Page 74 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 3. Measurements of interfacelinterlaminar properties 57

slice compression test to be discussed in Section 3.2.6 serves the same purpose. The
microindentation test utilizes a microhardness indenter with various tip shapes and
sizes to apply a compressive force to push against a fiber end into the metallograph-
ically polished surface of a matrix block. In the original version of the test
(Fig 3.12(a)), a selected fiber is loaded using spherical indenters in steps of increasing
force, and the interface bonding is monitored microscopically between steps, until
debonding is observed (Mandell et al., 1980). The IFSS, Zb, is calculated from

where Qd is the average compressive stress applied to the fiber end at debonding.
Zmax/q is the ratio of the maximum interface shear stress to the applied stress
determined in the finite element method (FEM).
In the second approach shown in Fig 3.12(b), a force is applied continuously using
a Vickers microhardness indenter to compress the fiber into the specimen surface
(Marshall, 1984). For ceramic matrix composites where the bonding at the interface
is typically mechanical in nature, the interface shear stress, qr, against the constant
frictional sliding is calculated based on simple force balance (Marshall, 1984):

Composite
+slice

Fig. 3.12. Schematic drawings of indentation (or fiber push-out) techniques: using (a) a spherical indenter;
(b) a Vickers microhardness indenter; (c) on a thin slice. After Grande et al. (1988).