Effect of Moisture and Temperature on the Compressive Failure of CCF300/QY8911 Unidirectional Laminates

CCF300/BMI composites are relevant materials for supersonic aircraft due to their high specific properties. However in aeronautical applications, the composites are exposed to severe environmental conditions, and it is known that hot and humid environments can degrade some aspects of the material performance especially the compressive strength. In this paper, the effect of moisture and temperature on the compressive failure of unidirectional CCF300 carbon fiber reinforced bismaleimide(BMI) matrix composites were studied. Also scanning electron microscope (SEM) was employed for fractographic investigations. It is observed that the plastic deformations at the fiber/matrix and interlaminar interface as well as residual stresses lower the compressive strength of the material. The failure of specimens tested in hot and wet conditions always occurs as a result of out-of-plane microbuckling that is attributed to the reduction of matrix strength. In addition, the fiber microbuckling model, fiber kinking model and combined model were employed for the compressive strength prediction of the UD CCF300/QY8911 composites subjected to different environment conditions. The comparison was done between these models. Results show that the combined model is more suitable for the compressive strength prediction of CCF300/QY8911 composite systems when suffering severe environment conditions.     

Influence of moisture absorption on the flexural properties of composites made of epoxy resin reinforced with low-content iron particles

In this work, the effect of moisture absorption on the mechanical properties of particulate composite materials is studied. Moisture absorption constitutes a main parameter affecting the thermomechanical behaviour of composites, since it causes plasticization of the polymer matrix with a concurrent swelling. In the present work, the influence of water absorption on the flexural properties of particle-reinforced composites was thoroughly investigated. It was found that during the process of moisture absorption there exists a variation of the flexural properties closely related to the degradation of the mechanical behaviour of the composite, as well as the percentage amount of moisture absorbed. Experiments were carried out with composite made of epoxy resin reinforced with low-content iron particles. The variation of ultimate stress, breaking strain, deflection, elastic modulus and Poisson ratio due to water absorption was examined.     

Hygrothermal effects on the dynamic compressive properties of graphite/epoxy composite material

The effects of temperature and moisture on the response of graphite/epoxy laminated composites to high strain rate penetration loading using the Split Hopkinson Pressure Bar apparatus was investigated. The results show that in the thickness direction loading under extreme temperature, moisture and combined moisture and temperature conditions, the compressive strength, elastic modulus, and energy absorbed decrease exponentially. Failure strain and displacement increase linearly with temperature and moisture with particle velocity increasing linearly with temperature but independent of moisture content. The combined effect of temperature and moisture on the damage process was more apparent than the effect of temperature or moisture acting alone. At the same impact energy, the results show the failure properties to be sensitive to the strain rate, with energy absorbed increasing linearly with strain rate at low temperature and remaining relatively constant at high temperature. The compressive yield strength increases as the strain rate increases both at low and high temperatures while the ultimate strength (maximum strength) decreases slightly with strain rate.     

Effect of post-curing on properties of carbon fibre–epoxy composites

Abstract

The effect of post-curing on the moisture absorption characteristics of Fibredux 914/T300 carbon fibre–epoxy composites, and hence on their thermomechanical behaviour, has been examined. Laminates 1 mm thick were post-cured at 190 or 210°C for 4 or 10 h. The various cross-link densities thus established had almost no effect on the moisture absorption behaviour. Interlaminar shear strength and torsion pendulum tests gave similar results, in that the cross-link density had almost no influence on the dynamic shear modulus or the mechanical dissipation factor. From these findings, environmental degradation of the composite is shown to depend on the content of absorbed water. The behaviour of the composite in hot, humid conditions therefore cannot be improved by post-curing treatment.

