Numerical investigations of microscopic characteristic influences on the mechanical properties of polymer‐matrix composites

In this study, an isoparametric micro‐mechanical model is developed to study the failure behaviors under biaxial loadings for composites with different fiber arrangements. For validating the method, the numerical results of the unidirectional (UD) composite lamina with consideration of two different failure criteria (the constituent failure criterion and UD failure criterion) are compared with the experimental results. On this basis, the fiber arrangements and fiber off‐axis angles influence on the failure envelopes for the UD composite lamina are studied. Furthermore, the failure strength of a typical composite laminates is also investigated. The results show that the failure strength of composite laminates is closely dependent on the fiber arrangements. Compared with the hexagonal packing, the square diagonal packing provides the higher failure strengths. POLYM. COMPOS., 38:2734–2742, 2017. © 2015 Society of Plastics Engineers     

Failure of composite laminae under biaxial loading

The failure of thin, fiber-reinforced composite laminae is considered. A parametric failure criterion applicable to plane stress loading conditions is presented. Both static and fatigue-type loadings are treated. In general, the formulation of failure criteria for composite materials has closely followed the development of yield criteria in metal plasticity. The parametric failure criterion presented here was inspired by a general formulation recently proposed for the plastic yielding of sheet metals. The criterion satisfies the essential conditions that must be met in developing phenomenological strength criteria for composite materials. This entirely general formulation encompasses all previously proposed failure theories for fiber-reinforced orthotropic laminae. The inherent advantages and drawbacks of the proposed parametric approach are discussed. An example of application for static loading is presented in detail. Possible extensions of the approach to fatigue-type loading are also suggested.     

复合材料层合板贴补修理失效模式分析与修理参数选择

在对复合材料层合板进行渐进失效分析时,Zinoviev刚度退化模型是最常用的刚度退化模型之一,但是该模型涉及的参数较多并且只能用于二维问题,对其进行简化和改进后,不仅参数减少而且可以扩展至三维,基于此改进的Zinoviev刚度退化模型,结合Shokrieh改进的三维Hashin准则,采用INTER205单元对胶层进行模拟,利用ANSYS软件建立了承受面内拉伸荷载的含圆孔缺陷的复合材料层合板双边贴补修理渐进失效三维有限元模型。此有限元模型的结果和试验结果拟合得比较好,验证了改进的Zinoviev退化模型和有限元模型的有效性。利用此有限元模型分析了承受面内拉伸荷载的双边贴补修理复合材料层合板的失效模式,发现主要破坏模式为胶层的脱胶加速了母板中的纤维失效,导致结构最终失效。最后分析了主要修理参数对修理效果的影响,结果表明:增大补片尺寸可以明显提高修补结构的强度;当补片刚度与母板刚度相同时,修补效果较好;当补片厚度为母板厚度一半时,修补效果最好。     

A strain-based parametric biaxial failure criterion for fiber-reinforced composites

Orthotropic, fiber-reinforced composite laminae under biaxial plane stress loading conditions are considered. A new strain-based strength criterion for such laminae is proposed. This failure criterion, which is cast in parametric form, is completely general. A parametric equation that is expressed as a Fourier series is proposed for describing the failure surface for the laminae. This equation is shown to be valid for most fiber-reinforced composite laminae through comparisons with experimental data taken from the literature. The advantages of the failure theory are discussed in comparison with the quadratic form of the tensor polynomial theory. Also presented is a biaxial apparatus for testing cruciform specimens under biaxial loading conditions. Efforts under way to characterize composite laminae and laminates under complex stress states are discussed.     

A general strength theory for orthotropic fiber-reinforced composite laminae

Thin, orthotropic fiber-reinforced composite laminae under plane stress loading conditions are considered. The essential conditions that must be satisfied in developing phenomenological strength criteria for such materials are first established. A new, parametric failure criterion is then presented. It is shown that this entirely general formulation encompasses all previously proposed failure theories (e.g., tensor polynomial criterion) for orthotropic laminae. Examples of application and the inherent advantages and drawbacks of the proposed parametric approach are discussed. In the past, the formulation of strength criteria for composite materials has closely paralleled the development of yield criteria in metal plasticity. The parametric approach advocated here for the failure of composite materials was inspired by a general formulation proposed by Budiansky (1) for the plastic yielding of sheet metals.     

