A finite element model for hygrothermal analysis of masonry walls with FRP reinforcement

Modeling of moisture migration and heat transfer in fiber reinforced polymer (FRP) composite upgraded masonry structures is of great importance, since the interfacial adhesive between the reinforcing FRP laminate and the host masonry is prone to moisture damages. In this paper, a generic theoretical formulation was first developed to model moisture and heat transport in a layered structure consisting of distinct materials. This formulation was based on the framework of the hygrothermal model presented by Philip and De Vries for a monolithic porous medium. Finite element implementation of the formulation was subsequently used to model moisture and heat transport in an FRP reinforced masonry block. Analytical results were then compared with experimental data to validate the model. Parametric studies were then performed for a concrete block with a reinforcing FRP laminate partially covering one surface. The results showed that changing temperature gradient affects the moisture distribution considerably. This effect was found particularly significant at the concrete/FRP interface where a drastic change in local temperature gradient is present.     

Experiments,modelling and simulation of adhesive curing processes in bonded three dimensional curved piezo metal composite structures

This contribution deals with the modelling and simulation of curing phenomena in adhesively bonded piezo metal composites which consists of a piezoelectric module enclosed by an adhesive layer which in turn is surrounded by two metal sheets. A short survey on the neccessary experimental investigations to characterise the adhesive's material behaviour is given and important aspects on the corresponding phenomenological modelling approach are presented. Both steps take into account the curing reaction, changes of volume, like chemical shrinkage, and inelastic mechanical behaviour which is temperature and curing dependent. Finally, the simulation strategy for the modelling within a finite element environment is depicted. By this, residual stresses, secondary deformations and loads on the piezo modules can be predicted, which is exemplified by a comparative study verifying a novel manufacturing strategy. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Description of the mechanical behaviour of micropolar adhesives

The paper describes the mechanical behavior of two solids—the adherends—adhesively bonded by a thin elastic adhesive considered as a polar material when the parameter associated to the thinness of the adhesive goes to 0. The adherends are considered as elastic nonpolar materials. The limit analysis for a thin adhesive is performed using first an asymptotic expansion of the solution, based on a mixed variational formulation of the equilibrium of the three bodies. The first- and second-order solutions are such that the adhesive behaves as a material surface. The implication of the additional rotational degrees of freedom on the kinematics of the adhesive is then studied. In the second part of the paper, the convergence of the solution of the three-dimensional problem to the limit solution is obtained, using a variational formulation of the equilibrium of the three body system and an epi-convergence argument.     

Application of a Viscoelastic Model to simulate the Interface of a Metal/Fibre-Reinforced Polymer Joint

A viscoelastic model with the Lemaitre-type damage is applied to simulate an interfacial adhesive zone in light weight engineering structures, like aluminum/fiber-reinforced polymer specimens. The evolution of irreversible deformation and damage progression are investigated. The joint of aluminium alloy 5754 (AA5754) and carbon fibre reinforced thermoplastic composite CF-PA66 is manufactured by means of an epoxy (1K) adhesive. The adhesive zone is considered as an interface material. The aim of the research is to study the influence of the interface geometry on the mechanical characteristics of the structure. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Prediction and Measurement of Residual Strains for a Composite Bonded Joint

A quasi-isotropic composite laminate/adherend of IM6/3501-6 and a composite bonded specimen were manufactured and tested. The bonded specimen was fabricated by postbonding composite adherends together using a 177°C adhesive resin. Predictions for the residual curing strains in the composite adherends and the adhesively bonded composite specimen were performed using a thermomechanical linearly elastic analysis. The analysis was performed using a computer program based on a polynomial spline displacement approximation method [1]. The residual strains of the specimens were measured using the moiré interferometry technique. Diffraction gratings were replicated at room temperature onto the edges of polished laminated adherends and on the edge of a fully cured adhesively bonded specimen. The specimens were cut through their entire thickness in the middle of the diffraction grating area, resulting in a redistribution of the residual curing stresses, with corresponding changes in the strain field at the edges of the cut. A full-field deformation pattern was obtained in the grating area by analyzing the recorded fringe patterns. The deformation field induced by the cut in the laminated adherends and the adhesive bondline were estimated by the linear thermomechanical analysis. A good agreement between the analysis and the experimental results was obtained.     

