A Revisit to the Frozen Stress Phenomena in Photoelasticity

Stress freezing in photoelasticity is regularly employed for 3-D elastic stress analysis of geometrically complex models, wherein the models under load are soaked at the stress freezing temperature and slowly cooled to room temperature. During this process, a model may distort or fail undergoing large deformations at the stress freezing temperature owing to thermo-mechanical interactions. The contribution of thermal deformation to the model distortions is neglected in the available literature of stress freezing. This aspect of stress freezing is investigated in this paper, wherein Digital Image Correlation (DIC) is used to study the strain behavior during a complete stress freezing cycle for an epoxy made of CY230 resin cured with HY951 hardener. The results show that the thermal contributions to the model distortions at the critical temperature must be taken care of to estimate the failure margins. The distortions and failures would mainly depend upon the thermal and mechanical response of the model material and the complexity of the model.     

Models in the Engineering Mechanics of Polymer-Matrix Composite Systems

This paper discusses the complete system of structural, thermochemical, and mechanical-mathematical models that describe all the phenomena accompanying the formation of polymer-matrix composite materials (PCMs) and structures made of them. The issues of optimizing design engineering and modeling the postprocess behavior of PCM structures are addressed     

Equivalent models of corrugated panels

The design of corrugated panels has wide application in engineering. For example corrugated panels are often used in roof structures in civil engineering. More recently corrugated laminates have been suggested as a good solution for morphing aircraft skins due to their extremely anisotropic behaviour. The optimal design of these structures requires simple models of the panels or skins that may be incorporated into multi-disciplinary system models. Thus equivalent material models are required that retain the dependence on the geometric parameters of the corrugated skins or panels. An homogenisation-based analytical model, which could be used for any corrugation shape, is suggested in this paper. This method is based on a simplified geometry for a unit-cell and the stiffness properties of original sheet. This paper outlines such a modelling strategy, gives explicit expressions to calculate the equivalent material properties, and demonstrates the performance of the approach using two popular corrugation shapes.     

基于材料相变的声子晶体带隙可调结构设计与性能分析

提出一种基于材料相变的穿孔型带隙可调声子晶体结构.其结构形式为含缝隙的形状记忆合金和环氧树脂的组合体,通过温度变化诱发相变引起的形状记忆合金材料性质的变化,实现声子晶体的带隙变化;通过合理布置缝隙与形状记忆合金相变材料的位置,实现声子晶体带隙性质的可调设计.基于有限元方法,建立了可调声子晶体的分析模型,分析了形状记忆合...     

Fracture testing of a self-healing polymer composite

Inspired by biological systems in which damage triggers an autonomic healing response, a polymer composite material that can heal itself when cracked has been developed. In this paper we summarize the self-healing concept for polymeric composite materials and we investigate fracture mechanics issues consequential to the development and optimization of this new class of material. The self-healing material under investigation is an epoxy matrix composite, which incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is triggered by contact with an embedded catalyst. The effects of size and concentration of the catalyst and microcapsules on fracture toughness and healing efficiency are investigated. In all cases, the addition of microcapsules significantly toughens the neat epoxy. Once healed, the self-healing polymer exhibits the ability to recover as much as 90 percent of its virgin fracture toughness.     

Towards stress freezing in birefringent orthotropic composite models

Birefringent composite models are fabricated using epoxy resin reinforced with unidirectionally oriented glass fibers. The mechanical and photoelastic properties of the material at room temperature are determined. To explore the possibility of application of stress-freezing technique to birefringent composite models, the behavior and properties of this material are studied at elevated temperature (at stress-freezing temperature of the resin). The properties of the material at room and at elevated temperatures are reported. The feasibility of stress freezing glass-fiber-reinforced epoxy composites with low-fiber-volume fraction is discussed.     

