Interlaminar fracture toughness and associated fracture behaviour of bead-filled epoxy/glass fibre hybrid composites

To investigate enhancement of matrix-dominated properties (such as interlaminar fracture toughness) of a composite laminate, two different bead-filled epoxies were used as matrices for the bead-filled epoxy/glass fibre hybrid composites. The plane strain fracture toughness of two different bead-filled epoxies have been measured using compact tension specimens. Significant increases in toughness were observed. Based on these results the interlaminar fracture toughness and fracture behaviour of hybrid composites, fabricated using bead-filled epoxy matrices, have been investigated using double cantilever beam and end notch flexure specimens for Mode I and Mode II tests, respectively. The hybrid composites based on carbon bead-filled matrix shows an increase in both G _IC initiation and G _IIC values as compared to a glass fibre reinforced plastic laminate with unmodified epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15% and 20%. However, the unmodified epoxy glass-fibre composite shows a higher G _IC propagation value than that of hybrid composites, due to fibre bridging, which is less pronounced in the hybrids as the presence of the beads results in a matrix-rich interply region.     

Mode I interlaminar fracture behavior and mechanical properties of CFRPs with nanoclay-filled epoxy matrix

The mechanical properties and fracture behavior of nanocomposites and carbon fiber composites (CFRPs) containing organoclay in the epoxy matrix have been investigated. Morphological studies using TEM and XRD revealed that the clay particles within the epoxy resin were intercalated or orderly exfoliated. The organoclay brought about a significant improvement in flexural modulus, especially in the first few wt% of loading, and the improvement of flexural modulus was at the expense of a reduction in flexural strength. The quasi-static fracture toughness increased, whereas the impact fracture toughness dropped sharply with increasing the clay content.Flexural properties of CFRPs containing organoclay modified epoxy matrix generally followed the trend similar to the epoxy nanocomposite although the variation was much smaller for the CFRPs. Both the initiation and propagation values of mode I interlaminar fracture toughness of CFRP composites increased with increasing clay concentration. In particular, the propagation fracture toughness almost doubled with 7 wt% clay loading. A strong correlation was established between the fracture toughness of organoclay-modified epoxy matrix and the CFRP composite interlaminar fracture toughness.     

Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates

The reinforcement effects of two nanofillers, i.e., multi-walled carbon nanotube (MWCNT) and vapor grown carbon fiber (VGCF), which are used at the interface of conventional CFRP laminates, and in epoxy bulk composites, have been investigated. When using the two nanofillers at the interface between two conventional CFRP sublaminates, the Mode-I interlaminar tensile strength and fracture toughness of CFRP laminates are improved significantly. The performance of VGCF is better than that of MWCNT in this case. For epoxy bulk composites, the two nanofillers play a similar role of good reinforcement in Young’s modulus and tensile strength. However, the Mode-I fracture toughness of epoxy/MWCNT is much better than that of epoxy/VGCF.     

Effect of ionomer thickness on mode I interlaminar fracture toughness for ionomer toughened CFRP

Effect of ionomer thickness on mode I interlaminar fracture toughness was investigated for the ionomer-interleaved CFRP. Laminates were fabricated with Toho UT500/111 prepregs. Ethylene-based ionomer, which has high ductility and good adhesion to epoxy resin, was used as an interleaf material in this study. Thickness of the ionomer film selected was 12, 25, 100 and 200 μm. EPMA analysis showed the existence of the interphase region between the interleaf film and the base prepreg lamina where ionomer and epoxy were mixed. Mode I fracture toughness tests were carried out using DCB specimens. Precracks were introduced into all of the specimens. Fracture toughness values were much improved by interleaving the ionomer films. The fracture toughness value increased sharply by inserting thin ionomer film; however, the additional increase with the increase of the ionomer thickness was smaller. The thickness effect of the ionomer interleaf differs from that of the other kinds of thermoplastic-interleaf. Microscopic observation revealed that the crack path depended on the thickness of the ionomer region. Crack propagated in the interphase/ionomer interfaces for thinner-ionomer-interleaved CFRP, and in the interphase region, at the interphase/base lamina interface and interphase/ionomer interface for thicker-ionomer-interleaved CFRP. Ionomer resin deformed largely only near the crack surfaces, and this fact is responsible for the nonlinear increase of the fracture toughness with the increase of the ionomer thickness.     

