Short thermal fatigue crack propagation behavior of alumina short fibre reinforced aluminum matrix composites

Thermal fatigue resistance is one of the most important parameters to design engine materials. The thermal fatigue crack growth behavior of alumina short fibre (V _f = 18 vol.%) reinforced AlSi12CuMgNi aluminum alloy composite has been investigated under thermal cycling condition between room temperature and 280 °C. Initiation and propagation of thermal fatigue crack have also been discussed. The results show that in the range of short crack, the fibres play an important role in the path of thermal fatigue crack, and the crack propagation rate of composites is much larger than that of the matrix alloy.     

Experimental characterisation of fibre failure and its influence on crack growth resistance in fibre reinforced titanium metal matrix composites

Abstract

The present paper addresses the effects of fibre failure on the fatigue crack growth resistance of a Ti-6AI-4V (wt-%) alloy matrix unidirectionally reinforced with continuous Sigma (SM1240) SiC fibres. Fibre fracture was monitored in situ using a PAC Locan acoustic emission (AE) analyser, and the exact spatial locations of the individual fibre failure events were identified using novel experimental techniques. A fibre probe technique has been illustrated to be a viable method with which to identify whether a fibre is broken or remains intact within a testpiece. Examination of exact spatial locations of fibres is possible, and evidence suggests that individual fibre failure is of ten followed by another fibre failure within the same row of a single mat lay up. Experimental observations and AE data reveal that crack arrest occurs if relatively few fibres fail in the crack wake as they are breached by matrix fatigue crack growth, and that fibre failure occurs only in the crack wake and behind the growing fatigue crack tip.     

Fracture toughness and reliability in high-temperature structural ceramics and composites: Prospects and challenges for the 21st century

The importance of high fracture toughness and reliability in Si₃N₄, and SiC-based structural ceramics and ceramic matrix composites is reviewed. The potential of these ceramics and ceramic matrix composites for high temperature applications in defence and aerospace applications such as gas turbine engines, radomes, and other energy conversion hardware have been well recognized. Numerous investigations were pursued to improve fracture toughness and reliability by incorporating various reinforcements such as particulate-, whisker-, and continuous fibre into Si₃N₄ and SiC matrices. All toughening mechanisms, e.g. crack deflection, crack branching, crack bridging, etc essentially redistribute stresses at the crack tip and increase the energy needed to propagate a crack through the composite material, thereby resulting in improved fracture toughness and reliability. Because of flaw insensitivity, continuous fibre reinforced ceramic composite (CFCC) was found to have the highest potential for higher operating temperature and longer service conditions. However, the ceramic fibres should display sufficient high temperature strength and creep resistance at service temperatures above 1000°C. The greatest challenge to date is the development of high quality ceramic fibres with associate coatings able to maintain their high strength in oxidizing environment at high temperature. In the area of processing, critical issues are preparation of optimum matrix precursors, precursor infiltration into fibre array, and matrix densification at a temperature, where grain crystallization and fibre degradation do not occur. A broad scope of effort is required for improved processing and properties with a better understanding of all candidate composite systems.     

Fracture toughness characterisation of a glass fibre‐reinforced plastic composite

By examining the state of the art, it can be realised that few research works have been done on the fracture behaviour of plastic composites reinforced with continuous glass fibres. Therefore, the present paper deals with the fracture toughness of a unidirectional glass fibre‐reinforced plastic (GFRP), such a parameter being analytically determined by means of the modified two‐parameter model (MTPM). The input data of the MTPM are obtained from an experimental campaign related to three‐point bending tests on single edge‐notched specimens characterised by different sizes. The novelty of this research work is that the MTPM, originally proposed for isotropic materials, is here employed to estimate the fracture toughness of GFRPs characterised by orthotropic mechanical properties.     