Buckling experiments on stiffened cast-epoxy conical shells

This paper describes a casting technique for fabricating high-quality plastic structural models and presents results on the use of such specimens to parametrically study the effect of base-ring stiffness on the critical buckling pressure of a ring-stiffened conical shell. The fabrication technique involves machining a metal mold to the desired configuration and vacuum drawing the plastic material into the mold. A room-temperature-curing translucent thermoset epoxy was the casting material selected. The casting technique allows many high-quality specimens to be produced and each specimen is capable of being repeatedly tested without failure. The conical shell was modified for successive tests by machining the epoxy base-ring configuration to reduce its stiffness. A shell-of-revolution computer program which uses a nonlinear axisymmetric prebuckling strain field to obtain a bifurcation-buckling solution was used to guide the selection of configurations tested. The shell experimentally exhibited asymmetric collapse behavior and the ultimate load was considerably higher than the analyticalbifurcation prediction. The asymmetric buckling-mode shape, however, initially appeared at a pressure near the analysis-bifurcation solution. Comparison of experimental and analytical prebuckling strains at pressure magnitudes below the initiation of asymmetric collapse showed good agreement.     

海水环境中环氧值对环氧树脂涂层摩擦学性能的影响研究

环氧树脂是一类具有优异的粘接、耐腐蚀和电气绝缘等性能的高强度热固性高分子材料,在黏结剂、防腐涂料和复合材料方面展现出广阔的应用前景,其摩擦学行为与环氧值密切相关.本研究中采用线棒涂布器在铸铁表面分别制备了3种具有不同环氧值的环氧树脂涂层.借助傅里叶红外光谱仪(FT-IR)、X射线光电子能谱仪(XPS)、扫描电镜(SEM)、纳米压痕仪、热重/差热综合热分析仪(TG/DTA)、UMT-3摩擦磨损试验机和表面轮廓仪研究了涂层的结构、硬度、耐热性及摩擦学性能.研究结果表明:涂层在干燥条件和海水环境中的摩擦系数和磨损率均随环氧值的增大而升高;海水环境中涂层的摩擦系数和磨损率均较干燥条件下低.涂层的磨损机理在干燥条件下包括了疲劳磨损、黏着磨损和磨粒磨损,而在海水中仅为疲劳磨损.本研究成果为高性能防腐耐磨涂层的设计与应用提供理论指导.     

Postbifurcation Equilibrium Paths Modified by Nonlinear Configuration-Dependent Conservative Loading Using Nonsmooth Analysis*

ABSTRACT

This paper discusses the effect of deformation-sensitive loading devices because the nature of loading is generally not perfectly dead, being independent of the deflections that occur. This paper presents the effect of nonlinear variable load. Postbifurcation equilibrium paths and structural tangent stiffness are modified on the basis of a polygonal approximation and nonsmooth analysis. The effects of dead and variable loads are compared.

Configuration-dependent loading devices can be characterized by some load-deflection functions, much like the nature of material behavior can be characterized by stress-strain functions. The effect of a deformation-sensitive load is similar to that of the material. Consequently, in the stability analysis of structures, a configuration-dependent loading program can be handled like material behavior. Thus, in the tangent stiffness of the structure, much like the tangent modulus of the material, the tangent modulus of the load appears.

Previous research has shown that nonlinear material behavior can be handled using nonsmooth analysis—approximating nonlinear material functions by polygonals. In this paper, this method and its results are extended to the case of nonlinear loading programs. The present analysis is based on earlier work in which a complete stability analysis was introduced for dead-loaded structures that have polygonal constitutive law, namely nonsmooth material functions and non-smooth internal potential. The aim of this paper is to extend these results to cases involving nonlinear configuration-dependent conservative loading by introducing nonsmooth loading functions.     

Damping characterization of a filled epoxy used for dynamic structural modeling

This paper presents information on the usefulness of a particular aluminum-particle-filled epoxy for dynamic modeling. Damping characteristics in terms of the loss factor are presented for this epoxy and some structural metals. Though the damping of the epoxy was larger than that of any metal tested, it can still be considered small. Portal frames were modeled using the epoxy. Natural frequencies of vibration for the metal frames and epoxy models were determined. The scaled-epoxy-model frequencies accurately predicted the metal prototype frequencies. A possible area of error is pointed out with respect to the modeling of the shear modulus.     

