Stress state of a fiber composite with curved structures with a low fiber concentration

International Applied Mechanics -     

Prediction of the thermal conductivity of the constituents of fiber reinforced composite laminates

Three empirical formulas are developed to predict the thermal conductivities of fiber-reinforced composite laminates (FRCL) and its constituents. The inherent two or three-dimensional problem that is common in composites is simplified to a one-dimensional problem. The validity of the models is verified through finite element analysis. This method utilizes the parallel and series thermal models of composite walls. The models are tested at different fiber-to-resin volume ratios (30:70–75:25) and various fiber-to-resin thermal conductivity ratios (0.2–5). The predicted thermal conductivity of the fiber can be accurately predicted throughout the spectrum via two models. The first model is a first-order formula (R ² = 0.94) while the second model is a second-order formula (R ² = 0.976). These two models can be used to predict the fiber thermal conductivity based on the easily measured resin and laminate values. A third model to predict the overall laminate thermal conductivity is introduced. The thermal conductivity of the composite panel is predicted with very high accuracy (R ² = 0.995). The thermal conductivity predicted via the use of these models has an excellent agreement with the experimental measurements. Another use of these models is to determine the fiber-to-resin volume ratio (if all thermal conductivities of fiber, resin and laminate are known).     

Stress concentration around reinforced holes in shells made of composite materials

Institute of Mechanics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Prikladnaya Mekhanika, Vol. 25, No. 1, pp. 88–93, January, 1989.     

Stress concentration and surface instability of anisotropic solids with slightly wavy boundary

This paper presents a first order perturbation analysis of stress concentration and surface morphology instability of elastically anisotropic solids. The boundary of the solids under consideration is periodic along two orthogonal directions. The magnitude of the undulation is sufficiently small so that a half-space model can be used for simplification. We derive expressions for the stress concentration factors and the critical wavelength of the perturbation in terms of the remote stresses, surface energy anisotropy and the elastic anisotropy of the solid. Numerical applications to cubic materials using Barnett–Lothe integrals are also given.     

Coupled twoscale analysis of fiber reinforced composite structures with microscopic damage evolution

The simultaneous twoscale analysis of unidirectionally fiber reinforced composite structures with attention to damage evolution is the objective of the contribution. The heterogeneous microstructure of the composite represents the microscale, whereas the laminate or the structural component are addressed as the macroscale. The macroscale is conventionally discretized by the finite element method (FEM). The generalized method of cells (GMC) in its efficient stress based formulation serves as the discrete microscale model. The stiff and brittle fibers behave linearly elastic. The epoxy resin is described by the nonlinear-elastic model of Ramberg–Osgood. By introducing microcrack models, the damage of the epoxy matrix under combined tensile and shear loading is taken into account. The cell boundaries of the micromodel are used to locate microscopic cracks deterministically. Interface models for the representation of damage in the matrix phase as well as for the weakening of the fiber–matrix-bond are used. This approach circumvents the need for the regularization, as it would be necessary for continuum damage models with softening characteristics. Hence, the micromodel is numerically stable and convergent. The GMC allows to obtain the consistently linearized constitutive tensor in the case of nonlinear material behavior in a simple and straight forward manner which is easily implemented in comparison to micromodels based on the finite element technique. The damage evolution on the microscale manifests itself macroscopically in the degradation of the homogenized stiffnesses.     

Residual strength of a fiber reinforced metal matrix composite with a crack

The residual strength of a cracked unidirectional fiver reinforced metal matrix composite is studied. We propose a bridging model based on the Dugdale strip yielding zones in the matrix ahead of the crack tips that accounts for ductile deformations of the matrix and fiber debonding and pull-out in the strip yielding zone. The bridging model is used to study the fracture of an anisotropic material and its residual strength is calculated numerically. The predicted results for a SiC/titanium composite agree well with the existing experimental data. It is found that a higher fiber bridging stress and a larger fiber pull-out length significantly contribute to the composite's residual strength. The composite's strength may be more notch-insensitive than the corresponding matrix material's strength depending on several factors such as fiber-matrix interface properties and the ratio of the matrix modulus to an ‘effective modulus’ of the composite.     

Compression-induced stiffness in the buckling of a one fiber composite

We study the buckling of a one fiber composite whose matrix stiffness is slightly dependent on the compressive force. We show that the equilibrium curves of the system exhibit a limit load when the induced stiffness parameter gets bigger than a threshold. This limit load increases when the stiffness parameter is increasing and it is related to a possible localized path in the post-buckling domain. Such a change in the maximum load may be very desirable from a structural stand point.     

The influence of fiber length and fiber orientation on damping and stiffness of polymer composite materials

This paper describes the theoretical analysis, the experimental results and the curve-fitting of the analytical model to the experimental results on the influence of fiber length and fiber orientation on damping and stiffness of polymer-composite materials. The experimental results show that, as predicted, very low fiber aspect ratios are required to produce significant improvements in damping. Measurements and predictions also indicate that the control of lamina orientation in a continuous fiber-reinforced laminate may be a better approach to the improvement of damping than the control of the fiber aspect ratio. Paper was presented at 1985 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 9–14, 1985.     

