Impact fatigue behaviour of carbon fibre-reinforced vinylester resin composites

Two types of unidirectional carbon fibre, one of high strength (DHMS) and another of medium strength (VLMS) reinforced vinylester resin composites have been examined for their impact fatigue behaviour over 10⁴ impact cycles for the first time. The study was conducted using a pendulum type repeated impact apparatus specially designed and constructed for the purpose. A well-defined impact fatigue behaviour (S-N type curve) curve has been demonstrated. It showed a plateau region of 10–10² cycles immediately below the single cycle impact strength, followed by progressive endurance with decreasing impact loads, culminating in an endurance limit at about 71% and 85% of the single impact strength for DHMS-48 and VLMS-48, respectively. Analysis of the fractured surfaces revealed primary debonding, fibre breakage and pull-out at the tensile zone of the samples and a shear mode of fracture with breakage of fibre bundles at the compressive zone of the samples. The occurrence of a few major macrocracks in the matrix with fibre breakage at the high load-low endurance region and development of multiple microcracks in the matrix, coalescing and fibre breakage at the low-load-high endurance region have been inferred to explain the fatigue behaviour of the composites examined.     

Effects of moisture on the mechanical properties of glass fibre reinforced vinylester resin composites

Glass fibre reinforced vinylester resin composites incorporating varying amounts of fibres (63.5, 55.75, 48.48, 38.63 and 27.48 wt%) were characterized for their mechanical properties both as prepared and after treatment with boiling water for 2, 4, 6, 8 and 24 h. Weights of the samples were found to increase to a saturation at about 8 h with boiling water treatment. In keeping with the composite principle, the mechanical properties improved with fibre loading. However, the properties were relatively inferior when treated with boiling water for longer hours attributing to ingress of moisture by capillary action through the interface between the fibre and the resin matrix. Considering the rates of moisture absorption and correlating with the mechanical properties, it was observed that the deteriorating effects were predominant up to 4 h treatment with boiling water. Estimation of defect concentrations for 63.5 wt% of nascent fibre reinforced composites as well as those composites treated with boiling water for 24 h were 56.93% and 64.16% respectively. Similarly, 27.48 wt% nascent fibre reinforced composites and those composites with boiling water treatment showed the estimation of defect concentrations of 39.94% and 50.55% respectively. SEM study of the fractured surfaces showed heavy fibre pull-out in the tensile zone whilst shear fracture of the fibre bundles was predominant at the compressive zone of the samples tested for flexural strength properties.     

The fatigue properties of weft-knit fabric reinforced epoxy resin composites

Carbon and glass tows were fabricated into Interlock, Full-Cardigan, Milano and Rib fabric stitches by weft-knitting. The fatigue strength of composites made from these fabrics and epoxy resin was studied in terms of the relationship of the knitting stitch and the applied direction, the stress number of the fatigue distribution, hysteresis heating, lost strength, and fatigue damage propagation rate. In addition, by using the above composites, we compared the fatigue strength with plain-weave fabric composites.

The weft-knit composites loaded by fixed cantilever bending showed that the wale direction had a greater stiffness than the course direction. The Interlock fabric composite provides the highest fatigue resistance in these weft-knit stitches. However, the fatigue strength of a plain-weave fabric composite is higher than that of the weft-knit fabric composites. In a fatigue test, the hysteresis heating is below 60°C. On the other hand, the fatigue resistance ability can be increased about 40–60% by a sandwich lamination method.     

纤维增强聚合物基复合材料的低温性能

对纤维增强聚合物基复合材料在低温领域的实际应用进行了分类介绍,通过对纤维增强聚合物基复合材料的低温性能、性能影响因素和作用机理、低温应用安全性等方面的研究工作进行总结,突出各类纤维增强聚合物基复合材料低温下的性能优势,阐明了材料性能的不足之处及相应改进措施.对于实际低温应用中纤维增强聚合物基复合材料的选择、性能设计优化,系统安全性的增强提供了参考作用.     

