层合板复合材料的层间剪切强度评价方法及其改进研究

针对国际上目前主要使用的双切口拉伸/压缩式层间剪切试验方法存在的问题,吸收了拉伸与压缩式试验方法各自的优点,对试验夹具及试验片形状与尺寸进行了改进设计,提出了双切口开孔拉伸式新的层间剪切强度测试、评价方法.以玻璃纤维平纹织物/环氧树脂复合材料层合板为试验材料对改进后的试验方法进行了实验及有限元分析.实验发现,表观层间剪切强度随槽间距的减小而有增大的趋势.通过有限元分析,对实验结果进行了解释,并确立了可获得真实层间剪切强度的最佳试验片尺寸.     

Effects of Matrix Porosity on the Mechanical Properties of a Porous-Matrix, All-Oxide Ceramic Composite

The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregnation and pyrolysis of a ceramic precursor solution. Mechanical tests are performed to assess the role of the matrix in both matrix-dominated and fiber-dominated loading configurations. The results demonstrate a loss in damage tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are obtained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a consequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double-notched shear specimen are broadly consistent with the limited values obtained by the short beam shear method, although the former exhibit large variability. In addition, effects of precursor segregation during drying on through-thickness gradients in matrix properties and their role in composite performance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabling damage tolerance in porous-matrix composites. Deficiencies in the understanding of the mechanisms that enable damage tolerance in this class of composites are discussed.     

风电叶片复合材料压缩损伤破坏声发射监测

研究了风电叶片单向复合材料的压缩力学特性及其声发射响应特征.结果表明,复合材料的横向和纵向压缩力学性能及其声发射响应特性明显不同,纵向压缩强度、模量高,失效应变小,对应的声发射相对能量、幅度高,但撞击累积总数少.复合材料具有脆性破坏的特点,横向压缩以45°剪切失效为主,纵向压缩以层间劈裂为主.风电叶片复合材料压缩损伤破...     

Through Thickness Mechanical Properties of Chemical Vapor Infiltration and Nano‐Infiltration and Transient Eutectic‐Phase Processed SiC/SiC Composites

The through thickness (interlaminar) shear strength and trans‐thickness tensile strength of three different nuclear‐grade SiC/SiC composites were evaluated at room temperature by the double‐notched shear and diametral compression tests, respectively. With increasing densification of the interlaminar matrix region, a transition in failure locations from interlayer to intrafiber bundle was observed, along with significant increases in the value of the interlaminar shear strength. Under trans‐thickness tensile loading, cracks were found to propagate easily in the unidirectional composite. The 2D woven composite had a higher trans‐thickness tensile strength (38 MPa) because the failure mode involved debonding, fiber pull‐out and fiber failure.     

Mechanical response and damage mechanism of C/SiC composites impacted by high-velocity water jet

Rain erosion is a potential hazard for supersonic vehicles, with severe damage to materials that may be impacted by raindrops. In this paper, a series of impact tests of 413–572 m/s are carried out on a 3 mm-thick 2D C/SiC composite specimen using a single impact waterjet apparatus. The typical morphology of C/SiC specimen is obtained by single jet impact test. Under the multi-drop impact, the stress wave interaction is enhanced, and the internal damage of the specimen is severe, showing a funnel-shaped damage. Moreover, the C/SiC specimen is penetrated after 5 drops of impact. Quasi-static tensile tests were employed to quantify the post-impact strength of the specimen, during which the digital image correlation (DIC) method was used to obtain the strain value, at the same time acoustic emission (AE) signal was detected and processed by the K-Means to reveal the damage evolution.     

Development and experimental validation of explicit dynamics simulation of composite structures using a stacked thick-shell methodology

The stacked thick-shell modelling approach is investigated in the frame of explicit dynamics FE method for the simulation of composite structures. The methodology is developed for static and dynamic loading conditions and demonstrated in the case of three-point bending of laminated strips. For the validation of the stacked thick-shell modelling approach, experimental testing using laminated short beam shear coupons of the AS4/8552 composite material system is performed and the interlaminar shear strength under impact loading is determined. The specimen dynamic tests were performed using a drop tower apparatus and a specially designed three-point loading fixture. In parallel, conventional three-dimensional solid models are also analysed for comparison purposes. Test results correlate well to the respective numerical predictions, demonstrating the accuracy of the stacked thick-shell approach and the efficiency it provides in interlaminar stresses prediction, which makes the proposed approach suitable for large-scale composite structures simulation, with emphasis in delamination damage propagation.     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

碳纤维编织复合材料损伤变形与破坏实验研究

通过对碳纤维编织复合材料的拉伸实验,利用声发射技术(Acoustic Emission,简称"AE")和数字图像相关(Digital Image Correlation,简称"DIC")方法研究碳纤维复合材料的损伤演化规律。通过采集试件在拉伸过程中的声发射信号、损伤变形与应变场信息,分析碳纤维编织复合材料的力学加载、变形场和声发射特征参数的关系。结果表明复合材料的位移场、应变场信息以及AE信号特征参数能良好地描述复合材料在拉伸状态下的损伤累积和破坏过程。在加载前期,以40~60 dB低幅度信号为主;随着载荷增加,撞击累计数急剧升高,高幅度、高持续时间信号增多。通过DIC测得的位移场和应变场信息,发现对于相同的载荷增量,加载方向的位移和最大拉应变呈先增加后减小的趋势。     

