Fiber fragmentation

As fibers are crushed in the composite, their strength increases. So long as the fibers interact with each other they can stand a load. When, however, the fiber length becomes so small that the shear stresses between the fiber and the matrix become about as low as the shear strength at the interphase the crushing process will stop and fiber fragments will be pulled out of the matrix. 

The fiber fragmentation test is at present one of the most popular methods to evaluate the interface properties of fiber-matrix composites. Although the loading geometry employed in the test method closely resembles composite components that have been subjected to uniaxial tension, the mechanics required to determine the interface properties are the least understood. 

Fig. 3.2. (a) Dog-bone shape fiber fragmentation test specimen (b) fiber fragmentation under progressively increasing load from (i) to (iii) with corresponding fiber axial stress of profile. 

The fiber fragment length can be measured using a conventional optical microscope for transparent matrix composites, notably those containing thermoset polymer matrices. The photoelastic technique along with polarized optical microscopy allows the spatial distribution of stresses to be evaluated in the matrix around the fiber and near its broken ends. 

Fig. (a) Typical load-displaccnicnt curve and (b) acoustic emission events for a fiber fragmentation test on an AS4 carbon fiber PEEK matrix composite. After Vautcy and Favre (1990). 

Fig. 3,4. Ln-Ln plot of fiber fragment length as a function of fiber stress (a) for Kevlar 29 fiber-epoxy matrix composite and (b) for a carbon fiber-epoxy matrix composite. Yabin et al. (1991).

Apart from the mechanical properties of the composite constituents that dominate the fiber fragment length, peculiar structural properties of the fiber may... 

It has been noted in a round robin test of microcomposites that there arc large variations in test results for an apparently identical fiber and matrix system between 13 different laboratories and testing methods (Pitkethly et al., 1993). Table 3.1 and Fig 3.15 summarize the IFSS values of Courtaulds XA (untreated and standard surface treated) carbon fibers embedded in an MY 750 epoxy resin. It is noted that the difference in the average ISS values between testing methods, inclusive of the fiber fragmentation test, fiber pull-out test, microdebond test and microindentation test, are as high as a factor of 2.7. The most significant variation in ISS is obtained in the fiber pull-out /microdebond tests for the fibers with prior surface treatments, and the microindentation test shows the least variation. 

Fig. 3.15. Interface shear strength. Xb, of (a) untreated and (b) treated LXA500 carbon fiber-epoxy matrix system measured at 10 different laboratories and using different testing methods. (O) fiber pull-out test ( ) microdebond lest ( ) fiber push-out lest (A) fiber fragmentation test. After Pitkelhly el al. (1993).

Curtin, W.A. (1991). Exact theory of fiber fragmentation in a single filament composite. J. Mater. Sci. 26, 5239-5253. 

Favre, J.P. and Jacques, D. (1990). Stress transfer by shear in carbon fiber model composites Part I Results of single fiber fragmentation tests with thermosetting resins. J. Mater. Sci. 25, 1373-1380. 

Kim, J.K. (1997). Stress transfer in the fiber fragmentation test, part IFF. Effects of interface debonding and matrix yielding. J. Mater. Sci. 32, 701-711. 

Netravali, A.N., Topoleski, L.T.T., Sachse, W.H. and Phoenix, S.L. (1989c). An acoustic emission technique for measuring fiber fragment length distributions in single fiber composite test. Composites Sci. Technol. 35, 13-29. 

Scherf, J. and Wagner, H.D, (1992). Interpretation of fiber fragmentation in carbon/epoxy single fiber composites Possible fiber pre-tension effects. Polym. Eng. Sci. 32, 298-304. 

Whitney, J.M. and Drzal, L.T. (1987). Axisymmetric stress distribution around an isolated fiber fragment. In Toughened Composites, ASTM STP 937, (N.J. Johnston ed.), ASTM, PA, pp. 179-196. 

Fig. 4.6. Schematic drawing of a partially debonded single fiber composite model subject to external stress, (Ta, in the fiber fragmentation test.

Fig. 4.7. Distributions of (a) fiber axial stress, a, (b) matrix axial stress, Om., and (c) interface shear stress. T along half the embedded fiber length, L, in the fiber fragmentation test.

Therefore, combining Eqs. (4.41)-(4.44), the fiber fragmentation criterion is derived in terms of the applied stress, [Pg.107]

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