单向纤维增强陶瓷基复合材料界面滑移规律

建立了分析单向纤维增强陶瓷基复合材料力学特性的界面摩擦模型。采用基体应变能准则对基体损伤状态进行预测; 由Weibull分布模型拟合出纤维断裂分数; 将界面磨损处理为纤维/基体相对滑移历程的函数τ_i=τ_i (Δδ), 很好地表征了不同位置纤维/基体相对滑移历程不同所引起的界面磨损程度的区别。运用该模型分析了准静态加载和拉-拉循环载荷下的应力-应变特性, 预测结果与实验数据吻合较好。最后采用此模型研究了任意载荷历程下界面的滑移规律。     

单向纤维增强陶瓷基复合材料界面滑移规律

建立了分析单向纤维增强陶瓷基复合材料力学特性的界面摩擦模型.采用基体应变能准则对基体损伤状态进行预测;由Weibull分布模型拟合出纤维断裂分数;将界面磨损处理为纤维/基体相对滑移历程的函数τi=τi(△δ),很好地表征了不同位置纤维/基体相对滑移历程不同所引起的界面磨损程度的区别.运用该模型分析了准静态加载和拉-拉循环载荷下的应力-应变特性,预测结果与实验数据吻合较好.最后采用此模型研究了任意载荷历程下界面的滑移规律.     

Dynamic delamination fracture toughness in unidirectional polymeric composites

A modified end-notched flexure (ENF) specimen was used to determine Mode-II-dominated dynamic delamination fracture toughness of fiber composites at high crack propagation speeds. A strip of FM-73 adhesive film was placed at the tip of the interlaminar crack created during laminate lay-up. This adhesive film with its greater toughness delayed the onset of crack extension and produced crack propagation at high speeds. Dynamic delamination experiments were performed on these ENF specimens made of unidirectional S2/8553 glass/epoxy and AS4/3501-6 carbon/epoxy composites. Crack speed was measured by means of conductive aluminum lines created by the vapor deposition technique. A finite-element numerical simulation based on the measured crack speed history was performed and the dynamic energy release rate calculated. The results showed that the dynamic fracture toughness is basically equal to the static fracture toughness and is not significantly affected by crack speeds up to 1100 m/s.     

Enhancing compressive strength of unidirectional polymeric composites using nanoclay

Morphology of nanoclay dispersed in resin and suspended in acetone was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images show intercalation of resin in the gallery spaces of nanoclay and regions of exfoliated clay with random orientation. A vacuum assisted wet lay-up (VAWL) process was used for the inclusion of nanoclay in conventional fiber reinforced composites. The VAWL specimen displayed improvement in off-axis compressive strength for nanoclay enhanced fiber composites. Addition of nanoclay produced a substantial increase in longitudinal compressive strengths (extracted from off-axis tests) of glass fiber reinforced composites. An elastic–plastic model was used to predict the compressive strength of fiber reinforced composites based on the matrix properties. The model predictions matched well with the experimental results.     

碳化硅纤维增强磷酸铝基复合材料的制备和性能研究

采用不同的磷酸铝盐基体制备了单向碳化硅纤维增强磷酸铝基复合材料,对其力学性能进行了对比,结果表明：基体对复合材料的力学性能和微观结构有极大的影响,磷酸铝基体随温度的升高脱水、相变残留下一定的气孔,填料的加入可以提高复合材料的整体力学性能;采用磷酸二氢铝基体、α-Al2O3填料制备的复合材料具有最好的性能,其弯曲强度为310MPa、弯曲弹性模量为47GPa,并通过扫描电镜对材料的微观结构形貌进行了分析研究。     

Predictions of viscoelastic strain rate dependent behavior of fiber-reinforced polymeric composites

A viscoelastic damage model for aligned and 3D randomly oriented discontinuous fiber-reinforced polymeric composites is proposed. The model, which predicts the effective viscoelastic stress-strain behavior of the composites, is based on a combination of the Laplace-transformed superposition principle and the ensemble-averaged micromechanics. The Weibull’s damage function is incorporated into the model for the modeling of the evolution of damaged fibers. An inverse analysis based on experimental data is adopted to simulate the strain rate sensitivity of the model. A series of numerical simulations based on the proposed model are performed to examine the influence of damage parameters, fiber orientations, strain rates, and the aspect ratio of discontinuous fibers on the behavior of the composites. In addition, experimental comparisons are made to illustrate and assess the predictive capability of the proposed model.     

An analysis of cracks emanating from a circular hole in unidirectional fiber-reinforced composites

This paper presents an analytical study of cracks emanating from a circular hole in an off-axis unidirectional fiber-reinforced composite. A convenient and accurate method of analysis is formulated on the basis of conservation laws of elasticity and of fundamental relationships in anisotropic fracture mechanics. The problem is eventually reduced to a system of linear algebraic equations in mixedmode stress intensity factors. Superiority of the current analysis to other approaches in investigating the problem with very complicated crack geometry and material anisotropy is demonstrated when used in conjunction with any numerical method such as a finite element analysis. Mixed-mode stress intensity factors and the associated energy release rates in the crack problem are determined for the composites with various fiber orientations. Solutions for both single and double cracks emanating from the edge of a hole in the composites are presented also to illustrate the fundamental nature of the problem.     