Experimental and numerical investigations of the interlaminar shear properties of carbon/carbon composites

Experimental tests and numerical simulations were implemented to investigate the interlaminar shear properties of carbon/carbon composites (C/Cs). A unit‐cell model, according to the microstructure of the C/Cs, was used to predict material properties of the C/Cs. A three‐dimensional finite element model was established to investigate the damage behavior of C/Cs on the basis of Linde failure criterion and damage evolution. Good agreement, in terms of the load force history and failure modes, was observed between the experimental and numerical results; this provided the applicability of the numerical simulation. The test results show that the interlaminar shear strength of the C/Cs was 10.52 MPa and the value of the simulation result was 10.89 MPa, with the relative error being less than 4%. Damage contours and stress distribution analysis of the simulation results are discussed. Fiber damage occurred at the bottom of the specimen, and matrix damage was found in the upper half of the specimen; this was similar to the appearance of the tested specimens. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44783.     

FRP复合材料拉伸过程的声发射特性实验研究

用声发射技术研究了FRP复合材料的拉伸损伤与断裂行为。宽带传感器记录了FRP复合材料试样在拉伸破坏过程中的声发射信号,运用声发射参数分析方法对单向FRP复合材料的声发射历程图进行分析,得出复合材料在拉伸过程中的损伤类型以及各损伤阶段所呈现出来的特性。用扫描电子显微镜（SEM）观察了试样的几种典型的损伤破坏断面,对比分析了不同类型的损伤机制。实验分析表明,拉伸过程中破坏机制对声发射信号的特征具有显著影响,不同损伤模式的信号频谱特征存在明显的差异。     

风电叶片复合材料层间剪切破坏声发射监测

采用声发射技术对含分层缺陷风电叶片多轴向复合材料的层间剪切破坏实验进行实时监测,研究分层缺陷对复合材料层间力学性能的影响规律及其损伤破坏过程的声发射响应特征.结果表明,具有不同分层面积的两类复合材料试样破坏载荷相近,当分层缺陷位于剪切面中间位置时,分层缺陷大小对界面承载能力影响不大,损伤演化主要集中在剪切面上偏离中心两...     

Fracture characterization of C/C composites under various stress modes by monitoring both mechanical and acoustic responses

C/C composites with different porosities, produced by chemical vapor infiltration have been mechanically tested under quasi-static loading in bending modes using uniform and notched specimens. The acoustic emission (AE) method was used to monitor the damage accumulation profile during loading up to fracture, supported by optical and scanning electron microscope characterization. Three stages in the damage buildup up to fracture were observed: Stage I, with no AE activity, Stage II, gradual growth in AE counts up to an abrupt jump and Stage III, sharp increases in AE counts. Moreover, the similarity in the profile between the cumulative AE counts vs. strain data and the predicted crack density vs. strain by the micro mechanical model suggested for interlaminar cracking, indicates the importance of AE in monitoring the damage evolution in composites in terms of AE counts. Fast Fourier transform analysis of the AE waves revealed three characteristic frequencies in Stage III, which is a sign of three main micro-mechanisms of failure which control the failure progress: fiber fracture, debonding and matrix cracking seem to be the active mechanisms.     

Fatigue behavior and damage analysis of PIP C/SiC composite

In this work, we study the fatigue behavior of a C/SiC composite produced by several cycles of polymer infiltration and pyrolysis (PIP). Fatigue tests were performed with maximum stresses corresponding to 60–90% of the tensile strength of the composite. During the fatigue tests, acoustic emission (AE) monitoring was performed and the measured AE energy was utilized to quantify the damage and distinguish possible damage mechanisms. Most of the fatigue damage in the form of matrix cracking, interface damage and fiber breakage occurs in the first cycle. As loading cycles proceeded, damage in form of matrix crack re-opening and interfacial friction constantly accumulates. Nevertheless, all samples survived the run-out of 1,000,000 cycles. After the fatigue tests, an increase of the tensile strength is observed. This phenomenon is associated with the relief of process-induced internal thermal stresses and the weakening of the fiber-matrix interface. In general, the studied material shows very high relative fatigue limit of 90% of its tensile strength.     

Characterization of Interlaminar Shear Strength of Ceramic Matrix Composites

The interlaminar shear strengths of three ceramic matrix composites have been characterized using a double-notch shear (DNS) test. The material systems investigated are plain woven C/SiC, plain woven SiC/SiC, and cross-plied SiC/calcium aluminosilicate-II. The use of the double-notch shear test for measuring the interlaminar shear strength of ceramic matrix composites is evaluated first. Numerical stress analyses are performed to investigate the effect of DNS specimen length, notch distance, and specimen supporting jig on the stress distribution in the expected fracture plane and the interlaminar shear strength. The numerical findings are then compared with an analytical model proposed elsewhere and correlated with the experimental results. The validity of this test technique has been established.     