Fracture toughness improvement of CFRP laminates by dispersion of cup-stacked carbon nanotubes

Several techniques are introduced to enhance the interlaminar fracture toughness of CFRP laminates using cup-stacked carbon nanotubes (CSCNTs). Prepared CSCNT-dispersed CFRP laminates are subject to Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests in order to obtain mode-I and mode-II interlaminar fracture toughness. The measured fracture toughnesses are compared to that of CFRP laminates without CSCNT to evaluate the effectiveness of CSCNT dispersion for the improvement of fracture toughness. All CSCNT-dispersed CFRP laminates exhibit higher fracture toughness, and specifically, CSCNT-dispersed CFRP laminates with thin epoxy interlayers containing short CSCNTs have three times higher fracture toughness than CFRP laminates without CSCNT. SEM observation of fracture surfaces is also conducted to investigate the mechanisms of fracture toughness improvement. Crack deflection mechanism is recognized in the CSCNT-dispersed CFRP laminates, which is considered to contribute the enhancement of interlaminar fracture toughness.     

高温下双搭接钢-CFRP板胶粘界面力学性能试验

高温环境下钢-碳纤维增强聚合物复合材料(CFRP)板的胶粘界面是CFRP粘贴加固钢结构的薄弱环节。为掌握温度对钢-CFRP板胶粘界面力学性能的影响,制作了双搭接接头试件,开展了3种胶粘剂在4种温度下(25℃、55℃、70℃和90℃)的静力拉伸试验。探索了接头试件的破坏模式、荷载-位移关系、CFRP板表面应变分布、界面剪应力分布和粘结-滑移关系等。结果表明:当温度低于55℃时,试件的破坏模式与胶粘剂种类相关性更大,当温度高于70℃时,不同胶粘剂的破坏模式具有相似性,且均出现了CFRP板撕裂。温度对不同胶粘试件的承载力影响存在差异,HJY-4105高韧性环氧树脂结构胶粘剂(HJY胶)试件的承载力随温度的升高而增大,LICA-100A/B 环氧树脂结构胶粘剂(LICA胶)试件的温度稳定性较差,Sikadur-30 CN双组份环氧结构加固碳板胶(SIKA30胶)试件在55℃时承载力最高。随着温度升高,胶粘层的剪切强度、界面剪应力峰值和剪切刚度下降,胶粘剂的延性增加,峰值剪应力不影响试件的抗拉强度。温度对粘结-滑移关系的影响显著,HJY胶随着温度的升高,粘结-滑移本构的延性增加,破坏模式由脆性破坏变为延性破坏。研究表明:合理的耐高温胶应用于钢结构加固,能适应自然高温环境的不利影响。      

Mechanical properties of carbon fiber/fullerene-dispersed epoxy composites

This study examined the effect of fullerene dispersion on the mechanical properties of carbon-fiber reinforced epoxy matrix composites (CFRPs). Mechanical properties such as tension, compression, open-hole compression, comparession after impact (CAI), binding, short beam shear, and interlaminar fracture toughness were evaluated for [0]₈, [90]₁₆, [45/0/?45/90]_2S laminates. Tension and compression strengths increased 2–12% by dispersing 0.5% of fullerene into the matrix resin. Furthermore, interlaminar fracture toughness of the composite was improved by about 60%. It was revealed that a small amount of fullerene (0.1–1 wt.%) increased the failure strain of epoxy resin itself, thereby improving the CFRP strength.     

碳纤维无纺布对CFRP层板层间的增韧作用及机制

为了揭示短纤维无纺布对碳纤维增强树脂基复合材料(CFRP)层板层间韧性的影响规律,测试了不同面密度(1.95、3.90、7.80和15.60 mg/cm2)和不同纤维平均长度(0.8 mm和4.3 mm)的碳纤维无纺布增韧的CFRP层板I型层间断裂韧性。实验结果表明:对于不同短纤维增韧的CFRP层板,平均长度为0.8mm的短纤维增韧效果优于平均长度为4.3mm的短纤维,并且面密度为7.8mg/cm2、厚度约为150μm、平均长度为0.8mm的碳纤维无纺布显著提高了CFRP层板的层间断裂韧性,与未改性的CFRP层板相比,其能量释放率最大可提高99%。光学显微镜观察结果表明环氧基体中长度为0.8mm的短纤维具有三维交织结构,该结构可以有效地阻止裂纹的扩展;SEM观察结果表明短纤维从环氧基体中的脱粘和拔出以及短纤维周围环氧基体的塑性变形是CFRP层板的主要增韧机制。研究结论为层板短纤维增韧技术的应用奠定了基础。     

