Fractographic study of transverse cracks in a fibre composite

Transverse fracture of unidirectional fibre composites was studied in a model glass/epoxy composite in which 1 mm-diameter rods had been used in place of fibres. The fracture surface resulting from transverse cracking in this model system was studied by scanning electron microscopy (SEM). The interaction of the crack with the epoxy matrix resin and the glass rods was the following: Cracks in the resin appeared to have effected a debonding at the glassmatrix interface before reaching the glass. The debonding then propagated along the interface and induced secondary cracks ahead of the primary debonding crack. The confluence of the secondary and primary cracks resulted in sharp ridges being formed on the matrix resin surface, produced by plastic deformation of the rigid epoxy resin. These appeared as a field of parabolic marks. Considering the brittleness of the resin, the amount of plastic deformation indicated by the ridges was astonishing. As the debonding continued around the glass rod, a transverse corrugated texture developed on the resin surface, again produced by plastic deformation. Finally, the cracks reentered the matrix from small patches of polymer adhering especially strongly to the glass surface. The overall fracture energy of transverse cracking of unidirectional fibre composites is suggested to consist, therefore, of the following elements in addition to crack propagation in the matrix resin: (a) the glass-resin debonding before the incoming cracks reach the glass, (b) the initiation of secondary cracks or debonds at the interface, (c) the plastic deformation in generating the ridges on the rigid resin surface, appearing both as the paraboloids and the transverse corrugation, and (d) cracking of the matrix reinitiated at the opposite side of the glass. The use of an enlarged glass reinforcement in this study provided a more direct observation of the properties of transverse crack propagation in composite materials than would have been possible with the small, roughly 10m fibres.     

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.     

Metallographic investigation of the damage caused to GRP by the combined action of electrical,mechanical and chemical environments

The use of glass fibre-reinforced polymers in electrical insulator components has gradually been taking place. Problems may arise where such insulators are in service at very high voltage, e.g. 200 kV and above, are under significant mechanical loads, and the environment (rain, and various pollutants) is able to gain access to the surface of the GRP. With the aid of optical and scanning electron microscope techniques, a detailed examination has been carried out on the nature of damage which has taken place in GRP pultruded rods that have operated for various periods of time in the above service conditions. These pultruded rods can receive significant levels of damage under the action of electrical fields, and the attendant environment; this takes the form of erosion, melting, burning and displacement of both glass and polymer phases. When a mechanical stress, which may be less than 10% of the breaking stress of the rod, is applied in conjunction with the above conditions a different form of insulator breakdown can take place. Instead of material displacement on the scale mentioned above, brittle failure of the GRP takes place. Such a failure mode can be compared with the process of stress corrosion which takes place when GRP is tested in 0.1 N acid solutions. It is suggested that the combined action of electrical activity and the presence of minor amounts of pollutants are able to influence the surface of glass fibres and promote stress corrosion in an analogous fashion to that described for concentrated acid solutions.     

Mechanism of fracture of short glass fibre-reinforced polyamide thermoplastic

The mechanism of fracture of short glass fibre-reinforced polyamide 6.6 thermoplastic was studied by means of optical and electron microscopy and acoustic emission methods. It was found that there were three stages in the failure, i.e. initiation of the interfacial cracks at fibre ends, propagation of the interfacial cracks along fibre sides, and propagation of the crack into the matrix leading to the failure of the composite. On the fracture surface, fibres were almost pulled-out from the matrix, not broken. The close correspondence between the crack initiation and propagation and the amplitude of AE signals was observed. The AE signals of lower amplitude occurring under a relatively low stress were considered to be made in association with the initiation and propagation of the interfacial cracks. The AE signals of higher amplitude observed prior to the failure of the composite were considered to be made in association with the occurrence of the matrix cracks. Furthermore, in order to analyse the effect of the stress state in the composite on crack occurrence and propagation, the stress levels in matrix, fibre and interface were estimated for the composite stressed to the failure stress. The calculation was based on the equivalent inclusion method proposed by Eshelby and on an assumption of a perfect bond between the matrix and the fibres. The result was found to be consistent with the mechanism of the fracture, the occurrence of the interfacial cracking in the initial stage and the matrix cracking in the final one.     

A study of the corrosion resistance of glass fibre reinforced polymers

Specific interactions between chemical environments (hydrochloric acid, sulphuric acid and distilled water) and glass fibre cause stress corrosion cracking in the glass fibre surface. The etching of the glass fibre gives rise to an extraction process. Axial or spiral cracks can then be observed. These effects depend on the fibre diameter, the etching time and the chemical environment and cause a drop in tensile stresses. The glass fibre crumbles with increasing etching time.Strict etching procedures lead to definite extraction processes and crack structures in the glass fibres and will be discussed in connection with strength tests.In addition to investigations of individual elements, e.g. glass fibres, it is also possible that whole glass fibre reinforced composites are damaged during service under the influence of aggressive surrounding media. In such cases, circular or spiral-shaped cracks can also be observed preferentially in the glass fibre. The fibres can then no longer contribute to an increase in strength and the result is the untimely failure of the composite material.     

