Fibre fracture tensile

In consequence, when a tensile stress acts transversely to the fibres, fracture occurs as sketched in Figure 6.20(a), without the need to break any fibres. Indeed, they now serve as stress raisers and actually reduce the strength a- to below that of the pure matrix a. Similarly, a low strength r 2 is obtained under shear paraUel to the fibres (Figure 6.20(b)). 

An early view of fracture of para-aramid fibres was given by Yang (1993, p. 97), who refers to three basic forms. The caption to his fig. 3.28 describes fracture morphology of Kevlar aramid fibre in tensile breaks as Type (a), pointed break type (b) fibrillated break type (c) kink-band break. The kink-band breaks, which extend over a length approximately equal to a fibre diameter can be attributed to fibres that have been weakened by axial compression and will be discussed in a later section. 

Naaman and Homrich [24] studied the strengthening mechanisms in SIFCON and found that the mode of fa Mure was by fibre pull-out without fibre fracture. The tensile strength achieved with hooked and deformed fibres was roughly similar. 

There are less exotic ways of increasing the strength of cement and concrete. One is to impregnate it with a polymer, which fills the pores and increases the fracture toughness a little. Another is by fibre reinforcement (Chapter 25). Steel-reinforced concrete is a sort of fibre-reinforced composite the reinforcement carries tensile loads and, if prestressed, keeps the concrete in compression. Cement can be reinforced with fine steel wire, or with glass fibres. But these refinements, though simple, greatly increase the cost and mean that they are only viable in special applications. Plain Portland cement is probably the world s cheapest and most successful material. 

Fig. 25.4. Load transfer from the matrix to the fibre causes the tensile stress in the fibre to rise to peak in the middle. If the peak exceeds the fracture strength of the fibre, it breaks.

Its = tensile strength parallel to fibres d] = fracture strength of fibres d = yield strength of matrix. 

As shown in Fig. 3.4 stress-strain tests on uniaxially aligned fibre composites show that their behaviour lies somewhere between that of the fibres and that of the matrix. In regard to the strength of the composite, Ocu, the rule of mixtures has to be modified to relate to the matrix stress, o at the fracture strain of the fibres rather than the ultimate tensile strength, o u for the matrix. 

For fibres made from the same polymer but with different degrees of chain orientation the end points of the tensile curves, a5, are approximately located on a hyperbola. Typical examples of this fracture envelope are shown in Figs. 8... 

Fig. 8 Tensile curves of cellulose II fibres measured at an RH of 65% (1) Fibre B, (2) Cor-denka EHM yarn, (3) Cordenka 700 tyre yarn, (4) Cordenka 660 tyre yarn and (5) Enka viscose textile yarn [26]. The solid circles represent the strength corrected for the reduced cross section at fracture. The dotted curve is the hyperbola fitted to the end points of the tensile curves 1,3 and 5. The dashed curve is the fracture envelope calculated with Eqs. 9,23 and 24 using a critical shear stress rb=0.22 GPa... 

The relation between the end points of the tensile curve, ab and eh (= b), can be calculated with Eqs. 9,23 and 24. This relation is now by definition taken as the fracture envelope. Note that these equations only hold for elastic deformation. In order to account for some viscoelastic and plastic deformation, a value gv is used, which is somewhat smaller than the value for elastic deformation g. The dashed curves in Figs. 8 and 9 are the calculated fracture envelopes (neglecting the chain extension) for the cellulose II and the POK fibres, respectively. These figures show a good agreement between the observed and calculated fracture points. 

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