Fracture Toughness of Hybrid Carbon Fibre/Epoxy Enhanced by Graphene and Carbon Nanotubes

Carbon-based nanoparticles have attracted considerable attention in materials science and engineering fields as they can significantly improve the electro-thermo-mechanical properties of polymer-based materials. With the need of enhancing the mechanical property through the thickness direction of a carbon fibre reinforced polymer (CFRP) system, this study investigates the effect of graphene nanoplatelets (GNP), multi-walled carbon nanotubes (MWCNT) and their hybridisations on its Mode I interlaminar fracture toughness. Various nanoplatelet sizes and weight percentages are compared to prohibit their agglomeration in epoxy which can drastically reduce the mechanical properties of CFRP. The smallest GNP size, 1 μm, dispersed in the n-methyl-2-pyrrolidone solvent leads to an advanced 146% enhancement of Mode I interlaminar fracture toughness on the CFRP system. The acetone solvent is found less surface compatible with the nanoplatelets, but provides a simple and environmentally friendly manufacturing process. The hybrid GNP/MWCNT with 1wt% content dispersed in acetone solvent shows the synergistic effect and reaches a 120% enhancement of Mode I interlaminar fracture toughness of CFRP. Additionally, the application of the thin film hot press technique on nanoplatelets enhanced CFRP demonstrates an effective and promising solution to manufacture homogeneous multi-phase composites.

     

Characterization of transverse tensile, interlaminar shear and interlaminate fracture in CF/EP laminates with 10 wt% and 20 wt% silica nanoparticles in matrix resins

The transverse tensile properties, interlaminar shear strength (ILSS) and mode I and mode II interlaminar fracture toughness of carbon fibre/epoxy (CF/EP) laminates with 10 wt% and 20 wt% silica nanoparticles in matrix were investigated, and the influences of silica nanoparticle on those properties of CF/EP laminates were characterized. The transverse tensile properties and mode I interlaminar fracture toughness (G_IC) increased with an increase in nanosilica concentration in the matrix resins. However, ILSS and the mode II interlaminar fracture toughness (G_IIC) decreased with increasing nanosilica concentration, especially for the higher nanosilica concentration (20 wt%). The reduced G_IIC value is attributed to two main competing mechanisms; one is the formation of zipper-like pattern associated with matrix microcracks aligned 45° ahead of the crack tip, while the other is the shear failure of matrix. The ratio of G_IIC/G_IC decreased with the concentration of silica nanoparticles, comparable with similar CF/EP laminates with dispersed CNTs in matrix. Fractographic studies showed that interfacial failure between carbon fibre and epoxy resin occurred in the neat epoxy laminate, whereas a combination of interfacial failure and matrix failure occurred in the nanosilica-modified epoxy laminates, especially those with a higher nanosilica concentration (20 wt%).     

碳纤维增强树脂复合材料-热成型钢超混杂层合板层间力学性能

通过双悬臂梁试验(DCB)研究了金属表面处理和界面插层协同作用对碳纤维增强树脂复合材料(CFRP)-热成型钢超混杂层合板层间力学性能的影响。试验结果表明,采用金属表面处理与界面插层协同增韧方案,可以极大地提升层合板的I型层间断裂韧性。其中,喷砂/界面胶膜插层试件(GB36#/AF)的I型层间断裂韧性相比于脱脂试件提高了343%;喷砂/界面纯树脂插层试件(GB36#/EP)相比于脱脂试件,其Ⅰ型层间断裂韧性提高了129%。并基于内聚区模型对CFRP-热成型钢超混杂层合板分层失效进行了有限元模拟。最后借助激光共聚焦扫描显微镜(LSM)、接触角测量仪(CAG)、扫描电子显微镜(SEM)等对热成型钢表面形貌和试件的断裂面进行了表征并揭示了层间增韧的机制。      

Monitoring of delamination onset and growth during Mode I and Mode II interlaminar fracture tests using guided waves

