Experimental assessment of fatigue life and failure modes in a SiC/Ti composite

Abstract

An experimental investigation of the fatigue life and the associated failure modes in a fiber-reinforced titanium-matrix composite (TMC) has been conducted. Tests have been performed over a wide range of loading conditions, characterized by stress ratios (minimum/maximum) of R=0–0·8, and the results presented in the form of a Goodman diagram. The failure modes include: (i) catastrophic fracture following damage initiation; (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 similar damage but without the occurrence of fiber bundle fracture. There exists a damage-initiation threshold, associated with a critical value of the stress amplitude, below which there is no apparent microstructural damage and the fatigue life is effectively infinite. There is also a fracture threshold following matrix cracking which is dictated by the fiber strength distribution. The fibers degrade somewhat upon cycling in the presence of matrix cracks, as manifested in a slight reduction (∼10%) in the mean strength of extracted fibers as well as the occurrence of composite fracture well after the attainment of a saturation in the matrix crack density. The results indicate that the damage-initiation threshold is likely to be the single most important property from the viewpoint of design and life prediction under high cycle fatigue.