Specimen design,manufacturing and testing procedures for flat carbon fiber reinforced plastic laminates under biaxial loading

The optimization of a specimen design allowing the investigation of the biaxial strength of composite laminates over the full range of failure strain was the primary objective of this work. Multiaxial strength criteria are often found unreliable mainly as a result of the inherent complexity of biaxial tests and, in many cases, as a result of inefficient specimen designs. As a result of a development program combining numerical simulations and experimental measurements, a flat cruciform-shaped specimen has been developed for carbon fiber reinforced plastic (CFRP) laminates. The design fulfills the basic criteria for such a specimen, namely allowing for a uniform biaxial stress/strain state to exist in the gauge area and for testing the virgin material up to failure in both the tension-tension and tension-compression quadrants of the strain/stress space. The fabrication of the specimen is described and a three-step testing procedure for generating biaxial strength data is proposed. Typical results obtained from specimens of the proposed configuration tested in accordance with this procedure are presented. Results compare well with those obtained from tubular specimens, thus confirming the effectiveness of the proposed design. Experimental data obtained for the AS4/3501-6 carbon/epoxy composite system are finally compared against strength predictions of recognized failure theories.     

Mechanical behavior of textile composite materials using a hybrid finite element approach

A novel analytical solution is presented to describe the mechanical behavior and failure of textile reinforced composites. This solution is constructed from two parts. First a geometrical model, based on a processing science approach coupled with graphical rendering, is utilized to quantify the spatial characteristics of the fabric preform inside the composite. A 3-D iterative hybrid finite element analysis is then used, utilizing data acquired from the geometrical model, to predict the stressstrain behavior of the composite. Inelastic behavior is modeled using an expanded Hahn and Tsai model and maximum stress criterion is used to predict initial failure. Damage progression is predicted based on a stiffness reduction approach. AS4 carbon/epoxy plain weave composite laminates, with a range of fiber volume fractions, have been tested. A 3-D woven E-glass/poly-vinyl-ester (PVE) angle interlock composite was also tested. Analytical results are compared with results from this experiment as well as other analytical models and experimental data in the literature. Theoretical model predictions are in agreement with most experimental data.     

Research on the mechanical properties prediction of carbon/epoxy composite laminates with different void contents

The influence of the porosity on the static mechanical strength of the carbon fiber fabric reinforced epoxy composites laminates was investigated. The tensile, compressive, bending, and interlaminar strength test on the CFRP laminates with porosity of 0.33% and 1.50% were conducted and simulated by a finite element analysis model. The article proposes the failure criterion of the static mechanical strength of the fabric fiber reinforced composites based on the improved Hashin failure criterion that is suitable for the undirectional composite laminates. The basic composite strength parameters are used to evaluate the mechanical properties of CFRP laminates with different porosities. A finite element analysis model is established by using software ABAQUS™ combined with the sudden stiffness degradation model. The experiment results show that the tensile, compressive, bending, and interlaminar strength decrease with the increasing porosities. The tensile, compressive, bending, and interlaminar strength of the fabric carbon fiber reinforced epoxy composites laminates are simulated accurately by the finite element model. POLYM. COMPOS., 14–20, 2016. © 2014 Society of Plastics Engineers     

Modified Tsai-Wu failure criterion for fiber-reinforced composite laminates

The Tsai-Wu Quadratic Failure Criterion provides satisfactory strength predictions for fiber-reinforced composite materials but requires five experimental tests to determine the strength parameters. This paper presents two modifications of this criterion, which employ micromechanics to determine these parameters. Experiments on uniaxial and multiaxial vinyl ester/fiberglass composite laminates show that the first modified failure criterion, which is based solely on fiber and resin properties, predicts strength within an average absolute error of 25.4% in comparison to the experimentally determined strength. The addition of a single longitudinal tensile test to the modified expression (second modified failure criterion) reduces the average absolute error to 15.9%. This compares well with the Tsai-Wu Failure Criterion, which gives an average absolute error of 9.4%. The proposed modified criteria are shown to provide satisfactory failure predictions, while greatly reducing the amount of testing required.     

