Fiber pull-out test

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 

8 shows the interface shear bond strength, tb, determined from Eq. (3.7), which is not a material constant but varies substantially with embedded fiber length, L. However, to evaluate all the relevant interface properties properly, which include the interface fracture toughness, Gic, the coefficient of friction, p, and the residual clamping stress, qo, it is necessary to obtain experimental results for a full range of L and plot these characteristic fiber stresses as a function of L. More details of the 

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 

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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. 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.

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).

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. 4.21. 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... 

Microcomposite tests including fiber pull-out tests are aimed at generating useful information regarding the interface quality in absolute terms, or at least in comparative terms between different composite systems. In this regard, theoretical models should provide a systematic means for data reduction to determine the relevant properties with reasonable accuracy from the experimental results. The data reduction scheme must not rely on the trial and error method. Although there are several methods of micromechanical analysis available, little attempt in the past has been put into providing such a means in a unified format. A systematic procedure is presented here to generate the fiber pull-out parameters and ultimately the relevant fiber-matrix interface properties. 

To analyze the stress transfer in the fiber pull-out test of a multiple fiber composite, the specimen is treated as a three-cylinder composite (Zhou and Mai, 1992) where a fiber is located at the center of a coaxial shell of the matrix, which, in turn, is surrounded by a trans-isotropic composite medium with an outer radius fl. 

The results presented in Section 4.3.6 suggest that the shear lag models based on a single fiber composite is inadequate for modelling a composite with a high fiber f). From the experimental viewpoint, to measure the relevant fiber-matrix interface properties, the fiber volume fraction in single fiber pull-out tests is always very low (i.e. Ff <0.01). This effectively means that testing with these specimens has the... 

There are many features in the analysis of the fiber push-out test which are similar to fiber pull-out. Typically, the conditions for interfacial debonding are formulated based on the two distinct approaches, i.e., the shear strength criterion and the fracture mechanics approach. The fiber push-out test can be analyzed in exactly the same way as the fiber pull-out test using the shear lag model with some modifications. These include the change in the sign of the IFSS and the increase in the interfacial radial stress, (o,z), which is positive in fiber push-out due to expansion of the fiber. These modifications are required as a result of the change in the direction of the external stress from tension in fiber pull-out to compression in fiber push-out. 

For the cylindrical coordinates of the fiber push-out model shown in Fig. 4.36 where the external (compressive) stress is conveniently regarded as positive, the basic governing equations and the equilibrium equations are essentially the same as the fiber pull-out test. The only exceptions are the equilibrium condition of Eq. (4.15) and the relation between the IFSS and the resultant interfacial radial stress given by Eq. (4.29), which are now replaced by ... 

In the same procedure as that employed for the fiber pull-out test, the solutions for stress distributions are obtained in the bonded region, which are exactly the same as those given in Eqs. (4.90) 4.92). The solutions for the stress distributions in the debonded regions are ... 

Butler, E.P., Fuller, E.R. and Chan, H.M. (1990). Interface properties for ceramic composites from a single-fiber pull-out test. In Tailored Interfaces in Composite Materials. Mat. Res. Soc. Symp. Proc, Vol. 170, Materials Research Society, Pittsburg, PA. pp. 17 24. 

Kallas, M.N., Koss, D.A., Hahn, H.T. and Hellmann, J.R. (1992). Interfacial stress state present in a thin slice fiber pull-out test. J. Mater. Sci. 27, 3821-3826. 

Marotzke, Ch. (1994). The elastic stress field arising in the single fiber pull-out test. Composites Sci. Technol. 50, 393 5. 

Fig. 5.27. Variation of tensile strength of copper coated carbon fibers as a function of coating thickness determined by single fiber pull-out test. After Abraham et al. (1992).

An implication of Eq. (6.3) is that debonding only occurs when < trj , or Gic < (7 d/SE[, otherwise the fiber will break prior to debonding. Other criteria for fiber fracture in single fiber pull-out test, refer to Section 4.2.4. 

Kim, J.K.. Zhou, L.M. Bryan, S.J. and Mai, Y,W. (1994b). Effect of fiber volume fraction on the stress transfer in fiber pull-out tests. Composites 25, 470-475. 

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