Page 79 - Engineered Interfaces in Fiber Reinforced Composites

62 Engineered interfaces in fiber reinforced composites
Table 3.1
Collated data obtained from all laboratories in a round robin test programmea
Testing method Pull-out Microdebond Fragmentation Push-out

Fiber treatment None Std. None Std. None Std. None Std
No. of laboratories 3 3 4 4 6 I 2 3
ISS, T~ (MPa) 64.6 84.1 48.3 69.1 23.8 41.3 47.8 49.5
SD 8.2 19.4 14.1 19.7 6.6 15.4 0.5 9.1
cv (Yo) 13 23 29 28 28 33 1 18.9

Std., standard surface treatment; ISS, interfacial shear strength; SD, one standard deviation; CV,
coefficient of variation.
"After Pitkethly et al. (1993).
laminated composites reinforced with continuous and long fibers, whether unidi-
rectional or cross-plied.
Apart from the short beam shear test, which measures the interlaminar shear
properties, many different specimen geometry and loading configurations are
available in the literature for the translaminar or in-plane strength measurements.
These include the Iosipescu shear test, the [f45"], tensile test, the [lo0] off-axis
tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled
tube torsion test. Since the state of shear stress in the test areas of the specimens is
seldom pure or uniform in most of these techniques, the results obtained are likely to
be inconsistent. In addition to the above shear tests, the transverse tension test is
another simple popular method to assess the bond quality of bulk composites. Some
of these methods are more widely used than others due to their simplicity in
specimen preparation and data reduction methodology.
Testing on bulk composite materials has a more serious limitation than in
microcomposite tests in that the actual locus and modes of failure have to be
consistent with what are originally designed for the composite in order for a specific
test to be valid. Judgment of validity of the test by examining the onset of failure
during the experiment is a tedious task, which cannot be assumed to have taken
place for a given loading condition. Even in an apparent interlaminar shear failure,
the failure may occur at the fiber-matrix interface, in the matrix or in a combination
of these, depending on the loading direction relative to the interface concerned and,
more importantly, on the relative magnitudes of the fiber-matrix interface bond
strength and the shear strength of the matrix material. This makes the interpretation
of experimental data more complicated since this requires proper micromechanics
analysis to be developed together with prior knowledge of the matrix properties (Lee
and Munro, 1986; Pindera et al., 1987).

3.3.2. Short beam shear test

The short beam shear test designated in ASTM D 2344 (1989) involves loading a
beam fabricated from unidirectional laminate composites in three-point bending as