汽车复合材料层合板准静态力学性能的试验测定

以应用于某新能源电动汽车的复合材料层合板为研究对象,利用万能试验机和静态应变测试分析系统等提出了可靠的复合材料层合板准静态拉伸和压缩力学性能试验测定方法,从而为复合材料结构在汽车轻量化中的设计和应用提供了试验依据。该层合板结构采用±45°交叉铺层方法,由2层碳纤维、1层芳纶纤维和2层玻璃纤维层叠构成。试验结果表明,该复合材料层合板在准静态拉伸时呈现沿±45°方向和层间分离挤压的断裂失效模式,这与其内部纤维铺层方向是一致的。同时,由于在复合材料板材中加入了增韧和板材失效时起连接作用的芳纶纤维和玻璃纤维铺层,该复合材料层合板的整体力学性能较常见碳纤维增强复合材料板材,其弹性模量和强度性能均有所降低。     

Steady-State Mechanics of Delamination Cracking in Laminated Ceramic-Matrix Composites

A fracture mechanics delamination cracking model has been developed for brittle-matrix composite laminates. The near-tip mechanics is discussed in the context of material orthotropy and composite material inhomogeneities. A fracture mechanics framework based on the near-tip energy release rate and the associated phase angle Ψ has been adopted. In the case of steady-state delamination cracking in a prenotched cross-ply symmetric laminated beam, analytical expressions for the steady-state energy release rate, _ss, have been obtained for the combined applied loading of an axial force and a bending moment. Parameter studies assessing the effects on _ss of load coupling, crack location, and lamination morphology which includes the total number of layers, layer thickness, and material properties are presented. Thus, composite homogenization criteria with respect to the total number of layers placed along the beam height can be obtained for a wide range of material selection. The associated phase angle Ψ at the delamination crack tip is discussed in the context of existing solutions. The analysis has been developed based on a theory for structural laminates. The delamination model can be used in conjunction with experimental data obtained from model geometries to extract the mixed-mode transverse composite fracture toughness. Thus, conditions for stable delamination crack growth can be established and design criteria based on toughness for composite laminates and composite fasteners can be obtained.     

Strength of discontinuous reinforced composites: I. Fiber reinforced composites

The strength of randomly oriented short fiber composites has been modeled by a quasi-isotropic laminate. Lamination theory and a failure criterion will be used to approximate the stress-strain response of a composite as it is loaded to failure. Experimental data are presented and compared with the maximum-strain failure criterion.     

准静态压痕作用下复合材料层合板损伤阻抗与损伤容限实验研究

参照标准实验方法,开展了复合材料层合板对准静态压痕力的损伤阻抗和损伤容限实验研究,获取了接触力、压痕深度、压头位移等实验数据,并对含静压痕损伤层合板进行了剩余压缩强度试验。研究了压痕深度-接触力与剩余压缩强度-压痕深度的变化关系,并讨论了准静态压痕过程中的损伤演变过程和层合板的压缩破坏模式。结果表明:当层合板表面出现目视勉强可见压痕时,初始损伤发生,压痕深度随接触力增大而明显增大,同时剩余压缩强度随压痕深度增加而明显降低;当达到最大接触力时,层合板失去承载能力,背面可看到大量纤维断裂。对于含静压痕损伤的层合板,压缩破坏模式为贯穿损伤区域的层合板断裂。     

Finite Element Based Design and Adhesion Failure Analysis of Bonded Tubular Socket Joints Made with Laminated FRP Composites

