Compressive fracture in undirectional glass-reinforced plastics

The shear mode of compressive failure in unidirectional fibre composites is discussed. A mechanism is described in which the shear deformation is restricted to a band of material inclined to the plane normal to the fibre axes. The relationship between the orientation of the failed band of material and the limiting shear deformation in the band is explained in terms of volumetric strains. Tests are described which demonstrate that, in GRP, this type of failure can propagate from a notch and this notch sensitivity is put forward as an explanation for the apparent inadequacy of the theoretical model. The sequence of events in the propagation of compressive failure is studied by examining serial sections of an arrested failure. It is found that fibre fracture at the boundaries and interlaminar failure within the band follow as a result of increasing shear deformation in the band.     

MODELLING THE TENSILE FRACTURE BEHAVIOUR OF THE REINFORCING FIBRE YARNS IN CERAMIC MATRIX COMPOSITES

Abstract— Using experimentally determined data on fibre radius distributions, yarn geometry, matrix and fibre elastic moduli and frictional shear stress at the matrix/fibre interface (obtained by nano-indentation experiments), the failure probability of the composite fibre yarns (after matrix cracking) is estimated. Each fibre is divided into a fixed number of segments above and below the matrix crack. The failure probability on every segment of each fibre is computed using Weibull fibre strength statistics. A fibre is assumed to be broken when the cumulative failure probability for the complete yarn reaches a value of 0.5. The segment and fibre are then selected at "random", according to their individual failure probabilities. After fibre failure, the broken fibre can only carry the frictional load and the load drop is transferred to its neighbours according to their distances to the broken fibre. The remote stress is then modified to match again the cumulative failure probability of 0.5 and a new fibre is broken. This procedure is repeated until all the fibres are broken. In this way, it is possible to obtain the "characteristic" load carried by the yarn and its corresponding elongation. Fibre extraction and pull-out behaviour are also considered. The roles of different load-transfer laws (from global to highly localised) are examined. The model is applied to simulate the fracture tensile behaviour of individual yarns of SiC/SiC ceramic-matrix composites. The results are compared with those obtained from tensile experiments on SiC/SiC individual yarns. The computed fracture morphology, in terms of individual pull-out lengths, is also compared to the actual SEM fractography of a woven SiC/SiC composite.     

Mechanical performance of composite-strengthened concrete structures

The interest of using fibre reinforced plastic (FRP) materials in rehabilitating damaged concrete structures respectively has been increased rapidly in recent years. In this paper, the structural behaviours of the glass–fibre composite strengthened concrete structures subjected to uni-axial compression and three point bending tests are discussed through experimental studies. Two types of concrete structure are used in present study, they are concrete cylinder and rectangular concrete beam. Discussion on the environmental effects of composite strengthened reinforced concrete (RC) structures is also addressed. Experimental results show that the use of glass–fibre composite wrap can increase the load carrying capacity of the plain concrete cylinders with and without notch formation. The flexural load capacity of the concrete beam increases to more than 50% by bonding 3 layers of glass–fibre composite laminate on the beam tension surface. Direct hand lay up method gives better strengthening characteristic in term of the ultimate flexural load compared with pre-cured plate bonding technique. The flexural strengths of composite strengthened RC beams submerged into different chemicals solution for six months are increased compared with the RC beams without strengthening. The strength of the concrete structure is seriously attacked by strong acids.     

In situ fibre fracture measurement in carbon-epoxy laminates using high resolution computed tomography

High resolution Synchrotron Radiation Computed Tomography (SRCT) has been used to capture fibre damage progression in a carbon-epoxy notched [90/0]_s laminate loaded to failure. To the authors knowledge this provides the first direct in situ measurement of the accumulation of fibre fractures for a high performance material under structurally relevant load conditions (i.e. fractures within the bulk of an essentially conventional engineering laminate). A high level of confidence is placed in the measurements, as the failure processes are viewed internally at the relevant micromechanical length-scales, as opposed to previous indirect and/or surface-based methods. Whilst fibre breaks are the dominant composite damage mechanism considered in the present work, matrix damage, such as transverse ply cracks, 0° splits and delaminations, were also seen to occur in advance of extensive fibre breaks. At loads where fibre break density levels were significant, splitting and delamination were seen to separate the central 0° ply in the near notch region from the 90° plies. Fibre breaks were initially observed in isolated locations, consistent with the stochastic nature of fibre strengths. The formation of clusters of broken fibres was observed at higher loads. The largest clusters observed consisted of a group of eleven breaks and a group of fourteen breaks. The large clusters were observed at the highest load, at sites with no prior breaks, indicating they occurred within a relatively narrow load range. No strong correlation was found between the location of matrix damage and fibre breaks. The data achieved has been made available online at www.materialsdatacentre.com for ongoing model development and validation.     

