Creep stress asymmetry

Another explanation of the creep asymmetry is based on the rate of approach (during compression) or separation (during tension) of adjacent grain facets controlled by the viscous secondary phase and the fact that the number of grain facets supporting compressive stresses is less than those supporting tensile stresses.19... 

Results on other composite materials are similar to those obtained by Morrell and Ashbee.56 Creep asymmetry has been demonstrated for two grades of siliconized silicon carbide,35,60,61 SiC whisker-reinforced silicon nitride,53 HIPed silicon nitride,29 and vitreous-bonded aluminum oxide.29 Again, stresses required to achieve the same creep rate were at least a factor of two greater in compression than in tension. In two grades of siliconized silicon carbide,35,58-61 the stress exponent changed from 4 at creep rates below... 

Solution-precipitation theory cannot be used to justify creep asymmetry or high tensile stress exponents for ceramic matrix composites. The theory suggests that creep is symmetric in stress and that the stress exponent is equal to 1. Justification of creep asymmetry by solution-precipitation would require other parameters in Eqn. (4) to depend on the sign of the applied stress. A nonlinear dependence on stress would be required. Diffusion and devitrification may play a role in this regard however, the data needed to support this possibility have yet to be obtained. 

In this paper, the importance of particle and whisker reinforcement to creep and creep rupture behavior of ceramics is discussed. Particle and whisker additions generally increase both the fracture toughness and creep resistance of structural ceramics. These additions also act as nucleation sites for cavities. Cavities form preferentially in tensile specimens. This results in a creep asymmetry, in which composites creep faster in tension than in compression. As a consequence of cavitation, the stress exponent for creep in tension 6-10,... 

Figure 13.7 Creep asymmetry in SN 88 silicon nitride at 1350 and 1400°C depends on stress. Tensile creep rates correspond to exponential dependence on stress, while compression creep follows the power law [43],...

Tensile creep in silicon nitride ceramics are best described by meso-mechanical models based on the dilatation of granular solids. These models provide a rationale for the exponential dependence of creep rate on applied stress, creep asymmetry, and the role of cavitation in the creep process. Meso-mechanical models are based on the assumption that grains of silicon nitride are rigid during deformation, so that displacements between adjacent grains can only occur along the grain boundaries. 

Cavitation creep follows an exponential rather than power-law dependence on stress [30]. This dependence, together with the understanding that cavitation only contributes to axial tensile strain, explains creep asymmetry and the unusually high stress exponents measured in tension at high stresses. 

Finally, due to the stress—strain cycle asymmetry induced by creep strain, average stresses develop positive for holding in compression and negative for holding in tension. The measured mean stresses range between —25 and 4-25 MPa, which can affect life in the case of cyclic loadings at low amplimde. 

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