圆弧形单向纤维增强树脂复合材料的I型层间断裂韧性测试及分析

层合板的I型层间断裂韧性的测量方法通常为单向纤维增强树脂复合材料的末端切口(End notched flexure, ENF)试样的双悬臂梁(Double cantilever beam, DCB)试验。为了得到带有弧度的层合复合材料结构的I型层间断裂韧性,对圆弧形末端切口(Arc-ENF)试样进行DCB试验。基于梁的弯曲理论和Irwin-Kies公式得到Arc-ENF试样的柔度公式与I型临界能量释放率G_IC公式,并且利用ABAQUS软件对DCB试验进行数值模拟。最终,通过对比分析理论公式计算结果、数值模拟结果和DCB试验结果来验证柔度公式和G_IC公式的合理性和有效性,对带有任意弧度的DCB试样的I型层间断裂韧性的测试与分析具有参考价值。      

Improving the prediction of tensile failure in unidirectional fibre composites by introducing matrix shear yielding

A Monte Carlo simulation is established to predict the failure strain of unidirectional fibre composites. The effect of matrix shear yielding of a high performance epoxy resin is introduced into the model through load sharing factors between the fibres adjacent to fibre-break(s). Strain concentration factors (SCF) of fibres are obtained using Finite Element Methods (FEM) in a three dimensional multi-fibre unit cell containing one, two and three adjoining fibre-break(s). The tensile strains of the surviving adjacent fibres are intensified as a function of their distances from the fracture. A statistical simulation is carried out to predict the failure strain of a single layer of unidirectional (UD) fibre composites with the thickness of the fibre ineffective length. Using the weakest link theory, the ultimate failure strain of a real size UD composite is predicted.     

Dynamic caustics in unidirectional fiber-reinforced composites with mode I cracks: Experiment and numerical simulation

The stress singularities of a dynamic crack tip in orthotropic composites were studied through caustics. The parametric equations of the caustic and its initial curve surrounding the dynamic crack tip were derived through the elastic dynamic crack solutions of orthotropic composites and the basic principle of reflective caustics. Theoretical caustics and initial curves for three kinds of orthotropic composites were simulated, and the effects of crack velocity on the caustics and initial curves were analyzed. In comparison with numerical results, the dynamic caustic experiment was performed for dynamic cracks along the material axes in unidirectional, fiber-reinforced composites under drop-hammer, three-point-bend loading.     

More on the effects of thermal residual and hydrostatic stresses on yielding behavior of unidirectional composites

The influence of manufacturing process thermal residual stresses and hydrostatic stresses on yielding behavior of unidirectional fiber reinforced composites has been investigated when subsequently subjected to various mechanical loadings. Three-dimensional finite element micro-mechanical models have been used. The results of this study reveal that the size of the initial yield surface is highly affected by the thermal residual and hydrostatic stresses. It was also found that effects of a uniform temperature change on the initial yield surface in the composite stress space is not equivalent to a solid translation of the surface in the direction of the hydrostatic stress axis. At the micro-level, magnitudes of various stress components within the matrix due to the thermal residual and hydrostatic stresses are different. However, at a macro-level, both temperature change and hydrostatic loading of composites show similar effects on the initial yield surface in the composite stress space. In an agreement with experimental data, results also show that residual stresses are responsible for asymmetric behavior of composites in uniaxial tension/compression in the fiber direction. This asymmetric behavior suggests that the existing quadratic yield criteria need modification to include thermal residual stress effects.     

Prediction model for determining the optimum operational parameters in laser forming of fiber-reinforced composites

Composite materials are widely employed in various industries, such as aerospace, automobile, and sports equipment, owing to their lightweight and strong structure in comparison with conventional materials. Laser material processing is a rapid technique for performing the various processes on composite materials. In particular, laser forming is a flexible and reliable approach for shaping fiber-metal laminates (FMLs), which are widely used in the aerospace industry due to several advantages, such as high strength and light weight. In this study, a prediction model was developed for determining the optimal laser parameters (power and speed) when forming FML composites. Artificial neural networks (ANNs) were applied to estimate the process outputs (temperature and bending angle) as a result of the modeling process. For this purpose, several ANN models were developed using various strategies. Finally, the achieved results demonstrated the advantage of the models for predicting the optimal operational parameters.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00304-3     

A tensile strength model for unidirectional fiber-reinforced brittle matrix composite

A model for the ultimate tensile strength of unidirectional fiber-reinforced brittle matrix composite is presented. In the model, transverse matrix crack spacing and change in debonding length between the fiber and the matrix is continuously monitored with increasing applied load. A detailed approximate stress analysis, together with a Weibull failure statistics for fiber fracture, are used to determine the probability of fiber fracture and fiber fracture location in the composite. Results of the model are consistent with experimental data. It is suggested from the results that the strength and toughness of the composite are significantly influenced by the Weibull modulus of the fiber and the fiber/matrix interfacial shear stress. A higher fiber Weibull modulus results in a lower composite strength while a higher fiber/matrix interfacial shear stress results in a composite with higher strength but lower toughness. A moderate variation in matrix strength and fiber/matrix interfacial shear strength does not significantly affect the strength of the composite.     

