Acoustic emission from stress corrosion cracks in aligned GRP

Acoustic emission (AE) produced by the propagation of stress corrosion cracks in an aligned glass fibre/polyester resin composite material has been recorded. Tests have been carried out over a range of crack growth rates and the variation of AE with crack velocity/applied stress intensity has been examined. The main source of AE is fibre fracture and there is a one-to-one relationship between the number of fibre fractures and the number of high-amplitude AE signals. This enables crack growth to be monitored directly from acoustic emission. The amplitude of AE signals produced by fibre failure appears to be proportional to the fracture stress of the fibres, although further analysis requires a greater understanding of the generation, transmission and detection of AE signals. This work demonstrates that stress corrosion cracking is an ideal source for the study of AE produced by fibre fracture without complications caused by interface effects, such as fibre debonding or pullout.On leave from the Technical University of Wroclaw, Wroclaw, Poland.     

Fatigue damage development in Al2O3/Al composite

Fatigue damage development in two aluminium matrix (Al7SiO.6Mg and Al5Si-3Cu1Mg) composites reinforced with discontinuous Al₂O₃ fibres has been monitored by means of acoustic emission (AE). The AE signals (RMS) recorded during the tests clearly exhibit three distinct stages which correspond to crack initiation, dominant crack formation and stable propagation. Generally speaking, the cracks initiated at a high load level form close together and a dominant crack forms easily. By contrast, at a low load, initiated cracks are widely separated and the formation of a dominant crack is difficult. If there are large defects in the composite, the first stage is absent, even at low load. In the first stage, little change in microstructure and modulus of the composite is observed; in the second, fibre fracture, interface debonding and matrix cracking occur and there are often sinusoidal cracks in the matrix; in the last stage, the principal characteristic is stable propagation of the dominant crack. The degradation of the elastic modulus of the composite in the last two stages is small.     

Effects of inter-fibre spacing and matrix cracks on stress amplification factors in carbon-fibre/epoxy matrix composites. Part I: planar array of fibres

When the loading on a composite is sufficient to cause fracture of an individual fibre, the resulting stress amplification in the adjacent intact fibres may be large enough to cause failure of these fibres. In this work, 3D elasto-plastic finite element analysis was used to investigate the effect of inter-fibre spacing on the stress amplification factor in a composite comprising a planar array of fibres. A Progressional Approach was used in the FE analysis to simulate the constituent non-linear processes associated with the generation of thermal residual stresses from fabrication, the fibre fracture event and the subsequent initiation and propagation of conical matrix cracks induced with incremental tensile loading. As the inter-fibre spacing increases, the effect of fibre fracture on the stress distribution in the neighbouring intact fibres is reduced, whereas the effect on the matrix material is increased, thereby inducing localised yielding. The presence of a conical-shaped matrix crack was found to increase both the stress amplification factor and the positively affected length in neighbouring fibres. For a large inter-fibre spacing, a longer matrix crack is required to obtain good agreement with LRS measurements of fibre stress.     

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.     

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.     

PROGRESSIVE MICROCRACKING IN UNIDIRECTIONAL BRITTLE MATRIX COMPOSITES

Abstract

In this work we investigate the initiation of microcracking in a unidirectional Nicalon^TMfibre, glass-ceramic (CAS) matrix composite under flexural loading. We find that damage develops and grows in these composites as a continual initiation of new transverse cracks in a more-or-less random manner, as well as growth of existing cracks, within the matrix. Initial cracks are usually of limited extent, being stopped when they encounter the nearest fibres, in a mode idealized by the so-called full-cell cracking model. Each new crack that initiates under increasing load yields another data point, provided the precracked state of stress and crack geometry at the initiation site can be determined. The latter conditions are satisfied since the zone of influence of each crack can be determined analytically and the full-cell cracking mode geometry receives at least partial validation. Analysis is accomplished by use of an axisymmetric micro mechanical model based on Reissner's variational principle in which variable fibre spacing can be recognized. In a semi-empirical failure model, the matrix axial stress just prior to crack initiation is computed and assumed to act on an unbounded matrix containing a penny-shaped crack having a radius dictated by the local fibre spacing, s. This model is geometrically appropriate for the case of large localized fibre spacings. Statistical information on s and the critical matrix mode I stress intensity factor is presented, and the average value seems to be consistent with expectation based upon measurements on monolithic samples of the matrix material. Application of the micromechanical model shows that, at least for the predominant range of s, sufficient energy is available to mobilize annular matrix cracks at stresses predicted by the semi-empirical model, and also that interfacial crack deflection is expected rather than fibre penetration. In accordance with this observation, very few fibre breaks were seen in the test programme. Finally, some comments regarding the practical consequences of this work are offered and the conventional view of a constant microcracking stress level in a steady-state model is dismissed, at least for the class of composites considered.     

