Thermal Failure of Polymeric Structural Elements under Monoharmonic Deformation

Thermal-failure problems are formulated for three-dimensional, thin-walled structural elements made of passive and piezoactive materials and subjected to monoharmonic deformation. Methods are described for calculation of critical electric and mechanical, quasistatic and dynamic monoharmonic loads and for analysis of postcritical behavior. The influence of various factors on the critical load is analyzed. Theoretical and experimental data are compared and found to be in good agreement, which is indicative of the reliability of the models used to describe the thermal failure of polymeric structural elements     

THE USE OF VACUUM PRESSURE IN STRUCTURAL TESTING

This paper presents an experimental setup for the method of simulating uniformly and varying distributed loads on structural members and roofing materials. The applications of this setup can be used for the study of beams, such as the lateral, torsional buckling behavior of cold-formed thin-walled structures. In addition, the method of measuring bending stiffness of composite materials in semimonocoque structures is illustrated. With a little modification to the setup, the study could possibly be extended to the investigation of elastic foundations and beam-column interaction.     

Experimental evaluation of incremental theories for nonproportionate loading of thin-walled cylinders

An experimental investigation was undertaken to evaluate incremental-strain theories which have been proposed in the literature to predict the loads on thin-walled cylinders subjected to nonproportionate loading which follow prescribed strain histories. Test data were obtained for two materials, annealed SAE 1035 steel and normalized 4340 steel. Material-property tests for the SAE 1035 steel indicated that the stress-strain diagram was flat topped and the material followed the Tresca flow condition. Similar tests for the SAE 4340 steel indicated that this steel was a linear strain-hardening material that followed the von Mises flow condition. Two incremental-strain theories were developed for thin-walled cylinders made of SAE 1035 steel. Both were based on the Tresca flow condition. One theory called the Tresca-Tresca theory used the stress-strain relations for the Tresca theory. The other theory called the Tresca-Mises theory used the Prandtl-Reuss stress-strain relations. In general, the test data fell between the two theories. The incremental theory developed for thin-walled cylinders made of the SAE 4340 steel, called the Mises-Mises theory, was based on the von Mises flow condition and the Prandtl-Reuss stress-strain relations. The agreement between theory and experiment was poor.     

Convergence of Computational Methods and Stability of Self-Balanced Stresses under Shrinkage of Spherical Inclusions of a Damageable Material

Some iterative methods for calculating self-balanced stresses under shrinkage of a ball inclusion enclosed in a spherical matrix of a physically nonlinear damageable material. The stability of this system was studied using methods of catastrophe theory. It has been established that the beginning of divergence of the proposed iterative processes coincides with the moment of transition of the system to an unstable position of equilibrium.     

Parameter identification in viscoelastic strain models for composite materials by analyzing impulsive loading of shells of revolution

An analytical-experimental method for identifying the material constants and functions in the constitutive relations of viscoelastic strain for homogeneous composite materials is proposed. The method is based on the minimization of the discrepancy between the results of numerical and experimental modeling of nonstationary deformation processes in shells of revolution made of the materials under study. The approach was tested and its adequacy was shown in problems of determining the rigidity and rheological characteristics of composite materials from the results of comparative analytical-experimental study of nonstationary deformation of spherical and cylindrical shells under impulsive loading.     

A simple model of the mechanical behavior of ceramic-like materials

This paper presents a macroscopic mechanical theory for ceramic-like materials undergoing isothermal deformations. The proposed model describes an elastic brittle material which is damageable only under tensile loading. The damage lowers the elastic stiffness in traction simulating hence the softening and the fracture (zero stillness) of the material. The basic idea is to consider the continuum as a mixture of two phases—a linear elastic phase and a masonry phase (which shows a linear elastic behavior under compression but cannot hold tractive loads at all). The damage is then related to the volume fraction β of the clastic constituent. The constitutive relations are derived from macroscopic thermodynamics with the volume fraction β and its gradient β taken as state variables.     