MST/400     

Effect of hot water immersion on the performance of carbon reinforced unidirectional poly(ether ether ketone) (PEEK) composites: Stress rupture under end-loaded bending

This paper describes the behaviour of AS4 and T700SC reinforced PEEK composites (SUPreM™ and ACP-2) under applied compressive bending strain. The effect of an increased molecular weight of the polymer matrix on the residual time under endloaded compression bending conditions is studied. Generally for a given composite material, the higher the testing temperature and the applied strain the faster the failure occurs. At test temperatures exceeding the glass transition temperature or at high strain ratios the time-to-failure for CF/PEEK composites follows a master curve. The residual times under endloaded compression bending conditions increase with increasing toughness of the PEEK matrix but decrease with increasing tensile strength of the reinforcing fibres. It seems that the better the fibre/matrix adhesion the lower is the time to failure of an endloaded composite, because more load is transferred from the matrix into the fibres.In order to simulate composite applications under ‘harsh’ conditions the CF/PEEK composites have been exposed to boiling water. PEEK is known to be highly resistant to environmental effects, but water uptake significantly influences the overall performance of CF/PEEK composites under endloaded compression bending conditions. The tensile properties of the composites have been measured as function of exposure time in boiling water. The fibre dominated uniaxial tensile strength is not/or only slightly affected by the boiling water conditioning even after extended exposure times but the transverse tensile strength decreases significantly after exposure to boiling water. The performance of SUPreM™ CF/PEEK-150 and 450 composites under endloaded compression bending conditions are positively affected by water conditioning whereas APC-2 fails at shorter residual times. The fracture behaviour under endloaded conditions is also affected by the ingress of water into the composite.The obtained results show clearly that applications of thermoplastic composites leading to large out of plane deformations can only be ‘safe’ if the maximum service temperatures of the finished part will be well below the glass transition temperature of the polymer matrix otherwise even at low bending radii a dramatic failure of the material cannot be excluded.     

Effect of humidity during manufacturing on the interfacial strength of non-pre-dried flax fibre/unsaturated polyester composites

The influence of moisture content in the environment during manufacture of a novel cobalt-free UP matrix reinforced with flax fibres, on the fibre–matrix adhesion was studied. Flax surface energy was experimentally determined by measuring contact angles on technical fibres, using the Wilhelmy technique and the acid–base theory. The mechanical strength of the interface under different humidity conditions was characterized by the critical local value of interfacial shear stress, τ_d, at the moment of crack initiation, which was assessed by single-fibre pull-out tests. Differential scanning calorimetry and X-ray photoelectron spectroscopy analysis gave further insight into the topic. The results suggest that the effect of humidity during manufacturing on the composite interface might be limited. However, longitudinal composite strength decreased somewhat for composites produced in humid conditions, showing that there is some detrimental effect of high levels of moisture during cure on the fibre mechanical performance, likely caused by some fibre degradation.     

Hot-drawing of single and multiwall carbon nanotube fibers for high toughness and alignment

We report a new hot-drawing process for treating wet-spun composite fibers made of single- and multiwall carbon nanotubes and poly(vinyl alcohol). As shown in previous reports, untreated composite nanotube fibers exhibit a very large strain-to-failure, and their toughness, which is the energy needed to break the fibers, exceeds that of any other known materials. However, untreated composite nanotube fibers absorb a very small amount of energy at low strain and become degraded in humid conditions. In this work, we use hot-drawing treatments, a concept inspired from textile technologies, to improve the properties of nanotube/PVA fibers. This treatment yields a crystallinity increase of the PVA and an unprecedented degree of alignment of the nanotubes. These structural modifications lead to a markedly improved energy absorption at low strain and make the fibers resistant to moisture. Hot-drawn nanotube/PVA fibers hold great potential for a number of applications such as bulletproof vests, protective textiles, helmets, and so forth.     

Effect of temperature and humidity on transient hygrothermal stresses during moisture desorption in laminated composite plates

Polymer matrix composites are relevant materials for future supersonic aircraft due to their high specific properties. However, in such aeronautical applications, the material is exposed to severe environmental conditions. The present paper aims at assessing an approximate model to evaluate hygrothermoelastic stress in composite laminated plates during moisture desorption taking into account the change of mechanical characteristics induced by the variation of temperature and moisture. The developed method permits us to calculate such stresses during desorption phase without the computation of the moisture concentration, through laminated plates. It observed through this study that the variation of elasticity modulus due to the temperature causes a stress relaxation. These stresses have to be taken into account for the design of composite structures submitted to a moist environment.

Through the presented study, we hope to contribute to the understanding of hygrothermal behaviour of composite laminated plates.     

