subjected to external pressure.     

On the vibration and stability of shear deformable FGM truncated conical shells subjected to an axial load

The aim of present study is to investigate the vibration and stability of functionally graded (FG) conical shells under a compressive axial load using the shear deformation theory (SDT). The basic equations of shear deformable FG conical shells are derived using Donnell shell theory and solved using Galerkin's method. The novelty of this study is to achieve closed-form solutions for the dimensionless frequencies and critical axial loads for freely-supported FG truncated conical shells on the basis of the SDT. Parametric studies are made to investigate effects of shear stresses, compositional profiles and conical shell characteristics on the critical parameters. Some comparisons with the various studies have been performed in order to show the accuracy of the present study.     

考虑发动机推力影响的机翼颤振分析

发动机安装在机翼上时,其推力具有典型的随动特征,并对机翼颤振产生重要影响.基于有限元分析软件MSC/Nastran的DMAP开发,提出了一种考虑发动机推力和几何非线性影响的机翼颤振分析方法.作为验证,分析了推力对某高空长航时飞行器机翼颤振速度的影响,与已有结果吻合良好.对一带有两个发动机的复杂机翼结构进行了结构建模和颤振分析,重点分析了推力大小及作用位置对颤振速度的影响.结果表明,发动机的推力效应在颤振分析中是不应忽略的.     

Nonlinear dynamic response and vibration of imperfect shear deformable functionally graded plates subjected to blast and thermal loads

Based on Reddy's higher-order shear deformation plate theory, this article presents an analysis of the nonlinear dynamic response and vibration of imperfect functionally graded material (FGM) thick plates subjected to blast and thermal loads resting on elastic foundations. The material properties are assumed to be temperature-dependent and graded in the thickness direction according to a simple power-law distribution in terms of the volume fractions of the constituents. Numerical results for the dynamic response and vibration of the FGM plates with two cases of boundary conditions are obtained by the Galerkin method and fourth-order Runge–Kutta method. The results show the effects of geometrical parameters, material properties, imperfections, temperature increment, elastic foundations, and boundary conditions on the nonlinear dynamic response and vibration of FGM plates.     

超音速弹翼非线性颤振分析与控制

摘　要：研究超音速弹翼非线性气动弹性稳定性与主动控制问题。采用三阶活塞理论建立含间隙弹翼非线性气动弹性动力学方程,分别利用Hopf分岔理论及数值方法给出系统的线性与非线性颤振速度,应用基于微分几何法及二次型最优控制相结合的方法,设计非线性系统控制器。最后分析控制器及系统参数对控制效果的影响。仿真结果表明设计的控制器可有效地实现对非线性系统颤振的抑制。     

Fatigue failure of an aircraft wings due to fitting error in hi-locks

Two hi-locks were fractured during a flight of a carrier aircraft, causing the leakage of fuel in the right wing section. The failed and a new hi-lock were subjected to failure investigation.Material of the hi-locks was AMS-6270. The hardness of both hi-locks was also comparable to each other. The investigations entailed that the failed hi-locks were loosened or improperly preloaded. Thence the fastened sections exerted bending fatigue loads on the hi-locks. Therefore, two propagating beach mark fields were observed on the fracture surface, which were advancing towards each other under high cycle fatigue (HCF). When the crack reached at the threshold level, the hi-lock(s) failed catastrophically.     

Dynamic response of deformable structures subjected to shock load and cavitation reload

The dynamic response of deformable structures subjected to shock load and cavitation reload has been simulated using a multiphase model, which consists of an interface capturing method and a one-fluid cavitation model. Fluid–structure interaction (FSI) is captured via a modified ghost fluid method (Liu et al. in J Comput Phys 190: 651–681, 2003), where the structure is assumed to be a hydro-elasto-plastic material if subjected to a strong shock load. Bulk cavitation near the structural surface is captured using an isentropic model (Liu et al. in J Comput Phys 201:80–108, 2004). The integrated multiphase model is validated by comparing numerical predictions with 1D analytical solutions, and with numerical solutions calculated using the cavitation acoustic finite element (CAFé) method (Sprague and Geers in Shocks vib 7:105–122, 2001). To assess the ability of the multiphase model for multi- dimensions, underwater explosions (UNDEX) near structures are computed. The importance of cavitation reloading and FSI is investigated. Comparisons of the predicted pressure time histories with different explosion center are shown, and the effect on the structure is discussed.     

