Methods of determining the temperature dependence of primary creep

The service life of high-temperature composites and reinforcement fibers can be limited by creep, therefore it is desirable to understand the relationships between the creep strain and external variables. Due to load sharing between constituents or internal microstructural changes, a steady-state creep rate is never observed in some materials. Usually in these materials the creep rate decreases with increasing time. This is known as primary creep. Determination of the temperature dependence during primary creep is complicated by its dependence on time and/or strain. The suitability of various methods of analyzing primary creep data has not been addressed in detail before. In this paper, three methods of determining the temperature dependence during primary creep are compared: the Arrhenius, the cross-cut, and the Sherby-Dorn methods. The methods are applied to creep data for silicon carbide fibers intended as reinforcements in high-temperature ceramic or metal matrix composites. The sensitivity of each method to measurement resolution and the capability of the creep laws associated with each method for predicting measured creep behavior are evaluated. The reliability of the computer software fitting routine is independently confirmed. The Sherby-Dorn and cross-cut methods are mathematically similar but the Sherby-Dorn method does not involve an arbitrary selection of experimental conditions, is less cumbersome to apply to large amounts of data, and uses a broader range of the available data.