Effect of fiber length variation on tensile properties of carbon-fiber cement composites

Unlike other fiber types, carbon fibers tend to possess a distribution of fiber lengths even before mixing into a cementitious matrix. During mixing, some amount of fiber rupture may be expected. Along with other fiber parameters, fiber length distribution may be expected to play an important role in governing the composite properties. This paper describes the results of an experimental program which measures the length distribution of carbon fibers both before and after mixing. A database of composite tensile stress-strain behavior is established for a number of commercially available carbon fibers in a cementitious matrix. Special emphasis is placed on pseudo strain-hardening behavior and the associated ultimate tensile strength beyond first cracking. A theoretical model of this ultimate strength which explicitly account for fiber length distribution is developed and shown to account for the reduced tensile strength of experimentally observed results reasonably well. Results of this research suggest that for the purpose of brittle cementitious matrix reinforcement, carbon fibers with higher strain capacity may be more important than high modulus typically demanded for polymer matrix composite reinforcement.