Characterization of the aramid/epoxy interfacial properties by means of pull-out test and influence of water absorption

Single fiber pull-out tests were carried out to investigate the influence of water absorption on the interfacial properties of aramid/epoxy composite. The fiber/matrix interfacial strength was severely decreased between 4 and 7 week immersion time in deionized water at 80 °C, and thereafter showed a plateau. This change with immersion time did not correspond with that of the water gain of the pull-out specimens, because the water gain did not reflect the one in the fiber/matrix interface. As a result of the degradation of the fiber/matrix interfacial strength, the pulled-out fiber surfaces of 7, 10 and 13 week wet specimen were smooth. In situ observations of interfacial crack propagation by a video microscope and an analysis of acoustic emission (AE) signals showed that AE signals obtained during the pull-out process were classified into four types according to fracture modes. AE signals detected at final unstable crack propagation and fiber breakage had high amplitude and long duration.     

Stress transfer in the fibre fragmentation test: Part III Effect of matrix cracking and interface debonding

A theoretical stress analysis has been developed for the fibre fragmentation test in the presence of matrix cracks at sites of fibre breaks. The strain energy release rates for both matrix cracking and interface debonding are calculated for a carbon fibre/epoxy matrix composite. By comparing these strain energy release rates with the corresponding specific fracture resistances, the competition between matrix crack growth and interface debonding has been studied. The distributions of fibre axial stress and interfacial shear stress obtained from the present analysis show that the matrix crack substantially reduces the efficiency of stress transfer from the matrix to the fibre. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

Acoustic emission analysis of composite pressure vessels under constant and cyclic pressure

The use of acoustic emission (AE) for the detection of damage in carbon fibre composite pressure vessels was evaluated for constant and cyclic internal gas pressure loading conditions. AE was capable of monitoring the initiation and accumulation of damage events in a composite pressure vessel (CPVs), although it was not possible to reliably distinguish carbon fibre breakage from other microscopic damage events (e.g. matrix cracks, fibre/matrix interfacial cracks). AE tests performed on the carbon fibre laminate used as the skin of pressure vessels revealed that the development of damage is highly variable under constant pressure, with large differences in the rupture life and acoustic emission events at final failure. Numerical analysis of the skin laminate under constant tensile stress revealed that the high variability in the stress rupture life is due mainly to the stochastic behaviour of the carbon fibre rupture process.     

Fracture mechanism of unidirectional carbon-fibre reinforced epoxy resin composite

The object of this study was to investigate the fracture mechanism of unidirectional carbon-fibre reinforced epoxy resin composite. For this purpose, the failure process of the composite under load was observedin situ by scanning electron microscopy and the matrix deformation around the broken fibre tip was examined by polarized transmission optical microscopy using a thin section of the composite. The failure process was shown to proceed through the following four stages: (1) fibre breakage began to occur at a load of about 60% of the failure load; (2) as the applied load was increased, plastic deformation occurred first from the broken fibre tip along the fibre sides, followed by final matrix cracking in the plastic region; (3) just before failure, partial delamination occurred, originating from fibre breakage and matrix cracking; (4) finally, a catastrophic crack propagation occurred from the delamination, leading to composite failure. Acoustic emission monitoring was also carried out for non-destructive evaluation, which indicated that internal failure began to occur at a load of 60% of the failure load and propagated remarkably before composite failure. A close correspondence between the acoustic signal and crack formation was obtained. The acoustic signal at lower amplitude, occurring over whole load range, corresponded to fibre breakage and matrix cracking while that at higher amplitude, occurring only just before failure, corresponded to partial delamination. From these experimental studies, the fracture mechanism of the composite has been clarified.     

Mechanical behaviour of a cross-weave ceramic matrix composite

The deformation and fracture processes of a cross-weave carbon fibre/SiC composite prepared by a chemical vapour deposition process has been explored by interrupted-loading tests and SEM examination of cracking and fracture processes. The tensile stress-strain curves show non-linear behaviour associated with progressive matrix cracking and spalling, and the occasional fracture of a fibre. Re-loading curves and compressive stress-strain curves show linear behaviour. The fracture process does not involve cracking by a single dominant crack but occurs by the development of multiple damage sites operating around the transverse fractures of groups of four to eight fibres followed by longitudinal cracking at their fibre-matrix interfaces and temporary arrest of the cracks, until specimen failure occurs and there is massive fibre debonding and pull-out.     

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.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

Interface Cohesive Elements to Model Matrix Crack Evolution in Composite Laminates

In this paper, the transverse matrix (resin) cracking developed in multidirectional composite laminates loaded in tension was numerically investigated by a finite element (FE) model implemented in the commercially available software Abaqus/Explicit 6.10. A theoretical solution using the equivalent constraint model (ECM) of the damaged laminate developed by Soutis et al. was employed to describe matrix cracking evolution and compared to the proposed numerical approach. In the numerical model, interface cohesive elements were inserted between neighbouring finite elements that run parallel to fibre orientation in each lamina to simulate matrix cracking with the assumption of equally spaced cracks (based on experimental measurements and observations). The stress based traction-separation law was introduced to simulate initiation of matrix cracking and propagation under mixed-mode loading. The numerically predicted crack density was found to depend on the mesh size of the model and the material fracture parameters defined for the cohesive elements. Numerical predictions of matrix crack density as a function of applied stress are in a good agreement to experimentally measured and theoretically (ECM) obtained values, but some further refinement will be required in near future work.     