This paper deals with Finite Element Analysis of bonded Tubular Socket Joints (TSJs) made with laminated Fibre Reinforced Plastic (FRP) composite structures. The effective coupling length for suitable performance of the joint is determined based on the Tsai–Wu failure criterion. The analysis revealed the three-dimensional nature of the stresses and are found to be concentrated in the close vicinity of the free edges and junction of the adherends in the coupling region of the bonded TSJ. Shear stress ( τ_r ), though comparatively small in magnitude, is found to be extremely sensitive to three-dimensional effects as compared to stresses τ_zr and σ_r . Failure indices at different critical interfaces are determined using Quadratic Failure Criterion (QFC) within the adhesive and Tsai–Wu coupled stress criterion for the adherend–adhesive and socket–adhesive interfaces. Based on the latter criterion, locations prone to adhesion failure initiation are identified to be existing near the free edges of the adherend–adhesive interfaces in the coupling region of the bonded TSJ. Strain Energy Release Rate (SERR) calculated using Modified Crack Closure Integral (MCCI) vis-à-vis Virtual Crack Closure Technique (VCCT) has been used as the characterizing parameter for assessing the growth of adhesion failures. The adhesion failure damages have been observed to propagate at the same rate in a self-similar manner mainly in the in-plane shearing mode. Quasi-isotropic and angle-ply orientations of the FRP composite laminates are more resistant to opening mode growth of failure, whereas cross-ply and unidirectional oriented socket/adherends offer better resistance to in-plane shearing mode of adhesion failure damage growth. Plies oriented in the direction of the applied load, especially Graphite/Epoxy (Gr/E) [90]₁₆, are found to offer the best resistance to all types of adhesion failure growth modes and hence are the most preferred fibre orientations for the bonded TSJ under tension. Increasing the degree of anisotropy of the composite socket/adherends improves the adhesion failure damage growth resistance of the bonded TSJ. Boron/Epoxy (B/E) FRP composites are found to be the best in slowing down the growth rate of the adhesion failures among the various FRP composite socket/adherends considered in the present study.     

Comparison of mechanical properties of epoxy composites reinforced with stitched glass and carbon fabrics: Characterization of mechanical anisotropy in composites and investigation on the interaction between fiber and epoxy matrix

The primary purpose of the study is to evaluate and compare the mechanical properties of epoxy‐based composites having different fiber reinforcements. Glass and carbon fiber composite laminates were manufactured by vacuum infusion of epoxy resin into two commonly used noncrimp stitched fabric (NCF) types: unidirectional and biaxial fabrics. The effects of geometric variables on composite structural integrity and strength were illustrated. Hence, tensile and three‐point bending flexural tests were conducted up to failure on specimens strengthened with different layouts of fibrous plies in NCF. In this article, an important practical problem in fibrous composites, interlaminar shear strength as measured in short beam shear test, is discussed. The fabric composites were tested in three directions: at 0°, 45°, and 90°. In addition to the extensive efforts in elucidating the variation in the mechanical properties of noncrimp glass and carbon fabric reinforced laminates, the work presented here focuses, also, on the type of interactions that are established between fiber and epoxy matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, were interpreted in an attempt to explain the failure mechanisms in the composite laminates broken in tension. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Progressive failure analysis of carbon-fibre/epoxy composites laminates to study the effect of stitch densities under in-plane tensile loading

Through thickness reinforced stitched laminates with different stitch densities (0.11 and 0.028?mm^?2) were studied in order to analyse effects on laminate behaviour, under in-plane tensile loading based on continuum mechanics. Multi-layered stitched laminates with the stacking sequence [+45/90/?45/0₂/+45/90₂/?45/0]_s were modelled on a lamina-wise basis to analyse the macroscopic damage and local stress–strain constitutive behaviour. Interfaces between lamina and stitch yarns were assumed to be perfectly glued and were modelled by the contact capability. Discretisation procedures using the principle of virtual work were applied in addition to discretisation of the contact traction. Progressive failure analysis with Puck’s failure criteria was conducted to characterise the failure behaviour of the laminate. This analysis showed that reinforcement density is one of the key factors affecting strength, stiffness and crack propagation in composite laminates. By suppressing the damage initiation, densely stitched laminates showed 15.2% higher in-plane stiffness than moderately stitched laminates. The results obtained by the finite element technique are consistent with the experimental results.     