纤维增强复合材料声发射的Felicity效应

本文对正交铺层的层间剪切试样和含表面裂纹试样的玻璃纤维增强复合材料板进行了声发射试验研究,记录了在单向拉伸下声发射的总数,计数率和幅度分布曲线。结果表明,两种试样的声发射幅度分布几乎相似,只是在加载的最后阶段才显示出由纤维破坏而引起的差别。这说明含表面裂纹板的破坏方式主要是以层间剪切破坏为主。两种试样的声发射总数—载荷关系曲线表明:在低载荷水平时,试样显示Kaiser效应,而在高载荷水平时则显示Felicity效应。      

The mechanical properties and fracture behaviour of unidirectionally reinforced Nylon 6

The tensile and shear properties of Nylon 6 polymerizedin situ around unidirectionally aligned carbon and glass fibres have been investigated and the fracture behaviour characterized by optical and scanning electron microscopy. The tensile strengths are found to lie within the limits predicted by the law of mixtures and deviations from the predicted strengths have been correlated with fibre type and surface treatment. The shear strength values follow the same trend and an important mode of fracture in bending is shown to be the compressive failure which accompanies a yield drop in the load deflection curve. Depending upon the fibre type and the properties of the matrix this compressive damage need not lead to catastrophic failure of the composite as, in certain cases, the matrix can undergo substantial deformation before failure.     

Damping Factors and Fatigue Notch Factors of Pseudoelastic NiTi‐Shape Memory Alloys

Pseudoelastic NiTi‐ shape memory alloys (SMAs) provide a high damping capacity and can be used in order to achieve a reduction of peak loads being caused by unexpected shock loading. These “pseudoelastic” properties are related to the formation of martensite M from austenite A, which has been induced by stress; they allow to refer to SMAs as functional materials. Furthermore, these functional materials can operate at high stresses and thus, have to withstand severe mechanical loadings like classical structural materials. In combination, these characteristics provide opportunities for technical applications, e.g., to reduce vibrations or to reduce peak loads caused by shock loading. An extensive knowledge of the functional and structural fatigue behaviour of the material is required to design SMA components. NiTi hollow shaft samples and solid shaft samples have been tested under cyclic torsional loading conditions in a load‐controlled mode. By using these two geometries the influence of the sample geometry on the fatigue behaviour can be investigated. In addition, a test programme has been elaborated in order to investigate the behaviour of the material when subjected to bending. The experimental data have been evaluated describing the transformation behaviour induced by stress concerning transformation stress, apparent shear modulus of the austenite G_A and apparent stiffness τ_Ms (describing the slope of the shear stress‐strain‐curve in the transformation range G_A‐M). These parameters naturally depend on the cycle number, the load amplitude as well as the temperature. Engineering failures are often associated with the presence of notches. In this context, torsion tests on notched samples are planned to be carried out in order to assess the resulting data based on the results obtained from the notch free samples. This will allow to derive simple design rules based on fatigue notch factors, which are needed for engineering design.     

Study of the interlaminar shear strength of unidirectional high-modulus polyethylene fibre composites

The interlaminar shear strength of a unidirectional high-modulus polyethylene fibre-epoxy resin composite, measured using a short-beam bend test, increases when the fibre has been plasma treated but decreases with increasing fibre content or increasing filament diameter. Attempts have been made to explain these observations. From observation of the sample failure and the bending load/deflection curves, it was found that there are two possible failure modes dependent on fibre type, fibre volume fraction and fibre plasma treatment. These are (1) brittle failure of the resin and (2) failure at the fibre/resin surface. The present results appear to be consistent with the interlaminar shear strength data obtained from high-modulus polyethylene fibre samples made by the hot compaction process developed at Leeds.     

Damage characterisation of fibre metal laminates under interlaminar shear load

Transverse composite plies are part of the fibre metal laminate Glare^®4B and were investigated under interlaminar shear load. Double-notched shear (DNS) tests were performed and deformation and damage were in situ observed by Scanning Electron Microscopy (SEM) equipped with a loading apparatus. Interlaminar shear strength as well as shear stress values corresponding to the onset of the fibre/matrix-debonding were determined.Although a cross-ply lay-up within the laminate has been interlaminar shear loaded, damage and failure could only be found within the transverse plies. Over their thickness, fibre/matrix-debonding proved to be pronounced near the ply boundaries of the transverse plies, where exceptionally high shear strains could be found. Nevertheless, single fibre/matrix-debonding phenomena were also observed within the centre area of these transverse plies. Although interlaminar shear strain within latter regions is reduced, single events of fibre/matrix-debonding could be attributed to local high stress concentrations due to the fibre arrangement and to small inter fibre distances.     