Divergence and flutter instability of elastically restrained structures under follower forces

The instability of elastically restrained simple structures acted upon by compressive follower forces is discussed by using a static stability analysis. From this investigation it is concluded that depending on the amount of elastic restraint: (a) divergence or flutter type instability is possible and (b) the critical load of a divergence type non-conservatively loaded structure may be higher or smaller than the critical load of the corresponding structure subjected to a conservative load. Moreover, a lower bound theorem is presented according to which under a certain condition the load carrying capacity of a non-conservatively loaded structure of divergence type is higher than the load carrying capacity of the corresponding conservatively loaded structure. From the foregoing findings a better insight into the actual mechanism of loss of stability of structures under follower forces is gained.     

Al-Cu合金析出相在等径角挤压中的演变

为了研究Al-Cu合金中两种不同析出相(θ′和θ相)在ECAP变形过程中的变化.采用透射电镜(TEM)和硬度测试方法研究了析出相形貌变化以及对合金性能的影响.结果表明:在本实验中,θ′和θ相其破碎、回溶速度明显不同,两者的破碎方式也不同.θ′相先是与基体失去取向关系,随后从其内部产生位错使其破碎,而θ相是被外部基体位错所切割、破碎.θ′相与位错的相互作用方式类似于绕过机制,θ相与位错的作用方式类似于切割机制.两种状态样品的硬度在变形过程中的变化趋势相同,但在第1道次后θ相状态样品的硬度增加值高于θ′相状态.     

Semi-rigid––simply supported shear deformable pultruded GRP beams subjected to end moment loading: comparison of measured and predicted deflections

A single span shear deformable beam is analysed. One end is simply supported and subjected to an external moment and the other is supported by a semi-rigid joint with a linear moment––rotation characteristic. An expression for the mid-span deflection is derived in order to predict deflections recorded in tests on pultruded GRP beams with four types of semi-rigid joint. Using rotational stiffnesses derived from moment––rotation tests on similar joints it is shown that semi-rigid, shear deformable beam analysis is generally able to predict the measured deflections to within less than 10%. The mid-span deflection formula is recast to allow a back analysis to be carried out to determine the actual stiffness of two of the semi-rigid joints. It is shown that the joint tests significantly under-estimate the rotational stiffness of the bolted web cleat joint, whereas the predicted stiffness of the bolted web and flange cleat connection differs only by about 20% from that determined in one of the joint tests on this connection.     

Dissipative heating of a hollow viscoelastic cylinder subjected to the action of normal loads moving along its surface with constant angular velocity

We consider a two-dimensional quasistatic problem of the stress-strain state and dissipative heating of a hollow circular viscoelastic cylinder subjected to the action of two, normal diametrically opposite loads moving along the surface of the cylinder with constant angular velocity. It is assumed that the physicomechanical properties of the material are independent of temperature. In this case, the combined problem of thermoviscoelasticity splits into two linear problems: the problem of the stress-strain state of the cylinder without axial symmetry and the axisymmetric problem of heat conduction with temperature averaged over a cycle and a known heat source. The exact solutions of these problems are obtained. The influence of both the coefficient of heat transfer on the inner boundary of the cylinder and the sizes of the cylinder on the temperature of dissipative heating are investigated. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 67–73, March–April, 1999.     

Failure analysis of UHPFRC panels subjected to aircraft engine model impact

Using the finite element approach, this paper evaluates the punching resistance of ultra-high performance fiber reinforced concrete (UHPFRC) panels subjected to the impact of an aircraft engine. The models are analyzed using LS-DYNA, a commercially available software program. The structural components of the UHPFRC panels, aircraft engine model, and their contacts are fully modeled. Included in the analysis is material nonlinearity, which considers damage and failure. The analysis results are then verified with the test results. A parametric study with varying fiber contents is carried out to investigate the punching behavior of the UHPFRC panels under aircraft engine impact. The penetration depth, residual velocity of the aircraft engine, scabbing area, and failure mode of various UHPFRC panels are examined. Punching resistance capacities of reinforced concrete (RC) and steel plated concrete (SC) panels are also investigated in this study.     

Design of reinforced concrete panels subjected to combined tension and shear

Abstract

This paper presents an analytical solution applying the principle of principal stress to the reinforced concrete panel subjected to an in‐plane loading with a combined shear and tension. The results are compared with the design equations specified in the current ACI Code (318–83). It is found that the amount of shear reinforcement in vertical and horizontal directions is controlled by the angle of the principal plane θ ₁. Based on the results of this study, coefficients C_k and C_v, are developed for the calculation of shear reinforcement in horizontal and vertical directions.     

Compliance of a microfibril subjected to shear and normal loads.