A delamination monitoring method was proposed to characterize Mode I and Mode II delamination onset in carbon fiber/epoxy (CF/EP) composite laminates through interrogation of guided waves activated and captured using piezoelectric actuators and sensors in a pitch–catch configuration. Mode I and Mode II interlaminar fracture tests were conducted using double cantilever beam (DCB) and end notch flexure (ENF) specimens to evaluate the proposed method. The changes in wave propagation velocity and wave magnitude (or attenuation), and the degree of waveform similarity between excitation and response signals, were calculated as delamination-sensitive wave parameters and plotted versus displacement recorded using a materials testing system. The kink points determined from wave parameter–displacement curves agreed well with the deviation from linearity (NL), visual observation (VIS) and maximum load (Max) points, which are often used in conventional methods for determining interlaminar fracture toughness. The propagation characteristics of the A₀ wave mode in a low frequency range were demonstrated to have high sensitivity to Mode I and in particular Mode II delamination onset in CF/EP composite laminates. It was concluded that the guided waves propagating in the DCB and ENF specimens were capable of determining Mode I and Mode II interlaminar fracture toughness, complementing current practices based on visual inspection or trivial interrogation using load–displacement curve alone.     

Improvement of interlaminar mechanical properties of CFRP laminates using VGCF

In order to improve the interlaminar mechanical properties of CFRP laminates, hybrid CFRP/VGCF laminates have been fabricated by using a newly-developed method, i.e., powder method, where the powder of vapor grown carbon fiber (VGCF) is added at the mid-plane of [0°/0°]₁₄ CFRP laminates. Experimental results of double cantilever beam (DCB) tests indicate the improvement on the interlaminar mechanical properties of Mode-I fracture behavior with much higher critical load P_C and fracture toughness G_IC with VGCF interlayer. Crack propagation and fracture surface have also been observed to interpret this improvement mechanism. Moreover, based on experimental G_IC, numerical simulations using finite element method (FEM) with cohesive elements have been carried out to analyze the delamination propagation. The interlaminar tensile strength of hybrid CFRP/VGCF laminates, which is obtained by matching the numerical load–COD (crack opening displacement) curves to experimental ones, is higher than that of base CFRP laminates.     

Multiscale carbon fibre–reinforced polymer (CFRP) composites containing carbon nanotubes with tailored interfaces

Pristine and functionalized multiwalled carbon nanotubes (MWCNTs) with tailored interfaces were efficiently dispersed in an epoxy matrix using a three‐roll mill and further reinforced with carbon fibres. 1.3‐Dipolar cycloaddition of azomethine ylides was used for the chemical modification of MWCNTs by a solvent‐free approach. The influence of different loadings and types of MWCNTs on the final properties of the epoxy matrix was studied. Moreover, the most promising formulations were selected for manufacturing of prepreg sheets. The transversal tensile properties and the interlaminar fracture toughness under mode I loading (G_IC) of multiscale carbon fibre–reinforced polymer (CFRP) composites were characterized. The results point out that it is not straightforward to transfer the remarkable intrinsic properties of MWCNTs to the composite level, although an overall positive trend was found. Double cantilever beam experiments showed that G_IC of CFRP composites was improved 44% at ultralow content of functionalized MWCNTs (0.043 wt%).     

Mechanical Properties and Interlaminar Fracture Toughness of Glass‐Fiber‐Reinforced Epoxy Composites Embedded with Shape Memory Alloy Wires

The effects of the content and position of shape memory alloy (SMA) wires on the mechanical properties and interlaminar fracture toughness of glass‐fiber‐reinforced epoxy (GF/epoxy) composite laminates are investigated. For this purpose, varying numbers of SMA wires are embedded in GF/epoxy composite laminates in different stacking sequences. The specimens are prepared by vacuum‐assisted resin infusion (VARI) processing and are subjected to static tensile and three‐point‐bending tests. The results show that specimens with two SMA wires in the stacking sequence of [GF₂/SMA/GF₁/SMA/GF₂] and four SMA wires in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] exhibit optimal performance. The flexural strength of the optimal four‐SMA‐wire composite is lower than that of the pure GF/epoxy composite by 5.76% on average, and the flexural modulus is improved by 5.19%. Mode‐I and II interlaminar fracture toughness tests using the SMA/GF/epoxy composite laminates in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] are conducted to evaluate the mechanism responsible for decreasing the mechanical properties. Scanning electron microscopy (SEM) observations reveal that the main damage modes are matrix delamination, interfacial debonding, and fiber pullout.

