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A finite element-based phase field method for modeling the complex failure mechanism of a unidirectional fiber composite under transverse tensile loading was developed. An interface layer with a finite width was incorporated between a fiber and the matrix to implement interfacial failure. Different processes such as onset of interfacial cracking and kinking, matrix cracking, and their interactions were modeled within a unified framework. The sensitivity of the onset of interfacial cracking to the fracture mode was assessed by incorporating a phenomenological model of the interface fracture toughness in the computational model. It was found that the onset of interfacial cracking involved a combination of cracking modes I and II over a finite incremental length. The single-fiber composite model was verified by comparing its predictions of the stress distribution along the interface and the critical stress at the onset of interfacial cracking with those from the analytical solutions. The effects of the stiffness and fracture properties of the constituents on the critical stress at the onset of interfacial cracking were also evaluated, as well as the effects of the interface width and the fiber radius. Finally, the validated model was applied to the fracture analysis of a multi-fiber composite. The complex failure mechanisms were found to agree well with experimental observations.

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The effects of complex state of stress on the compressive behaviour of 3D carbon/carbon composites are investigated by application of uniaxial and biaxial loadings using a specially developed Zwick cruciform testing facility. The shape of the biaxially loaded cruciform specimen is optimised to avoid premature fracture outside the gauge section. A semi-analytical method is proposed to determine the stress components in the gauge section of the biaxial specimen. The experimentally obtained failure stress relation, which traces an elliptical path in the principal stress space, can be well represented by the Tsai criterion with a stress interaction parameter of F 12 =-0.85. Macro-fracture morphology and SEM micrographs are examined and the results show that the failure mechanisms of the composites vary with the loading ratio. The results also suggest that the biaxial stress interaction effect is represented by a domain in the biaxial specimen, which is characterised by torsion and bending fractures in the dislocated fibres between two adjacent Z yarns.

Virtual tests for a single-fiber–reinforced composite model subjecting to transverse tension are carried out based on a phase-field method considering a varying interface toughness parameter. Without pre-treating the crack initiation location and propagation path, the complete fracture process is realized for the first time in a three-dimensional numerical model, including nucleation cracks on the fiber/matrix interface at the free end, tunneling cracks along the fiber axis, and kinked interface cracks deviating from the interface and penetrating into the matrix. The numerically calculated crack propagation process is in good agreement with the in situ observations in the literature, indicating that the present model provides a good real-time quantitative numerical method for three-dimensional fracture analysis of fiber-reinforced composites. Tunneling cracks tend to cause macroscopic interface debonding and fiber pull-out. The interface tunneling crack initiation and the transition to the steady state inside the model are captured and analyzed in the numerical model. Kinked interface cracks can merge with other matrix cracks, forming a macroscopic transverse crack fracture mode. The kinking behaviors affected by the initial crack size and the interface properties are investigated. This study for the detailed crack propagation is helpful in understanding the toughening mechanism of fiber-reinforced composites under transverse tension.