Static and fatigue characterisation of new basalt fibre reinforced composites

Basalt reinforced composites are recently developed materials. These mineral amorphous fibres are a valid alternative to carbon fibres for their lower cost, and to glass fibres for their strength. In order to use basalt reinforced composites for structural applications, it is necessary to perform a mechanical characterisation. With this aim in the present work experimental results of several static and fatigue tests are described. Two polymeric matrices are taken into account, vinylester and epoxy, to assess their influence on the evaluated parameters. In parallel to these mechanical tests, also the thermal answer of the specimens to mechanical loads is evaluated by means of thermography. This experimental technique allows defining the composite local heating during the application of mechanical loads and its behaviour in details. Final discussion on obtained results is proposed focussing the attention on basalt fibre composite behaviour, and comparing mechanical properties of BFRP with other composite materials in glass and carbon fibres.     

Effects of static–fatigue (tension) on the tension–tension fatigue life of glass fibre reinforced plastic composites

The effects of static–fatigue interaction on tension–tension fatigue life of glass fibre reinforced plastic (GFRP) composites were investigated. This paper proposed a new static–fatigue model, which is capable of predicting residual strength after a period of static loading. Also an algorithm is proposed to calculate fatigue lives with the inclusion of static–fatigue interaction. Predictions from the proposed static–fatigue model show a good agreement with the experimental results. Static–fatigue interaction has shown a considerable effect on fatigue lives of GFRP composites at intermediate and lower applied stress levels possibly due to a longer exposure to applied loads. At higher load levels approximately greater than 65% of ultimate stress, and higher stress ratios range like 0.5 < R < 0.9, fatigue lives shown to be closer to material’s static–fatigue limits which is shorter than the expected lifetime by cyclic fatigue.     

碳纤维/环氧树脂复合材料高速冲击性能

采用树脂传递模塑(RTM)工艺制备碳纤维/环氧树脂复合材料,通过空气炮冲击实验研究树脂韧性和碳纤维类型对复合材料抗高速冲击性能的影响,并对高速冲击后的试样进行压缩性能测试,研究高速冲击损伤对复合材料剩余压缩性能的影响。结果表明:树脂的韧性可以降低复合材料遭受高速冲击时的内部损伤程度,大幅提高复合材料的抗高速冲击性能和冲击后剩余压缩性能;T700S碳纤维增强复合材料抗高速冲击性能优于T800H碳纤维增强复合材料;复合材料的破坏模式与冲击速率有关,冲击速率较低时,复合材料弹击面出现圆形凹坑,背弹面出现鼓包;冲击速率较高时,复合材料弹击面出现圆形通孔,背弹面出现沿纤维方向撕裂断口。     

Recycling of carbon fibre reinforced composites using water in subcritical conditions

In this paper, a method of chemical recycling of thermosetting epoxy composite was discussed. Water was used to be reaction medium and the decomposition of carbon fibre reinforced epoxy composites was studied. Experiments were devised in order to identify the significant process parameters that affect fibre reinforced composite recovery potential including temperature, time, catalyst, feedstock, and pressure. Experiments were performed in a batch-type reactor without stirring. Under the condition that the temperature was 260 °C and the ratio of resin and water was 1:5 g/mL, the decomposition rate could reach 100 wt.% and the carbon fibres were obtained. The results from the Scanning Electron Microscopy (SEM) and Atomic Force Microscope (AFM) measurements showed that the fibres were clean and no cracks or defects were found. The average tensile strength of the reclaimed fibres was about 98.2% than that of the virgin fibres.     

Low-velocity impact response of non-woven hemp fibre reinforced unsaturated polyester composites: Influence of impactor geometry and impact velocity

In this study, the influence of varying impactor geometries on the impact damage characteristics of hemp fibre reinforced unsaturated polyester composites were subjected to a low-velocity impact loading using an instrumented falling weight impact test setup. The three varying tup geometries: hemispherical, 30° and 90°, at four different impact velocity levels: 2.52 m/s, 2.71 m/s, 2.89 m/s and 2.97 m/s were assessed. The experimental results to investigate the influence of impactor geometry suggest that HFRUP composites were able to withstand higher loads when tested with hemispherical impactor and also absorbed more energy than that for 90° and 30° shaped tup geometry. The post impact damage patterns and failure mechanisms of impacted samples were further characterised by ultrasonic (UT) inspection. Impact induced damage characterised by scanning electron microscope (SEM) suggests that damage induced by the impact included a typical failure mechanisms showing matrix cracking, fibre breakage and fibre pullout. As the impact velocity increases the damage to back face of the laminate increased for laminates tested with a hemispherical impactor while it decreased to certain extent for laminates tested with 90° and 30° impactor geometries.     

