Interlaminar shear strength of cloth-reinforced composites

A test specimen to define the interlaminar shear strength of cloth-reinforced composite materials is developed. The specimen is a hollow circular cylinder which is subjected to torsion. The experiment is employed for the determination of the warp-normal shear strength of a graphite-fiber carbon-matrix composite material. It is demonstrated that the proper failure mode takes place, while the problems associated with the use of strain gages on this porous material, nonlinearity, difference in tension and compression properties, and the influence of clamping effects are all discussed. The proposed experiment appears to be ideally suited to study the interlaminar shear response of cloth-reinforced composites.     

Spall strength of glass fiber reinforced polymer composites

In the present paper results of a series of plate impact experiments designed to study spall strength in glass–fiber reinforced polymer composites (GRP) are presented. Two GRP architectures are investigated—S2 glass woven roving in Cycom 4102 polyester resin matrix and a balanced 5-harness satin weave E-glass in a Ciba epoxy (LY564) matrix. The GRP specimens were shock loaded using an 82.5 mm bore single-stage gas-gun. A velocity interferometer was used to measure the particle velocity profile at the rear (free) surface of the target plate. The spall strength of the GRP was obtained as a function of the normal component of the impact stress and the applied shear-strain by subjecting the GRP specimens to normal shock compression and combined shock compression and shear loading, respectively. The spall strengths of the two GRP composites were observed to decrease with increasing levels of normal shock compression. Moreover, superposition of shear-strain on the normal shock compression was found to be highly detrimental to the spall strength. The E-glass reinforced GRP composite was found to have a much higher level of spall strength under both normal shock compression and combined compression and shear loading when compared to the S2-glass GRP composite. The maximum spall strength of the E-glass GRP composite was found to be 119.5 MPa, while the maximum spall strength for the S2 glass GRP composite was only 53.7 MPa. These relatively low spall strength levels of the S2-glass and the E-glass fiber reinforced composites have important implications to the design and development of GRP-based light-weight integral armor.     

Failure surface of epoxy-modified fiber-reinforced composites under transverse tension and out-of-plane shear

The mechanical behavior of uniaxially fiber-reinforced composites with a ductile rubber-toughened epoxy matrix was studied through the finite element analysis of a RVE of the composite microstructure. The fibers were represented by elastic and isotropic solids, while the rubber-modified epoxy matrix behaved as a elasto-viscoplastic solid. The matrix flow stress followed the model developed by Jeong [Jeong, H.-Y., 2002. A new yield function and a hydrostatic stress-controlled void nucleation model for porous solids with pressure-sensitive matrices. International Journal of Solids and Structures 39, 1385–1403.], which included the inherent pressure-sensitivity of the yield stress in the epoxy matrix, the damage due to the cavitation of the rubber particles and subsequent void growth, and the particular features of elastic–viscoplastic behavior in glassy polymers, particularly the intrinsic softening upon yield followed by hardening. Composites with either perfect or weak fiber/matrix interfaces (the latter introduced through cohesive elements) were studied to assess the influence of interface strength on the composite behavior. Simulations under transverse tension and out-of-plane shear were carried out to establish the effect of loading conditions on the dominant deformation and failure micromechanisms. In addition, the corresponding failure locus was obtained and compared with the predictions of current phenomenological failure criteria for composites. The range of validity of these criteria and the areas for further improvement were established by comparison with the numerical results.     

2D编织陶瓷基复合材料应力-应变行为的试验研究和模拟

本文对2D编织陶瓷基复合材料拉伸应力-应变行为进行了试验研究和理论模拟。将2D编织结构简化为：正交铺层结构和纤维束波动结构。基于基体随机开裂、纤维随机断裂的统计分布理论,得到正交铺层结构的应力-应变关系;基于体积平均方法,将纤维束波动部分分割为若干子单元;由于纤维束的波动使各子单元材料方向与加载方向不一致,因此考虑了各子单元的线性行为和非线性行为对材料响应的影响,同时引入强度分析模型,得到纤维束波动部分的应力-应变关系。结合正交铺层部分和纤维束波动部分的应力-应变关系,得到2D编织结构的应力-应变行为,理论与试验吻合较好。     

二维编织陶瓷基复合材料应力-应变行为

对二维编织陶瓷基复合材料拉伸应力-应变行为进行了试验研究和理论模拟. 将二维 编织结构简化为：正交铺层结构和纤维束波动结构. 基于基体随机开裂、纤维随机断裂分布 理论,得到正交铺层结构的应力-应变关系;基于体积平均方法,将纤维束波动部分进行分割, 引入强度分析模型,得到纤维束波动部分的应力-应变关系. 结合正交铺层部分和纤维束波动 部分的应力-应变关系,得到二维编织结构的应力-应变行为,理论与试验吻合较好.     