Investigation of the mechanical behaviour of magnesium composites

Stress/strain and fracture toughness behaviour of a commercial heat-treatable magnesium alloy reinforced with up to 20 volume% short alumina fibres was studied at room and elevated temperatures. Microscopic examination of the composites, which were prepared by conventional squeeze casting, revealed damage of a small portion of the fibres during the infiltration process. Sufficient chemical reaction between the matrix alloy and alumina reinforcement tends to produce a good bond at the fibre/matrix interface. The tensile-related properties of the composites increased at room and elevated temperatures with increasing content of the reinforcement. The ductility and fracture toughness of the composites decreased at room temperature with increasing reinforcement content. While failure strains of the composites were slightly improved at higher testing temperatures, the fracture toughness decreased significantly as the testing temperature exceeded 100°C. Examination of the fracture surfaces of specimens tested at room temperature showed a mixed mode fracture appearance with predominantly brittle cleavage fracture. The fracture surfaces of specimens tested at temperatures above 100°C revealed increasing fibre/matrix interface debonding and fibre pull-out with increasing testing temperature. Micromechanism examinations of crack initiation and propagation indicated that the fracture process of the composites may be matrix controlled.     

The fracture toughness of composites reinforced with weakened fibres

Fibre fractures which occur near, but not at, the plane of matrix failure in a composite, lead to fibre pull-out during fracture. Energy absorbed in this process contributes directly to the work of fracture and hence to the toughness of the composite.Factors which determine the mean length of fibre pulled out during fracture are discussed for the case of composites reinforced with continuous fibres having variously spaced points of weakness. The presence of such weak points also affects the strength of the composite, but not all composites of the same strength have the same toughness. The greatest toughness for a given strength is always found in composites reinforced with discontinuous fibres.     

The impact strength of fibre composites

Two models have been developed which predict the crack initiation energy, notched impact strength and unnotched impact strength of fibre composites. One is applicable to composites containing short fibres and the other to composites containing long fibres. Data obtained with randomly oriented short fibre composites were consistent with the one model. The other model has been verified using composites containing uniaxially oriented long fibres and long fibres oriented randomly in a plane. The success of the model demonstrates that the high notched impact strength with long fibres is due to the redistribution of stress away from the stress concentrating notch, the extra stress that can be held by the fibre relative to the matrix and the work required to pull fibres out of the matrix during crack propagation. The parameters which have been shown to control the fracture energy are composite modulus, fibre length, fibre volume fraction, effective fibre diameter, fibre tensile strength and the coefficient of friction during fibre pull-out from the matrix. The matrix toughness on the other hand usually has no effect at all for composites containing fibres randomly oriented in two dimensions and only a minor effect in exceptional cases. The shear strength of the fibre-matrix bond has only an indirect effect in that it controls the number of fibres which pull out rather than fracture.     

Comparison of Interlaminar Fracture Toughness in Unidirectional and Woven Roving Marine Composites

This paper investigates the effect of fibre lay-up and matrix toughness on mode I and mode II interlaminar fracture toughness (G_Ic and G_IIc) of marine composites. Unidirectional and woven roving fibres were used as reinforcements. Two vinyl ester resins with different toughness were used as matrices. Results from both modes showed toughness variation that is consistent with matrix toughness. Values of G_Ic were not significantly influenced by fibre lay-up except at peak load points in the woven roving/brittle-matrix composite. Each peak load point, caused by interlocked bridging fibres, signified the onset of unstable crack growth. For unidirectional specimens, crack growth was stable and G_Ic statistically more reliable than woven roving specimens, which gave fewer G_Ic values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, G_IIc was higher in woven roving composites. This was due to fibre bridging, perpendicular to the crack growth direction, which encouraged stable crack growth and increased energy absorption. Mode II R-curves were obtained for the woven roving specimens. These R-curves provide additional information useful for characterising delamination resistance. The paper concludes that composites with woven roving fibres show similar mode I delamination characteristics to the unidirectional composites; but their mode II delamination characteristics, after crack initiation, are quite different.     

Fracture toughness of unidirectional fibre reinforced composites in MODE II

Experimental investigations have been performed on unidirectional glass fibre reinforced/epoxy composites in Mode II (Forward shear) with the presence of crack parallel to the fibres direction through the use of end-cracked beam. A concentrated load at the Centre of the beam produced bending-induced shear deformation at the crack tip. Calibration factors for Mode II have been obtained. The stress-intensity factor at instability $\text{K}$_IIR(INR) is obtained by experiments on a small end cracked beam through a compliance matching procedure. The crack growth resistance at instability and the corresponding critical strain energy release rate are independent of initial crack in the range of crack length investigated. In composite materials, fibre-matrix interfacial shear stress play an important role in load transfer mechanism: hence Mode II study may be very useful to analyse the interfacial mechanisms and to understand the fracture behaviour of unidirectional fibre reinforced composites in Mode I when load is applied in the direction of the fibres.     

