Creep-rupture accelerated tests

Time to rupture can be predicted by using the accelerated times generated by the creep data, and the creep-rupture characteristic generated by performing twelve of these tests over a range of loads. Conventional long-term creep strain and creep-rupture tests have so far confirmed the validity of the predictions for polyester fibres. Comments on the method have been published by Greenwood and Voskamp [10]. 

Most non-aerospace CMC applications require long service lives. For these applications CMC components must avoid creep rupture and must exhibit creep strains lower than 1 percent after 10,000 hours of service (e.g., at 1,200°C [2,192°F] and 100 MPa [14.5 ksi]) components must also be chemically and microstructurally stable. These stringent demands present major challenges to researchers and engineers, particularly for material development and accelerated testing. The performance objectives limit the material choices to polycrystalline oxides, SiC, or amorphous Si-C-N-B compositions (single-crystal fibers are not affordable). 

To prevent unrealistically short creep rupture times, one needs to maintain stress loads below about 60-70% of the measured tensile strength at the same test chamber temperature. This is an important criterion for selecting appropriate weights for accelerated tensile strength determination. 

Whereas most conventional creep and creep-rupture tests are performed at a reference temperature of 20—23°C, the use of elevated test temperatures is common to achieve accelerated creep behavior and subsequent extrapolation exercises to achieve long-term strain or rupture predictions. Temperature acceleration of creep curves uses the established method of time—temperature superposition of the creep of geotextiles without direct use of Arrhenius formula. A procedure employing accelerated temperatures follows. 

Engineering design often requires engineers to predict material properties at high temperatures where no experimental data are available. The creep deformation rate can be so slow that it might require 10 years test time to reach 1% deformation. Reliable predictions based on accelerated test data obtained over a shorter period of time are essential. Several theoretical parameters were proposed to predict long-term metal creep or stress rupture life based on short-term test data. One of the most utilized parameters is the Larson-Miller parameter, as defined by Equation 4.20 ... 

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