Page 62 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 3. Measurements of interface/interlaminar properties 45

Fig. 3. I. Single fiber compressive tests with (a) parallel-sided and (b) curved-neck specimen

for shear debonding in the parallel-sided specimen and for tensile debonding in the
curved-neck specimen, respectively. CTN is the net compressive stress at the smallest
cross-section obtained upon interface debonding. a = Ern/& is Young’s moduli
ratio of the matrix to the fiber, and vf and v, are Poisson ratios of the fiber and
matrix, respectively. The constant 2.5 in Eq. (3.1) is taken from the empirically
measured shear stress concentration factor.
The single fiber compression test has not been as popular as other microcomposite
tests because of the problems associated with specimen preparation and visual
detection of the onset of interfacial debonding. To be able to obtain accurate
reproducible results, the fibers have to be accurately aligned. With time, this test
method became obsolete, but it has provided a sound basis for further development
of other testing techniques using similar single fiber microcomposite geometry.

3.2.3. Fiber fragmentation test
The fiber fragmentation test is at present one of the most popular methods to
evaluate the interface properties of fiber-matrix composites. Although the loading
geometry employed in the test method closely resembles composite components that
have been subjected to uniaxial tension, the mechanics required to determine the
interface properties are the least understood.
This test is developed from the early work of Kelly and Tyson (1965) who
investigated brittle tungsten fibers that broke into multiple segments in a copper
matrix composite. Here a dog-bone shaped specimen is prepared such that a single
fiber of finite length is embedded entirely in the middle of a matrix (Fig 3.2(a)). The
failure strain of the matrix material must be significantly (Le., ideally at least three
times) greater than that of the fiber to avoid premature failure of the specimen due
to fiber breakage. When the specimen is snbjected to axial tension (or occasionally in
compression (Boll et al., 1990)), the embedded fiber breaks into increasingly smaller