DAMAGE MONITORING OF 2D SiC-SiC COMPOSITES UNDER MONOTONIC AND CYCLIC LOADING/UNLOADING USING ACOUSTIC EMISSION AND NATURAL FREQUENCY

In this paper, tensile damage and fracture of 2D SiC/SiC composites are investigated using damage monitoring of acoustic emission (AE) and natural frequency. Nonlinear damage and fracture are mainly attributed to damage mechanisms of matrix cracking, interface debonding, and fibers fracture. Monotonic tensile stress-strain curves are divided into three stages based on the analysis of AE count, amplitude, and energy. Under cyclic loading/unloading, hysteresis loops appear due to internal frictional slip between the fiber and the matrix, and the natural frequency and composite modulus are obtained for different peak stress. A micromechanical tensile and cyclic loading/unloading constitutive model is adopted to predict the tensile curves. Micro damage parameters of interface debonding ratio and broken fibers fraction are used to characterize tensile damage and fracture. Relationships between natural frequency, composite modulus, interface debonding, and fibers fracture are established. Under tensile loading, the AE signal is large and concentrated mainly in the stress range of 50 - 100 MPa and 150 - 180 MPa, which corresponds to matrix cracking and fiber fracture, respectively. When the degradation rate of natural frequency approaches 0.01, matrix cracking and interface debonding occur, however, fibers failure does not appear; when the degradation rate of natural frequency approaches 0.04, the composite modulus decreases approximately 47% and the interface debonding ratio approaches 0.8 and the broken fibers fraction is approximately 2.2%.