聚乙烯自增强复合材料损伤过程的声发射特征

复合材料在承受外载时, 声发射可产生于基体破裂、纤维-基体界面脱粘和纤维断裂等。测定了U HMWPE/ HDPE 复合材料在拉伸载荷作用下的声发射(AE) 振幅信号。对特殊试样, 即预测到断裂有明确方式, 如纤维-基体界面脱粘、基体破裂、纤维断裂和分层等的试样, 实施加载直至破坏。用扫描电子显微镜(SEM) 观测试样的断裂表面, 对产生于若干特殊损伤类型的AE 信号进行了鉴别。在相同加载条件下, 完成了不同种类的U HMWPE/ HDPE 准各向同性层合板声发射检测。结果在特殊试样损伤类型与声发射信号事件振幅之间建立了对应关系, 揭示了上述各种准各向同性层合板损伤扩展过程的AE 特征与损伤破坏机制。各种准各向同性层合板试样的声发射事件累计数对拉伸应力关系曲线相异, 其相同损伤类型发生时所对应的拉伸载荷水平不等, 表明它们的铺设角度和铺设顺序对损伤演变过程有显著的影响。结果证实了它们的最终破坏由严重层间分层造成。      

The role of matrix cracks and fibre/matrix debonding on the stress transfer between fibre and matrix in a single fibre fragmentation test

The single fibre fragmentation test is commonly used to characterise the fibre/matrix interface. During fragmentation, the stored energy is released resulting in matrix cracking and/or fibre/matrix debonding.Axisymmetric finite element models were formulated to study the impact of matrix cracks and fibre/matrix debonding on the effective stress transfer efficiency (EST) and stress transfer length (STL). At high strains, plastic deformation in the matrix dominated the stress transfer mechanism. The combination of matrix cracking and plasticity reduced the EST and increased STL.For experimental validation, three resins were formulated and the fragmentation of an unsized and uncoupled E-glass fibre examined as a function of matrix properties. Fibre failure was always accompanied by matrix cracking and debonding. With the stiff resin, debonding, transverse matrix cracking and conical crack initiation were observed. With a lower modulus and lower yield strength resin the transverse matrix crack length decreased while that of the conical crack increased.     

Evaluation of toughness of textile concrete

High Performance Fibre Reinforced Cementitious Composites (HPFRCC) are characterized by a stress–strain response in tension that exhibits strain-hardening behaviour accompanied by propagation of multiple cracks. This process is often referred to as pseudo-ductility due to multiple cracking with relatively large energy absorption capacity. The cracking characteristics are dependent on matrix strength, fibre/matrix bond, fibre volume fraction and the aspect ratio of the fibre used in the composite. The matrix cracking strength and interfacial bond vary with the degree of hydration of cement in the matrix, which is time and environment dependent. This study analyses the multiple cracking patterns formed in weathered Textile Concrete (TC) samples due to direct tensile testing, and links the cracking patterns to the tensile behaviour. The specimens used for the study were thin laminates which were produced by casting six layers of specially made polypropylene (PP) textile in fine-grained mortar. The samples were cured under controlled laboratory conditions for 28 days, and thereafter exposed to different weathering regimes for different periods. The weathered samples were tested in direct tension in a Universal Testing Machine (UTM) over a range of stresses. For all the samples tested, it was observed that the tensile behaviour was characterised by strain hardening and multiple cracking, which gave high tensile strains in excess of 20% at final failure. It was further found that the cracking patterns varied mainly with age, weathering history and stress levels. Other factors that contributed to the cracking characteristics were moisture state of the specimen and the fibre/matrix bonding strength. A strong bond and dense matrix resulted in wide crack spacings compared with samples with a weaker bond which developed closely spaced cracks. A general trend of increasing crack widths and crack spacings with ageing was observed which was accredited to increased hydration accompanied by an increase in fibre/matrix bond strength.     

Some aspects of fracture in brittle fibre composites

This paper describes some observations on the propagation of cracks through a brittle matrix reinforced with strong, stiff, unidirectional fibres. By means of a model material, observations are made of the interaction of a matrix crack with an isolated fibre normal to the crack. The experiments were extended to cover interaction with a row of fibres. The results of these tests are then compared with the behaviour of a real composite. One interesting observation concerns a mechanism whereby a single initial crack can initiate a series of parallel cracks which enable the opening displacement of the main crack to be distributed among this series of secondary cracks as fracture proceeds through the material.     

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.