Mechanical properties of polymer–paper laminates

Tensile mechanical properties of synthetic polymer–paper laminates were measured. The laminates were constructed by hot pressing a sandwich made of a sheet of paper between polymer films. There is complete penetration of the polymer inside the paper; no voids are left. Two different polymer matrices were used: poly(methyl methacrylate) and polyethylene. Several paper samples were utilized: an unoriented holocellulose paper (a strong paper), a highly oriented holocellulose paper, and an unoriented Whatman filter paper (a weak paper). The laminates contain from 0% to 50% of paper. Young's moduli and breaking strengths of the unoriented holocellulose paper laminates can be theoretically predicted from the properties of their constituents using laws of mixtures. The mechanical properties of the Whatman paper laminates are significantly higher than those predicted from the laws of mixtures. This indicates that the polymer increases the strength of the fiber-to-fiber bonds of the weaker sheets, although it does not change the bond strength of a stronger paper such as the holocellulose paper. For the oriented paper laminates, changes in Young's modulus with angle of measurement are explained by the composite materials theories if the angular variations in shear modulus are taken in to account. Changes in breaking strength with angle for the oriented laminates can be analyzed by Tsai and Azzi's theory for composite materials.     

Controlled energy dissipation in fibrous composites I. Controlled delamination

Delamination mechanisms in continuous fiber reinforced composites were investigated. The concept of controlled interlaminar bonding (CIB) is proposed as a guideline for preparing fiber-epoxy composite laminates with enhanced fracture toughness without significant degradation in strength properties. The interlaminar bonding was manipulated by several specialized techniques including insertion of delamination promotors and surface modification of laminae. Results indicated that the plane-strain fracture toughness of E-glass-epoxy laminates could be improved by inserting perforated interlaminar films of aluminum, paper, polyester and polyimide, and fabrics. Such interlayers were used to promote delamination which dissipate strain energy by blunting and diverting a propagating crack. The fracture resistance of a laminate was found to be dependent on the degree of delamination. The competition between the growth of delamination cracks and the propagation of a main crack is controlled by the relative magnitude of the interlaminar bonding strength and the lamina cohesive strength. The interlaminar bonding is controlled by the degree of interlayer perforation and the adhesion between interlayer and lamina. The loading direction was found to be very important in dictating the failure processes. Experimental results from several composite systems are presented and discussed along with post-failure analysis data.     

Initial Failure Maps for Fibrous CMC Laminates

A simple micromechanics-based procedure is used to evaluate initial failure maps for brittle composite laminates under combined in-plane loads and temperature changes. The maps are derived from local stresses in the fiber, matrix and at their interfaces, and from selected magnitudes of the respective strengths. In a particular loading plane or space, the maps indicate the damage-free load range of the laminate, and the source of likely initial failure by fiber or matrix cracking, or by fiber debonding. An application to Al₂O₃/MoSi₂ laminates with unidirectional and (0/±45)_slayups is presented. In this system, the thermal stresses are very small in the 1200°–20°C range; hence laminate failure is dominated by mechanical loads. Propensity to fiber debonding appears to limit the load magnitudes that can be safely applied to the angle-ply laminate.     

Modified average stress criterion for open hole tension strength in presence of localised wrinkling

Abstract

This paper presents an analytical and experimental study to examine the behaviour of notched carbon/epoxy laminates containing out of plane fibre waviness defects. The open hole tension specimen geometry is modified to include carefully controlled waviness defects. The specimen is proposed to study the interaction between the notch and out of plane waviness. A new methodology using the modified average stress fracture criterion is developed for predicting the failure of notched composites containing localised waviness. The approach can be used to assess the quality of a given processing technique for the manufacturing of the composite system.