Elbridge Z. Stowell
The phenomenological theory previously proposed in NACA Technical Note 4000 for the behavior of metals at elevated temperatures has been modified to yield transient creep curves by assuming that the metal consists of two phases, each with its own elasticity and viscosity. The extended theory satisfies the basic requirements for a theory of transient creep at elevated temperatures: that the transient creep be closely connected with the subsequent steady creep, and that the apparent exponent of the time in the transient region be permitted wide variations between 0 and 1. From this theory it is possible to construct nondimensional creep curves which extend continuously from the transient region into the steady-state region. The corresponding family of creep curves for any metal may be obtained from the nondimensional family by use of appropriate constants. The constants required are those obtained from steady creep measurements, together with two additional constants which represent the difference between the phases. The transient creep curves resulting from this theory are compared with the experimental curves for pure aluminum, gamma iron, lead, and 7075-T6 aluminum alloy; good agreement is found.
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