CFRP修复缺陷钢板应力解析模型

在使用碳纤维复合材料(carbon fiber reinforced polymer, CFRP)修复钢结构腐蚀缺陷的修复方式中,CFRP应力及胶层应力是确定碳纤维修复结构承载能力的关键。基于平截面假设,得到弯矩作用下应力与应变分布;基于胶层剪切模型,得到胶层剪应力与CFRP和钢板位移间的关系;基于力的平衡,得到CFRP和钢板的应力关系。结合得到的各种材料之间关系,推导出轴力和弯矩联合作用状态下CFRP双面修复钢板的CFRP与胶层应力分布解析解。采用数值分析对CFRP双侧粘贴修复缺陷钢板进行分析,分析结果与解析结果具有一致性,同时获得了CFRP双侧粘贴修复缺陷钢板的应力分布特点,以及构件可能发生破坏的位置,为计算构件极限承载力提供了基础。     

Damage modelling and lifetime prediction of adhesively bonded joints under sustained loading with constant and variable amplitudes

The lifetime of adhesively bonded joints under service loading is predicted by a linear viscoelastic traction–separation model, which is enhanced by an isotropic damage approach. Therefore, a scalar damage variable is defined according to the concept of effective stresses based on the hypothesis of strain equivalence in the framework of continuum damage mechanics. The damage evolution is driven by a specific equivalent stress, adapted for ductile adhesives. Experimental data acknowledge the validity of the proposed model for the lifetime prediction of adhesive joints. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three-Dimensional Variational Analysis of Composite Structures Using Bernstein Polynomial Approximations. Report 2

The application of the previously developed 3D varionational analysis approach to the investigation of crack propagation in composite bonded joints is presented. In this application, the propagation of three different types of a 2D planar crack (adhesive, cohesive, and interfacial) is modeled by relaxing the respective continuity conditions for displacements between adjacent bricks in the mosaic structure. The crack propagation process is then characterized by the release rate of the total potential energy between two consecutive states of the mosaic body with different crack lengths. Numerical examples illustrate the 3D analysis of double-lap adhesively bonded joints with unidirectional and cross-ply laminated composite adherends. The numerical results provide an illustration of various characteristics of the crack propagation process. The values of the ultimate failure load predicted by analyzing the initial stage of crack propagation are found to be in a good agreement with experimental data.     

An improved closed-form solution to interfacial stresses in rc beams strengthened with a composite plate

The strengthening of concrete structures in situ with externally bonded fiber-reinforced plastic (FRP) composite sheets is increasingly being used for the repair and rehabilitation of existing structures. However, debonding along the FRP-concrete interface can lead to premature failure of the structures. The interfacial stresses have played a significant role in understanding this premature debonding failure of such repaired structures. In this paper, an improved theoretical analysis of the interfacial stresses is presented for a simply supported concrete beam bonded with a FRP plate. The shear strains of the adherends have been included in the present theoretical analysis by assuming a parabolic distribution of shear stress across their thickness. Contrary to some existing studies, the assumption that both adherends have the same curvature is not used in the present investigation. The results of this numerical study are beneficial for understanding the mechanical behavior of material interfaces and for the design of hybrid FRP-reinforced concrete structures.     

Interfacial model for crash resistant adhesives of ductile-modified epoxy resins

Debonding in adhesive joints is modelled and analyzed with the concept of interfacial mechanics. Material equations are presented for the inelastic behaviour of ductile-modified epoxy resins. A two surface function for the onset of yielding is advantageously expressed in terms of the stress vector on the interface. The theory may be extended to material softening due to damage and to rate dependency. This simple constitutive model is not stable in the sense of Drucker's postulate. Therefore, the non-associated flow rule is modified with a quadratic stress-dependent plastic potential. The material parameters are identified by means of the finite-element simulation of the experimental setup for a bluntly glued double steel tube sample. The numerical performance of this modified model is tested at an adhesively bonded joint in the form of a T-intersection. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

颗粒弥散增强型核辐射屏蔽材料强度模型研究

基于微观力学的均匀化理论,旨在从核辐射屏蔽材料的微观结构、物理特性的角度出发,通过多尺度方法研究了材料宏观的机械力学性质.主要研究对象为颗粒弥散增强的孔隙基体材料,推导出了此类复合材料（金属基材料、非金属类材料）的强度准则模型,可预测微观孔隙率与颗粒相体积分数对材料宏观强度的影响.在塑性极限分析法的理论框架下,在介观上成功引入了速度场跳动来描述两相界面间的力学特性,利用刚性核的球体胞元模型进行求解.最后,选用了界面速度为0的速度场对模型进行研究,并初步探讨了界面效应对材料性能的影响.     

