Creep exponent

Nomenclature n Shape parameter for matrix flaw size distribution Creep exponent... 

Length of unbridged matrix nf Creep exponent for fiber... 

GS-44 was creep tested at 1200°C in vacuum with neutron diffraction measuring the microstructural effects in situ for the first time. The a and c lattice parameters are seen to fork with greater magnitude at greater stress in such a way as to obtain and maintain a constant unit ceil volume during steady state creep. The creep exponent was determined from extensometer data to be 3.18, higher than prior reported values due to the vacuum furnace used in the present study. 

A—instantaneous deformation when loaded, X—creep exponent,... 

The exponential dependence of the strain rate on the temperature has been confirmed experimentally. The relation between strain rate and stress is found to follow a power law, as predicted by the equation (see also section 11.1), but in reality the creep exponent typically takes values between 3 and 8. Due to the large variations of the creep exponent in different materials, the value of the factor A can differ by several orders of magnitude. The activation energy in equation (11.13) is frequently stated per mole in the units kJ/mol. In this case, Boltzmann s constant k has to be replaced by the gas constant R in the equation as explained in appendix C.l. 

As usual, Agbs is a material parameter, S is the thickness of the grain boundary, a the externally applied stress, Dqb the diffusion coefficient of grain boundary diffusion, and d the diameter of the grain. The creep exponent n of grain boundary sliding usually takes values between 2 and 3. 

The various creep mechanisms discussed so far differ in their temperature dependence because the activation energy of the mechanisms is different. Furthermore, they differ in their stress-dependence. The creep exponent takes values... 

At low external stresses and low temperatures, the material deforms elastically. At higher temperatures, diffusion creep starts, being stronger at small stresses than dislocation creep because of its lower creep exponent. Because of the lower activation energy for grain boundary diffusion, this mechanism is more important than bulk diffusion at low temperatures. Since the creep exponent is the same in both cases, the two regions are separated by a vertical line. 

If the external stress is not sufficient, part of the required energy has to be provided by thermal activation. Because the required energy depends on the external stress, which thus enters the Boltzmann factor in the exponent, the dependence on the stress is unusually strong and can be approximately described by a power law with large creep exponent. 

Discuss various methods to find the creep exponent in the power law. Is it ever a function of stress Temperature Explain. 

The model should produce statistical statements on the lifetime in terms of the overall applied stress field, the overall volume of material, and boundary effects. Important input parameters are fiber packing geometry, fiber strength, matrix and interface creep exponents, rate factors in the stress-corrosion chemistry, and the applied stress level. 

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