纤维增强复合材料热隐身斗篷设计

基于变换热动力学原理可获得具有热隐身性能的隐身结构(隐身斗篷)所需要的材料性质的空间分布。但这种材料性质的复杂分布形式以及局部热传导性能无限大等极值性质需求,使得隐身斗篷设计的实现非常困难,需要研究基于常规材料的隐身斗篷设计。本文基于常规材料的热隐身结构实现问题,提出了基于纤维增强复合材料圆环结构的实现热隐身的结构形式。首先,基于变换热动力学原理获得热隐身所需的热传导系数沿半径方向的变化规律;进而,通过设计复合材料不同位置的纤维铺设方式(含量和铺设方向)实现热隐身对材料性能的需求。选择金属银作为纤维,空气作为基体,设计出了具有热隐身性能的复合材料圆环结构纤维含量和铺设方向沿径向的分布方案。对该设计方案进行数值仿真,结果显示所设计的隐身结构具有良好的热隐身性能。由于设计方案基于常规材料,因此具有容易实现的优点。     

The effective moduli of composite reinforced by spherical coating particles

1.IntroductionThecompositereinforcedbysphericalparticlesisanilllportantengilleel-lugmaterial.Itiswidelyusedinvariousfieldsuchasaviationandspaceflight.Withl'egardtoitsefttctivemodulusproblenl,maily1llodelsandmethodswereproposedillpilotyeal's.Somethed.eticalpredictionlbrnlulaehavebeenobtailledtitpresent,whicharequiteinagreementwithexpel.imelltalresultslll.FIowcver,becausedemandingl'orcompositeincllglneeri11gtlpplicationbecomeslargel'daybyday,theSurftlcecoatillgofreinforcednlatel.iLtlincomposit…     

单向纤维复合材料粘弹性性能预测

建立了基于均匀化理论的单向纤维复合材料粘弹性性能预测方法。对单向纤维增强复合材料粘弹性问题的控制方程进行Laplace变换,在像空间中利用均匀化理论建立宏观松弛模量的Laplace变换与微结构描述参数以及变换参数间的关系。用Prony级数模拟松弛模量随变换参数的变化形式,并根据像空间中一系列变换参数对应的松弛模量的数值,采用函数拟合技术确定Prony级数的形式,从而确定用显示形式表示的松弛模量的Laplace变换随变换参数的变化规律。对显式表达式的逆变换获得时间域内的松弛模量。该方法利用拟合函数的逆变换避开了复杂的数值Laplace逆变换,使单向纤维增强复合材料的粘弹性性能的确定变得容易。文中给出了单向纤维复合材料松弛模量的数值预测结果并同有限元法模拟试验的结果对比,验证了预测结果的准确性以及本文方法的有效性。     

Constitutive modeling of viscoelastic-viscoplastic behavior of short fiber reinforced polymers coupled with anisotropic damage and moisture effects

In this paper, a combined viscoelasticity-viscoplasticity model, coupled with anisotropic damage and moisture effects, is developed for short fiber reinforced polymers (SFRPs) with different fiber contents and subjected to a variety of strain rates. In our model, a rate-dependent yield surface for the matrix phase is employed to identify initial yielding of the material. When an SFRP is loaded at small deformation before yielding, its viscoelastic behavior can be described using the generalized Maxwell model, while when plasticity occurs, a scalar internal state variable (ISV) is used to capture the hardening behavior caused by the polymeric constituent of the composite. The material degradation due to the moisture absorption of the composite is modeled by employing another type of ISV with different evolution equations. The complicated damage state of the SFRPs is captured by a second rank tensor, which is further decomposed to model the subscale damage mechanisms of micro-voids/cracks nucleation, growth and coalescence. It is concluded that the proposed constitutive model can be used to accurately describe complicated behaviors of SFRPs because the results predicted from the model are in good agreement with the experimental data.     

Elastic equilibrium and effective elastic constants of a cross-reinforced fiber composite

Institute of Very Hard Materials, Ukrainian Academy of Sciences, Kiev. Translated from Prikladnaya Mekhanika, Vol. 28, No. 1, pp. 47–56, January, 1992.     

CONSTITUTIVE RELATION AND STIFFNESS DEGRADATION OF CRACKED ANISOTROPIC COMPOSITE LAMINATES(II)─DETERMINATION OF RESOLVED STIFFNESS AND INVESTIGATION OF STIFFNESS DEGRADATION

The study on properly degradation of damaged,.onlj,osile laminates is extendedto anisotropic laminates with matrix cracking. In (II) of the paper, a solution forpartitioned stiffness is given to complete the constitutive relations developed in (I) Thestiffness degradation in (θ_m/90_n)_s cracked laminates is calculated and the results arediscussed.