Compression strength of aligned carbon fibre-reinforced thermoplastic laminates

Compression strength measurements have been made on a number of unidirectional carbon fibre-reinforced thermoplastics fabricated by film stacking or by melt impregnation and comparisons made with a standard aerospace epoxide system. The results indicate that polyetheretherketone based laminates, despite having a lower matrix shear modulus, have similar compression strengths to the epoxide system.     

Effect of strain rate on interlaminar shear properties of carbon/epoxy composites

In this paper, the effect of strain rate on interlaminar shear properties of laminates is studied. The material tested was a T300/5208 carbon/epoxy composite, and the range of strain rates explored was about 10⁻³ − 10³ s⁻¹. The specimens used were designed and optimized by finite element analysis, and the calculations are presented here. One of the specimens permitted the determination of the interlaminar shear modulus, G₁₃, and the other permitted the determination of the interlaminar shear strength, S₁₃. No influence of testing speed on interlaminar properties was observed at low, intermediate and high strain rates. Fracture surfaces were studied by scanning electron microscopy: a slight difference was observed between specimens tested at low and high strain rates.     

Effects of carbon nanotubes on the interlaminar shear strength and fracture toughness of carbon fiber composite laminates: a review

Journal of Materials Science - The interlaminar mechanical properties of composites are important parameters for the application of laminates, and many scholars have applied carbon nanotubes (CNTs)...     

A liquid droplet measurement technique as a means of assessing the interlaminar shear strength of fibre-reinforced composites

A liquid droplet measurement technique was used to measure the contact angle for low modulus, medium modulus and high modulus pitch-based carbon fibres. A phase transfer catalytic oxidation treatment and an electrochemical oxidation treatment both resulted in falls in the observed glycerol contact angle. Increases in surface energy values were estimated using an equation-of-state approach. The electrochemical oxidation treatment increased the concentration of oxygen- and nitrogen-bearing groups on the fibre surface (as shown by Auger electron spectroscopy), resulting in a fall in the surface energy values estimated using the droplet measurement technique, and corresponding improvements in composite interlaminar shear strength (ilss) were observed. Thus it was shown that the surface energy values determined using this method could be used to assess changes in ilss due to surface treatment of the fibre, provided only chemical changes resulted from the fibre treatment.     

孔隙对碳纤维/环氧复合材料层合板层间剪切疲劳性能的影响

研究了孔隙对碳纤维增强环氧树脂基复合材料层合板［(±45)/0₄/(0, 90)/0₂］_S的静态层间剪切强度和层间剪切疲劳性能的影响。采用不同的热压罐压力制备了孔隙率为0.4%~6.6%的试样。采用显微照相法和图像分析技术对孔隙率和孔隙的微观形貌进行了分析。研究结果表明, 随着热压罐压力的降低, 大孔隙(S>7.85×10-3mm2)所占的比例逐渐增加, 平均孔隙率增加。在孔隙率为0.4%~6.6%时, 每增加1%, 复合材料层压板的层间剪切强度下降2.4%。随着孔隙率的增加, 层压板的疲劳寿命降低。与静态试验相比, 孔隙率对层压板疲劳性能的影响比对静态性能的影响大。大孔隙的存在促进了疲劳裂纹的产生和扩展。      

基于纤维束增强树脂基复合材料测试的单向层合板层间剪切性能的预估方法

纤维束增强树脂基复合材料（FBC）及其单向层合板在标准Iosipescu剪切实验中表现出非常相似的破坏特征,然而测量得到的剪切强度却有明显差异。本文使用两种碳纤维和两种环氧树脂制备了3种FBC和单向层合板,对FBC剪切强度和单向层合板层间剪切强度进行了测量与分析。应用界面单元方法分析了纤维束与基体之间的界面应力场,发现FBC剪切试件中纤维束/基体界面附近的应力状态为拉剪耦合,而单向层合板中界面处于纯剪切应力状态,这一差异导致FBC剪切实验测量的强度低于单向层合板的剪切强度。本文基于Yamada-Sun强度理论建立了FBC剪切强度与单向层合板剪切强度之间的关系模型,应用该模型预测的单向层合板剪切强度与实测强度之间达到良好的一致性,相对偏差为10%左右。根据本文提出的方法,通过制样较简单的FBC试验能够预测和评估相应单向层合板的层间剪切性能。     

Influence of titanium carbide on the interlaminar shear strength of carbon fibre laminate composites

The potential use of carbon fibre laminate composites is limited by the weak out-of-plane properties, especially delamination resistance. The effect of incorporating titanium carbide to the mesophase pitch matrix precursor of carbon fibre laminate composites on interlaminar shear strength is studied both on carbonised and graphitised composites. The presence of titanium carbide modifies the optical texture of the matrix from domains to mosaics in those parts with higher concentrations and it contributes to an increase of fibre/matrix bonding. This fact produces an increase of the interlaminar shear strength of the material and changes the fracture mode.     