Early fatigue damage in carbon-fibre composites observed by electrical resistance measurement

Early fatigue damage during the first tenth (or less) of the fatigue life was observed in carbon fibre composites by d.c. electrical resistance measurement. The damage was most severe in the first loading cycle and the incremental damage in each subsequent cycle diminished cycle by cycle. For the continuous carbon fibre carbon-matrix composite, the resistance increased irreversibly during early fatigue due to matrix damage and possibly fibre fracture as well. For the short carbon fibre polymer-matrix and cement-matrix composities, the resistance decreased irreversibly during early fatigue due to matrix damage near the junction of adjacent fibres and the resulting increase in the chance that adjacent fibres touched one another.     

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.     

Influence of residual stresses and interfacial shear strength on matrix properties in fibre-reinforced ceramic matrix composites

Zircon matrix composites, uniaxially reinforced with a variety of SiC fibres were fabricated in order to create composites with different interfacial properties. Interfacial properties were varied by changing the nature of fibre coatings. The effect of changes in interfacial shear strength on important matrix properties, such as hardness and fracture toughness, was studied on a micro-scale using the microindentation technique. In addition, the relative orientation of the indented cracks with respect to the fibres was varied to investigate the existence of anisotropic behaviour of the matrix. The results indicated that the crack growth in the matrix was influenced by the presence of residual radial and axial stresses, such that relatively higher crack lengths were seen in certain directions in the matrix with respect to other directions. This asymmetric nature of the crack formation upon indentation was the reason for the observed anisotropic fracture toughness of the matrix. The residual stresses also led to anisotropic hardness and a critical load for crack initiation in the matrix.     

Impact fatigue behaviour of vinylester resin matrix composites reinforced with alkali treated jute fibres

An impact fatigue study has been made for the first time on 35% jute/vinylester composites containing both untreated and alkali treated fibres. Longer alkali treatment removed the hemicellulose and improved the crystallinity and gave better fibre dispersion. The flexural strength properties of the composites made from treated fibre were superior. 4 h alkali treated jute fibres gave the optimum combination of improved interfacial bonding and fibre strength properties. However this was not reflected in their impact fatigue behaviour. On the contrary, the composites reinforced with 8 h alkali treated fibres displayed superior impact fatigue properties. Here, the fibres suffered catastrophic fracture with microfibrillar pull-out at some places and improved the fatigue resistance property of the composites as evident from SEM micrographs.     

钢纤维对水泥基复合材料抗起裂特性的影响

通过不同钢纤维体积分数及不同试件尺寸的预制缺口三点弯曲梁断裂试验,研究了普通乱向及定向钢纤维增强水泥基复合材料的抗起裂特性。利用试验测得的荷载-裂缝口张开位移曲线,分析了钢纤维对水泥基复合材料断裂性能的影响,并基于线性相关系数陡降法计算了起裂韧度。结果表明,定向钢纤维增强水泥基复合材料的起裂韧度明显高于普通乱向钢纤维增强水泥基复合材料;起裂韧度随钢纤维体积分数的增加而逐渐增大,当钢纤维体积分数达到0.9%左右时,定向钢纤维增强水泥基复合材料的起裂韧度值趋于稳定;在本试件高度范围内(40~100mm),起裂韧度随试件尺寸增加而逐渐增大,且定向钢纤维增强水泥基复合材料的增长趋势较为平缓。此外,从裂缝尖端夹杂改变其应力强度因子的角度解释了钢纤维的掺入及定向对起裂韧度的提高作用。     

Fatigue fracture of nylon polymers

The influence of glass fibres on the fatigue crack propagation rates of injection-moulded nylons has been determined. In contrast to previous results for unreinforced nylons, the cracking kinetics are independent of the oscillating load frequency. The fact that the crack growth rate per cycle is constant, when expressed in terms of the time under load, demonstrates that the contribution of creep crack extension is minimized by the glass fibres. Thus a true fatigue process is suggested for the fatigue fracture of the reinforced system, even when the glass fibres are preferentially aligned parallel to the crack growth direction. A complicating factor in characterizing the fatigue resistance of the glass-reinforced nylons is the tremendous influence of fibre orientation on crack growth rate. It is shown that the anisotropy problem can be handled by simply expressing the crack growth rate data in terms of the strain energy release rate rather than the usual stress intensity factor representation. Results for four different glass-filled nylons show that the diverse crack growth rates for cracking parallel versus perpendicular to the glass-fibre axes collapse on to individual strain energy release rate curves. Each single relationship therefore characterizes the fatigue fracture of the filled material and furthermore permits a prediction of the cracking rates for any glass-fibre orientation based upon the expected change in modulus. Finally it is demonstrated that the increased stress dependence of fatigue crack propagation (slope of the Paris plot) in filled nylons can be duplicated in unfilled samples under certain conditions. It is concluded that the fatigue fracture mechanism is matrix dominated in these chopped glass-fibre reinforced materials.     