The crack evolution on the atomistic scale during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

The development of the crack patterns during the pyrolysis of carbon fibre reinforced plastics (CFRP) to carbon/carbon composites as the second manufacturing step in the liquid silicon infiltration (LSI) process was investigated. In the basic examination reported previously, it was discovered that a substantial amount of cracking occurs beyond 650 °C, when the mesoscopic crack pattern has already fully developed. This additional cracking could not be visualized by using standard microscopy. Thus additional investigations were conducted by using conventional and high-resolution transmission electron microscopy to obtain information on the atomistic scale on the assumed cracking activity.It was found that the crack development starts at pores that develop as a compensation for the rough fibre surface. Crack propagation takes place by evolution of new nanoscopic cracks caused by fibre–matrix-debonding in the tensile stress field in front of the crack tip and subsequent connection with the main crack. Thus the interconnection mechanism – cracking as the connection of cracks on a subordinated scale to form a new crack – is the second main cracking mechanism beside transversal cracking (leading to a regular mesoscopic crack pattern) active during the carbonization of CFRP components.     

Rißbildung in Elementarglasfasern durch Einwirkung korrosiver Medien und erhöhter Temperatur

Crack Initiation in Glass Fibres Under the Influence of Chemical Environment and High Temperature Specific interactions between chemical environments (distilled water, hydrochloric acid, and sulfuric acid) and the glass fibre generate stress corrosion cracking in the glass fibre surface. The etching of glass fibre effects either axial or spiral cracks in the glass fibre surface. These effects depend on the fibre diameter, the etching time and the chemical environment. Crack initiation generates a drop of tensile stresses. At least the glass fibre crumbles with increasing etching time. The reasons for this phenomenon are residual stresses in the glass fibre. Therefore it is necessary to know something about residual stresses. Strict etching procedures lead to definite crack structures in glass fibres and should indicate the kind of residual stresses in connection with strength tests.     

The mechanical properties of carbon fibre reinforced Pyrex glass

The mechanical properties of carbon fibre reinforced Pyrex glass are discussed in terms of the volume fraction of fibre, the orientation of the fibres, fibre damage during fabrication, matrix porosity, matrix critical strain, interface properties and the mode of failure in bend tests. The stress at which matrix cracking occurs increases with fibre concentration indicating that the critical strain of the matrix increases as the fibre separation decreases. The ultimate strength of the composite is considerably greater than the stress at which the matrix begins to crack. Preliminary stress cycling experiments at stresses above that at which matrix cracks are formed suggest that propagation of these cracks is inhibited by the fibres.     

FATIGUE CRACK GROWTH BEHAVIOUR OF FIBRE-METAL LAMINATE GLARE-1 AND METAL LAMINATE 7475 WITH DIFFERENT BLUNT NOTCHES

Abstract— The fatigue crack growth behaviour of the fibre metal laminate "GLARE-1" has been investigated for different blunt notches in Constant Amplitude (CA) tests. In order to investigate the influence of the fibres, the same laminate material but containing no fibres (Laminate 7475) was also tested. The fatigue crack growth properties of GLARE-1 are superior to those of Laminate 7475. GLARE-1 shows lower crack growth velocities at the same K_nom values and in addition the crack growth rates decrease with increasing crack length. The Laminate 7475 shows typical metal behaviour for single crack propagation and accelerating crack growth with increasing crack length. In GLARE-1, multiple crack propagation takes place. The cracks propagate independent of each other and have similar crack growth rates, in part due to closure effects caused by the unbroken fibre layers.
The crack growth rates of specimens having a small root radius are higher in both materials than in specimens with a large notch radius. In GLARE-1, the superiority of a larger notch radius is more pronounced than in the Laminate 7475 and is attributed to a stronger crack closure effect owing to fibre bridging. The reason for the higher bridging capability in specimens containing larger notches is that less fibres are broken or damaged in the notch vicinity.     

