Fibre fracture brittle

At low densities of fibres, fracture proceeds by brittle failure or debonding of the fibres a fully linear elastic loading curve is observed (type I loading curve in Table II). 

Brittle failure of homogeneous fibres can be modelled, at a continuum level, using the tools of linear elastic fracture mechanics (LEFM) because usually the main hypotheses on which LEFM is based are satisfied when the rupture of a fibre is brittle, i.e. the... 

Some of these questions have been answered in a recent paper (Guinea et al., 2002b) for a very simple family of defects inner circular cracks and surface circular cracks, both in planes normal to the fibre axis. The main result is plotted in Fig. 5, showing the dependence of minimum rupture stress on crack radius. This plot may be used to set boundaries for some problems in brittle fibre fracture analysis, i.e. to evaluate the maximum circular defect size when the rupture stress is known or, conversely, to estimate the rupture stress when defects, of unknown location, can be modelled as circular cracks. 

If the load is increased further, the failure behaviour depends mainly on the fracture toughness of the matrix and the properties of the interface. If the matrix is brittle and the fracture toughness of the interface is large, the stress concentration in front of the crack tip is transferred to the fibre, causing it to break. In this case, the crack propagates on load increase, starting from the site of first fibre fracture. If, on the other hand, the stress concentration in front of the crack tip is not sufficient to cause fibre fracture, another weak fibre somewhere else in the material will fail first, at a position that is completely independent of the first failure position. Thus, fibres will fracture at arbitrary positions in the material, and the load on the material will increase... 

The bundled structure may also provide a reinforcing unit which is flexible and can be engaged in some local bending as it bridges a crack. Such local bending in a brittle matrix composite may produce premature fracture when the fibre is brittle but with a bundled reinforcement, even with brittle filaments, local bending capacity can be provided by the inner filament that are only loosely bonded to the matrix and can slide one relative to the other [91,92]. These special characteristics play an important role in the case of glass fibre reinforced cements and will be further discussed in Chapters 5 and 8. 

Many fibrous composites are made of strong, brittle fibres in a more ductile polymeric matrix. Then the stress-strain curve looks like the heavy line in Fig. 25.2. The figure largely explains itself. The stress-strain curve is linear, with slope E (eqn. 25.1) until the matrix yields. From there on, most of the extra load is carried by the fibres which continue to stretch elastically until they fracture. When they do, the stress drops to the yield strength of the matrix (though not as sharply as the figure shows because the fibres do not all break at once). When the matrix fractures, the composite fails completely. 

The glass-fibre nylons have a resistance to creep at least three times as great as unfilled polymers. In the case of impact strength the situation is complex since unfilled nylons tend to break showing tough fracture whereas the filled polymers break with a brittle fracture. On the other hand the glass-filled polymers are less notch sensitive and in some tests and service conditions the glass-filled nylons may prove the more satisfactory. 

This is because, with brittle fibres, failure of the composite will occur when the fibres reach their fracture strain. At this point the matrix is subjected to the full applied load, which it is unable to sustain. 

Recently, Oldfield Ellis (1991) have examined the reinforcement of glass-ionomer cement with alumina (Safil) and carbon fibres. The introduction of only small amounts of carbon fibres (5% to 7-5% by volume) into cements based on MP4 and G-338 glasses was found to increase considerably both the elastic modulus and flexural strength. There was an increase in work of fracture attributable to fibre pull-out. A modulus as high as 12-5 GPa has been attained with the addition of 12% by voliune of fibre into MP4 glass (Bailey et al, 1991). Results using alumina fibre were less promising as there was no fibre pull-out because of the brittle nature of alumina fibres which fractured under load. 

Fibres are added to a plastic or metal matrix, mainly to increase the strength of the material. In case of a ceramic matrix this is done to increase the fracture toughness of the brittle matrix. 

In addition, it has been shown (Boccaccini etal., 2001 Chlup etal., 2001) that the chevron-notched specimen flexural technique (the CN-technique) can be a reliable method of assessing fracture properties (fracture toughness, work of fracture) in thermally shocked brittle matrix composites reinforced by brittle fibres. 

Although fracture toughness can be increased, particle- or whisker-reinforced sialon composites generally show brittle behaviour and low damage tolerance this is in contrast to fibre-reinforced sialons which exhibit non-catastrophic failure. 

Long before a brittle hair crack has come to its complete development, it is already present as a nucleus, and the material has already undergone irreversible damage. Microscopic cracks are often partially filled with material in the form of very thin fibres, forming bridges between the fracture surfaces. These cracks are denoted as... 

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

Brittle fracture in glassy polymers occurs by the prior formation of crazes which then fracture. In the craze, fibrils of highly oriented polymer are produced and it is the fracture of these fibrils that almost certainly produces the observed radicals. Even in this case, then, the signal derives ultimately from a fibre in which... 

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