The effect of resin failure strain on the tensile properties of glass fibre-reinforced polyester cross-ply laminates

An investigation has been made of the effect of resin properties on the transverse cracking behaviour of glass fibre-reinforced polyester resin three-ply laminates. The polyester resin properties were modified by the addition of a flexibilizing resin to produce five resin systems with failure stresses ranging from 1.75 to 11.1%. The mechanical properties of the resins which were determined, are observed to affect the stress level at which transverse cracking is initiated and the nature of the cracking behaviour. If fibre bunching is taken into account the Kies strain magnification theory can predict the general trend of the results. However, it is concluded that strain-rate effects associated with fibre bunching are worthy of further investigation.     

Electron irradiation effects on interlaminar shear strength of glass or carbon cloth reinforced epoxy composites

Interlaminar tensile shear tests are conducted to study the degradation mechanisms of electron irradiated glass or carbon cloth reinforced epoxy laminates. Interlaminar shear strength decreases significantly after the dose exceeds 3000 Mrad for glass/epoxy, but remains constant up to 12 000 Mrad for carbon/epoxy. SEM photos reveal that debonding of glass fibres and epoxy matrix (or degradation of silane coupling agents) plays an important role in the dose-dependent strength reduction of glass/epoxy laminates. The decrease in the interlaminar shear strength corresponds to that in the three-point bending strength. On the other hand, the SEM fracture appearance is almost dose-independent for carbon/epoxy laminates. In addition, some preliminary irradiation tests are conducted at –120° C to observe the effects of irradiation temperatures.     

Effect of phenoxy-based coating resin for reinforcing pitch carbon fibers on the interlaminar shear strength of PA6 composites

Carbon fiber-reinforced thermoplastic composites have not been considered as constituent materials for structural parts due to the poor interfacial adhesion between the fiber and the thermoplastic matrix. In this work, polyamide 6 (PA6) composites with pitch carbon fibers (pCF) were fabricated by alternatively stacking PA6 films and pCF fabrics followed by being pressed. In order to improve the interfacial adhesion, phenoxy resin-based materials were coated on the surface of the fiber. The surface analyses of the fiber were carried out by XPS, TGA and dynamic contact angle method. Interlaminar shear strength (ILSS) of the composites was measured to evaluate the effect of the coating materials. The results showed that the composites with the coated pCF had higher ILSS than that with neat pCF by more than 20%. This indicated that a proper coating material can improve mechanical properties of the PA6 composites, which can be applied to the structural parts.     

A size effect law for notched tensile strength of woven carbon/epoxy laminates

Specimen-size effect and notch-size effect on the tensile strength of woven fabric carbon/epoxy laminates are evaluated and modeled. For two different layups of [(0/90)₁₂] and [(±45)₂/(0/90)₅]_S, respectively, static tension tests were performed on two-dimensional geometrically similar unnotched and double-edge notched specimens scaled to three different sizes. Experimental results demonstrate that the notched strength of the woven CFRP laminates depend on the size of specimen as well as the size of notch. The ratio of notched strength to unnotched strength decreases as the length of notch increases, regardless of the size of specimen. For a given size of notch, the notch strength ratio becomes larger with decreasing size of specimen. A notch-size effect law is derived by means of the Neuber interpolation method. A specimen-size effect is embedded into the notch sensitivity parameter involved by the notch-size effect law to establish a size effect law that can cope with these two kinds of size effect. The engineering size effect law proposed can adequately describe the specimen-size effect as well as notch-size effect on the tensile strength of the woven CFRP laminates. It is also demonstrated that the size effect law allows determining the size independent fracture toughness on the basis of notched strengths of small specimens that fail in a quasi-brittle manner.     

Design and analysis of the crack rail shear specimen for mode III interlaminar fracture

The crack rail shear (CRS) specimen is a proposed test method to characterize the Mode III interlaminar fracture toughness of continuous-fiber-reinforced composite materials. The specimen utilizes the two-rail shear test fixture, and contains embedded Kapton film placed symmetrically about the midplane to provide starter cracks for subsequent characterization. Otherwise, specimen length and width are identical to the ASTM shear test specimen geometry. An analytical expression for the strain energy release rate is developed based on a strength of materials approach. The model illustrates the important material and geometric parameters of the test, and provides a simple data reduction scheme for experiments. A quasi three-dimensional, linear elastic finite element code, CCMQ3D, is employed to verify the pure Mode III fracture state and to determine admissible crack lengths. Deformation of the model shows that only the out-of-plane displacement is non-zero, indicating that a pure Mode III fracture state does indeed exist within the constraints of the Q3D assumption. Compliance and strain energy release rate predictions are in good agreement with the strength of materials model over the range of crack lengths, 0·15<a/w<0·85. A fully three-dimensional, linear elastic finite element analysis of the CRS is employed to quantify the effect of finite length on the fracture state. Only intermediate crack lengths are investigated. Crack closure techniques are utilized to determine the components of the strain energy release rate present. Results indicate that a small boundary layer of mixed mode behavior exists at the free edges that diminishes to a pure Mode III fracture state. Compliance and strain energy release rate predictions by the 3D model show good agreement with the Q3D and strength of materials models.     