FATIGUE CRACK PROPAGATION AND CRACK CLOSURE BEHAVIOR IN POLYCARBONATE AND FIBRE REINFORCED POLYCARBONATE

Abstract— Fatigue crack propagation was investigated in polycarbonate and glass fibre reinforced polycarbonate and the effect of stress ratio and glass fibre content determined. The addition of glass fibre increases the tensile strength, but does not always contribute to an increase in fatigue crack propagation resistance. For polycarbonate the effect of stress ratio can be partly explained by using crack closure concepts as other researchers have suggested, but for glass fibre reinforced polycarbonate this was not possible. Fractography revealed a void growth process, which occurred by decohesion at the interface of the glass fibres and the base material, which was dependent on the maximum stress intensity factor. The process of linking the voids and the main crack growth behavior depended on the stress intensity factor range, Δ K. A proposed crack propagation model can explain the effect of stress ratio on crack propagation in fibre reinforced polycarbonate.     

TWINTEX纤维增强塑料的应力腐蚀开裂

研究了TWINTEX纤维增强塑料在酸中的应力腐蚀,并与玻璃纤维增强塑料的应力腐蚀进行了比较.结果表明,TWINTEX纤维增强塑料具有比玻璃纤维增强塑料好得多的耐应力腐蚀性能,而且这种好的耐应力腐蚀性主要表现在长的裂纹孕育期.聚丙烯纤维良好的耐蚀性是造成长的裂纹孕育期的主要原因.外加载荷的变化对TWINTEX纤维增强塑料的应力腐蚀裂纹扩展速率的影响不如其对玻璃纤维增强塑料的应力腐蚀裂纹扩展速率的影响显著.     

Polypropylene composite materials oriented by solid-state drawing: low-temperature impact behaviour

Isotactic polypropylene samples, both neat and containing short glass fibres, with two different interfacial adhesions, were oriented by solid state drawing with typical neck propagation. A microscopic examination showed that the glass fibres turned to the orientation direction rather suddenly in the narrow region at the neck shoulder. Such extensive movement of fibres up to 1 mm long indicates high mobility of the polypropylene matrix in this region. Thus, these results seem to support the hypothesis of stress-induced melting of semicrystalline polymers in the propagating neck. The resulting oriented materials showed high resistance to crack propagation at cryogenic temperatures. Inspection of the crack surfaces after impact testing revealed that macroscopic failure was accompanied by multiple fractures and splitting of the specimens along the orientation direction. Such a mechanism effectively blunts the crack tip and dissipates mechanical energy even at very low temperatures.     

Compressive fatigue crack growth behaviour of alumina and glass

Cylindrical specimens of polycrystalline alumina and sodium glass with a deep circumferential notch were loaded cyclically under compression, and the fatigue crack-propagation processes in the notch root were studied. The studies showed that debris and a fatigue crack are formed in the notch root of alumina, but not in glass. There, only fragmentation takes place at the crack tip; glass chips are formed which surround the notch. The influence of microstructural inhomogeneities on fatigue crack formation and propagation as well as the consequences for material scientists and engineers are discussed on the basis of a simple model.Dedicated to the 60th birthday of Professor Dipl. Ing Dr. Peter Ettmayer, Technical University, Vienna.     

In situ observation of crack propagation in ESP (engineered stress profile) glass

Important features of the ESP (engineered stress profile) glasses are the crack arrest and multiple cracking phenomena that occur even in an unstable stress field. In this work a detailed “in situ” observation of crack observation and analysis was performed with the aim to examine crack propagation in detail and relate it to the residual stress field produced by ion exchange and to the final mechanical performances of the material. The results showed that the peculiar residual stress field with a maximum below the surface is responsible for the formation of a multitude of stable cracks on the tensile surface of the glass that evolved into through-thickness flaws. The propagation within the material is limited by the increasing compressive residual stress, which also leads to kinking of the cracks in a direction parallel to the surface. The observed fracture phenomena are also responsible for a shielding effect that makes the measured failure resistance of ESP glass larger than predicted by simplistic single crack models.     

In situ analysis of delayed fibre failure within water-aged GFRP under static fatigue conditions

A stress corrosion model has been applied to the microscopic analysis of the delayed fibre failure processes occurring within a water-aged unidirectional glass/epoxy composite under static fatigue loading (i.e. relaxation). By means of in situ microscopic observations, the individual fibre failures within an elementary volume located on the tensile side of the flexural specimens have been quantified as a function of time under various applied strain levels. It was found that the time dependence of the in situ fibre failure processes obeyed a stress corrosion model. From the microscopic observations, it was possible to assess consistent values of the parameters characterising the in situ fibre strength distribution and the subcritical crack propagation law. A comparison with separate static fatigue experiments using unimpregnated fibre bundles demonstrated that the specific physico-chemical environment encountered by the glass fibres within the aged epoxy matrix can induce significant changes in the subcritical crack propagation rates, as compared to stress corrosion cracking data collected in humid air.     

