The fracture toughness for first matrix cracking of a unidirectionally reinforced carbon/carbon composite material
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The fracture toughness for first matrix cracking of a uniaxially reinforced C-fiber/C-matrix composite is investigated using a modified controlled surface flaw method. The theoretical models for first matrix cracking of brittle matrix composites including the stress intensity and the potential energy approaches are reviewed in the light of the experimental results. The sharing of the applied load between the reinforcing fibers and the brittle matrix along with extensive crack front debonding enhance the fracture toughness for first matrix cracking.
I. INTRODUCTION Linear elastic fracture mechanics and fracture toughness test methods developed for metallic materials apply readily to a number of monolithic ceramics with relatively fine microstructures and no phase-transformation toughening.1"4 The fracture toughnesses (KIc) and the strengths (07) of these fine-grain-size ceramics are not only defined unambiguously but also determined precisely in fracture mechanics tests using the critical load or stress for crack initiation. However, various types of high performance composite materials, including ductile or brittle matrices, exhibit extremely complicated and nonlinear fracture behaviors that lead to a number of critical issues in the definitions and the experimental determinations of strength and fracture toughness.5 A simple application of the rule-of-mixtures to the mechanical properties of brittle matrix composites with unidirectionally reinforcing fibers reveals that the first cracking appears in, and propagates through, the matrix in the first stage of crack growth process (first matrix cracking).6 9 If the interface is strong enough for stress transfer, yet weak enough to debond, the crack tip acuity is reduced by interface debonding. The postmatrix-cracking accompanied by interface debonding forms an extensive crack bridging of intact fibers and leaves fiber pull-out on the fracture surface behind the propagating crack tip, thus "toughening" the composite. In the present paper, the fracture toughness and the microfracture processes for first matrix cracking of fiberreinforced composite materials will be addressed. Other microfracture processes and mechanisms (fiber bridging and fiber pullout) are also substantial for providing higher resistance to crack extension in fiber-reinforced composites, and have been discussed9'10 or will be reported elsewhere.11 Two different theoretical models have been proposed that predict the fracture toughness for first matrix cracking in brittle matrix composites. One is the stress 2312
J. Mater. Res., Vol. 6, No. 11, Nov 1991
http://journals.cambridge.org
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intensity approach of Marshall, Cox, and Evans,12 and the other is the energy approach of McCartney.13 In the model of Marshall et al, the fracture toughness for first matrix cracking is introduced after stating intuitively that the matrix and composite stress intensities scale with stresses in the ratio Em/Ec: K -
Ec
(i)
Here Kc and K™ are the first matrix cracking fracture toughness of the c
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