Accelerated creep 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). 

Averaged experimental creep compliance data are represented by dots in Figure 12.12 as a function of log time. The tests show that even after 14 months the creep continued and deformations kept increasing. After tests of 10000 h the compliance of the materials examined exceeds the instantaneous elastic compliance by a factor varying from 3.8 (for pure PP) to 3.2 (for the 80% PP + 20% PLC blend). This shows that the inelastic strain of these materials manifests itself substantially methods of accelerated testing and prediction have to be subjected to very detailed scrutiny. 

Xj(,(,ei is the response (creep from scribe) from the accelerated test Xfleid is the response from field exposure taccei is the duration of the acceleration test tfleid is the duration of the field exposure... 

The results for creep of SAC solder suggest that the transition from one dominant creep mechanism to another occurs at a temperature within the range of common operating or accelerated testing conditions, somewhere near 75 °C (167 °F). This temperature-related transition implies that accelerated thermal cycling profiles should be designed carefully... 

Creep data for a variety of SAC solders shows that SAC is more creep resistant than Sn-Pb under low- to medium-stress levels. Under high enough stresses, as may be encountered under accelerated test conditions, SAC creep rates are similar to Sn-Pb creep rates, and even higher for some SAC compositions. 

Fatigue life corresponds to different accelerated tests performed on PBGA packages and FCOB with and without underfill. Strain and energy refer only to creep conponents, and are evaluated using FEA. 

Service lifetime prediction of polymers and/or polymer based materials may be undertaken from different types of tests, such as creep behavior tests (linear and non-linear creep, physical aging, time-dependent plasticity), fatigue behavior tests (stress transfer and normalized life prediction models, empirical fatigue theories, fracture mechanics theory and strength degradation) and standard accelerated aging tests (chemical resistance, thermal stability, liquid absorption) [32]. 

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. 

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