Fracture fibre

Note that as shown below, this must be the matrix stress at the fibre fracture strain. 

Fig. 15 Initiation of fibre fracture by a crack oriented parallel to the chain direction in a domain. It is proposed that a circular crack with a radius q releases the strain energy in a sphere around the crack with the same radius. Note that in this two-dimensional drawing only the circular crack is shown in perspective...

So far we have employed in this discussion a critical shear stress as a criterion for fibre fracture. In Sect. 4 it will be shown that a critical shear strain or a maximum rotation of the chain axis is a more appropriate criterion when the time dependence of the strength is considered. 

Hearle JWS, Lomas B, Cooke WD (1998) Atlas of fibre fracture and damage to textiles. Woodhead, Cambridge... 

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). 

Hearle J W S, Fibre fracture and textile durability-part 2 , Textiles Magazine, 1999, 28, 15-21. 

Atlas of fibre fracture and damage to textiles Second edition... 

Some modes may dominate for example, for large bending strains in a flexible structure, fibre fracture in tension and fibre kinking in compression wdl dominate near both surfaces. Matrix cracks can cause delamination when they reach a ply interface. If the structure is stiff enough to resist with a significant force, then local indentation damage, and shear-driven delamination in the interior, wdl occur. Figure 9.2 shows schematically the different modes of fadure in three zones of a laminate. The peanut shape deformations (3) have this shape because the compression under the impact force suppresses the delaminations. 

In compression, the mechanism of failure changes from fibre fracture to fibre buckling. Here, the modulus of the matrix is a critical factor in supporting the fibre and by prevention of kinking (of the fibres) in a shear band. Soutis [53] has discussed the modelling of these types of failure using Eqns (12.17) and (12.18) ... 

Figure 13.5 The variation of the length of fibre fracture with impulse for 2.2-, 4.2- and 6.2-mm-thick CF/PEI plates (laminates A to C, respectively) and a 4.5-mm-thick GF/PEI plate (laminate F) [24]. CF, carhon fibre GF, glass fibre PEI, polyether imide.

Acoustic emission utilizes the fact that a material s response to stress is accompanied by the release of energy as sound or ultrasound. Different kinds of events - notably matrix cracking, debonding and fibre fracture -can be distinguished. Thus the mechanical condition of a composite can be examined. 

If the interfacial bond between fibres and resin is broken, whether by mechanical stress, hot water immersion or thermal cychng, the fibres will not be able to support the load adequately. As a consequence, there can be localized matrix cracking and the cracks in the matrix eventually propagate by a sequence of events which ultimately leads to large scale fibre fracture or to delamination, or both. 

A common hazard for reinforced plastics is damage from stones on vehicles, from aircraft runways on airplanes, from collisions between boats and driftwood or between boats and the quayside, or from dropped tools on chemical plant and aircraft wings, and so on. The resulting damage is often difficult to see with the naked eye, but it can include delamination, matrix cracking, fibre debonding and in severe cases, fibre fracture. Particulate... 

During stage III the first fibre fracture occurs in the fill threads at the centre of undulation. Later, total failure of a fill thread results in a stepwise stiffness reduction, as is documented in Fig. 5.9(c). 

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)). 

FIG. 8 SEM micrographs of plasma polymer coated carbon fibres Fracture (a) of Grafil a fibre and (b) of a Hercules fibre... 

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