     

Prestandardization study on mode I interlaminar fracture toughness test for CFRP in Japan

This paper summarizes results from a series of interlaboratory round robin tests (RRTs) performed in order to establish a JIS standard for mode I interlaminar fracture toughness test using double cantilever beam (DCB) specimens. For the case of unidirectional laminates, brittle and toughened CF/epoxy, and CF/PEEK systems were used. Only a brittle CF/epoxy system was used for woven laminates. The round robin tests were conducted with two main aims: first, to examine the influence of starter films and the precracking condition on the initial mode I fracture toughness values; and second, to establish the definition of initial fracture toughness. Polyimide starter films stuck to the epoxy matrix, and caused unstable crack growth from starter films. Comparison of the tests with and without mode I precracks from starter films indicated that tests with precracks gave lower values of initial fracture toughness. The definition of initial fracture toughness values was discussed, based on the reproducibility. A 5% offset point was recommended as the initial fracture toughness from the RRT results. The influence of loading apparatus, data reduction methods, etc. was also discussed.     

Prestandardization study on mode II interlaminar fracture toughness test for cfrp in japan

A three year long cooperative research programme has been carried out to establish a Japanese standard for an interlaminar fracture toughness test method for CFRPs. For mode II fracture toughness tests, the end-notched flexure test was employed. Two series of round robin tests (RRTs) were carried out using conventional brittle epoxy CFRPs, a toughened epoxy CFRP and APC-2. The following points became clear in the investigation: 1) crack shear displacement measurement gives precise information on crack initiation; 2) the fracture toughness values at the start of non-linear behaviour showed a considerably large scatter; 3) the toughness at the 5% offset points has a lower scatter and is still conservative; and 4) it is necessary to take account of the increase in crack length for the toughness calculation after the 5% offset point. Considering this information, together with other important observations obtained in the RRTs, a proposal for the mode II fracture toughness test was made and it was established as Japanese Industrial Standard K 7086 in March 1993.     

低温环境下测试复合材料Ⅰ型层间断裂韧性的简易方法

目前ASTM复合材料Ⅰ型层间断裂韧性测试标准需不断观测裂纹扩展长度。然而在低温环境下,裂纹扩展长度不易测量且过程繁琐。为克服这一缺陷,本文采用双柔度法测试复合材料低温环境下Ⅰ型层间断裂韧性,该方法的步骤与ASTM标准基本相同,但不需观测裂纹扩展长度便能获得低温下Ⅰ型层间断裂韧性。为了验证该方法的可靠性和精度,采用5件碳纤维增强环氧树脂基复合材料双悬臂梁（DCB）试样在-10℃环境下进行Ⅰ型层间裂纹扩展实验,应用ASTM标准所推荐的三种方法及本文的双柔度法进行数据处理获得复合材料Ⅰ型层间断裂韧性。结果表明:ASTM标准的三种方法与双柔度法得到的Ⅰ型层间断裂韧性结果一致,相对差别小于5%,而本文的双柔度法不需测量裂纹扩展长度,因此更简单,为测试低温环境下Ⅰ型层间断裂韧性提供了一种准确、简单的新方法。      

Improved fracture toughness in carbon fibre epoxy composite through novel pre-preg coating method using Epoxy Terminated Butadiene Nitrile rubber

A novel pre-preg coating method was used to improve the interlaminar fracture toughness in carbon fibre epoxy composite laminates, using reactive liquid rubber. The Epoxy Terminated Butadiene Nitrile (ETBN) liquid rubber incorporated between pre-pregs using automatic draw bar coating technique. Experimental test results reveal that by adding ETBN in small quantities in the range of 15.55–22.66 g/m², inter laminar critical energy release rates (G_IC and G_IIC) can be improved up to 140% in mode-I loadings and 32% in mode-II loadings respectively. It was confirmed that the effect of ETBN rubber concentration in carbon epoxy pre-preg system on interlaminar fracture toughness under mode-I and mode-II loadings, was discussed by on the bases of fractographic observations and mechanism considerations using SEM.     

Experimental study on mechanical properties and fracture toughness of structural thick plate and its butt weld along thickness and at low temperatures

The thick plate induces the variation of mechanical properties and fracture toughness, especially in cold regions. At the low temperature, the brittle behaviour of steel becomes worse. A series of tests (such as uniaxial tensile test and three‐point bending test) were carried out at low temperature to investigate the mechanical properties and fracture toughness of structural steel plates of Q345B with thickness of 60 to 150 mm, as well as the fracture toughness of 150 mm thick butt welded plate. The test specimens are all manufactured from plates along thickness with small size, and the tensile test specimens included through‐thickness specimens additionally. The ductility index (percentage reduction of area) and the fracture toughness index (critical CTOD values) all decrease with the temperature decreases and the distance from plate surface increases. The results obtained in this paper provide technical basis for preventing brittle fracture of thick plate steel structures in cold regions.