Fibre pull-out

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. 

As you can see in the above figure, some fibres are somewhat pulled out of the matrix when a crack grows this occurs under the influence of prevailing forces. This is called the fibre pull-out effect. 

The photographs 14.3 and 14.5 clearly show the fibre pull-out, both in coated and non-coated specimens. In photograph 14.4 little pull-out can be seen because the temperature is high. 

Single fibre pull-out test 179 Site energy distribution 47 Skin 124... 

This sharp variation in toughness has been interpreted as a transition between different fracture mechanisms. Matrix fracture and fibre debonding are the prevailing fracture mechanisms at low values of the fibre orientation factor, while at high values the main fracture mechanism is fibre pull-out. Thus, besides a critical fibre length, also a critical fibre angle should be considered. 

In this method, a thin layer between fibre and matrix with proper interfacial bonding plays critical role for the crack deflection and fibre pull-out as shown in Figure 1.12. The CVI technique possesses outstanding advantages to deposit thin coatings on the surface of individual fibres within the preform then followed by the subsequent infiltration of the matrix phase. 

Figure 1.12 Crack deflection and fibre pull-out in 3-D C/SiC composites (a) crack deflection along the interface [60] and (b) fibre pull-out [58]...

The toughening mechanism of fibre-reinforced ceramic-matrix composites is due to the pull-out phenomenon of the fibre from the matrix. For 3-D C/SiC composites three kinds of pull-out are observed from the fracture, namely the fibre pull-out, the fibre cluster pull-out and the bundle pull-out illustrated in Figure 5.12a, b and c respectively. 

Figure 5.12. Fracture surface of 3D C/SiC composites [24] (a) fibre pull-out, (b) fibre...

The boron-doped pyrocarbon sublayer of C(B) is used as the mechanical fuse which ensures crack deflection and fibre pull-out. A hydrocarbon (CxHy) and BXn(X=F or Cl) are used as reactant precursors to deposit the C(B) layer. Both Bi3C2 and SiC are employed as the glass former to improve the oxidation resistance of the composite. B13C2 is deposited with the same precursors as for C(B) but in different conditions. CH3SiCl3 and H2 are used as the precursor gases for the SiC deposition. 

FIGURE 14. Crack deflection and fibre pull-out in a 2D C/C-SiC composite, loaded under bending stresses... 

FIGURE 4. SEM micrograph of the fracture surface of a soda-lime glass matrix composite reinforced by stainless steel fibres. The fracture surface exhibits fibre pull-out and partial plastic deformation of the fibres. (Micrograph reproduced with permission from Ref. [6]). 

As a consequence of this reaction, a strongly bonded Si02 oxidation layer is formed at the interface impeding crack deflection and fibre pull-out, thus rendering the composite brittle [3, 126]. 

Gao SL, Kim JK, Interphase morphology and fibre pull-out behaviour of carbon fibre PEEK composites. Key Eng Mater, 145(1,2), 811-816, 1998. 

The effect of AAPP treatment on the properties of namral fibre reinforced cellulose acetate butyrate (CAB) composites was then further studied [66]. At a fibre loading fraction of 30 wt%, the storage modulus of the short fibre composites improved by as much as 370% (Fig. 6.4). This is due to the enhanced interfacial adhesion between the fibres and the matrix as a result of AAPP treatment (as measured by single fibre pull-out test) [79], In addition to this, the increment in the mechanical glass transition temperature and a reduction in the height of tan 5 showed better fibre-matrix bonding and fibre-in-matrix distribution. It is evident that AAPP treatment is able to improve the fibre-matrix interface, resulting in composites with improved mechanical properties. 

Tensile properties of the HDPE/RET blend are shown in Table 8.2. The HDPE 100/0 carbon-fibre composite showed complete linear stress-strain behaviour up to its ultimate tensile strength and fracture at 10.3% strain. No definitive fracture was seen in the HDPE blends. This is due to the interfacial de-bonding between the constituents within the polymer. The apparent loss of cohesive strength of the matrix material resulted in fibre pull-out and interlaminar slip between the carbon-fibre plies. 

Fig. 6.104 Scanning electron micrograph showing the zone characterized by oxidized fibres marked A) and the final fibre pull-out zone on the creep fracture surface of a 0/90° SiCf/Al203 crept to failure in air under a tensile stress of 79 MPa at 1573 K [94], With kind permission of Elsevier...

Heppenstall-Butler, M., Bannister, D.J. and Young, R.J. (1996) A study of transcrystalline polypropylene/single-aramid-fibre pull-out behaviour using Raman spectroscopy. Composites A, 27A, 833-838. 

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