Micromechanical theory and uniaxial tensile tests of fiber reinforced cement composites
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The mechanism of fracture arrest in brittle-matrix composites with strong, long fibers is analyzed by using the inclusion method. The maximum stress contribution of the matrix in composites is discussed in this paper. A critical volume fraction of fibers fc is theoretically derived. If the volume fraction / is less than fc, then debonding between fibers and matrix occurs before the crack propagates through the whole section. If / is greater than fc, then no debonding occurs before the crack propagates through the whole section. The value of fc depends on the matrix and fiber properties and the bond character of the interface. To verify the analytical predictions, experiments on fiber reinforced cement composites subjected to uniaxial tension were conducted. The results of the theoretical predictions were also compared satisfactorily with other published experimental data.
I. INTRODUCTION Studying the uniaxial tensile stress-strain curve of fiber reinforced cement composites is a way to understand the behavior of this composite material. From the results of tensile tests, three stages of behavior are observed (see inset, Fig. 1). In the first stage between points A and B in Fig. 1, the first cracking occurs. The second stage between points B and C can be termed as the multiple cracking stage. Finally, beyond point C, the additional increase in external load is sustained entirely by fibers. The maximum stress contribution of the matrix is normally referred to as the bend-over-point (BOP) that is shown in Fig. 1. A major crack tends to propagate through the whole section of the specimen at the BOP. It is known that when quasi-brittle materials are reinforced with strong continuous fibers, the stress at the bend-over-point cannot always be predicted by the law of mixtures. This is because the matrix in the composite exhibits a higher toughness than that of a monolithic sample of the matrix material. This toughening effect can come from two possible sources. One is the bridging action of the fibers, which prevents crack opening. The other is due to sliding and debonding along fiber-matrix interfaces. The energy dissipation due to debonding and sliding increases the effective resistance due to crack extension. Using the inclusion method,1 Mori and Mura2 examined the effects of crack arrest in a fiber reinforced composite when a crack is completely bridged by fibers. Recently this analysis was extended to the cases where sliding along matrix-fiber interfaces occurs in composites with short fibers3 and composites with strong long fibers.4'5 A similar subject has been treated J. Mater. Res., Vol. 6, No. 11, Nov 1991 http://journals.cambridge.org
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0.1 0.2 0.3 0.4 0.5 0.6 Strain (percentage)
Steel fiber reinforced cement Number of steel wires = 30 Diameter of steel wires =0.4 mm Volume ratio = 1.534%
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Strain (percentage) FIG. 1. A tensile stress-strain response of steel wire reinforced cement-based composite is shown. In addition to the composite response, the stress-strain response of
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