A lower bound on snap-through instability of curved beams under thermomechanical loads

A non-linear finite element formulation (three dimensional continuum elements) is implemented and used for modeling dynamic snap-through in beams with initial curvature. We identify a non-trivial (non-flat) configuration of the beam at a critical temperature value below which the beam will no longer experience snap-through under any magnitude of applied quasi-static load for beams with various curvatures. The critical temperature is shown to successfully eliminate snap-through in dynamic simulations at quasistatic loading rates. Thermomechanical coupling is included in order to model a physically minimal amount of damping in the system, and the resulting post-snap vibrations are shown to be thermoelastically damped. We propose a test to determine the critical snap-free temperature for members of general geometry and loading pattern; the analogy between mechanical prestress and thermal strain that holds between the static and dynamic simulations is used to suggest a simple method for reducing the vulnerability of thin-walled structural members to dynamic snap-through in members of large initial curvature via the introduction of initial pretension.     

Crack growth induced by thermal-mechanical loading

Advanced aerospace structures are often subjected to combined thermal and mechanical loads. The fracture-mechanics behavior of these structures may be altered by the thermal state existing around the crack. Hence, design of critical structural elements requires the knowledge of stress-intensity factors under both thermal and mechanical loads. This paper describes the development of an experimental technique to verity the thermal-stress-intensity factor generated by a temperature gradient around the crack. Thin plate specimens of a model material (AISI-SAE 1095 steel) were used for the heart transfer and thermal-mechanical fracture tests. Rapid thermal loading was achieved using high-intensity focussed infrared spot heaters. These heaters were also used to generate controlled temperature rates for heat-transfer vertification tests. The experimental results indicate that thermal loads can generate stress-intensity factors large enough to induce crack growth. The proposed thermal-stress-intensity factors appear to have the same effect as the conventional mechanical-stress-intensity factors with respect to fracture.     

Stability of layered coatings with outer layer made of ceramics

The stability of layered coatings with a ceramic layer under biaxial loading is studied. The three-dimensional linearized theory of stability and piecewise-homogeneous material model are used. Specific problems for layered bodies of various structure are solved. The critical loads and wave numbers responsible for loss of stability of layered bodies are determined     

遗传递增演化算法配筋优化设计

递增演化结构算法(AESO)是在初始结构上,逐步增加有效材料,进而得到优化拓扑形状,其运算速度快,但容易陷入局部最优解。为提高寻找全局最优解的能力,把递增演化结构算法(AESO)和遗传算法(GA)相结合,提出遗传递增演化算法(GAESO),在增加有效材料的选择性上引入生物进化遗传理论,并以ANSYS有限元分析软件的非线性分析为平台,采用钢筋混凝土分离式模型,探讨遗传递增演化算法(GAESO)在钢筋混凝土复杂应力构件配筋优化设计上的应用,直观地完成在荷载作用和应力约束条件下简支深梁、简支开洞深梁和开洞剪力墙等钢筋混凝土复杂应力构件的配筋优化设计,所得结果符合受力机理,演化方向正确,钢筋布置明确,与遗传演化优化算法(GESO)所得结果进行比较验证,证实了算法的可行性、速敛性和稳定性,能为钢筋混凝土配筋优化设计提供参考。     

ENERGY ABSORPTION CONTROL CHARACTERISTICS OF AL THIN-WALLED TUBES UNDER IMPACT LOAD

An experimental investigation was carried out to study the energy absorption characteristics of thin-walled square tubes subjected to dynamic crushing by impact loading to develop the optimum structural members. Here, the controller is introduced to improve and control the absorbed energy of thin-walled square tubes in this paper. When the controller were used, the experimental results of crushing of square tubes controlled by the controller＇s elements showed a good candidate for a controllable energy absorption capability in impact crushing.     

Experiments on the stability of spherical and paraboloidal shells

This paper presents the experimental part of a study of the elastic stability of clamped and hinged spherical and paraboloidal shell caps subjected to external uniform pressure loading. Ninety-six poly-vinyl-chloride (PVC) shells of 15-in. base diameter and formed by thermovacuum process, were tested and their critical loads recorded. The effects of geometry, clamped and hinged boundary conditions, height/span and radius of curvature/thickness ratios on the critical pressure were investigated. The results are compared, where possible, with experimental data obtained by previous investigators.     

