Downloaded from https://www.everyspec.com V STRUCTURES a straight bar is attached to a rigid structure action of these bending moments the column -' at its upper end and is loaded by a weight at deforms in creep bending. The increasing. its lower end, the bar elongates continuously curvatures in turn lead to increasing lever with time. As creep deformations of metals arms which again result in increasinir hpnd-± take place substantially without any change ing moments. This is a vicious circle which in the total volume of the material, ;.
one can end only in the deveiopment o very large percent increase in the length of the bar curvature and compiete collapse of the must be accompanied by a one percent de- column. When the behavior of the materiA is crease in its cross-setiona area. However, such that the creep rates increase niore rap- constant load applied to a !:.-aller cross- idly than th-bnn~r , - -_ the time sectional area causes an increased stress in necessary for creep buckling to occur is e- - - the bar which in turn increases the rate at ways finite (not infinitely long). Consequent- which creep deformations take place. The ly we can properly talk of a criticP'1* - e .
creep xtrain rate is generally a non-linear Depending upon the material, the 1oda, . 4 nd function of the stress, increasing much more the temperature, this critical time may be a rapidly than the stress. The consequence is few years, a few months, a few days, a few a very rapid acceleration ,f the creep proce;s minutes or even a few seconds. It is therefore which can end only in a reduction of the most important that the engineer be con- cross-sectional area to the value at which the versant with the facts of creep buckling material will rupture instantaneously under when designing missile structure-. the applied load.
The time at which this rup- ture takes place is known as the critical tinie. 1-3.4. Melting and Ablation. Other phe- This characteristic exista for all structural nomena which must be treated, if a complete materials at elevated temperatures and must picture of the problems of ballistic missile P be known to the structural designier. structures is to be give,, are even farther re- A second instability of importance is moved from conventional structural analysis. known as creep buckling.
This type of buck- The heating rates experienced by ballistic ling differs ini many respects from elastic and missiles on re-entry are so large that the inelastic buckling as these phenomena are surface of the material may melt, sublime or known to the structural engineer. The main even burn. The designer of the missile must difference is that in creep buckling one can- therefore take the necessary rrecautions in not utilize the concept of a efriical load. this regard.
Some of these precautions are of Buckling takes place under any compressive an entirely aerodynamic nature; if the con- load however small if one waits long enough tour.; of the missile are properly shaped, the after the A pplication of the load. On the other heating that the missile will experience can hand, jL.St as in the case of tension, a new be minimized. A second precautionary meas- concept must be introduced, namely that of a ure is more in the realm of the structural critical ime. An explanation of this phenorn- designer; it consists of provisiorr for ther- enon can be givea without ditlhcuity.
mal protection through shielding the struc- Every practicai column, as distinguished tural materivl with insulation, through cool from the 'dealized ones underlying some ing the structure by means of the evaporation theoretical calculations, is imperfect. This of a coolant, or. through other means. means that the column's axis is never coin- In some designs the external metal cover- pletely straight and the load is never applied ing of the missile is used as a heat sink. When to it completely centrally, Consequently the this is the case, information is reqLired azout column is subjected to bending motmenii.
A the thermal conductivity, the thermal diffu- bending moment can be calculated as the ap- sivity, and the specific heat of the material. plied !oad multiplied by its lever arm, or the The form.r quantities determine how rapidly distance of the centroid of the cross-section heat will 1.e transmitted from the surf;)e of from the line of load application, Under the the missile Vx its interior, while the last item 4