Effect of strain rate on tensile behavior of carbon fiber reinforced aluminum laminates

In this paper, the tensile behavior of carbon fiber reinforced aluminum laminates (CRALL) has been determined at a strain rate range from 0.001 s^− 1 to 1200 s^− 1. Experimental results show that CRALL composite is a strain rate sensitive material, and the tensile strength and failure strain both increased with increasing strain rate. A linear strain hardening model has been combined with Weibull distribution function to establish a constitutive equation for CRALL at different strain rates. The analysis of the model shows that the Weibull scale parameter, σ₀, increased with increasing strain rate, but Weibull shape parameter, β, can be regarded as a constant.     

Effect of manufacturing methods on the shear strength of composite single-lap bonded joints

The present paper experimentally addresses the effect of manufacturing methods on the strength of composite bonded joints. A total of 391 specimens, manufactured by four different fabrication methods, were tested. For each method, various overlap lengths, adherend thicknesses and lay-up patterns were examined. The failure strength was higher in thicker adherend joints and lower in specimens with larger overlap length. Results showed that the secondary bonded joints had higher strength than the co-bonded and adhesively cocured joints and yielded similar strength compared with the non-adhesive cocured case. Changes in the stacking sequence also affected the interlaminar stresses and failure loads.     

Inluence of water up-take on interlaminar fracture properties of carbon fibre-reinforced polymer composites

Composite materials in practical use can be subjected to a wide variety of different loading conditions. The most important conditions are mechanical stresses and environmental attacks. An issue of major concern in the utilization of composites is associated with the occurrence of delaminations or interlaminar cracks, which may be related to manufacturing defects or are induced in service by low-velocity impacts. The main environmental attacks are temperature, humidity, radiation, and chemical exposure. Three materials were investigated; two thermosetting matrices (unmodified and toughness-modified epoxy, EP and EP_mod) and one thermoplastic matrix (semicrystalline polyetheretherketone, PEEK), all reinforced with unidirectional continuous carbon fibres. Samples of these materials were exposed to water in baths of different temperatures; they were taken for mechanical testing after various time periods. As a result of absorbed moisture, G _IC-values increased with moisture content of the samples, whereas G _IIC-values decreased. By means of scanning electron microscopy, fracture surfaces were examined. Evidence was found that the increase of G _IC-values was due to a greater ductility of the matrix (as a result of the moisture absorbed) and hence more energy-consumptive fibre-bridging. On the other hand, interface failure, as well as a loss of shear strength of the epoxy with increasing amount of moisture absorbed, were responsible for the decrease in the G _IIC-values. The thermoplastic matrix system (CF/PEEK) exhibited no influence of moisture on the Mode I property, but a decrease of the values for Mode II.     

A new specimen geometry to determine the through-thickness tensile strength of composite laminates

The tensile strength in thickness direction is one of the dimensioning parameters for composite load introductions, which are exposed to complex three-dimensional stress states, like e.g. composite lugs. In the present paper a simple test setup which introduces the load into the specimen by a form fit was chosen to determine the through-thickness tensile strength of quasi-isotropic carbon/epoxy laminates. By means of detailed finite element analyses a new quadrilaterally waisted specimen geometry was developed and validated by mechanical testing. The influence of the manufacturing process on the location of failure was investigated and recommendations for future tests are made. Compared to alternative state of the art methods the proposed test method leads to higher accuracy and reproducibility of the determined through-thickness tensile strength.     

Effect of damage on the interlaminar shear properties of hybrid composite laminates at cryogenic temperatures

This paper presents the experimental and numerical characterization of the interlaminar shear failure of hybrid composite laminates at cryogenic temperatures. Cryogenic short beam shear tests were performed on hybrid laminates consisting of woven glass fiber reinforced polymer (GFRP) composites and polyimide films to evaluate their interlaminar shear strength. Microscopic observations of damage accumulation and failure mechanisms were also made on failed specimens. In addition, a progressive damage analysis was conducted to predict the initiation and growth of damage in the specimens, and the interlaminar shear strength was determined from the maximum shear stress in the failure region. The damage effect on the interlaminar shear properties of hybrid laminates at cryogenic temperatures was examined based on the experimental and numerical results.     

Electrical conductivity and interlaminar shear strength enhancement of carbon fiber reinforced polymers through synergetic effect between graphene oxide and polyaniline

Utilizing synergetic effect of different ingredients is an important strategy to design new multi-functional composites. In this work, high-strength graphene oxide and conductive polyaniline were selected to dope into divinylbenzene to fabricate a new type carbon fiber reinforced polymer laminates, where a cooperative improvement of through-thickness electrical conductivity and interlaminar shear strength was observed. With addition of 15 wt% of PANI-GO at the optimized weight ratio of 60:1 in the CF/DVB-PANI-GO, 150% enhancement of the electrical conductivity compared to the CF/DVB-PANI, and 76% enhancement of the ILSS compared to the CF/DVB-GO were realized. Our laminates reach 66% in ILSS of that for the conventional CFRP made of epoxy, but the former features about 103 times higher AC conductivity. The mechanism for such a synergic enhancement for both electrical and mechanical performance was investigated by rheology measurement and scanning electron microscopy, where uniform 3-D network formed by PANI/GO has been clearly observed.