Effects of graphite filler loading and heat treatment temperature on the properties of phenolic resin based carbon–carbon composites

Carbon/carbon composites (C/Cs) were prepared through polymer pyrolysis using PAN based carbon fabric (Panex^(R) 35) and resol type phenolic resin having 0, 10, 20, 30, and 40 wt% of graphite fillers. These precursor composites were heat treated at 600, 900, and 1200°C. The effects of filler loading on the precursor composites and their C/Cs were investigated through density, microstructure, and mechanical properties. Since, the precursor composites were prepared under similar processing conditions and technique, at any particular filler loading when the heat treatment temperature increases, the bulk density of the samples decreases. The filler addition accelerates the formation of the carbon basal planes in the matrix supported by X‐ray diffraction studies. The properties such as tensile strength and strain decrease continuously mainly due to change in the matrix structure and decrease of density, whereas, the interlaminar shear strength (ILSS) and interlaminar fracture toughness (ILFT) increase mainly because of improvement in the modulus of matrix. At any particular heat treatment temperature, depending on the filler content and matrix type, the density, tensile properties, ILSS, and ILFT of the samples show different trends. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Experimental study of the material and bond properties of frost-damaged concrete

In an extensive experimental investigation, several types of tests were conducted on a reference specimen and frost-damaged concrete. Two levels of internal frost damage were quantified by the relative dynamic modulus of elasticity and compressive strength. Test results showed a significant influence of freeze–thaw cycles on the compressive strength and even more influence on the modulus of elasticity and the compressive strain at peak stress. Reduced tensile strength and increased fracture energy were measured. From inverse analysis of wedge splitting test results, a significant effect of frost on the shape of the tensile stress–crack opening relationship was observed: tensile strength was reduced, while the post-peak behaviour was more ductile for the frost-damaged concrete. Pull-out tests showed the influence of freeze–thaw cycles on bond strength and slip. The pull-out test results are compared with similar tests available in the literature and the effect of frost on bond behaviour is discussed.     

Interlaminar shear test by hybrid sandwich specimen under distributed load

A new method is proposed for the determination of the interlaminar shear strength of composites. The method is particularly pertinent to composites of high interlaminar shear strengths, where the ratio of tensile (compressive) strength to shear strength is relatively low. In such materials, including unidirectional composites with improved fiber/matrix bond strength and angle-ply laminates, an analysis based on a short beam interlaminar shear test is highly problematic and may, in fact, be erroneous. The test method is based on the use of a sandwich composite structure with a core made of layers of the tested composite and skins made of an elastic, strong unidirectional composite. A proper design procedure determines the choice of the skin material and of the relative thicknesses, so that flexural testing under distributed load leads to the intended core failure in shear. Calculations of the stress profile in a hybrid sandwich beam in bending and of the stress ratios under distributed load are presented. Also presented are experimental results recorded with sandwich hybrids made of unidirectional carbon-fiber-reinforced epoxy skins and a ±θ aramid-fiber-reinforced epoxy angle-ply core.     

G／Epoxy复合材料胶接强度研究

使用G／Epoxy作为底材研究了垫板、结构胶黏剂厚度和底材表面处理对拉伸剪切强度的影响。使用光学显微镜观察了断口形貌。结果表明加垫板能减小试验过程中由于加载偏心引起剥离应力,测试结果较大;结构胶黏剂的厚度和底材表面处理对拉伸剪切强度影响十分明显,随着厚度的增大而减小,经打磨表面裸露出纤维的试样拉伸剪切强度很低。结构胶黏剂厚度较小时以内聚破坏为主,随着厚度的增加破坏模式转变为粘接破坏。     

Reinforcing effects of aminosilane-functionalized graphene on the tribological and mechanical behaviors of epoxy nanocomposites

In this study, aminopropyl trimethoxysilane as an interfacial modifier was introduced on the surface of graphene (Gr) nanoplatelets. The effects of the silane-modified graphene (SGr) loading (0, 0.05, 0.1, 0.3, and 0.5 wt %) and silane modification on the tensile, compressive, interlaminar shear stress (ILSS), and tribological properties of the epoxy-based nanocomposites were investigated. Out of these specimens, the highest values of ILSS and compressive strength were related to the 0.3 wt % SGr–epoxy nanocomposite. The addition of SGr enhanced the tensile strength and strain to failure only at low contents (i.e., 0.05 wt %). Also, the tensile and compressive moduli were improved, and the highest values were observed at a 0.5 wt % SGr loading. In addition, decreases of approximately 40 and 68% in the coefficient of friction and wear rate, respectively, were observed at a 0.3 wt % SGr loading. Enhanced tensile, compressive, ILSS, and wear properties in the SGr–epoxy specimens were observed compared to those in the Gr–epoxy specimens. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47410.     

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.