EXPERIMENTAL ASSESSMENT OF FATIGUE LIFE AND FAILURE MODES IN A SICC TI COMPOSITE/

An experimental investigation of the fatigue life and the associated f ailure modes in a fiber-reinforced titanium-matrix composite (TMC) has been conducted. Tests have been performed over a wide range of loadin g conditions, characterized by stress ratios (minimum/maximum) of R=0- 0.8, and the results presented in the form of a Goodman diagram. The f ailure modes include: (i) catastrophic fracture following damage initi ation; (ii) the formation and propagation of multiple bridged cracks, leading to the development of hysteresis and permanent strain and, in some instances, fiber bundle fracture; and (iii) the development of si milar damage but without the occurrence of fiber bundle fracture. Ther e exists a damage-initiation threshold, associated with a critical val ue of the stress amplitude, below which there is no apparent microstru ctural damage and the fatigue life is effectively infinite. There is a lso a fracture threshold following matrix cracking which is dictated b y the fiber strength distribution. The fibers degrade somewhat upon cy cling in the presence of matrix cracks, as manifested in a slight redu ction (similar to 10%) in the mean strength of extracted fibers as wel l as the occurrence of composite fracture well after the attainment of a saturation in the matrix crack density. The results indicate that t he damage-initiation threshold is likely to be the single most importa nt property from the viewpoint of design and life prediction under hig h cycle fatigue. (C) 1998 Elsevier Science Ltd. All rights reserved.