Fiber pull-out

In the single fiber pull out test (SFPO), a small portion of the fiber is embedded in the bulky matrix and the interfacial strength is calculated from the peak load when the fiber is pulled out of the composite. 

Fig. 7. Fiber delamination, fiber pull-out and matrix cracks lead to a high energy dissipation and to good damage tolerance (SiC/SiC)ii...

Fig. 11. Fractured tensile specimen from 2D-C/SiC with marked fiber pull-out...

In the fiber pull-out test, a fiber(s) is partially embedded in a matrix block or thin disc of various shapes and sizes as shown in Fig 3.6. When the fiber is loaded under tension while the matrix block is gripped, the external force applied to the fiber is recorded as a function of time or fiber end displacement during the whole debond and pull-out process. There are characteristic fiber stresses that can be obtained from the typical force (or fiber stress). The displacement curve of the fiber pull-out... 

Fig. 3,6, Schematic illustrations of various specimen geometry of the fiber pull-out test (a) disc-shaped specimen with restrained-top loading (b) long matrix block specimen with fixed bottom loading, (c) double pull-out with multiple embedded fibers.

In view of the fact that the above techniques examine single fibers embedded in a matrix block, application of the experimental measurements to practical fiber composites may be limited to those with small fiber volume fractions where any effects of interactions between neighboring fibers can be completely neglected. To relate the interface properties with the gross performance of real composites, the effects of the fiber volume fraction have to be taken into account. To accommodate this important issue, a modified version of the fiber pull-out test, the so-called microbundle pull-out test, has been developed recently by Schwartz and coworkers (Qui and Schwartz, 1991, 1993 Stumpf and Schwartz, 1993 Sastry et al., 1993). In... 

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).

Kim, J.K., Baillic, C. and Mai, Y.W. (1992). Fnterfacial debonding and fiber pull-out stresses, part F. A critical comparison of existing theories with experiments. J. Mater. Sci. 27, 3143-3154. 

Qiu, Y. and Schwartz, P. (1991). A new method for study of the fiber-matrix interface in composites Single fiber pull-out from a microcomposite. J. Adhesion Sci. Technol. 5, 741-756. 

In an approach similar to that adopted in the work of Greszczuk (1969) on the fiber pull-out test, Piggott (1980) has obtained solutions for the stress fields in the fiber for several different cases of fiber-matrix interface, including the perfectly... 

The radial (compressive) stress, qo, is caused by the matrix shrinkage and differential thermal contraction of the constituents upon cooling from the processing temperature. It should be noted that q a, z) is compressive (i.e. negative) when the fiber has a lower Poisson ratio than the matrix (vf < Vm) as is the normal case for most fiber composites. It follows that q (a,z) acts in synergy with the compressive radial stress, 0, as opposed to the case of the fiber pull-out test where the two radial stresses counterbalance, to be demonstrated in Section 4.3. Combining Eqs. (4.11), (4.12), (4,18) and (4.29), and for the boundary conditions at the debonded region... 

Theoretical analyses of interfacial debonding and frictional pull-out in the fiber pull-out test were initially modeled for ductile matrices (e.g. tungsten wire-copper matrix (Kelly and Tyson, 1965, Kelly, 1966)) assuming a uniform IFSS. Based on the matrix yielding over the entire embedded fiber length, as a predominant failure mechanism at the interface region, a simple force balance shown in Fig. 4.19 gives the fiber pull-out stress, which varies directly proportionally to the cylindrical surface area of the fiber... 

Fig. 4.19. Fiber pull-out stress as a function of embedded fiber length, /./2a, for a tungsten wire embedded copper matrix composite system. Open symbols for pulled-out specimens solid symbols for fractured specimens. After Kelly and Tyson (1965).

Fig. 4.21. Schematic drawing of the partially debonded fiber in fiber pull-out test.

For the cylindrical coordinates r,9,z) in the fiber pull-out test, the basic governing equations and the mechanical equilibrium conditions between the composite constituents are essentially the same as those given in Section 4.2.3, i.e. Eqs. (4.8)-(4.18). The only exception is the equilibrium condition between the external stress and the axial stresses in the fiber and the matrix given by Eq. (4.11), which has to be modified to... 

Fig. 4.22. Distributions of (a) fiber axial stress, a, (b) matrix axial stress, and (c) interface shear stress, Zi, along the embedded fiber length in fiber pull-out. After Zhou et al. (1992a, b, c).

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