Anisotropic damage evolution in ceramic matrix composites

This thesis seeks to develop a better understanding of the damage evolution process and damage mechanisms in ceramic matrix composites, and to provide more insight into anisotropic constitutive modeling for component design. This is achieved using experimental techniques including laser-ultrasonics, acoustic emission measurements, surface replication studies, and electron microscopy. The laser-ultrasound approach was developed to monitor damage in composites in situ during uniaxial loading: by measuring ultrasonic velocities along various directions, anisotropic elastic stiffness degradation could be measured continuously during mechanical testing. This characterization method was applied to study damage evolution in a variety of CMC systems, including unidirectional and {dollar}0spcirc/90spcirc{dollar} cross-ply calcium aluminosilicate (CAS) composites, and unidirectional magnesium aluminosilicate (MAS), all reinforced by Nicalon SiC fibers. Acoustic emission and surface replica observations were used to relate degradation of the axial elastic stiffness to the accumulation of matrix cracks normal to the loading (and fiber) direction. A substantial loss of transverse elastic stiffness was observed and attributed to fiber/matrix interfacial debonding.; To investigate the influence of the interfacial strength and the residual stress state on damage evolution, the mechanical behavior of barium-stuffed magnesium aluminosilicate both undoped (MAS0) and doped with 5% borosilicate glass (MAS5), again both reinforced by unidirectional Nicalon SiC, were also studied. The interfacial bonding strengths of MAS0 and MAS5 are different. The residual stress states between the MAS and the CAS also differ. Applying the laser-ultrasonic technique, damage transverse to the fiber axis was characterized, and correlated to the degree of nonlinear deformation in the fiber axial direction. The influence of interfaces on inhibiting and deflecting matrix cracks was investigated. A micromechnaical cell model and the finite element method were both used to analyze the effect of interfacial debonding on the transverse elastic properties.; The nature of matrix crack initiation was investigated. Long transverse steady-state cracks, which are usually assumed to exist in CMGs, were not found in the MAS system. Instead, cracks are short, localized and distributed randomly. The deviation of the stress-strain curve from linearity is shown to be a result of the accumulation of short matrix cracks in this system. Furthermore, matrix crack initiation in the MAS system does not appear to be affected by the degree of bonding at the fiber/matrix interface. While experimental observations suggest that matrix cracking is the dominant damage mechanism in CAS, both fiber breakage and matrix cracking are important in MAS. These damage behaviors strongly depend on the residual stress states. This study leads to a better understanding of the damage evolution and mechanism in CMCs, significantly impacts the constitutive models used to design multiaxially loaded ceramic matrix composites, and establishes a unique laser-ultrasonic approach for anisotropic damage characterization. (Abstract shortened by UMI.)...