Page 61 - Engineered Interfaces in Fiber Reinforced Composites

44 Engineered interfaces in jber reinforced composites
3.2. The mechanical properties of fiber-matrix interfaces

3.2.1. Introduction

Test methods using microcomposites include the single fiber compression test, the
fiber fragmentation test, the fiber pull-out test, the fiber push-out (or indentation)
test and the slice compression test. These tests have a variety of specimen geometries
and scales involved. In these tests, the bond quality at the fiber-matrix interface is
measured in terms of the interface fracture toughness, Gi,, or the interface shear
(bond) strength (IFSS), Zb, for the bonded interface; and the interface frictional
strength (IFS), qr, which is a function of the coefficient of friction, 1.1, and residual
fiber clamping stress, 40, for the debonded interface. Therefore, these tests are
considered to provide direct measurements of interface properties relative to the test
methods based on bulk composite specimens.
Microcomposite tests have been used successfully to compare composites
containing fibers with different prior surface treatment and to distinguish the
interface-related failure mechanisms. However, all of these tests can hardly be
regarded as providing absolute values for these interface properties even after more
than 30 years of development of these testing techniques. This is in part supported
by the incredibly large data scatter that is discussed in Section 3.2.6.

3.2.2. Single jiber compression test

The single fiber compression test is one of the earliest test methods developed
based on microcomposites to measure the bond strength of glass fibers with
transparent polymer matrices (Mooney and McGarry, 1965). Two different types of
specimen geometry are used depending on the modes of failure that occur at the
fiber-matrix interface: one has a long hexahedral shape with a uniform cross-section
(Fig 3.1(a)); the other has a curved neck in the middle (Fig 3.1(b)). When the
parallel-sided specimen is loaded in longitudinal compression, shear stresses are
generated near the fiber ends as a result of the difference in elastic properties
between the fiber and the matrix, in a manner similar to the stress state occurring in
uniaxial tension. Further loading eventually causes the debond crack to initiate from
these regions due to the interface shear stress concentration (Le., shear debonding).
The curved-neck specimen under longitudinal compression causes interface
debonding to take place in the transverse direction @e. tensile debonding) due to
the transverse expansion of the matrix when its Poisson ratio is greater than that of
the fiber. The equations used to calculate the interface bond strengths in shear, Tb,
and under tension, Qb, are (Broutman, 1969):