Cooling rate influences in carbon fibre/PEEK composites. Part II: interlaminar fracture toughness

A study has been made of the effect of cooling rate on interlaminar fracture toughness of unidirectional carbon fibre–polyetheretherketone (PEEK) matrix composites. It is shown that the propagation values of both mode I and II propagation interlaminar fracture toughness increased with increasing cooling rate towards a saturation level for the range of cooling rate studied. The cooling rate dependency of the composite interlaminar fracture toughness is the result of complex interactions between two important properties, namely the matrix ductility and fibre–matrix interface bond strength. These two properties varied totally in an opposite manner against cooling rate through its effect on crystallinity: matrix ductility varied directly proportional to cooling rate while the converse is true for interface bond strength. The extent of plastic deformation of PEEK matrix contributed a predominant part to composite toughness, while an adequate interface bond is required to allow matrix deformation to take place to a full degree. A practical implication is that these two properties need to be optimised using an appropriate cooling rate to produce composites possessing high interlaminar fracture resistance.     

Interlaminar fracture (model II) of commingled yarn-based GF/PP composites

A 5050 wt % mixture of commingled glass/polypropylene fibre system was selected to study the correlations between the morphological details, mode II interlaminar fracture toughness and corresponding failure mechanisms. Mode II interlaminar fracture tests were performed by using the end-notched flexure test procedure. Compared to conventional composite laminates, mode II interlaminar crack extension in these commingled yarn-based composites was very stable, and extensive fibre nesting occurred along the main crack plane. Crack jumping and non-broken matrix links were observed.R-curve behaviour for these materials was identified and the toughness for initiation was much lower than that for propagation. Compared to mode I interlaminar fracture toughness, similar trends in effects of cooling rates and isothermal crystallizations on mode II interlaminar fracture toughness were observed. However, the effects were not as significant as those found for mode I interlaminar fracture toughness.Alexander von Humboldt Fellow.     

Mode I and mode II fracture toughness of E-glass non-crimp fabric/carbon nanotube (CNT) modified polymer based composites

In this study, mode I and mode II interlaminar fracture toughness, and interlaminar shear strength of E-glass non-crimp fabric/carbon nanotube modified polymer matrix composites were investigated. The matrix resin containing 0.1 wt.% of amino functionalized multi walled carbon nanotubes were prepared, utilizing the 3-roll milling technique. Composite laminates were manufactured via vacuum assisted resin transfer molding process. Carbon nanotube modified laminates were found to exhibit 8% and 11% higher mode II interlaminar fracture toughness and interlaminar shear strength values, respectively, as compared to the base laminates. However, no significant improvement was observed for mode I interlaminar fracture toughness values. Furthermore, Optical microscopy and scanning electron microscopy were utilized to monitor the distribution of carbon nanotubes within the composite microstructure and to examine the fracture surfaces of the failed specimens, respectively.     

Improving interlaminar fracture toughness of carbon fibre/epoxy laminates by incorporation of nano-particles

Double-cantilever-beam tests were applied to investigate the mode I interlaminar fracture toughness of carbon fibre/epoxy laminates, in which the epoxy matrices were incorporated with rubber and silica nano-particles, either singly or jointly. It is shown that the toughness is improved owing to the presence of these nano-particles although nano-rubber is more effective than nano-silica. Further, by keeping the total particle weight percentage constant in epoxies (e.g., at 8 and 12 wt.%) filled with equal amount of nano-silica and nano-rubber, the interlaminar toughness values of the hybrid laminates are always higher than those with nano-silica filled epoxies but lower than those with nano-rubber filled matrices. Scanning electron microscopy examination of the delaminated surfaces of composite laminates filled with nano-particles revealed that cavitation of nano-rubber particles/void growth and debonding of nano-silica from epoxy matrix are responsible for the improved interlaminar toughness observed. It is also shown that the bulk toughness of nano-particle filled epoxies cannot be fully transferred to the interlaminar toughness of composite laminates, being limited by the constraint effect imposed by the carbon fibres. Finally, the role of fibre-bridging on the delaminated crack and hence delamination toughness is discussed.     

Hybrid nanoreinforced carbon/epoxy composites for enhanced damage tolerance and fatigue life

Hybrid nano/microcomposites with a nanoparticle reinforced matrix were developed, manufactured, and tested showing significant enhancements in damage tolerance properties. A woven carbon fiber reinforced polymer composite, with the polymer (epoxy) matrix reinforced with well dispersed carbon nanotubes, was produced using dispersant-and-sonication based methods and a wet lay-up process. Various interlaminar damage tolerance properties of this composite, including static strength, fracture toughness, fatigue life, and crack growth rates were examined experimentally and compared with similarly-processed reference material produced without nanoreinforcement. Significant improvements were obtained in interlaminar shear strength (20%), fracture toughness (180%), shear fatigue life (order of magnitude), and fatigue crack growth rate (factor of 2). Observations by scanning electron microscopy of failed specimens showed significant differences in fracture surface morphology between the two materials, related to the differences in properties and providing context for understanding of the enhancement mechanisms.     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.     

