Dominant Creep Mechanism Research Articles

Stress relief and plastic flow in nickel oxide at elevated temperatures

Stress relief and plastic flow modes in nickel oxide over the temperature range 1323–1473 K have been studied, involving an assessment of its creep-in-bend behavior in the form of small sintered beams, of average grain sizes in the range 2–10 μ, and of densities of ∼70% of the theoretical maximum; conditions of three-point loading were adopted involving a range of maximum fiber stress of 2 × 106 −8×107 N · m−2, that is, of the magnitudes likely to arise during the growth of nickel oxide scales on metallic nickel. After subtraction of any observed threshold stresses, most specimens were found to exhibit a stress index of about unity. The grain-size index was determined to be −2.5. It is believed that the dominant creep mechanism was probably that of Nabarro-Herring.

The creep of alpha-iron at low stresses

The creep of α-Fe in vacuum has been studied for the ranges of stress from 0.3 to 4.6 MN/m 2, of temperature from 758 to 1103 K (0.42–0.61 T m ), of intercept grain size from 32 to 102 μm and of test duration from 2 to 550 hr. Helical specimens were self-heated by electric current and crept under their own weight so that each turn of the coil gave an increment in stress. The deflection of each turn was monitored in situ as a function of time by direct observation through a transparent vessel using a cathetometer. Creep rates were found to be proportional to stress in excess of a threshold stress decreasing from 0.6 MN/m 2 at 758 K to zero at 1103 K. Both grain size and temperature dependences of creep rate per unit stress point to Coble creep as the dominant diffusion creep mechanism, although observed creep rates were typically one hundred times those predicted from published radiotracer data for grain boundary diffusion in iron. This enhancement of creep rate is attributed to the transient operation of dislocations as sources, sinks and short-circuit paths for vacancies. Creep rates at stresses above the transition stress for slip creep show a stress exponent of six in agreement with previous work at higher stresses and an inverse dependence on grain size as found for other metals at similarly small grain sizes.

Dominant Creep Mechanism and the Descending Lithosphere

Summary The descending of the lithospheric slab into the asthenosphere is governed by gravitational body forces generated by thermal volume contraction and by phase boundary elevation in the colder slab. The dynamics of sinking depends on the rate controlling deformation mechanism in the lithospheric slab and in the adjacent mantle; it is a function of the thermal conditions, the state of shear stress, the grain size, and the elastic moduli. A finite element initial strain, incremental procedure has been used to model the sinking of the lithospheric slab into the asthenosphere. Both diffusional flow and power law creep have been incorporated as dominant deformation mechanisms separately. Computed principal stresses and maximum shear stresses are in good agreement with observations of seismicity and focal mechanisms in those models which involved power law creep mechanisms with a power of n= 3. Similarly, computed creep displacements and velocities support the view that the power law creep mechanism is the dominant mechanism in the mantle region of slab subduction. The resistance of the mantle to a slab sinking with a velocity of 8 cm/yr requires a stress-strain rate equation equivalent to the one based on experimental and theoretical results, involving constants, however, which render strain rates five orders of magnitude smaller than those in experiments. A corresponding strain rate map of the upper mantle based on the computed shear stress suggests a maximum rate of 3 × 10−14s−1 in the asthenosphere and an average rate of 10−19s−1 in the mesosphere, both taken at 100 bars shear stress. Translation into effective viscosities leads to a reasonable agreement with viscosities derived from glacio-isostatic rebound data.

Compressive Creep and Hot Hardness of U‐Pu Carbides

The compressive creep of carbides having the composition U0.79Pu0.21C1.02 and a sintered density of 11.8 g/cm3 was studied at 1300°, 1400°, and 1500°C and stresses of 2000, 4000, and 6000 psi. The equation which best fitted the data wasThe dominant creep mechanism was probably grain‐boundary sliding. The hardness of monocarbides with various U/Pu ratios was measured from room temperature to 1200°C. Probable mechanisms of deformation are discussed.

Thermodynamics of hindered adsorption and its implications for hardened cement paste and concrete

The dominant mechanism of shrinkage and creep of concrete at working stress levels is presently believed to consist in a coupling between change of thickness and diffusion in some sort of mono- or multi-molecular layers of water confined by an adsorbent solid. The classical solution thermodynamics and the usual formulation of surface thermodynamics are both inadequate for a rigorous treatment of this phenomenon. In the latter case this is due to the fact that a surface phase of limiting zero thickness is implied at the outset. In the present paper, thermodynamics of hindered adsorption which is consistent with the basic concepts of surface thermodynamics but free from the above shortcoming is developed.