Damage, Fracture, and Fatigue of Ceramic-Matrix Composites

Under tensile loading, the fiber-reinforced ceramic-matrix composites (CMCs) exhibit obvious nonlinear behavior, due to the multiple damage mechanisms of matrix multicracking, fiber/matrix interface debonding and fibers failure. In this chapter, the micromechanical approach to predict the tensile stress–strain curves of fiber-reinforced CMCs is developed. When matrix cracking, fiber/matrix interface debonding, and fibers failure occur, the shear-lag model is adopted to analyze the microstress field of the damaged fiber-reinforced CMCs, i.e., the fiber and matrix axial stress distributions. Combining the shear-lag model with damage models of matrix statistical cracking, fracture mechanics fiber/matrix interface debonding criterion and Global Load Sharing (GLS) fibers failure criterion, the matrix cracking spacing, fiber/matrix interface debonding length, and fibers broken fraction are determined. The tensile stress–strain curves of fiber-reinforced CMCs corresponding to different damage stages are modeled. The tensile stress–strain curves of unidirectional, cross-ply, 2D, and 2.5D woven CMCs are predicted.

The fatigue hysteresis behavior of unidirectional, 2D cross-ply and woven, and 2.5D woven fiber-reinforced ceramic-matrix composites (CMCs) are analyzed. Based on the fiber/matrix interface debonding and sliding behavior, the fiber/matrix interface debonding and sliding lengths are determined using the fracture mechanics approach. The fiber/matrix interface debonding ratio and interface sliding ratio are determined for different interface slip cases. The effects of fiber volume fraction, peak stress, matrix crack spacing, interface shear stress, interface debonded energy, fibers failure, fiber Poisson contraction, fiber strength, fiber Weibull modulus, matrix cracking mode, applied cycle number and fiber/matrix interface wear on the fatigue stress–strain hysteresis loops and the fiber/matrix interface debonding and sliding are discussed. The experimental cyclic fatigue stress–strain hysteresis loops of unidirectional SiC/CAS, SiC/1723 and C/SiC, 2D cross-ply SiC/CAS and woven SiC/SiC, and 2.5D woven C/SiC composites under cyclic loading/unloading tensile and tension–tension fatigue loading are predicted.

Under cyclic fatigue loading, the damage mechanisms of fiber/matrix interface debonding, interface sliding and interface wear degrade the fiber/matrix interface shear stress. The fiber/matrix interface shear stress plays an important role in the fatigue behavior of fiber-reinforced ceramic-matrix composites (CMCs). In this chapter, the fiber/matrix interface shear stress of fiber-reinforced CMCs with different fiber preforms, i.e., unidirectional, 2D cross-ply and woven, 2.5D woven and 3D braided, is estimated from the fatigue hysteresis dissipated energy at room and elevated temperatures. The experimental fatigue hysteresis dissipated energy versus the applied cycles and the theoretical fatigue hysteresis dissipated energy versus the fiber/matrix interface shear stress relationship are analyzed. With decreasing fiber/matrix interface shear stress, the fatigue hysteresis dissipated energy increases to the peak value, and then decreases to zero, corresponding to the fiber/matrix interface slip Case I, II, III, and IV. Using the experimental fatigue hysteresis dissipated energy, the fiber/matrix interface shear stress of unidirectional SiC/CAS, SiC/Si₃N₄ with the strong and weak fiber/matrix interface bonding, C/SiC at room temperature and 800 °C in air condition, cross-ply SiC/CAS and C/SiC at room temperature, 700, 800, and 850 °C in air condition, 2D C/SiC at room temperature, 550 °C in air and 1200 °C in vacuum conditions, 2D SiC/SiC at room temperature, 800 °C in air, 600, 800, and 1000 °C in inert, 1000, 1100, and 1200 °C in air and steam, 1300 °C in air conditions, 2.5D C/SiC at room temperature, 800 °C in air and 600 °C in inert conditions, and 3D braided SiC/SiC at 1300 °C in air conditions are obtained.

In this chapter, the fatigue lifetime of fiber-reinforced ceramic-matrix composites (CMCs) at room and elevated temperatures is predicted for different testing conditions and fiber preforms. The relationships among the broken fibers fraction, applied cycle numbers, and the fatigue peak stress are established. At room temperature, the damage mechanisms of fiber/matrix interface wear degrade the fiber/matrix interface shear stress and fibers strength; and at elevated temperature in oxidative atmosphere, the damage mechanisms of fiber/matrix interface wear and oxidation degrade the fiber/matrix interface shear stress and fibers strength. The fibers broken fraction in different damage regions is determined, i.e., the interface wear region, interface oxidation region, interface debonded region, and interface bonded region. When the fibers broken fraction approaches to the critical value, the composites fatigue fracture. The fatigue limit stress and fatigue life S–N curve of fiber-reinforced CMCs with different fibers preform are predicted, i.e., the unidirectional C/SiC, SiC/CAS, SiC/1723 and SiC/Si₃N₄, cross-ply C/SiC, SiC/CAS, SiC/1723 and SiC/BMAS, 2D woven C/SiC and SiC/SiC, 2/5D woven C/SiC, and 3D braided C/SiC composites.