Toughness of glass fibres reinforced glass-ionomer cements

Two fracture toughness parameters, the critical stress intensity factor, K _c and the work of fracture, W _f have been used to characterise the toughness of conventional and resin-modified glass-ionomer cements reinforced with glass fibres. The critical stress intensity factor was determined from the peak load, and the work of fracture was determined as the energy required to extend an introduced crack through the respective glass ionomers. For both materials, crack propagation became more stable as the weight fraction of glass fibres was increased. Additionally, when the weight percent of glass fibres was increased the work of fracture increased. Fibre bridging at the crack tip resulted in the increase in the work of fracture. As the percentage weight of fibres was increased, the critical stress intensity factor decreased proportionally to the increase in porosity.     

Numerical simulation of periodic cracking mechanism in microscopic surface films during roll bonding processes

The microscopic surface films existing on the top of metallic layers play an important role in the process of joining by plastic deformation. The bond formation during cold welding processes is basically associated with the fracturing of surface films to produce intimate metallic contacts. The present paper aims at providing a numerical model to describe the cracking pattern of brittle surface films bonded to the ductile substrates. A microscale finite element model is developed which takes into account the fracturing mechanisms of thin surface films in roll bonding processes. The presented model is calibrated by using the existing experimental data for an aluminum alloy covered by a thin layer of oxide film. The model is also validated against a well‐known analytical model for periodic cracking. The distribution of stresses within the fractured surface film demonstrates that the generated cracks in the surface film have essentially a periodic pattern. Moreover, it is shown that the crack spacing is highly dependent on the properties of the surface film. Finally, the obtained results for the roll bonding show that a crack density saturation takes place at the entry of the roll bite where a small surface expansion is applied to the rolled samples.     

Microfailure behaviour of randomly dispersed short fibre reinforced thermoplastic composites obtained by direct SEM observation

The microfailure behaviour of thermoplastic polyamide 6,6 composites reinforced with randomly dispersed short glass fibres was studied. Scanning electron microscopy was carried out on the surface of the composites under load to observe directly the behaviour. The microfailure proceeds following the steps (1) interfacial microfailure occurs at the fibre tips, (2) the microfailure propagates along the fibre sides, (3) plastic deformation bands of the matrix occurs from the interfacial one, (4) crack opening occurs in the band and the crack grows slowly through the band, (5) finally a catastrophic crack propagation occurs through the matrix with pulling-out fibres from the matrix. A model for the microfailure mechanism of the composites is proposed and some methods to improve the mechanical properties of the composites are discussed on the basis of the mechanism.     

Plastic shrinkage cracking of concrete incorporating mineral admixtures and its mitigation

In recent years, there has been a rapid increase in the use of mineral admixtures for high performance and durable concrete. Plastic shrinkage cracking in such concretes is a serious concern in large surface area/volume applications. The present study has two objectives: firstly, to investigate the influence of incorporating fly ash and granulated blast furnace slag (GGBS) on the susceptibility to such cracking; and secondly, to assess the techniques, such as fibre and shrinkage reducing admixture (SRA) addition, and spraying of curing compounds, to mitigate the cracking. The results indicate that replacement of ordinary Portland cement (OPC) with fly ash and GGBS increases the possibility of plastic shrinkage cracking significantly, with higher severity as the replacement level increases; 30% replacement of OPC with fly ash and GGBS doubled and quadrupled the crack area, respectively, mainly due to higher binder finesses, and the delay of setting and strength gain. Among the fibres tested, polypropylene and polyester fibres, at the recommended dosages of about 0.9 kg/m³, completely eliminated cracking in the most affected concrete (i.e., with 30% GGBS) while the dosages of the polyacrylonitrile and glass fibres had to be increased to provide a higher volume fraction. Two glycol-based SRAs, and two curing compounds based on acrylic resin and methacrylate mitigated cracking by significantly reducing evaporation from the surface of concrete.     

Study of the stress corrosion cracking of GFRP: effect of the toughness of the matrix resin on the fatigue damage and stress corrosion cracking of GFRP

The stress corrosion cracking of glass fibre-reinforced plastic (GFRP) accompanies a phenomenon of catastrophic failure as a result of the rapid fall in strength owing to corrosion breaking of the glass fibres. This produces a flat surface without pulling fibres out of the plane. Attack on the glass fibres can only occur by contact with an acid which must first diffuse into the matrix resin. It is confirmed, however, that no diffusion occurs or that it is too slow to be detected. The relationship between fatigue damage and stress corrosion in an acidic environment, has been investigated, focusing on the effect of matrix toughness on the resistance to stress corrosion failure of GFRP. Three types of GFRP, made from matrices with different toughness, were studied after subjecting them to fatigue damage at different levels.