Moisture absorption by structural epoxy-matrix carbon-fiber composites

The effect of absorbed moisture on the hygrothermal properties has been investigated by dynamic mechanical spectroscopy (dms) and flexural modulus testing for four commercial epoxy-matrix graphite-fiber composite materials. The dms technique was shown to be more convenient and accurate than conventional flexural testing in determining the ‘wet T_g’ of composites for materials qualification/acceptance or resin development work. The absorption kinetics followed Fickian behavior and the depression of T_g and the diffusion coefficients were comparable to the results reported in the past for similar epoxy-matrix composites. A thermodynamic analysis of the data indicated that plasticization was due entirely to conformational dilution of the matrix network.     

C/C复合材料销钉准静态和动态剪切性能

根据复合材料销钉剪切试验的需要,设计剪切试验装置。利用电子万能试验机和落槌冲击试验系统完成C/C复合材料销钉在准静态和动态加载工况下的面内剪切力学性能试验,并通过SEM试验系统分析其剪切失效模式和失效机制。结果表明:C/C复合材料销钉抗剪切强度具有明显的应变率效应,随着加载速率的增加,其抗剪切强度显著提高;C/C复合材料在不同加载速率下失效模式不同,准静态加载工况下纤维与基体严重剥离,纤维束丧失整体承载能力,其破坏过程表现出“伪塑性”失效特征;动态加载工况下纤维与基体未发生明显剥离,纤维束整体承载,其破坏过程表现为“脆性”失效特征。C/C复合材料在不同加载速率下剪切失效模式的不同可归结为内部缺陷扩展的应变率效应。      

Processing and hygrothermal effects on viscoelastic behavior of glass fiber/epoxy composites

Fiber reinforced epoxy composites are used in a wide variety of applications in the aerospace field. These materials have high specific moduli, high specific strength and their properties can be tailored to application requirements. In order to screening optimum materials behavior, the effects of external environments on the mechanical properties during usage must be clearly understood. The environmental action, such as high moisture concentration, high temperatures, corrosive fluids or ultraviolet radiation (UV), can affect the performance of advanced composites during service. These factors can limit the applications of composites by deteriorating the mechanical properties over a period of time. Properties determination is attributed to the chemical and/or physical damages caused in the polymer matrix, loss of adhesion of fiber/resin interface, and/or reduction of fiber strength and stiffness. The dynamic elastic properties are important characteristics of glass fiber reinforced composites (GRFC). They control the damping behavior of composite structures and are also an ideal tool for monitoring the development of GFRC’s mechanical properties during their processing or service. One of the most used tests is the vibration damping. In this work, the measurement consisted of recording the vibration decay of a rectangular plate excited by a controlled mechanism to identify the elastic and damping properties of the material under test. The frequency amplitude were measured by accelerometers and calculated by using a digital method. The present studies have been performed to explore relations between the dynamic mechanical properties, damping test and the influence of high moisture concentration of glass fiber reinforced composites (plain weave). The results show that the E’ decreased with the increase in the exposed time for glass fiber/epoxy composites specimens exposed at 80^∘C and 90% RH. The E’ values found were: 26.7, 26.7, 25.4, 24.7 and 24.7 GPa for 0, 15, 30, 45 and 60 days of exposure, respectively.     

High-temperature mechanical response of A359–SiCp–30%: torsional loading (II)

Abstract

Aluminum metal–matrix composites are lightweight materials that have the potential to supplant steel in many applications. The current work helps to identify the parameters that confer maximal strength and ductility. Torsion tests were performed on the as-cast aluminum metal–matrix composite A359–SiCp–30% at a variety of temperatures and twist rates. Dependence of material properties on temperature and strain rate were identified from equivalent stress–strain curves constructed from the reduced data. Examination of the microstructure was performed on the as-cast material and on fracture surfaces. A temperature- and strain rate-dependent constitutive model was applied to simulation of the mechanical response of the torsion specimen. Trends in material properties corroborate and extend trends identified previously under tensile loading with regard to temperature and strain rate dependence. Shear properties of simulated specimens agree with properties obtained through experimentation.     

