Bond Properties of Micro-Fibers in Cementitious Matrix
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ABSTRACT Using a new technique for pullout test of microfibers, the interfacial bond properties of two carbon fibers having a diameter of 10 jtm and 46 ltm were tested and compared with those of high modulus polyethylene and steel fibers having a diameter of 42 lim and 18 [tm, respectively. The fibers were embedded in cement matrices of different water to binder ratio and silica-fume content. By studying the complete pullout load-pullout displacement curve and by using Environmental Scanning Electron Microscope (ESEM), it was found that the bond mechanism is mainly of friction for the fine and smooth carbon and polyethylene fibers. For the large carbon fiber and steel fiber, mechanical anchorage is the main bond mechanism.
INTRODUCTION Interfacial fiber-matrix properties serve as an important parameter with the use of fibers in composite materials. Most composite models are based on the pullout behavior of a single fiber, assuming a constant frictional bond between the fiber and the matrix, independent of slip. P-u
(pullout load vs. displacement) relationship was developed by Li [1], assuming constant friction (Fig. 1). The pre-peak P-u relationship typically shows a non-linear behavior due to the debonding process and fiber stretching with increasing load, while the post-peak relationship shows a linear decrease as the embedded length of the fiber becomes shorter when the fiber is pulled out (line a in Fig. 1). However, studies done by Wang et al. [2] on Nylon fibers and Naaman et al. [3] on steel fibers showed slip-hardening or slip-weakening behavior, respectively, as the fiber-matrix interaction is changed due to damage of the interface, associated with the relative stiffness of fiber and matrix. For stiff matrix and relatively weak fiber it might be slip-hardening behavior (line c in Fig. 1), due to peeling of the fiber by the stiff matrix (Wang et al. [2]). For a stiff fiber and a relatively weak matrix it might be slip-weakening behavior (line b in Fig. 1), due to rapid destruction of the matrix around the fiber as it is pulled out (Naaman et al. [3]).
1Present address: National Building Research Institute, Technion - Israel Institute of Technology, Haifa 32000,
Israel. 529
1995 Materials Research Society Mat. Res. Soc. Symp. Proc. Vol. 370 ©
In this work, the bond properties of micro-fibers were studied by using the direct pullout technique developed by Katz and Li [4] and by using an Environmental Scanning Electron Microscope (ESEM) for different matrix compositions and micro-fiber types, in order to determine the pullout behavior of this kind of fibers for different matrices. EXPERIMENTAL Four micro-fiber types were tested in the experimental program: Fiber A - high modulus polyethylene fiber ('Spectra 900') having a diameter of d=-42 pim and modulus of elasticity of E t =120 GPa; Fiber B - steel fiber, d=l&8gm and Ef=210 GPa; Fiber C - carbon fiber, d=10 pim and E/==240 GPa; and Fiber D - carbon fiber, d=-46 gm and E/=175 GPa. Two water to binder (cementitious materials) ratios were tested: low (w
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