A continuum theory for fiber-reinforced elastic-viscoplastic composites

A higher order continuum theory with microstructure is derived for the modeling of the 3-dimensional motion of fiber-reinforced composites in which both the matrix and fibers constituents are assumed to be elastic-viscoplastic work-hardening materials. The fibers are unidirectional with rectangular cross section and are imbedded in the matrix in the form of a doubly periodic array. The derivation of the theory is systematic and can be applied to various types of non-elastic composites to the desired degree of expansion. An appropriate reduction of the theory gives the average behavior of the viscoplastic composite in the form of effective rate-dependent stress-strain curves. In the special case of perfectly elastic constituents the reduction gives the approximate effective moduli of the composite.     

Coefficients of nonlinear thermal expansion for fiber-reinforced composites

In the present study, a micromechanics model is proposed to predict the coefficients of nonlinear thermal expansion (CTEs) of fiber-reinforced composites. The influence of fiber aspect ratio on the CTEs is also investigated. It is noted that the parameters of fiber aspect ratio have a significant effect on both the longitudinal CTEs and transverse CTEs. The CTEs of composites are also very sensitive to the different fiber volume fractions. Moreover, the Young’s modulus and Poisson’s ratio of composites are taken into account in the present analysis. The theoretical derivations are applicable for the composites under mechanical or thermal environment conditions. The present model offers a direct prediction of CTEs and can account for the effects of fiber aspect ratio and volume fractions.     

Creep behavior of unidirectional composites

The creep behavior of the continuous fiber reinforced unidirectional composites due to the viscoelasticity of the resin matrix is investigated assuming that the constituent matrix obeys the nonlinear creep law and the fiber is the linear elastic materials. Utilizing a quasi three-dimensional finite element method, the macroscopic creep behavior of the composites with regular fiber packing is obtained, giving the orthotropic creep law for the composites. Then, the creep of the composites with random fiber packing is estimated applying the random model proposed by Kondo and Saito in which the neat matrix cylinders are embedded in the regular array composites. The theoretical predictions for the creep behavior are compared with the experimental results.     

Influence of filler/reinforcing agent and post-curing on the flexural properties of woven and unidirectional glass fiber-reinforced composites

The aim of this study is to investigate the reinforcing effect of woven and unidirectional glass fibers and the effect of post-curing on the flexural strength and flexural modulus of glass fiber-reinforced composites. A series of composites containing 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)-phenyl]propane and triethyleneglycol dimethacrylate matrices and different reinforcements of unidirectional or woven glass fibers were prepared. The samples, 25 × 2 × 2 mm, were cured with a halogen curing lamp, followed by additional curing by thermal treatment at 135 ± 5 °C temperature and 60 psi pressure. Samples were tested before and after post-curing in order to determine the flexural strength and flexural modulus. The degree of reinforcement with glass fibers was varied between 14 and 57 wt% or 7.64 and 38.44 vol% by changing the number of unidirectional bundles or woven glass fiber bands in the composites, respectively. The obtained flexural strength values were in the range of 95.20–552.31 Mpa; the flexural modulus ranged between 2.17 and 14.7 GPa. The highest flexural strength and flexural modulus values were recorded for samples with unidirectional glass fibers. The mechanical qualities of the glass fibers-reinforced composites increased after post-curing treatment. Increasing of the glass fiber amount in the experimental composites improves both flexural strength and modulus. SEM micrographs of fractured composites indicate a strong interfacial interaction between the glass fibers and the polymer matrix.     

Fractography of unidirectional CFRP composites

The morphology of fracture surfaces of unidirectional carbon fibre reinforced plastic composites has been studied to categorize the fractographic characteristics for several types of loading. Specimens were tested in compression and in tension, both in the axial and the transverse direction, in bending and in interlaminar shear. The influence of prior exposure to moisture and to elevated temperature on the fracture morphology of specimens tested at elevated temperature has also been studied. Optical and scanning electron microscopy were used for the fracture surface analyses. Fractographic features characteristic of each mode of stressing and directionality of specimen were identified.     

A theory for physically nonlinear elastic fiber-reinforced composites

A three-dimensional constitutive relationship for the fiber-reinforced composite with a nonlinearly elastic matrix material is derived. The composite is modeled by a medium consisting of thin nonlinear matrix layers alternating with effective linearly elastic fibrous layers. Shear wave propagations are investigated. The formation of shock wave, its stability and growth behaviors are discussed.