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National Advisory Committee for Aeronautics, Technical Notes - Creep Buckling Analysis of Rectangular Section Columns

A previous analysis of the creep behavior of a slightly curved pin-
ended H-section column under constant load is extended to the slightly
curved solid rectangular—section column. The analysis leads to a dif-
ferential equation for the plastic strains at the midheight cross
section. The form of the equation indicates the significant parameters
which may be useful in plotting test data on the creep life of columns.
These are a lifetime parameter t’cr, an initialsstraightness param—
eter S or S', and the ratio of the average applied stress to the Euler
stress E/UE. A numerical method of solving the differential equation,
suitable for use with a highgspeed digital computer, is described, and
typical computed results are given. The existence of a finite lifetime,
although not evident from the differential equation, is argued intuitively
and confirmed by the numerical computations.

The high temperatures that can develop in aircraft during supersonic
flight make it important to consider the possible limitations due to
creep on the useful service life of the structural components. In a
previous paper (ref. l), the creep of a slightly curved pin-ended ideal—
ized H-section column under a constant load and constant temperature was
studied theoretically. The material of the column was characterized by
an assumed creep law, for constant uniaxial compressive stress and con—
stant temperature, of the form

where e is the total compressive strain, 0 is the constant compressive
stress, t is the time after application of the stress, and E, A, B,
and K are material constants. This form was selected because it
applied to at least two alloys: the 758-T6 aluminum alloy at 6000 F
(ref. 2) and a low—alloy steel at 8000 F and, possibly, 1,1000 F (ref. 5),
Shanley's engineering hypotheses of creep (ref. 4) were used in order to
generalize the assumed constant—stress creep law to cover situations in
which the stress varies with time, the condition encountered in the
fibers of a column undergoing continuous lateral deflection because of
creep.