On the Torsion of a Class of Nonlinear Fiber Composite Cylinders

This paper presents an analysis of the torsion of a solid or annular circular cylinder consisting of nonlinear material in the form of an elastic matrix with embedded unidirectional elastic fibers parallel to the cylinder axis. The specific class of composite considered is one for which nonlinear fiber-matrix interface slip is captured by uniform cohesive zones of vanishing thickness. Previous work on the effective antiplane shear response of this material leads to a stress–strain relation depending on the interface slip together with an integral equation governing its evolution. Here, we obtain an approximate single mode solution to the integral equation and utilize it to solve the torsion problem. Equations governing the radial distributions of shear stress and interface slip are obtained and formulae for torque–twist rate are presented. The existence of singular surfaces, i.e., surfaces across which the slip and the shear stress experience jump discontinuities are analyzed in detail. Specific results are presented for an interface force law that allows for interface failure in shear.     

热环境中旋转运动功能梯度圆板的强非线性固有振动

研究热环境中旋转运动功能梯度圆板的非线性固有振动问题．针对金属－陶瓷功能梯度圆板,考虑几何非线性、材料物理属性参数随温度变化以及材料组分沿厚度方向按幂律分布的情况,应用哈密顿原理推得热环境中旋转运动功能梯度圆板的非线性振动微分方程．考虑周边夹支边界条件,利用伽辽金法得到了横向非线性固有振动方程,并确定了静载荷引起的静挠度．用改进的多尺度法求解强非线性方程,得出非线性固有频率表达式．通过算例,分析了旋转运动功能梯度圆板固有频率随转速、温度等参量的变化情况．结果表明,非线性固有频率随金属含量的增加而降低;随转速和圆板厚度的增大而升高;随功能梯度圆板表面温度的升高而降低．     

几种高性能纤维束的冲击动力学性能实验研究

利用直拉式Hopkinson装置研究了碳纤维、无碱E玻璃纤维、Kevlar 4 9/96 4 /96 4c、Twaron2 0 0 0、DyneemaSk6 6等纤维的动态拉伸性能。与准静态加载条件下相比 ,纤维束的拉伸强度基本与应变速率无关(玻璃纤维除外 ) ,而纤维束的弹性模量和失效应变随应变率的升高而明显变大。从高分子物理以及两种无机纤维的内部微观结构特征对纤维的力学性能与加载速率的关系进行了初步的物理阐释。讨论了实验数据的发散原因。     

Behavior of Pin-loaded Laminated Composites

In this study, an investigation was carried out to determine the effects of joint geometry and fiber orientation on the failure strength and failure mode in a pinned joint laminated composite plate. Behavior of pin-loaded laminated composites with different stacking sequence and different dimensions has been observed experimentally. E/glass–epoxy composites were manufactured to fabricate the specimens. Mechanical properties of the composites were characterized under tension, compression and in-plane shear in static loading conditions. Laminated composites were loaded through pins. Single-hole pin-loaded specimens were tested for their tensile response and width-to-hole diameter (W/D) and edge distance-to-hole diameter (E/D) ratios evaluated. A series of experiments was performed with six different material configurations ([0/±45]_s–[90/±45]_s, [0/90/0]_s–[90/0/90]_s and [90/0]_2s–[±45]_2s), in all, over 120 specimens. E/D ratios and W/D ratios of plates were changed from 1 to 5 and 2 to 5, respectively. Failure propagation and failure type were observed on the specimens. The influence of the joint geometry on the strength of the pin-loaded composites was assessed. When laminated composite plates were loaded to final failure, three basic failure modes consisting of net-tension, shear out and bearing failure were observed for the different geometric dimensions. All the connections tested showed that the fiber orientations have a definite influence on the position around hole circumference at which failure initiated. Net-tension failure occurred for specimens that had small width and large end distance. When the width was increased, the specimens which had small end distances failed in the shear-out modes. When the end distance was increased, bearing failure developed in addition to shear-out failure. The experimental results showed that the ultimate load capacities of E/glass–epoxy laminate plates with pin connection were increased by increasing W and E. However, increasing the E/D and W/D ratios beyond a critical value has an insignificant effect on the ultimate load capacity of the connection.     