Thermoelastic buckling of steel columns with load-dependent supports

The in-plane elastic buckling of a steel column with load-dependent supports under thermal loading is investigated. Two elastic rotational springs at the column ends are used to model the restraints which are provided by adjacent structural members or elastic foundations. The temperature is assumed to be linearly distributed across the section. Based on a nonlinear strain–displacement relationship, both the equilibrium and buckling equations are obtained by using the energy method. Then the limits for different buckling modes and the critical temperature of columns with different cases are studied. The results show that the proposed analytical solution can be used to predict the critical temperature for elastic buckling. The effect of thermal loading on the buckling of steel columns is significant. Furthermore, the thermal gradient plays a positive role in improving the stability of columns, and the effect of thermal gradients decreases while decreasing the modified slenderness ratios of columns. It can also be found that rotational restraints can significantly affect the column elastic buckling loads. Increasing the initial stiffness coefficient α or the stiffening rate β of thermal restraints will increase the critical temperature.     

Stability and load-bearing capacity of smooth and ribbed shells with local dents

A method for analysis of the stability and load-bearing capacity of imperfect smooth and ribbed shells is developed. This method is based on the finite-difference method and is implemented as an algorithm for fast calculation of critical forces, as opposed to the finite-element method. The theoretical results discussed include both early and recent results. The emphasis is on shells with local dents. The numerical results are successively corrected and compared with available experimental data for shells with a single dent and with other data. The method enables us to discover new features in the behavior of thin-walled structures under loading: development of precritical state, change in the dent shape, and exhaustion of load-bearing capacity. The lower local critical loads and upper stresses are determined. They correspond to general buckling and agree well with available experimental data.Translated from Prikladnaya Mekhanika, Vol. 40, No. 10, pp. 35–64, September 2004.     

热-力联合作用下柱壳结构变形的计算分析

本文研究了激光连续加热和恒内压作用下柱壳结构的变形规律。以4340钢材料作为研究对象，其本构方程选用热粘塑性本构模型（Johnson-Cook模型）；选取某一应变率为临界值（本文中临界应变率取为1s^-1)。考察了激光功率密度，预载荷大小，激光作用时间等对结构变形规律的影响。主要结论有：结构失稳是导致激光辐照下充内压柱壳破坏的重要原因；激光功率密度和内压越高，结构破坏时间越短；根据辐照时间的长短，预测到了结构急速破坏，延迟破坏和不破坏三种模式。本工作对于深入认识激光作用下预载结构的热－力联合破坏有一定意义。     

多晶金属材料的三维组集式弹塑性本构模型

本文从分析多晶金属材料的细观组织在弹塑性变形中的贮能和耗能机制入手,提出一个三维组集式弹塑性本构模型。该模型将材料单元抽象成沿三维空间各方向均匀分布组件的集合体,方向组件反映材料的细观性态并在宏观上协调变形,所有方向组件的内力总效应构成宏观应力。文中导出了显式的弹塑性本构关系,并与Budiansky的复杂加载试验结果及其它塑性模型进行了对比。     

Applied creep theory for bodies with anisotropy due to plastic prestrain

Plastic strains in structures at the stages of manufacturing, testing, and approaching the operation regime cause anisotropic variations in the mechanical properties of materials, including creep strength. We consider the following special but practically important class of loading processes for originally isotropic materials: a simple active plastic strain is followed by a long-term steady-state loading within the elastic limits. To describe the second stage, we present the creep strain deviator in the form of an additive orthogonal decomposition in the directions of the repeated loading and the vector anisotropy. The coefficients in the decomposition are material functions of time, of the intensities of the preliminary and repeated loadings, and of the angle between the directions of these loadings. We obtain conditions on the material functions under which, at any given time instant, there is a one-to-one continuous correspondence between the stress and strain tensors for the model proposed and the boundary-value problem in the generalized statement has a unique solution; we also prove the convergence of the iteration method of elastic solutions used to find this unique solution. The model is identified according to the creep diagrams (under steady-state stresses of different values) determined for the material in the original state and after the plastic prestrain at an angle (zero, extended, and intermediate) to the direction of the repeated loading. We show that our results are in good agreement with the results available in the literature concerning experiments in this class of processes for stainless steel at high temperature. We propose an engineering version of the theory in which only the experimental data for uniaxial tension are used. We discuss the versions of the model for the cases in which the plastic preloading is cyclic (one-dimensional or circular) and the repeated loading is unsteady.