Sustained Load Behavior of SCS-6/TIMETAL 21S Composite
Authors: Khobaib M , John R , Ashbaugh NE 
Published: March 2009

TIMETAL 21S matrix composite reinforced with silicon-carbide (SCS-6) fibers is a potential candidate material for the high temperature structural components of advanced aircraft. However, a considerable amount of mechanical property characterization is required before this material can be used for aerospace structures. This paper discusses the characterization of the sustained load behavior of SCS-6/TIMETAL 21S composite under simulated service conditions. TIMETAL 21S, reinforced with approximately 35% SCS-6 fibers (by volume) was used for this study. The tension creep specimens were fabricated from [0]4, [90]4, [0/90]s, and [0/±45/90]s laminates. Sustained load creep tests were conducted under cold grip conditions at 650, 760, and 815°C at various stress levels. The creep response for [90]4, [0/90]s, and [0/±45/90]s layups exhibited generic creep curves with three distinct stages: (1) an early region of primary creep, (2) a linear region similar to a steady stage, and (3) a relatively short region similar to a tertiary stage. In contrast, the creep response of the [0]4 layup showed hardly any tertiary region and the failure was abrupt just after the secondary stage. The total strain to failure for all orientations, except the [90]4, was near the ultimate strain for fiber failure. Accelerated rate of damage accumulation was observed with increasing stress and temperature that resulted in higher creep strain under these conditions. A simple creep model based on the relaxation of matrix stress in the primary and secondary creep stages was developed to predict the creep response of the unidirectional SCS-6/ TIMETAL 21S composite at 650°C. The model predictions correlated well with the data for stresses less than 700 MPa. The optical and scanning electron microscopic (SEM) analyses of the failure surfaces showed multiple crack initiation sites near fiber/matrix interface. Two distinct zones of failures were observed in all cases. The flat zone, with little or no fiber pull-out, represented the stable crack growth region. The rugged zone, with significant fiber pull-out, was caused by overload failure. In all cases, the damage (crack) initiated from the edge. For [90]4 and [0/90]s, the crack initiation sites were observed to be close to the 90° fiber/ matrix interface, while in the case of [0/±45/90]s, the crack initiated predominantly from the 45° fiber matrix interface. The fibers that were damaged during the fabrication process provided the crack initiation site for the [0]4 orientation. These edge cracks enhanced the environmental access to the fiber/matrix interface that resulted in progressive damage of the interface and the fiber. Data plotted on a Larson-Miller Parameter (LMP) chart enabled a direct comparison of the creep resistance of all the layups tested. The creep resistance of the [0]4 layup was significantly greater than that of the other layups. The creep rupture life of specimens with 0° fibers was governed by the failure of unidirectional fibers. An empirical model was developed to predict the creep rupture life of all layups containing 0° fibers based on the LMP approach assuming that only the 0° fibers carry the load for the majority of the life.