Multi-scale modeling of the mechanical response of plain weave composites and cellular solids

Multi-component materials with customized mechanical properties, such as textile composites and sandwich materials (cellular core with metallic or composite skin), show a great prospective for use in aerostructures. Understanding of the mechanical response of these materials is still in progress. In the present paper, the tensile response of plain weave composites as well as the compressive response of cellular solids are investigated using a multi-scale damage model. The model, implemented by means of the FE method, is based on homogenized progressive damage modeling of a representative unit-cell. Four failure modes have been considered in the failure analysis of the tows, while material property degradation was performed using a damage mechanics approach which takes into account strain softening. For the cellular solids, two different types of FE models were considered namely, a beam model and a shell model. Failure analysis and material property degradation of the struts were integrated into a bilinear material model. Simulations show a non-linear tensile response of the plain weave mainly attributed to matrix cracking and shear failures occurring at warp tows and resin-rich areas. For the cellular solid, preliminary elastic analyses show a customizability of the normal stiffness with regard to strut’s dimensions.     

Experimental evaluation of the Wyoming-modified two-rail shear test method for composite materials

In-plane unidirectional shear properties of carbon/epoxy and glass/epoxy materials were measured using unidirectional and cross-ply specimens utilizing the Wyomingmodified two-rail shear test fixture. Simple test specimens of trapezoidal and rectangular geometries, and specimens with tabs were tested. Various specimen sspect ratios were also considered. Untabbed unidirectional specimens exhibited premature shear failures but cross-ply specimens produced reasonably reliable results with acceptable failure modes. Both bonded tab and integral tab 0° specimens were found to give comparable results. Unidirectional and cross-ply PEEK specimens, and angle-ply and quasi-isotropic AS4/3501-6 specimens, were also tested.     

A failure criterion for fiber reinforced polymer composites under combined compression–torsion loading

A fracture mechanics based failure criterion for unidirectional composites under combined loading has been developed. The predictions from this criterion have been compared with experimental data obtained from combined compression–torsion loading of glass and carbon fiber reinforced polymer composites of 50% fiber volume fraction. The specimens were loaded under rotation control and displacement control in a proportional manner. Comparison of the Budiansky–Fleck kinking model, specialized to a solid circular cylinder, and the new failure model against experimental data suggests that the Budiansky–Fleck model predictions do not capture the variation of compressive strength as a function of shear stress for glass fiber composites. This is because these composites fail predominantly by compressive splitting. The Budiansky–Fleck model predictions are appropriate for composites that fail by compressive kinking. The new model predictions capture the experimental results for glass composites where the compression strength is initially unaffected by shear stress but undergoes a drastic reduction when a critical value of shear stress is reached.     

Quasi-static behavior of Mg-alloys with and without short-fiber reinforcement

Magnesium alloys AE42 and AZ91 reinforced with 23 vol.% carbon short fibers (D_f ≈ 7 μm, L_f ≈ 100 μm) were tested under quasi-static loading. The carbon fibers were quasi-isotropically distributed in the horizontal plane (reinforced plane) of the casting. Compression and tensile tests were carried out on both the matrix alloys and the composites at temperatures between 20 °C and 300 °C. Specimens were machined to be loaded either parallel or normal to the reinforced plane. Due to the reinforcement, the compression yield stress of the composite AE42-C increased to a value approximately three-fold greater than the yield strength of the matrix; for composite AZ91-C this parameter was approximately 2.5-fold greater than that of the AZ91 matrix. The improvement in tensile strength was less than that in compression, which could be related to early tensile fracture through decohesion at the matrix–fiber interface, as detected by SEM investigations conducted on failed tensile specimens. Flow curves for the matrix alloys at different temperatures were described by a modified Kocks–Mecking material law. An idealization of a 2-D mesomodel was used for finite-element simulation of the mechanical behavior of the composites. The fibers were first considered as elastic bodies and the behavior of the matrix material was set according to the material law determined from the flow curves for the matrix alloys. Other calculations were carried out by considering elasto-plastic behavior of the fibers for application of a failure initiation technique to simulate the behavior of the composite materials beyond the ultimate stress.     

Failure analysis of unidirectional fiber composites under combined axial tension and shear

A method of analysis is proposed for investigating the mechanics of failure of unidirectional, fiber composite materials subjected to axial tension and shear. The mechanisms of failure are assumed to result from the interaction of the applied shear stress and local matrix shear stress concentrations which arise as a result of the scattered fiber breaks that occur throughout the material. Two modes of failure are identified. One is associated with the unstable growth of shear failure regions in the matrix. The other is primarily a tensile failure mode which is influenced by the applied shear. The analysis predicts that a variety of shear-tensile stress failure surfaces are possible, depending on material properties. The results suggest that radical changes in the shape of failure surfaces can occur as the result of environmental effects. This has significant implications for reliability.     

