Page 83 - Engineered Interfaces in Fiber Reinforced Composites

66 Engineered interfaces in fiber reinforced composites

strength. Therefore, in-situ microscopic examination is often necessary to ensure
that interlaminar shear failure occurs at the maximum bending load.
Since the range of SDR that consistently produces interlaminar shear failure is
very small (i.e. four or five when the Young's moduli for the composites are greater
or less than 100 GPa, respectively, in accordance with the specification (ASTM D
2344, 1989)), the specimen has to be very thick, which is both expensive and more
difficult to fabricate. As an alternative, four-point bending of a long thin specimen is
suggested (Browning et al., 1983), whereby the sharing of the total load between two
loading noses can reduce the local stress concentration compared to three-point
bending (Cui and Wisnom, 1992).

3.3.3. Iosipescu shear test

The Iosipescu shear test (Iosipescu, 1967) is an ideal method that is relatively
simple to conduct with small and easily fabricated specimens, and it is capable of
measuring reliable shear strength and the modulus simultaneously (Lee and Munro,
1990). This test employs a double-edge notched specimen that is subjected to two
counteracting moments produced by force couples as shown in Fig 3.20(a). In a
qualitative photoelastic study, Iosipescu (1967) showed that when the depth of each
90" vee-notch is between 20% and 25% (typically 22% ) of specimen depth and the
notch tip radius is zero (Le., a sharp notch), the stress state across the notched
section is under pure and uniform shear for an isotropic material. This is a direct
result from the coincidence between the directions of the principle stresses at f45" to
the specimen axis and the 90" notch angle in the region of the zero bending moment.
In this case, there is no stress singularity at the notch tip because of the absence of
normal stresses at the point. The average shear stress in the middle section of the
specimen with width b is simply given by the applied load P, divided by the net
cross-sectional area

P
z=- (3.1 1)
bt '
To calculate the shear modulus, strain gauges are used to obtain the shear stress-
shear strain curve. Attracted by the almost pure shear state generated at the test
section, a number of researchers have studied the applicability of this test technique
to advanced composite materials, using FEMs as well as other experimental means.
Adams and Walrath (1982, 1987a, b) in particular have evaluated the shear stress
distribution as a function of notch depth, angle, notch tip radius, etc., which resulted
in redesigning the specimen geometry and test fixture. It is clearly shown that there is
a substantial stress concentration near the notch tip and the shear stress distribution
in the middle section of orthotropic specimen is not uniform as opposed to isotropic
materials. The stress concentration is found to be a function of the orthotropic ratio
(Le., Young's moduli ratio between two principal in-plane directions, E1 I /E22, which
is governed by the fiber orientation and the fiber volume fraction) and notch
geometry, and can be reduced by incorporating a large notch tip radius with a large