Investigation on fiber fracture mechanism of C/SiC composites by rotary ultrasonic milling

Abstract With wide application of continuous carbon fiber reinforced silicon carbide matrix (C/SiC) composites, machining damages have been paid more and more attention. Due to prominent anisotropy and heterogeneity, fiber damages affect the service performance of C/SiC parts directly. Surface quality is closely related to fiber fracture mechanism. This paper emphatically analyzed fiber removal process of rotary ultrasonic milling (RUM) of C/SiC composite. A calculation method for fiber fracture stress based on ultrasonic vibration was presented. High-frequency, low-amplitude oscillation varied fiber cutting angle and increased fiber shear stress through deflecting friction form the axial section to radial section of fiber. It promoted shear fracture mode to dominant removal mechanism greatly. The proportion of fiber cutting angle under shear fracture mode was enlarged by 30%. Then both RUM and conventional milling (CM) C/SiC tests were carried out. The principle of surface quality improvement during rotating ultrasonic milling was presented. Experimental results showed that fiber cutting angle affected C/SiC surface morphology directly. Surface of RUM had better machining quality with less delamination, fiber-matrix debonding length and fiber pulling-out damage. Its surface roughness Sa values were reduced by 26.8% and 40.6% during fiber forward cutting and backward cutting, respectively. It proved the rationality and effectiveness of rotary ultrasonic milling C/SiC composites.