Fiber push-out

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).

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

Fig. 4.20. Schematic presentation of rough fiber surface in a fiber push-out test. After Maekin et al.

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 ... 

Based on the same energy balance theory as employed for the fiber pull-out, a fiber-matrix interface debond criterion is derived for fiber push-out in a form similar to that for fiber pull-out... 

Fig. 4.37. Distributions of (a) fiber axial stress, a, (b) matrix axial stress, and (c) interface shear stress. Ti, along the embedded fiber length in fiber push-out. After Kim et al. (1994c).

Comparisons between fiber pull-out and fiber push-out... 

To evaluate the stability of the debond process, the instability parameter, z,nax, is compared, z ax values calculated based on Eqs. (4.104) and (4.139) respectively for fiber pull-out and fiber push-out give z ax = 6-5, 6.2 mm for coated steel wire-epoxy matrix and z ax = 0.5, 0.49 mm for the untreated SiC-fiber-glass matrix composite... 

Fig. 4.38. Comparisons of partial debond stress, (rj, between fiber pull-out and fiber push-out as a function of debond length, f, for (a) release agent coated steel fiber-epoxy matrix composites and (b) untreated SiC fiber-glass matrix composites. After Kim ct al. (1994c).

The analytical solutions derived in Sections 4.3 and 4.4 for the stress distributions in the monotonic fiber pull-out and fiber push-out loadings are further extended to cyclic loading (Zhou et al., 1993) and the progressive damage processes of the interface are characterized. It is assumed that the cyclic fatigue of uniform stress amplitude causes the frictional properties at the debonded interface to degrade... 

Fig. 4,40. Distributions of interface shear stress, r, along the fiber length at a constant applied stress o = 4.0GPa for carbon fiber-epoxy matrix composites in fiber pull-out and fiber push-out. After...

Fig. 4.41. Schematic drawings of loading and unloading processes measuring the relative displacements 5 and 6, in (a) fiber pull-out and (b) fiber push-out models under cyclic loading. After Zhou et al. (1993).

Eqs. (4.140) and (4.150)-(4.152) are used to evaluate the response of the model composites in cyclic loading and the displacements 6 and 8, can be expressed as a function of the alternating stress, Aff, and the number of cycles, N. In experiments, degradation of the interface properties, e.g., the coefficient of friction, p or A(= 2pjfc/a), can also be expressed in terms of the cyclic loading parameters, Aoptical methods (with a microscope) or by means of more complicated instruments (see for example Naaman et al. (1992)) in fiber pull-out. Alternatively, they can be directly determined from the load and load-point displacement records in the case of fiber push-out. 

Figs. 4.44 and 4.45 show the increase in the debond length, f, and displacement, as a result of the reduction of p (from Po = 0.22 to p = 0.07) under cyclic loading. It is interesting to note that both I and 5 remain constant until the coefficient of friction, p, is reduced to a critical value p. (= 0.144 and 0.166, respectively for fiber pull-out and fiber push-out). The implication is that the debond crack does not grow... 

Ananth, C.R. and Chandra, N. (1995). Numerical modelling of fiber push-out test in metallic and intermetallic matrix composites mechanics of failure process. J. Composite Mater. 29, 1488-1514. 

Shetty, D.K. (1988). Shear-lag analysis of fiber push-out (indentation) tests for estimating interfacial friction stress in ceramic-matrix composites. J. Am. Ceram. Soc. 71, C.107-109,... 

Singh, R.N. and Sutcu, M. (1991). Determination of fiber-matrix interfacial properties in ceramic-matrix composites by a fiber push-out technique. J. Mater. Sci. 26, 2547-2556. 

Zhou, L.M., Kim, J.K. and Mai, Y.W. (1992b). A comparison of instability during interfacial debonding in fiber pull-out and fiber push-out. In Proc. Second Intern. Symp. on Composite Materials and Structures (ISCMS-2) (C.T. Sun and T.T. Loo, eds.), Peking University press, Beijing, pp. 284-289. 

Fig. 5.5. Normalized irKerfacial shear strength of unsized (bare) and sized E-glass fiber-epoxy matrix eomposites measured from the interfaeial testing system (ITS, equivalent to fiber push-out test), short beam shear (SBS) test, 0° flexural test and 90° flexural test. After Drown et al. (1991).

Fig. 1.13 Some typical fiber push-out measurements on metal, ceramic, and intermetallic composites (a) A O/HAl with C/AI2O3 double coatings, (b) SiC/glass,...

In Eqn. (12), rd represents the dynamic interfacial shear stress, which may differ from that which would be measured from fiber push-out experiments, which are typically conducted at low sliding velocities. Equation (12) holds for partial sliding along the interface. When the minimum applied stress is equal to zero, the area of the hysteresis loop can also be calculated as the integral from zero to (Tmax of the difference between the strain paths for loading and unloading (Eqns. (3) and (4)) ... 

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