Page 126 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 4. Micromechanics of stress transfer 109

6

5-
4-

3-

2-
1-

.......... ...............................................
(4 Applied stress u, (GPa)

E . 7,
-6 ‘1 ,

. . ..I ................
0.05 0.1 0.15
(b) Applied stress 0, (GPa)

Fig. 4.9. Comparisons of mean fiber fragment length, 2L, as a function of applied stress, uar between
experiments and theory for carbon fiber-epoxy matrix composites with (a) XAI fiber and (b) XAlOO fiber:
(0) experiment; (-) prediction (-------) debond length. After Zhou et al. (1995a, b).

treatment (on the XA 100 fibers) gives a stronger interface bond with shorter
debond length. The strong interface bond in turn permits efficient stress transfer,
causing the FAS to reach more easily the tensile strength of the fiber (and eventual
fiber fracture). The average length of fractured fiber segments at a given applied
stress is a measure of efficiency of stress transfer across the fiber-matrix interface in
the fiber fragmentation test. The above observations regarding the different fiber
fragmentation response for the composites with different fiber surface treatments
confirm that the interface properties influence significantly the fiber fragment
behavior, as opposed to the suggestion of the dominant role of the Young’s modulus
ratio in determining the critical transfer length (see Section 3.2.3). Another
important implication is that, since there are debonded regions of substantial lengths
at both ends of the fiber, the efficiency of stress transfer at the interface cannot be