Measurement of creep mechanisms

The mechanisms in steady-state creep are usually studied experimentally using Eq. (7.6). There are four main variables strain rate stress temperature and 

Experimental creep data for ceramics have been obtained using mainly flexural or uniaxial compression loading modes. Both approaches can present some important difficulties in the interpretation of the data. For example, in uniaxial compression it is very difficult to perform a test without the presence of friction between the sample and the loading rams. This effect causes specimens to barrel and leads to the presence of a non-uniform stress field. As mentioned in Section 4.3, the bend test is statically indeterminate. Thus, the actual stress distribution depends on the (unknown) deformation behavior of the material. Some experimental approaches have been suggested for dealing with this problem. Unfortunately, the situation can become even more intractable if asymmetric creep occurs. This effect will lead to a shift in the neutral axis during deformation. It is now recommended that creep data be obtained in uniaxial tension and more workers are taking this approach. 

1) Estimate the activation energy for diffusion. (Hint for a single stress, determine the temperatures at which creep rates are 10 and 10 Vs,A =8.31 J/mol K.) 

2 The following steady-state ereep data were obtained for polyerystalline 

Microscopy did not reveal any grain boundary glassy phase. b) Determine the activation energies for steady-state ereep. c) Predict the creep rate for stresses of 20 and 52 MPa at 1600 °C, if the  

---