Experimental techniques for fracture instability toughness determination of unidirectional fibre metal laminates

ABSTRACT The aim of this work is to propose procedures for the measurement of the fracture toughness of fibre metal laminates (FMLs) reinforced with unidirectional fibres of aramid or glass. Experimental techniques for fracture toughness evaluation by using Compact (C(T)) and Single‐Edge Bend (SE(B)) specimens obeying ASTM standards are introduced. Procedures from the standard for thick metallic materials were modified in order to overcome problems, which can arise when testing FMLs – that is, specimen buckling, indentations and crack growth in planes other than the plane of the fatigue pre‐crack or notch. The methodology proposed was experimentally tested leading to satisfactory results.     

Evaluation of fibre bridging stress of short fatigue cracks in SCS-6/Ti-15-3 composite

Single‐edge notched specimens of a unidirectional SiC long fibre reinforced titanium alloy, were fatigued under four point bending. The propagation behaviour of short fatigue cracks from a notch was observed on the basis of the effects of fibre bridging. The branched fatigue cracks were initiated from the notch root. The fatigue cracks propagated only in the matrix and without fibre breakage. The crack propagation rate decreased with crack extension due to the crack bridging by reinforced fibres. After fatigue testing the loading and residual stresses in the reinforced fibres were measured for the arrested cracks by the X‐ray diffraction method. The longitudinal stresses in the reinforced fibres were measured using high spatial resolution synchrotron radiation. A stress map around the fatigue cracks was then successfully constructed. The longitudinal stress decreased linearly with increasing distance from a location adjacent to the wake of the matrix crack. This region of decreasing stress corresponded to the debonding area between the fibre and the matrix. The interfacial frictional stress between the matrix and the fibre could be determined from the fibre stresses. The bridging stress on the crack wake was also measured as a function of a distance from a notch root. The threshold stress intensity factor range, corrected on the basis of the shielding stress, was similar to the propagation behaviour of the monolithic matrix. Hence the main factor influencing the shielding effect in composites is fibre bridging.     

Fracture behaviour of fly-ash filled FRP composites

The major objective of this study was to determine the fracture toughness and fracture surface energy of epoxy, epoxy/fly-ash, epoxy/carbon fibre, epoxy/carbon fibre/fly-ash, epoxy/glass fibre and epoxy/glass fibre/fly-ash composites. The quality of composite specimens was evaluated by the ultrasonic method. The results show that a fly-ash particle can arrest the crack path and thus improve the fracture properties of fibre reinforced plastic (FRP) composites. The results of this study have further significance in view of the fact that fly-ash powder is far cheaper than carbon fibre, glass fibre and epoxy resin.     

The environmental fatigue behaviour of carbon fibre reinforced polyether ether ketone

The fatigue behaviour of carbon fibre/PEEK composite is compared with that of carbon/ epoxy material of similar construction, particularly in respect of the effect of hygrothermal conditioning treatments. Laminates of both materials were of 0/90 lay-up, and they were tested in repeated tension at 0° and at 45° to the major fibre axis. The superior toughness of the polyether ether ketone and its better adhesion to the carbon fibres results in composites of substantially greater toughness than that of the carbon/epoxy material, and this is reflected in the fatigue behaviour of the carbon fibre/PEEK. The tougher PEEK matrix inhibits the development of local fibre damage and fatigue crack growth, permitting a 0/90 composite with compliant XAS fibres to perform as well in fatigue as an epoxy laminate with stiffer HTS fibres. Hygrothermal treatments have no effect on the fatigue response of either material in the 0/90 orientation. The fatigue response of a cross-plied carbon/PEEK laminate in the ±45° orientation is much better than that of equivalent carbon/epoxy composites, again because the superior properties of the thermoplastic matrix.