Influence of silane coupling agents on interlaminar fracture in glass fibre fabric reinforced unsaturated polyester laminates

The relationship between the adhesive properties of the interphase of glass fibre/resin and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate (GFFL) was studied. The Mode I interlaminar fracture toughness of GFFL was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of GFFLs which have two silane coupling agents and three concentration finishes is discussed on the basis of interlaminar fracture toughness. The crack propagation behaviour of DCB testing was mainly divided into stable and unstable manners. The fracture toughness and the crack propagation behaviour were dependent on the types and concentration of silane coupling agents.     

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.     

碳纳米管加入方式对碳纤维/环氧树脂复合材料层间性能的影响

针对碳纤维/环氧树脂预浸料,对比了直接在树脂中加入碳纳米管(CNTs)后制备预浸料以及将CNTs喷涂在预浸料表面2种CNTs加入方式对CNTs-碳纤维/环氧树脂复合材料层合板I型与II型层间断裂韧性及层间剪切强度的影响。通过对树脂黏度、固化反应以及玻璃化转变温度的考察,分析了CNTs含量对树脂性能的影响,考察了添加方法对CNTs长度与形态的影响。分析了2种CNTs加入方式对CNTs-碳纤维/环氧树脂层合板断裂韧性及层间剪切强度的改善效果与作用规律。结果表明:CNTs的加入使树脂的黏度提高,固化反应程度下降;2种分散方法对CNTs的长度与形态无明显影响;直接在树脂中加入CNTs对CNTs-碳纤维/环氧树脂复合材料I型与II型层间断裂韧性的提高效果低于在碳纤维/环氧树脂预浸料表面喷涂CNTs的方式,后者的CNTs利用率较高;由于CNTs团聚及对树脂固化反应的影响,CNTs含量过高会使得其对CNTs-碳纤维/环氧树脂层合板的增韧效果下降。     

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%).     

苎麻短纤维层间增韧碳纤维/环氧树脂复合材料

采用热压机层压成型工艺制备了苎麻短纤维(SRF)层间增韧碳纤维/环氧树脂(CF/EP)复合材料层压板,研究了SRF的长度、面密度及其表面偶联处理对CF/EP复合材料层间断裂韧性的影响,并进一步研究了SRF的铺入对复合材料弯曲、拉伸性能的影响。研究结果表明,层间SRF的铺入明显改善了CF/EP复合材料的I型和II型层间断裂韧性(G_(IC)和G_(IIC)),当表面偶联处理的纤维长度为6mm、面密度为12g·m~(-2)时,增韧效果最佳,GIC由497.48J·m~(-2)增加到667.54J·m~(-2),提高了34.24%;GIIC由508.52J·m~(-2)增加到862.11J·m~(-2),提高了69.54%。此外,铺入SRF对复合材料的弯曲、拉伸性能也有一定程度的提高。通过SEM观察发现,SRF的增韧机制与其层间桥联以及裂纹扩展过程中从基体中拔出与劈裂等现象有关。     

有机黏土对碳纤维/聚醚砜-环氧复合材料层间断裂韧性的影响

采用溶剂法和热熔法制备了不同有机黏土质量分数的有机黏土/聚醚砜（PES）-环氧复合材料,通过对其微观形态和力学性能的研究,揭示了复合材料的增韧机制。在有机黏土/PES-环氧复合材料中添加T800H（12K）碳纤维,制备了T800H-有机黏土/PES-环氧复合材料预浸料单向带,采用热压罐工艺制备了复合材料单向板,对其I型、II型层间断裂韧性进行了研究。结果表明:T800H-有机黏土/PES-环氧复合材料的层间断裂韧性随有机黏土质量分数变化趋势与有机黏土/PES-环氧复合材料的断裂韧性趋势一致,证明了增韧机制的正确性。      

A new mixed mode test for carbon/epoxy composite systems

The present research examines analytically and experimentally the mixed mode interlaminar fracture toughness of a resin film infused (RFI) carbon fiber/epoxy laminate, namely a IM7-AS4/3501-6 hybrid composite system. The inability to develop representative interlaminar failure in composites with current mixed mode test configurations motivated this particular investigation. The paper is part of a more extensive research effort concerned with the effects of stitching upon the mixed mode fracture toughness of a RFI composite.A new mixed mode test configuration is suggested, the Single Leg Four Point Bend (SLFPB), which provides a robust method with small specimens and a simple apparatus. Closed form fracture mechanics-based strain energy release (SERR) calculations have been established for this configuration. Finite element analysis was conducted to validate the closed form solution. Results show a very good agreement between analytical solutions, numerical simulation and the newly designed SLFPB experimental test.     

