Design and Stress Analysis of Extraterrestrial ... - The Black Vault

According t; this formula, at constant stress, elongation
maintains a constant value and does not depend on time. (At low
temperatures and stresses, change in c will be slow and small and
can be disregarded.) In Fig. 1.25c this stress corresponds to point
0.
At high temperatures and stresses, the rise in e will be
significant; it will substantially depend upon time (Fig. 1.25c).
On this curve we can distinguish three stages of creep (I, II, and IU),
Stage I is characterized by a variable growth rate in creep flow
£ = de/dt. At the beginning of the stage the growth rate is not
high; then it drops to a minimum value, remaining approximately
constant in the I1 stage.
The ar•rc of creep I with decreasing rate is called unstable
creep. Thio stage is usually brief, although, in certain cases,
it can react-i s+,-veral tens of hours. This area of' creep is manifested
in calcularlons for parts of short-life rocket engines. In designing
parts for long-life rocket engines it can be disregarded.
Th. area of creep I1, characterizing constant deformation
rate is called steady creep. Depending upon temperature and stress
level, it can last from tens of minutes to many hundreds of hours.
In materials for ektraterrestrial electrical rocket engines this
zone can reach 10,000 and more hours. It is fundamental in analyzing
parts for creep.
The third, brief stage of creep is characterized by the formation
of a neck in the sample and fracture. The sharp increase in c at
the end of stage III is explained by the increase in sample stress.
It is difficult to keep stress ,onstant •in Rn ordinary experimental
machine. If we create such conditions, then creep stage III becomas
less pronounced (dashes on Fil;. 1.25c) and also terminates in the
fracture of the sample at point p. Time T characterizes the full
operating time of the material up to its fracture.
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