Charakterisierung des Festigkeits‐ und Bruchverhaltens von kohlenstofffaserverstärktem Kohlenstoff (C/C) und siliziertem kohlenstofffaserverstärktem Kohlenstoff (C/C‐SiC) unter Biegebeanspruchung

Characterisation of the strength and failure behaviour of carbon fibre reinforced carbon (C/C) and siliconized carbon fibre reinforced carbon (C/C‐SiC) under flexural load The mechanical investigation of different C/C and C/C‐SiC materials as well as the gaining of engineering data and the criteria for failure behaviour have already been widely discussed in literature. However, further developments of these materials show that an interpretation of the results can be very difficult, for example those obtained from flexural tests, especially when special fibre preforms or additives are used. The effects of geometry and machining are also examined. On the basis of conventional testing methods it is often not possible to report on the failure mode or on the origin of failure. Therefore, in order to obtain exact information, additional 3‐point in‐situ flexural tests were performed by means of scanning electron microscopy (SEM), offering the possibility of studying the failure behaviour of the materials under flexural load. Three standard materials (CF222, CF222P75 and CF222P76) of Schunk Kohlenstofftechnik GmbH, Giessen, were tested in the investigation.     

Gibt es eine Universalkerbkontur nach dem Vorbild der Natur?

Notch stresses are bending stresses due to the deformation of the notch contour line which are superimposed to the nominal stresses. This dual nature of the overall stresses allows to generate an uniform stress state along the notch contour by increasing the superimposed bending stress in the same way as the nominal stress decreases. This was possible till now by CAO (Computer Aided Optimization) [1,2] which simulates tree growth. However a FEM‐program and CAO‐software were necessary. By this deeper understanding of the nature of notch stresses a simple pocket calculator can do the job. Even a pair of compasses and a 45° angle can help. The optimized notch shapes are very similar, so it seems that a universal notch shape might exist under certain circumstances. Fatigue tests by swelling bending proof the success by drastic increase of the number of load cycles until failure.     

‘Bundle-debond’ technique for characterizing fibre/matrix interfacial adhesion

An experimental technique called bundle-debonding, has been developed for characterizing the interfacial adhesion of fibre bundles and matrix. The specimen is double-notched and contains a partially embedded fibre layer in between the notches. When a tensile load is applied at the specimen ends, the load transfer across the notch and between two pieces of matrix, occurs through the interface between a single layer of fibres and matrix. Kevlar-29 (Kelvar is a registered trademark of E.I. duPont de nemours) fibre tows were used in conjunction with a solid phenolic resin to fabricate the specimens. Experiments were conducted at various embedded lengths resulting in interfacial debond. A simple shear-lag analysis was carried out to determine the interfacial shear strength. The interfacial shear strength of Kevlar-29/phenolic resin has been determined to be 15 MPa. This technique is promising for application on several fibre/matrix systems, specially for fibres of extremely low nominal diameter, supplied as tows.     

Failure assessment of notched polycrystalline graphite under tensile-shear loading

The fracture load and the fracture initiation angle were experimentally measured for a V-notched specimen made of polycrystalline graphite under combined tensile-shear loading. The experimental results were obtained for several specimens with different notch angles and various notch tip radii. The experimental observations showed that for a constant notch tip radius, the fracture load in pure tensile loading conditions decreases as the notch angle increases. Moreover, for a constant notch angle, as the notch tip radius increases the fracture load in graphite specimens enhances in the entire domain between pure tensile and pure shear loading conditions. A recently developed failure criterion was then used to estimate the experimental values of the notch fracture resistance and the fracture initiation angle for the tested graphite specimens. The experimental results could be estimated very well by using the results of the proposed criterion.     

A micrographic study of bending failure in five thermoplastic-carbon fibre composite laminates

The local deformation and failure sequences of five thermoplastic matrix composites were microscopically observed while bending the samples in a small fixture attached to a microscope stage. The thermoplastics are polycarbonate, polysulphone, polyphenylenesulphide, polyethersulphone and polyetheretherketone. The composites made from these plastics contain a variety of carbon fibres, though all with similar properties, and have fibre volume fractions ranging from 32 to 66%. Comparison is made to an epoxy matrix composite, 5208-T-300. Laminates tested are (0/90)_2S, with outer ply fibres parallel to the beam axis. Four-point bending is used at a typical span-to-thickness ratio of 39:1. A shallow notch is put in the samples at mid-span to avoid failure under the loading pins. It was found that all the thermoplastic composites failed by abrupt longitudinal compression buckling of the outer ply. Very little precursory damage was observed. Micrographs reveal typical fibre kinking associated with longitudinal compression failure. Curved fracture surfaces on the fibres suggest they failed in bending rather than direct compression. Delamination was suppressed in the thermoplastic composites, and the delamination that did occur was found to be the result of compression buckling, rather than vice versa. Microbuckling also caused other subsequent damage such as ply splitting, transverse ply shear failure, fibre tensile failure, and transverse ply cracking.     