Compressive response of glass–fiber reinforced polymeric composites to increasing compressive strain rates

The utilization of composite materials instead of traditional materials in structural high-speed applications has induced the need for a proper knowledge of dynamic behavior as well as static behavior of them. The material and structural response vary significantly under dynamic loading as compared to static loading conditions. In order to investigate the dynamic responses of composite materials under dynamic loading at various strain rates, special testing machines are needed. Most of the researches in this field are focused on applying real loading and gripping boundary conditions on the testing specimens.The present study is carried out in order to characterize the compressive properties of unidirectional glass–fiber reinforced polymeric composites using a servo-hydraulic testing apparatus at varying strain rates, ranging from 0.001 to 100 s⁻¹. For performing practical tests, a jig and a fixture are designed and manufactured, which could insure the alignment of axial loads on the specimens. During of tests, the performance of the test jig is evaluated. It is found that the designed jig and the fixture perform very well during the test process. The results of the dynamic tests are compared with the results of the static tests carried out on specimens with identical geometry. Based on the experimental results obtained from the tests, empirical functions for the mechanical properties are proposed in terms of strain rates. The results of the study indicate that strain rate has a significant effect on the material response. It is found that the compressive strength and modulus both increased with increasing the strain rate. Also, the results show that the compressive strain to failure is generally insensitive to strain rate.     

构型参数及方式对Al2O3p/高锰钢球形网络复合材料压缩性能的影响

传统的耐磨金属基复合材料普遍存在塑韧性低的问题。对氧化铝颗粒(Al₂O₃p)增强高锰钢复合材料进行球形网络构型设计,研究了构型方式、参数及热处理对复合材料压缩性能的影响。制备了3种构型参数(球径?分别为6 mm、7 mm、8 mm)结合两种构型方式(平行、错落)的Al₂O₃p/高锰钢球形网络复合材料、均匀复合材料和基体材料。结果表明：同构型方式下,随着构型参数(复合区体积分数)的增加,材料的压缩性能降低,其中?6材料的屈服强度、抗压强度和(抗压强度下)应变最佳,相比于均匀复合材料分别提升203.8%、236.1%和134.8%,屈服强度相比于基体材料提升107.5%;同构型参数下,错落排布比平行排布的屈服强度、抗压强度和应变分别提升10.9%、28.5%和16.3%;水韧处理后,错落排布材料的屈服强度降低35.2%,抗压强度提升11.0%,应变提升163.1%。裂纹易在基体区与复合区界面处萌生并进行扩展,基体能够阻碍裂纹的扩展;错落排布增大了复合区的最小间距,提升了塑性。     

Effect of moisture absorption on the mechanical behavior of carbon fiber/epoxy matrix composites

A carbon fiber/epoxy unidirectional laminated composite was exposed to a humid environment and the effect of moisture absorption on the mechanical properties and failure modes was investigated. The composites were exposed to three humidity conditions, namely, 25, 55, and 95 % at a constant temperature of 25 °C. The carbon fiber–epoxy laminated composites for two different carbon fiber surface treatments were used. The results showed that the mechanical properties differ considerably for each fiber surface treatment. The application of a coupling agent enhanced the fiber-matrix adhesion and reduced dependence of the properties on humidity. The damage mechanism observed at micromechanical level was correlated to acoustic emission signals from both laminated composites. The untreated carbon fiber failure mode was attributed to fiber-matrix interfacial failure and for the silane-treated carbon fiber reinforced epoxy laminate attributed to matrix yielding followed by fiber failure with no signs of fiber-matrix interface failure for moisture contents up to 1.89 %.     

Environmental durability of z-pinned carbon fibre–epoxy laminate exposed to water

The influence of z-pins on the water absorption properties of a quasi-isotropic carbon fibre–epoxy laminate is assessed. Fibrous composite pins accelerate the moisture absorption rate and increase the total absorbed moisture concentration when the laminate is immersed in water. However, the moisture absorption properties of the laminate are not affected significantly by pins when exposed to hot and humid air. Water diffusion into the z-pinned laminate is aided by interfacial cracks between the pins and laminate. Also, the axial alignment of fibres within the composite pins in the through-thickness direction increases the water absorption rate. Pin pull-out tests reveal that water absorption reduces the mode I crack bridging traction load generated by pins by reducing the shear strength of the pin-laminate interface. This indicates that the mode I delamination toughness induced by pinning is weakened by moisture absorption.     