Impact and subsequent fatigue damage growth in carbon fibre laminates

The influence of the ply stacking sequence on the impact resistance and subsequent O-tension fatigue performance of carbon fibre laminates has been investigated. Drop-weight impact tests were conducted on a range of 16 ply carbon fibre laminates with either all non-woven plies or mixtures of woven and non-woven plies. Damaged coupons were tested in O-tension fatigue for up to 10⁶ cycles, scanned using an ultrasonic probe and then loaded in tension until failure.The impact resistance and subsequent fatigue performance have been found to be sensitive to the ply stacking sequence. The non-woven composites showed a marked sensitivity to impact loading, but increases in residual static strength were noted after cycling. The inclusion of a woven fabric served to improve the impact resistance of the laminates. Fatigue cycling resulted in considerably improved residual static strengths; by 10⁶ cycles any effect of the impact damage had been removed.     

The low velocity impact response of non-woven hemp fibre reinforced unsaturated polyester composites

Hemp fibre reinforced unsaturated polyester composites (HFRUPE) were subjected to low velocity impact tests in order to study the effects of non-woven hemp fibre reinforcement on their impact properties. HFRUPE composites specimens containing 0, 0.06, 0.10, 0.15, 0.21 and 0.26 fibre volume fractions (V_f) were prepared and their impact response compared with samples containing an equivalent fibre volume fraction of chopped strand mat E-glass fibre reinforcement. Post-impact damage was assessed using scanning electron microscopy (SEM). A significant improvement in load bearing capability and impact energy absorption was found following the introduction hemp fibre as reinforcement. The results indicate a clear correlation between fibre volume fractions, stiffness of the composite laminate, impact load and total absorbed energy. Unreinforced unsaturated polyester control specimens exhibited brittle fracture behaviour with a lower peak load, lower impact energy and less time to fail than hemp reinforced unsaturated polyester composites. The impact test results show that the total energy absorbed by 0.21 fibre volume fraction (four layers) of hemp reinforced specimens is comparable to the energy absorbed by the equivalent fibre volume fraction of chopped strand mat E-glass fibre reinforced unsaturated polyester composite specimens.     

连续玄武岩纤维增强环氧树脂基复合材料抗冲击性能研究

制备了连续玄武岩纤维增强的环氧树脂基复合材料靶板,并进行了抗冲击性能测试,研究了影响其抗冲击性能的主要因素及抗冲击机理.结果表明,表面处理会使复合材料抗冲击性能下降;而降低织物面密度、提高纤维体积含量可以使复合材料抗冲击性能得到提高.复合材料靶板的主要能量吸收形式为靶板局部变形、分层和纤维拉伸、剪切断裂及纤维拔脱.     

Interfacial damage in fibre‐reinforced composites subjected to tension fatigue loading

ABSTRACT The interfacial behaviour of fibre‐reinforced composites subjected to tension fatigue loading is studied based on the shear‐lag model. The governing equations of this problem are obtained and solved. In order to describe the interfacial debonding, the Paris fatigue crack growth formula as well as a modified degradation model for the coefficient of friction is adopted. Finally some important values related to interfacial debonding are obtained. In the present investigation, Poisson's contraction is considered.     

Effect of filler addition on the compressive and impact properties of glass fibre reinforced epoxy

Flyash is incorporated in glass fibre reinforced epoxies to study their response to the filler addition. Low cost of flyash can reduce the overall cost of the component. Only very low volume fractions of filler are investigated in the present study. To obtain further clarification of the observed phenomenon, another abundantly available low cost material, calcium carbonate is incorporated in one set of the specimens. Compressive strength of the material is found to decrease, whereas steep increase in impact strength is observed by introduction of very small quantity of fillers. Specimens containing calcium carbonate are tested for impact properties only. Effect of specimen aspect ratio on the compressive strength values is also studied by testing specimens of three different aspect ratios. Scanning electron microscopic observations are taken to develop a better understanding of the phenomena taking place in the material system at microscopic level.     

Comparison of low cycle fatigue performance of several [0] Sigma fibre reinforced titanium matrix composites

The low cycle fatigue behaviour of four SiC fibre reinforced titanium matrix composites (TMCs) is examined at temperatures relevant to gas-turbine compressor components. It is found that, in the high cyclic stress/short life regime, the fatigue performance of the four TMCs is similar at each temperature. At lower stresses, however, more relevant to the gas-turbine compressor application, fatigue lives of Ti-6-4/SM1240, Timetal 834/SM1140+ and Timetal 834/SM1240 TMC all become significantly inferior to those of Ti-6-4/SM1140+ at temperatures of 450°C and above. This inferiority is attributed to intergranular embrittlement of the Timetal 834 matrix, the high creep resistance of the Timetal 834 matrix and due to an as yet unexplained weakening of the SM1240 fibre.     

