Creep power law

The relaxation time (arbitrarily defined as the time taken for the stress to relax to half its original value) can be calculated from the power-law creep data as follows. Consider a bolt which is tightened onto a rigid component so that the initial stress in its shank is CTj. In this geometry (Fig. 17.3(c)) the length of the shank must remain constant - that is, the total strain in the shank e must remain constant. But creep strain can rqjiace elastic strain e - , causing the stress to relax. At any time t... 

As the stress is reduced, the rate of power-law creep (eqn. (19.1)) falls quickly (remember n is between 3 and 8). But creep does not stop instead, an alternative mechanism takes over. As Fig. 19.4 shows, a polycrystal can extend in response to the applied stress, ct, by grain elongation here, cr acts again as a mechanical driving force but, this time atoms diffuse from one set of the grain faces to the other, and dislocations are not involved. At high T/Tm, this diffusion takes place through the crystal itself, that... 

Designing metals and ceramics to resist power-law creep... 

If you are asked to select, or even to design, a material which will resist power-law creep, the criteria (all based on the ideas of this chapter and the last) are ... 

Metallic alloys are usually designed to resist power-law creep diffusional flow is only rarely considered. One major exception is the range of directionally solidified ( DS ) alloys described in the Case Study of Chapter 20 here special techniques are used to obtain very large grains. 

Ceramics, on the other hand, often deform predominantly by diffusional flow (because their grains are small, and the high lattice resistance already suppresses power-law creep). Special heat treatments to increase the grain size can make them more creep-resistant. 

F. W. Crossman and M. F. Ashby, The Non-Uniform Flow of Polycrystals by Grain Boundary Sliding Accommodated by Power Law Creep, Acta Metall., 23, 425-440 (1975). 

R. M. McMeeking, Power Law Creep of a Composite Material Containing Discontinuous Rigid Aligned Fibers, International Journal of Solids and Structures, 30, 1807-1823 (1993). 

T. K. Kim and R. M. McMeeking, Power Law Creep with Interface Slip and Diffusion in a Composite Material, Mechanics of Materials, to be published. 

Consider a sharp, Mode I crack in an elastic-power-law creeping solid (Eqn. (2)). The isotropic multiaxial generalization of Eqn. (2) is in terms of the Von Mises effective stress [Pg.337]

Cohesive Zone with Power-Law Creep and Damage... 

C.-Y. Hui, The Mechanics of Self-Similar Crack Growth in an Elastic Power-Law Creeping Material, Int. J. Solids Struct., 22(4], 357-372 (1986). 

Tsenn, M. C., Carter, N. L. (1987). Upper limits of power law creep of rocks. 

Figure 1. Schematic representation of the power-law creep (e a") in conjunction with the effects of the threshold stress Oo and load transfer.

According to figs. 10a and 10b, the stress exponents where calculated from the slope of the creep stress vs. strain rate curves. The n value for creep of the Ti5Si3 compound is n=3.0 0.2. This predicts a power law creep behavior based on viscous glide of dislocations sustained by diffusion... 

Figure 12.6 Summary of power law creep data for a number of ceramics. Data taken from W. R. Cannon and T. G. Langdon, J. Mat. Sci., 18 1-50 (1983).

The shrinkage rate due to power-law creep, 8creep can be evaluated based on the continuum theory of sintering, which deals with the dependences of normalized shear and bulk viscosity modules on porosity, 9, along with the effective sintering. 

Figure 5.9 shows an example of HIP diagrams which identify the dominant densification mechanism under various experimental conditions and shows the rate of densification that results from all the mechanisms acting together. As can be seen in the diagrams, diffusion is usually the dominant mechanism in ceramics even under a high external pressure while power-law creep can be an important densification mechanism in metals. 

Power-law creep can also be a major densification mechanism in pressure-assisted sintering. At the early stage of densification achieved by power-law... 

For final stage densification by power-law creep under HIP, where a hollow sphere model (an isolated spherical pore within a spherical particle) is acceptable, dp/dt can be expressed as ... 

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