Virtual improvement of ice cream properties by computational homogenization of microstructures

Optimal shape design of microstructured materials has recently attracted a great deal of attention in materials science. The shape and the topology of the microstructure have a significant impact on the macroscopic properties. This paper presents different computational models of random microstructures, to virtually improve the physical properties of ice cream. Several sensory properties of this heterogeneous material issued from food industry are directly controlled by the elastic and thermal conducting ones. The material effective elastic and thermal conducting properties are obtained through direct large scale numerical simulations. The different formulations address the problem of finding the shape of the representative microstructural element for random heterogeneous media that increase the elastic moduli and thermal conductivity compared to existing products. The computational models are established using finite element method and images of virtual microstructures. In this paper we propose a new model of microstructures. This model is constructed with hexagonal prismatic rods and plates with volume fractions around 0.7 for the hard phase represented by hexagons of ice. A comparison between three two-phase elastic heterogeneous microstructures models is drawn. This illustrates the concept of design of microstructures using computational homogenization tools.     

Model making and slicing for three-dimensional photoelasticity

Problems which can occur during casting of epoxy resin are high exotherm, mottle and residual stress. Some methods for reducing these effects are discussed. Techniques for sawing, holding and machining epoxy models are presented. Particular attention is given to avoidance of chipping or cracking by use of proper tool profiles, cutting speeds and feed rates. Several ways of assembling models are discussed, including inspection of machined models and cemented joints. The effect of cemented joints on stress patterns is demonstrated. Suggestions for laying out, cutting and flattening slices are presented. The effect of scribed lines on a stress pattern is shown. A procedure for surface subslice preparation is given. Two types of edge effect can occur in epoxy frozen-stress models-temporary edge effect due to humidity and permanent edge effect due to oxidation. Methods for avoidance and treatment of edge effect are presented.     

Model for Elastic Modulus of Multi-Constituent Particulate Composites

In this paper, a methodology has been developed to accurately predict the elastic properties of multi-constituent particulate composites by accounting for irreversible effects, such as energy loss that arises due to internal friction. The complex dependence on loading density and particle properties (i.e., size, shape, morphology, etc.) is investigated in terms of their effects on the effective elastic modulus of the composite. Confirmed by experimental data from the compression loading of individual Ni and Al particles dispersed in an epoxy matrix, it is believed that this approach captures the effects of internal friction, consequently providing a more accurate and comprehensive representation for predicting and understanding the material behavior of multi-constituent particulate reinforced composites. The present methodology provides a model to directly compare the elastic modulus from an uncomplicated test, such as dual-cantilever beam loading in dynamic mechanical analysis (DMA), to the modulus obtained by other more complex experimental methods such as quasi-static compression. The model illustrates an efficient method to incorporate input data from DMA to represent realistic elastic moduli, hence promising for the characterization and design of particulate composites.     

Stiffness of planar tensegrity truss topologies

This paper proposes and demonstrates a symbolic procedure to compute the stiffness of truss structures built up from simple basic units. Geometrical design parameters enter in this computation. A set of equations linear in the degrees-of-freedom, but nonlinear in the design parameters, is solved symbolically in its entirety. The resulting expressions reveal the values of the design parameters which yield desirable properties for the stiffness or stiffness-to-mass ratio. By enumerating a set of topologies, including the number of basic units, and a set of material distribution models, stiffness properties are optimized over these sets. This procedure is applied to a planar tensegrity truss. The results make it possible to optimize the structure with respect to stiffness properties, not only by appropriately selecting (continuous) design parameters like geometric dimensions, but also by selecting an appropriate topology for the structure, e.g., the number of basic units, and a material distribution model, all of which are discrete design decisions.     