Many synthetic bio-inspired adhesives consist of an array of microfibrils attached to an elastic backing layer, resulting in a tough and compliant structure. The surface region is usually subjected to large and nonlinear deformations during contact with an indenter, leading to a strongly nonlinear response. In order to understand the compliance of the fibrillar regions, we examine the nonlinear deformation of a single fibril subjected to a combination of shear and normal loads. An exact closed-form solution is obtained using elliptic functions. The prediction of our model compares well with the results of an indentation experiment.     

Finite deformations of compressible hyperelastic tubes subjected to circumferential shear

Finite deformations of homogeneous, isotropic, hyperelastic and compressible tubes subjected to circumferential shearing forces are studied using the theory of finite elasticity. The coupled, non-linear differential equations governing the problem are solved numerically to obtain the angle of twist and radial displacement. The qualitative difference of the present problem from the corresponding problem with incompressible material being the presence of radial deformations. the significance of this coupling is studied for various material constants. Limiting cases of infinitesimal deformations and incompressible material are compared with the findings of this work.     

Light-scattering properties of undiluted human blood subjected to simple shear

An experimental investigation was performed into the effect of simple shear on the light-scattering properties of undiluted human blood. Undiluted human blood was enclosed between two glass plates with an adjustable separation between 30 and 120 microns and with one plate moving parallel to the other. For various shear rates and layer thicknesses, the angular light distribution and the collimated transmission were measured for 633-nm light. For shear rates above 150 s-1, the transmission results directly yielded a total attenuation coefficient of 120 mm-1. At lower shear rates the total attenuation followed an irregular pattern. From the angular intensity distributions, the anisotropy for single scattering was deduced by inverse Monte Carlo simulations. A continuous increase of the average cosine g with the shear rate was observed, with g in the range 0.95-0.975.     

Nonlinearly viscoelastic analysis of asphalt mixes subjected to shear loading

This study presents the characterization of the nonlinearly viscoelastic behavior of hot mix asphalt (HMA) at different temperatures and strain levels using Schapery’s model. A recursive-iterative numerical algorithm is generated for the nonlinearly viscoelastic response and implemented in a displacement-based finite element (FE) code. Then, this model is employed to describe experimental frequency sweep measurements of two asphalt mixes with fine and coarse gradations under several combined temperatures and shear strain levels. The frequency sweep measurements are converted to creep responses in the time domain using a phenomenological model (Prony series). The master curve is created for each strain level using the time temperature superposition principle (TTSP) with a reference temperature of 40°C. The linear time-dependent parameters of the Prony series are first determined by fitting a master curve created at the lowest strain level, which in this case is 0.01%. The measurements at strain levels higher than 0.01% are analyzed and used to determine the nonlinear parameters. These parameters are shown to increase with increasing strain levels, while the time–temperature shift function is found to be independent of strain levels. The FE model with the calibrated time-dependent and nonlinear material parameters is used to simulate the creep experimental tests, and reasonable predictions are shown.     

Stretching and bending deformations due to normal and shear tractions of doubly curved shells using third-order shear and normal deformable theory

ABSTRACT

We analyze static infinitesimal deformations of doubly curved shells using a third-order shear and normal deformable theory (TSNDT) and delineate effects of the curvilinear length/thickness ratio, a/h, radius of curvature/curvilinear length, R/a, and the ratio of the two principal radii on through-the-thickness stresses, strain energies of the in-plane and the transverse shear and normal deformations, and strain energies of stretching and bending deformations for loads that include uniform normal tractions on a major surface and equal and opposite tangential tractions on the two major surfaces. In the TSNDT the three displacement components at a point are represented as complete polynomials of degree three in the thickness coordinate. Advantages of the TSNDT include not needing a shear correction factor, allowing stresses for monolithic shells to be computed from the constitutive relation and the shell theory displacements, and considering general tractions on bounding surfaces. For laminated shells we use an equivalent single layer TSNDT and find the in-plane stresses from the constitutive relations and the transverse stresses with a one-step stress recovery scheme. The in-house developed finite element software is first verified by comparing displacements and stresses in the shell computed from it with those from either analytical or numerical solutions of the corresponding 3D problems. The strain energy of a spherical shell is found to approach that of a plate when R/a exceeds 10. For a thick clamped shell of aspect ratio 5 subjected to uniform normal traction on the outer surface, the in-plane and the transverse deformations contribute equally to the total strain energy for R/a greater than 5. However, for a cantilever shell of aspect ratio 5 subjected to equal and opposite uniform tangential tractions on the two major surfaces, the strain energy of in-plane deformations equals 95–98% of the total strain energy. Numerical results presented herein for several problems provide insights into different deformation modes, help designers decide when to consider effects of transverse deformations, and use the TSNDT for optimizing doubly curved shells.