Influence of adsorption behaviour of a silane coupling agent on interlaminar fracture in glass fibre fabric-reinforced unsaturated polyester laminates

The relationship between the interphase consisting of physisorbed and chemisorbed silane on glass fibres and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate, was studied. The Mode I interlaminar fracture toughness of the laminate specimen was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of the laminate specimen finished with two silane concentrations and washed in methanol solvent, is discussed on the basis of the interlaminar fracture toughness. In order to determine the amount of physisorbed and chemisorbed silane on the glass fibre, the amount of total carbon was determined using an analysis instrument. The physisorbed silane migrated into the resin matrix and influenced the mechanical properties and interlaminar fracture of the laminate specimen. The amount of unsaturated polyester resin blended with a silane coupling agent was measured using dynamic mechanical spectroscopy, and a DCB specimen for mechanical properties and fracture toughness.     

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。      

聚酰亚胺颗粒层间增韧碳纤维/邻苯二甲腈树脂复合材料

邻苯二甲腈树脂是一种新型的高性能热固性树脂,具有优良的力学性能和耐高温性能,而邻苯二甲腈树脂本身的脆性限制了其用作结构材料方面的应用。本文采用热塑性聚酰亚胺(PI)颗粒对邻苯二甲腈树脂复合材料进行层间增韧改性,研究改性前后复合材料的耐热性能和力学性能。研究发现,使用PI对邻苯二甲腈复合材料进行改性时,随着掺入量的增加,复合材料的玻璃化转变温度降低。PI颗粒的引入会显著提高复合材料韧性,10wt% PI改性复合材料层间剪切强度提高了41.2%,15wt% PI改性复合材料的Ⅰ型层间断裂韧性提高了156.3%。复合材料的层间能够清晰地观察到颗粒的存在;PI的质量分数进一步提高时,出现粒子团聚缺陷,导致复合材料的层间剪切强度下降。此外PI增韧邻苯二甲腈树脂复合材料在380℃下的层间剪切强度与未改性复合材料数值相当,该温度下PI颗粒的含量已不是影响复合材料韧性的主要因素。      

Interlaminar fracture of carbon-thermoplastic polyimide composites

Mode I interlaminar fracture of a novel amorphous thermoplastic polyimide reinforced with unidirectional carbon fibre has been studied experimentally using double cantilever beam specimens and scanning electron microscopy. Three kinds of composite were manufactured from different monomeric reactant solutions which were prepared by using different alcohol solvents. The values of fracture toughness of these three composites were measured to construct the crack growth resistance curves (R curves). The contributions from various failure processes to the total fracture toughness were separated, and approximate calculations of these contributions were conducted based on several simplifying assumptions and some data obtained from the fracture surfaces. Though fibre peeling and fibre breakage are observed, interlaminar fracture in the composites studied is primarily controlled by fracture and deformation in the matrix. It is found that the measured fracture toughnesses of the composites differ from each other not only in the propagation values but also in the initial values. A possible reason for this may be variations of matrix ductility in the three composites.     

Delamination properties of z-pinned composites in hot–wet environment

The effect of hot–wet environment (75 °C and 85% relative humidity) on the delamination fracture properties and interlaminar toughening mechanisms of z-pinned carbon fibre–epoxy composite is investigated. The absorption rate of water from the hot–wet environment into the composite is accelerated slightly by z-pins, although the pins did not change the saturation limit of the material. Absorbed water weakens the pin/composite interface and this lowers the ultimate elastic traction load generated by z-pins under mode I interlaminar loading. However, once the pin/composite interface has failed, the traction load and energy required to pull-out the z-pins is not affected by absorbed water. The mode I interlaminar fracture toughness and low-energy impact damage resistance of z-pinned composites is not degraded significantly by exposure to hot–wet environment, and this is because absorbed water does not affect the pull-out traction properties of z-pins.     

Mechanical characterization of hierarchical carbon fiber/nanofiber composite laminates

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber–matrix adhesion and indications of CNF-induced matrix toughening.     

Interlaminar and intralaminar fracture modes in 0/90 cross-ply glass/epoxy laminate

Delamination of a cross-ply 0/90 glass fibre-reinforced composite laminate with an epoxy-phenol matrix was studied using a double cantilever beam test. Fracture toughness was determined by measurement of bend angle of the cantilever beams. Results obtained with this method were in agreement with those from conventional compliance and area methods. Two different fracture modes were observed: interlaminar and intralaminar. In the interlaminar fracture mode, crack jumps in the space between two neighbouring 0° and 90° plies were observed. With the interlaminar fracture mode, during crack initiation G_Ic decreased with crack length. Intralaminar fracture mode consisted of the gradual growth of a crack through a 0° ply. Fibres bridging the opposite sides of the crack were observed in this case, and fracture toughness G_Ic did not change with crack length. G_Ic (420 J m⁻²) at intralaminar fracture mode was approximately twice that at interlaminar fracture mode (220 J m⁻²). The difference in fracture toughness was explained by the dissipation of energy by fibres bridging the opposite sides of the crack at intralaminar fracture mode.