Deformation of a fibrous composite with physically nonlinear fibers and microdamageable matrix

The structural theory of short-term microdamage is generalized to a fibrous composite with a microdamageable matrix and physically nonlinear fibers. The basis for the generalization is the stochastic elasticity equations of a fibrous composite with a porous matrix. Microvolumes in the matrix material meet the Huber-Mises failure criterion. The damaged-microvolume balance equation for the matrix is derived. This equation and the equations relating macrostresses and macrostrains of a fibrous composite with porous matrix and physically nonlinear fibers constitute a closed-form system of equations. This system describes the coupled processes of physically nonlinear deformation and microdamage occurring in different components of the composite. Algorithms for computing the microdamage-macrostrain and macrostress-macrostrain relationships are developed. Uniaxial tension curves are plotted for a fibrous composite with linearly hardening fibers __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 1, pp. 38–47, January 2006.     

Uncertainties of unidirectional composite strength under tensile loading and variation of environmental condition

A probabilistic strength model is developed for unidirectional composites with fibers in hexagonal arrays. The model assumes that, a central core of broken fibers surrounded by unbroken fibers which are subjected to unidirectional tensile loading. The proposed approach consists in using a modified shear lag model to calculate the ineffective lengths and stress concentrations around fiber breaks. The main feature in the model lies in incorporating the variation of composite properties due to temperature and moisture, in order to predict degradation of fibers and matrix characteristics. The strength degradation is often seen as a result of changes in ineffective lengths at fiber breaks, leading to stress concentrations in intact neighboring fibers. As fiber breaks are intrinsically random, the variability of input data allows us to describe the probabilistic model by using the Monte-Carlo method. The sensitivities of the mechanical response are evaluated regarding the uncertainties in design variables such as Young’s modulus of fibers and matrix, fiber reference strength, shear yield stress, fiber volume fraction and shear parameter defining the shear stress in the inelastic region.     

玻璃纤维增强型复合材料圆筒高温高压动态冲击断口形貌分析

为了探究埋头弹火炮所用的玻璃纤维增强型(GFR)复合材料药筒在高温高压瞬态冲击条件下的结构强度,分别开展了圆筒静态整体拉伸和动态高温高压冲击实验,从拉伸/瞬态超高压破坏试样断口部分截取断口样品,在扫描电子显微镜下观察断口形貌,得到GFR复合材料在两种不同受力情况下的失效模式。结果表明:室温整体单轴拉伸断裂时,GFR复合材料的断面与轴线夹角接近45°, 失效模式为环氧树脂基体破坏和纤维拔出;在高压瞬态冲击作用下,试样主要失效模式为纤维的脆性断裂,同时由于火药燃烧产生的高温燃气使部分环氧树脂基体碳化,纤维与基体界面结合力降低,少数纤维熔融或软化附着在断口上,部分软化的纤维因瞬态超高压被拉细。     

Time to Creep Failure of Thin-Walled Cylindrical Pipes under Torsion

The time to creep failure is calculated for rectilinear thin-wall pipes subjected to pure torsion, torsion with uniaxial tension, and torsion with internal pressure. The problem is solved using the concept of equivalent stress. The equivalent stresses are found from the generalized mixed failure criterion whose form depends on the signs of the principal stresses. The criterion relates the maximum normal stress and the intensity of shear stress if the signs coincide, and the maximal shear stress and the octahedral shear stress if the signs are opposite. A technique for determining the material constants is developed. The calculated and experimental data are compared and found to be in satisfactory agreement     

纤维/金属网增强层板低速冲击损伤机理和演化

本文首先通过落锤低速冲击实验测试了纯玻璃纤维增强环氧树脂复合材料和304不锈钢丝网(SSWM)/玻璃纤维混杂复合材料的力学性能,探究了SSWM嵌入数量对混杂复合材料抗冲击性能的影响．随后采用Abaqus有限元软件建立了混杂复合材料的低速冲击模型,分别采用三维Hashin失效准则和Jason-Cook破坏准则模拟了纤维/基体和SSWM的损伤;建立了基于表面接触的内聚力模型来模拟界面分层;编写了VUMAT用户子程序定义混杂复合材料层合板的渐进失效过程．结果表明：相较于纯玻璃纤维增强环氧树脂层合板,SSWM/玻璃纤维混杂增强环氧树脂层合板的抗冲击性能更优,其中铺层形式为铺层III的混杂复合材料抗冲击性能最佳．通过对比发现有限元仿真结果与实验结果吻合良好,表明建立的模型适用于SSWM/玻璃纤维混杂增强环氧树脂复合材料低速冲击损伤的评估．通过分析仿真结果发现混杂复合材料的低速冲击损伤主要是冲击区域的纤维断裂、基体破坏和层间分层;SSWM通过吸收和传递冲击能量从而提升了混杂复合材料的抗冲击性能．     

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.