Two-dimensional Green's function of orthotropic three-phase material under a normal line force with application in the design of composite

Green's function of orthotropic three-phase material is an important and basic problem in the study of mechanics of materials. It is also the foundation of further theoretical researches and engineering applications. Most of adhesive structures in engineering can be well simulated by the mechanical model of orthotropic three-phase material, such as composite laminate, integrated circuit (IC) packaging, micro-electro-mechanical systems (MEMS) and biomedical materials, etc. In order to understand the mechanical properties of the adhesive structure, a two-dimensional Green's function of orthotropic three-phase material loaded with a normal line force is presented. Based on the Green's function proposed in this paper, the stress field of adhesive structure under arbitrary normal loadings can be obtained with superposition method. Besides, this Green's function is convenient to be used in further studies, because it is expressed explicitly in form of elementary functions. Numerical examples are proposed to study the mechanical properties of the adhesive structure in five difference aspects: (1) the distribution rule of stress fields of the adhesive structure; (2) the influence from fiber orientation of composite to the stress fields of the adhesive structure; (3) the influence from elastic modulus of adhesive layer to the stress transfer of the adhesive structure; (4) the influence from the thickness of adhesive layer to the stress transfer of the adhesive structure; (5) the reasonability of spring interface model.     

Strength of adhesion of epoxy polymer to carbon,glass, and steel fibers

A study has been made of the strength of adhesion of epoxy polymer to fibers of various chemical natures, and it has been shown that the magnitude of the adhesion depends not only on the nature of the fiber but also on its surface geometry. Questions are discussed on the effect of residual (internal) stresses arising during the process of heat-treating or cooling samples of bonded articles on the strength of the adhesive bond.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 1, pp. 37–42, January–February, 1974.     

Thermo-mechanical modelling of cellular hybrid refractories

Refractory materials are subjected to both quasi-static and dynamic thermal loading (thermal shock) causing damage up to mechanical failure. Typical refractories are magnesia carbon bricks consisting of periclase (MgO) and carbon inclusions. Recently, a significant improvement of the thermo-mechanical behaviour could be achieved by cellular hybrid composites made of periclase-filled carbon foams. The present contribution focuses on MgO-filled carbon foams and the investigation and optimisation of the structure-property relationship with respect to a reduction of thermally induced stresses and damage. It is a transient as well as static, fully coupled thermo-mechanical problem. According to the fact that, in general, refractories are brittle materials a linear elastic model, with a damage criterion was used. To optimise the structural morphology of the cellular refractories, the effect of micro structural changes has been determined. For the investigation of the thermal shock! behaviour, the results correlate very well with the experimentally motivated Hasselman relation. There is a significant size effect depending on the pore size. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Energetic modelling for carbon fibre reinforced carbon

In this contribution an energetic model for multi-phase materials is developed describing the influence of microstructure on different length scales as well as the evolution of phase changes. Restrictions on the energy functional are discussed. In such a non-convex framework, interfacial contributions serve for relaxing the total energy. Such models can be applied to describe the macroscopic material properties of carbon fibre reinforced carbon where phase transitions between regions of different texture of the carbon matrix are observed on nanoscale as well as columnar microstructures on microscale [2]. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and properties of magnetic fibrous polymeric materials

Characteristic features of the filtration of liquids by magnetic fibrous polymeric materials (MFPM) are addressed; these materials are a set of fibers of a polymeric material containing a ferritic filler; they are adhesively bound at points of their intersection. It is demonstrated that variations in structural parameters (fiber thickness, distribution of fiber throughout the material, and its concentration), as well as the degree and direction of magnetization of the MFPM exert an influence on the efficiency of the liquid filtration. Optimum parameters for MFPM designed to purify liquids of ferromagnetic and nonmagnetic contaminants are given.A paper to be presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 3, pp. 291–297, May–June, 1995.     

