Page 175 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 4. Micromechanics of stress transfer 157
I
80

0-
0 0.1 0.2 0.3 0.4 0.5 0.6
L-z (mm)
Fig. 4.40. Distributions of interface shear stress, q, along the fiber length at a constant applied stress
u = 4.0GPa for carbon fiber-epoxy matrix composites in fiber pull-out and fibcr push-out. After
Kim et al. (1994~).
gradually through wearing out or smoothing of the fiber surface roughness due to
abrasions under repeated loading and unloading. Experimental evidence on some
ceramic matrix composites containing SCS-6 Sic fibers (Jero and Keran, 1990; Jero
et al., 1991; Carter et al., 1991; Waren et al., 1992; Mackin et al., 1992a) and
sapphire fibers (Mackin et al., 1992b) has shown that the roughness interaction
contributes significantly to the interfacial clamping stress, as mentioned in Section
4.3.1. Frictional resistance is reduced when a fiber predisplaced in pull-out (or in
push-out), is then forced back to its original position, due probably to the fiber re-
seating in the matrix socket where the fiber surface roughness matches that of the
matrix.. Mode I fatigue tests on a meta-stable p-titanium alloy reinforced with
unidirectional SCS-6 Sic fibers also strongly indicate that degradation of the
interface properties allows large debonding and sliding.
Fatigue tests can be conducted on the same single fiber-matrix cylinder model as
used for monotonic pull-out and push-out tests. A simple alternating tensile (or
compressive) stress of magnitude Ao (= omax - omin where omin = 0) is applied
repeatedly to the fiber for each loading geometry, as schematically shown in Fig.
4.41. It is assumed here that the smoothed fiber surface due to repeated abrasion
eventually leads to a reduction in the frictional shear stress at the interface, which is
cquivalcnt to a dccrcasc in the cocfficicnt of friction p. Bascd on the thcorctical
results, a simple experimental method is proposed to evaluate the frictional
degradation of the interface.

4.5.2. Relative displacements and degradation function

Degradation of frictional resistance at the debonded interface will cause the
relative axial displacement between fiber and matrix to increase gradually. There are