Comparative assessment of stress transfer efficiency in tension and compression

Raman spectroscopy is used to get an insight into the microstructural aspects of the compressional behaviour of carbon fibre composites. This is done by a comparative assessment of the stress transfer efficiency in tension and compression in single-fibre discontinuous model geometries. It was found that the axial stress is transferred in the fibre through the generation of shear stresses at the interface. The mechanism of stress transfer is independent of the loading mode. Furthermore, the values of maximum interfacial shear stress are a function of the applied strain for both tension and compression loading. Significant differences were found, however, in the mode of failure of the two systems. In tension, interfacial failure initiates from the fibre ends at relatively high applied strains and the stress transfer efficiency is affected by the onset of matrix plasticity. On the other hand, in compression, deterioration of the stress transfer efficiency occurs prior to any noticeable interfacial failure at the fibre ends due to fibre collapse at low strains. Finally, it is worth noting that in compression, the fibre fragments remain in contact, and thus can still bear load.     

Fatigue failure transition analysis in load‐carrying cruciform welded joints based on strain energy density approach

This paper details a study of the application of notch stress intensity theory to the fatigue failure mode analysis of the transition in load‐carrying cruciform welded joints. The weldment fatigue crack initiation point is difficult to predict precisely because it usually occurs in the vicinity of the weld toe or weld root. To investigate the relationship between fatigue failure location and the geometry of the weldments, we analysed the weld toe and root asymptotic notch stress fields were analysed using the notch stress intensity factors on the basis of the Williams' solution in Linear Elastic Fracture Mechanics (LEFM). Numerous configurations of cruciform joints of various plate thicknesses, transverse plate thickness, weld sizes and incomplete penetration size were used to investigate the location of the fatigue failure. The strain energy density (SED) surrounding the notch tip was introduced to unify the scalar quantity and preclude the inconsistency of the dimensionality of the notch stress intensity factors for various notch opening angles. The results of the investigation showed that the SED approach can be used to determine the transition zone for a variety of joint geometries. The validity of the SED criteria was verified by comparing the experimental results of this study with the complied results for load‐carrying cruciform welded joints reported in literature.     

Property Optimisation in Fibre Metal Laminates

Fibre Metal Laminates (FMLs) are hybrid materials, which consist of thin metal sheets bonded together with alternating unidirectional fibre layers. This material concept has resulted in superior fatigue characteristics with respect to the metallic counterpart. Several static characteristics (specifically tension, shear, bearing, blunt and sharp notch behaviour) are however negatively influenced due to the fibre addition. This paper investigates the influence of constituent properties on these characteristics to define possible improvements. The available analytical models are reviewed and if necessary the specific failure mechanisms are described. Using these models and available test data, trend lines are obtained, which indicate the effects of the principal parameters and quantify potential improvements.     

Improving the prediction of tensile failure in unidirectional fibre composites by introducing matrix shear yielding

A Monte Carlo simulation is established to predict the failure strain of unidirectional fibre composites. The effect of matrix shear yielding of a high performance epoxy resin is introduced into the model through load sharing factors between the fibres adjacent to fibre-break(s). Strain concentration factors (SCF) of fibres are obtained using Finite Element Methods (FEM) in a three dimensional multi-fibre unit cell containing one, two and three adjoining fibre-break(s). The tensile strains of the surviving adjacent fibres are intensified as a function of their distances from the fracture. A statistical simulation is carried out to predict the failure strain of a single layer of unidirectional (UD) fibre composites with the thickness of the fibre ineffective length. Using the weakest link theory, the ultimate failure strain of a real size UD composite is predicted.     

Brittle fracture considerations of two external notches under combined loading

The interaction behavior of two external notches, modeled by plane hyperbolas, is analyzed using the strain energy density failure criterion[1,2]. It is shown that the pure shear and normal loadings may be combined to the case of inclined or off-axis loading of this double-notch geometry. The crack trajectory emanating from the notch surface is assumed for most cases to follow a path along which the material elements experience more volume change than shape change. This is determined by taking the minimum values of the strain energy density function. The failure loads are obtained by holding the critical strain energy density function constant and are shown to vary with the angle between the inclined load and the major axis of the notch. The variations will depend on the aspect ratios of the notches. Graphical plots of the crack trajectories for different loading angles are displayed. The double-notch geometry is such that only local trajectory information can be gleaned from the initial state. Numerical results of failure load, fracture angle, etc. are given for notches with different degrees of bluntness.