The effect on the tensile properties of PIP-processed SiC/SiC composite of a chemical vapor-infiltrated SiC layer overlaid on the pyrocarbon interface layer

A chemical vapor-infiltrated (CVI) SiC layer is often deposited on the pyrocarbon (PyC) fiber–matrix interface layer in SiC fiber-reinforced SiC matrix (SiC/SiC) composites. It is normally applied to protect the PyC layer from reacting with molten Si or sintering aids during manufacturing, and to guard against the effects of high temperature, oxidation and moisture during use. In this study, we investigated the effect of this SiC layer on the tensile properties of a composite. Tensile tests of our composite samples showed the SiC layer to have no noticeable effects on its ultimate load or fracture strain, whereas it decreased the load-to-strain ratio and proportional limit. The test results were analyzed by carrying out element tests on filaments and fiber bundle samples, fracture mirror analysis of pullout fibers, and finite element analysis (FEA) of residual thermal stress around the interface.     

Mechanical behaviour of glass and carbon fibre reinforced composites at varying strain rates

The choice of composite materials as a substitute for metallic materials in technological applications is becoming more pronounced especially due to the great weight savings these materials offer. In many of these practical situations, the structures are prone to high impact loads. Material and structural response vary significantly under impact loading conditions as compared to quasi-static loading. The strain rate sensitivity of both carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) are studied by testing a single laminate configuration, viz. cross-ply [0°/90°] polymer matrix composites (PMC) at strain rates of 10⁻³ and 450 s⁻¹. The compressive material properties are determined by testing both laminate systems, viz. CFRP and GFRP at low to high strain rates. The laminates were fabricated from 48 layers of cross-ply carbon fibre and glass fibre epoxy. Dynamic test results were compared with static compression test carried out on specimens with the same dimensions. Preliminary compressive stress–strain vs. strain rates data obtained show that the dynamic material strength for GFRP increases with increasing strain rates. The strain to failure for both CFRP and GFRP is seen to decrease with increasing strain rate.     

碳/环氧树脂复合材料应变率效应的实验研究

选择两种铺设方式( _S和 _S)的T300/Epoxy(炭纤维/环氧树脂)层合板, 利用MTS试验机以及Hopkinson拉伸杆分别对其进行了准静态拉伸试验(应变率为10-5~10-4 s-1)、 中应变率拉伸试验(应变率为100 ~101s-1)和高速冲击拉伸试验(应变率为102~104s-1)。静态、 动态实验的试件形状及尺寸均相同。获得了不同应变率加载条件下T300/Epoxy的应力-应变曲线。基于所获得的应力-应变曲线, 讨论了应变率对炭纤维增强复合材料力学性能的影响。研究结果表明: 复合材料T300/Epoxy是应变率相关的材料; 层合板的铺设方向对其应变率效应有着显著的影响; 随着应变率的增加, 材料的强度及弹性模量有较大程度的提高, 但破坏应变有所降低。通过对试验结果的数据拟合, 提出了材料应变率相关的动态本构模型。      

Fractographic interpretation of failure mechanisms in titanium matrix composites

Titanium matrix composites (TMC) offer a combination of good mechanical properties and high temperature durability that make them attractive candidate materials for advanced engine components and high temperature structural applications. In such applications the material will be subjected to changing mechanical loads and temperature fluctuations, resulting in complex stress states within the constituents of the composite. This study examines how the various loading conditions on the TMCs are reflected in the fracture behaviour to gain insight into the damage mechanisms active in these materials. A fractographic study was conducted on several different TMC specimens, fabricated from Ti-15-3 and Timetal 21S alloys and SCS-6 fibres, that have been subjected to various thermomechanical loading conditions. The analysis showed that the Ti-15-3 composites were more susceptible to damage during sustained load at elevated temperature than the Timetal 21S composites. For both materials, striations only appear during elevated temperature fatigue when the residual processing stresses are relieved. During the Generic Hypersonic Flight Profile (GHFP) tests, the higher temperatures of the Mission 1 profile induce more damage in the Timetal 21S composites. The striations appear in the composites containing centre holes, unlike the unnotched specimens, indicating that the combined effect of stress concentration due to the hole and an underdeveloped fibre bridging zone may have resulted in crack closure.