Influence of low velocity impact on fatigue behaviour of woven hemp fibre reinforced epoxy composites

The purpose of this work is to study the resistance to low velocity impact of woven hemp/epoxy matrix composites and the influence of impact damage on their residual quasi-static tensile and cyclic fatigue strengths. Impact characteristic parameters were evaluated and critically compared to those found in the literature for other similar composites. Damage mechanisms were analysed by using AE monitoring and microscopic observations. An analytical model is used to predict the fatigue lifetime of impacted specimens. Moreover a damage scenario is proposed, reduced to two phases in post-impacted fatigue behaviour, instead of three phases for non impacted specimens.     

Multiaxial fatigue of a short glass fibre reinforced polyamide 6.6 – Fatigue and fracture behaviour

The multiaxial fatigue behaviour of a short glass fibre reinforced polyamide 6.6 (PA66-GF35) is investigated on hollow tubular specimens in the range of fatigue lives between 10² and 10⁷ cycles. Fatigue experiments included pure tension, pure torsion, combined tension–torsion at different biaxiality ratios and phase shifting angles between the stress components. Tests were carried out with load ratio R = 0 and R = −1 at room temperature as well as at 130 °C. The influence of biaxiality ratio, phase angle between load components and load ratio is discussed.An extensive analysis of the fracture behaviour is performed on the specimens to recognise the crack nucleation and propagation mechanisms; failure modes were evaluated via optical and scanning electron microscopy.     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—II. impact behaviour

Quasi-static, low-hanging and high-velocity impact tests have been conducted in order to study the effect of fibre/matrix adhesion on the impact properties of fibre-reinforced metal laminates. Differences in fibre/matrix adhesion were achieved by using treated or untreated carbon fibres in an epoxy resin system. Chemical removal of the aluminium layers and a sectioning technique were applied to examine and characterize the impact damage in the laminates. The results show that the laminates with the weaker fibre/matrix adhesion exhibit larger damage zones, although the back face crack length and permanent indentation after impact are smaller for a given impact energy. Residual tensile strength after impact is also higher for the untreated fibre laminates due to increased fibre/matrix splitting in the composite layer.     

Numerical investigation on steel fibre reinforced cementitious composite panels subjected to high velocity impact loading

Steel fibre reinforced cementitious composite (SFRCC) panels are numerically investigated for their performances under high velocity impact of short projectiles. Numerical responses are obtained using advanced constitutive material model of Riedel–Hiermaier–Thoma (RHT) for cementitious materials and adopting appropriate modelling techniques. Effects of steel fibre volume and the thickness of panels on the impact performance are mainly highlighted in this paper. Various characteristics phenomenon during impact on cementitious composite panels namely, spalling, cracking, scabbing and perforation, are captured which is a difficult task. Scabbing is likely to occur when tensile stresses at the back face of the panel exceed dynamic tensile strength of the material. Various critical aspects in numerical modelling like boundary conditions, material input parameters, and handling severe distortion of the Lagrangian based finite elements are appropriately explained. Design chart is also developed to determine optimum fibre volume and thickness for an impact energy level up to 2.2 kJ. The numerically predicted impact responses are found to corroborate well with experimental results.     

Finite–element analysis of selectively SiC fibre–reinforced titanium composites (II)

Finite-element analysis has been carried out to predict the through-thickness stresses generated at the transition boundary between monolithic cladding layers and SiC fibre reinforced titanium matrix composite plates. Such stresses have been shown in studies elsewhere to promote premature transverse damage near the transition region ahead of a mode I crack propagating within the cladding layer. Results obtained in DEN tension are presented to demonstrate the influence of thermal residual stress, external loading, and the relative thicknesses of the cladding material and the composite regions, on the through-thickness stresses. A dimensionless parameter S has been constructed and is demonstrated to provide a useful tool for predicting the potential for through-thickness damage in a range of testpiece dimensions and levels of selective reinforcement. This revised version was published online in August 2006 with corrections to the Cover Date.