Thermal stress state of a bimaterial with a closed interfacial crack having rough surfaces

We have formulated the problem of thermoelasticity for a bimaterial whose components differ only in their shear moduli, with a closed interfacial crack having rough surfaces. The bimaterial is subjected to the action of compressive loads and heat flow normal to the interfacial surface. We have taken into account the dependence of thermal conductance of the defect on the contact pressure of its faces and heat conductivity of the medium that fills it. The problem is reduced to a Prandtl-type nonlinear singular integro-differential equation for temperature jump between the crack surfaces. An analytical solution of this problem has been constructed for the case of action of the heat flow only. We have analyzed the dependence of contact pressure of the defect faces, temperature jump between them, and the intensity factor of tangential interfacial stresses on the value of given heat flow, roughness of the surfaces, and ratio between the shear moduli of joined materials.     

Investigation on a rotating FGPM circular disk under a coupled hygrothermal field

In this paper, the distributions of the temperature, moisture, displacement and stress of a functionally graded piezoelectric material (FGPM) circular disk rotating around its axis at a constant angular velocity under a coupled hygrothermal field are presented by a numerical method. The material properties of the FGPM circular disk are assumed to vary along the radial coordinate exponentially. First, the coupled hygrothermal field along the radius of a rotating circular disk is achieved by solving the coupled hygrothermal equations, and then the dynamic equilibrium is solved by utilizing the finite difference method. Finally, numerical results show the effects of functionally graded index, inner radius, angular speed and hygrothermal index on the hygrothermal behaviors of the FGPM circular disk. The results can be useful for the optimal design of rotating FGPM circular disks under a coupled hygrothermal field.     

Correlation between the strength of fiber-reinforced plastics and the adhesive strength of fiber—Matrix joints

The relationship between the strength (σc) of unidirectional fiber-reinforced plastics in different stressed states and the interfacial strength of their components is investigated. The shear adhesive strength (τ0) of fiber—matrix joints determined by the pull-out technique is used as a measure of the interfacial strength. To obtain the correlation curves betweenσc andτ0, the experimental results are used, where both the plastic and adhesive strength change under the influence of a single factor. In this case, such factors are the fiber surface treatment, nature and composition of polymer matrices, and test temperature. It is shown that the strength of the glass, carbon, and boron plastics increases practically linearly with increased interfacial strength. Such a behavior is observed in any loading conditions (tension, shear, bending, and compression). Sometimes, a small (10–20%) increase in the adhesive strength induces a significant (50–70%) growth in the material strength. Therefore, the interface is the “weak link” in these composites. The shape of theσc—τ0 curves for composites based on the high-strength and high-modulus aramid fibers and different thermoreactive matrices depends on the nature of the fiber and the type of stress state. In many cases, the composite strength does not depend on the interfacial strength. Then, the fiber itself is the “weak link” in these composites. Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 3, pp. 291–304, May–June, 2000.     

Imperfect interface effect for nano-composites accounting for fiber section shape under antiplane shear

This paper investigates the elastic responses of fibrous nano-composites with imperfectly bonded interface under longitudinal shear. The proposed imperfect interface model is the shear lag (or the spring layer) model; the presented nano interfacial stress model is the Gurtin–Murdoch surface/interface model; and the three-phase confocal elliptical cylinder model is the geometry model accounting for the fiber section shape. By virtue of the complex variable method, a generalized self-consistent method is employed to derive the closed from solution of the effective antiplane shear modulus of the fibrous nano-composites with imperfect interface. Five existing solutions can be regarded as the limit form the present analytic expression. The influences of the interface elastic constant, the interfacial imperfection parameter, the size of the elliptic section fiber, the fiber section aspect ratio, the fiber volume fraction and the fiber elastic property on the effective antiplane shear modulus of the nano-composites are discussed. Particularly, numerical results demonstrate that the interfacial elastic imperfection will always cause a significant reduction in the effective antiplane shear modulus; and the fiber interface stress effect on the effective modulus of the fibrous nano-composites will weaken with the interfacial imperfection increases.