Thermo-elastic behavior of a polymeric layer bonded between rigid interfaces

A highly expandable polymeric material have been combined with a stiff skeleton material to form a powerful design of thermal micro-actuators. The bond interfaces with the skeleton laterally restrain deformation of the polymer and consequently direct its volumetric expansion in the transverse direction. A complete lateral constraint at the infinite bond width could maximize the apparent thermal strain of the bonded polymer. However, it is not sure how much strain enhancement can be achieved using a finite bond width. To answer this, we resort to an approximate thermo-elastic model and solve it using the mean-pressure method. This model leads to closed-form solutions to the thermally induced strains and stresses in a bonded polymer layer between rigid interfaces. The closed-form solution shows that the apparent strain of a bonded layer depends on the aspect ratio of the bond width to the layer thickness, besides Poisson’s ratio. Furthermore, it further shows that a bond width five times the thickness of the SU-8 epoxy layer is sufficient to attain 95% of the maximum apparent strain, which is obtained at the infinite width.     

Review of Drucker’s postulate and the issue of plastic stability in metal forming

Drucker’s postulate defines a class of stable work hardening materials that are classified as non-energetic and is equivalent to the associated flow rule (AFR). The postulate has been shown to be a sufficient condition for plastic stability. However, experiments indicate that plastic deformation of aluminum and steel alloys does not adhere to the constraints of the AFR. Therefore, the requirement for accuracy suggests that the metal forming industry should also consider material models that are based on non-associated flow. But Drucker’s work raises the issue of stability when considering the use of non-associated flow in material models. While this concern is merited and many types of instability arises from certain types of non-associated flow, this has led to a widely accepted view that Drucker’s postulate is a necessary condition for stability. This perception is inhibiting the acceptance or consideration of more accurate material models that are suggested from the experimental observations about violations of the AFR. This paper proposes a specific class of material models based on non-associated flow and derives the constraints on this class of models to ensure stability. The existence of this class of non-AFR models proves that Drucker’s postulate is a sufficient but not necessary condition for stability. Furthermore, the class of models described in this paper is quite general and provides a framework for consideration of potentially more accurate material models while guaranteeing the same level of stability as typically associated with materials that satisfy Drucker’s postulate.     

Rheological models for unified curves for simplified design calculations in polymer processing

A method has been proposed earlier by Shenoy et al. to estimate complete flow curves depicting the variation of melt viscosity over industrially relevant range of shear rate and temperature through the use of unified curves characteristic of each generic type of polymer. In the present paper, the unified curves have been fitted through modified forms of already known generalized Newtonian models. The advantage of fitting such rheological models to unified curves lies in the fact that calculations involving entrance length estimates, heat transfer, die design, mold design, etc. are greatly simplified. Knowing the model parameters, one needs to know only the melt flow index, which is a relatively easy parameter to obtain, to make any engineering design calculations. The modified generalized Newtonian models have been fitted to a number of polymers covering one or more from each group of olefinics, styrenics, cellulosics, vinyls and engineering thermoplastics.NCL-Communication No. 3108     

结构破坏的尺度律

文中综述了结构破坏的尺度律和尺寸效应的研究进展,尤其将重点放在准脆性材料的分析上,因为它们的尺寸效应是重要和复杂的．在回顾了尺寸效应研究的悠远发展史以后。着重讨论了三种主要类型的尺寸效应,即由于强度随机性引起的统计尺寸效应、能量释放的尺寸效应和由于微裂纹或断裂的分形特性可能引起的尺寸效应．得出了这些理论应用的明确结论．之后讨论了如何运用已知的尺寸效应律来测量材料的断裂特性,并采用内聚裂纹模型（cohesivecrackmodel）、非局域化有限元模型和离散元模型等对尺寸效应进行模化．文中还进而分析了尺寸效应在压缩失效和车相关材料行为下的有关问题,并讨论了在断裂扩展区描述含微裂纹材料所需的损伤本构关系．最后也讨论了尺寸效应对准脆性材料的多种应用,这些材料包括,如混凝土、海冰、纤维复合材料、岩石和陶瓷等．本文包含了参考文献377篇