Downloaded from https://www.everyspec.com INTRODUCTION ) shot" nature of the ballistic missile intro. structures and the thermal stresses of non- duces n design philosophy which cannot be uniformly heated structures. tolerated in other structure.. Thermal stresses are of the nature of the secondary stresses of conventional structural 1-3. ELEVATED TEMPERATURE EFFECTS engineering. They are a consequence of the plied loads. It is unlikely they will cause most immediate effect of elevated tempera- failure except, perhaps, in fatigue.
However, ture -s the deterioration of the load-carrying failur e excelt p a e fati H er, capaityof he ateial.Man tets ave they may cause unduly large plastic deforma- capacity of the material. Many tests have tions and, in particular, compressive thermal been carried out to obtain detailed informa- stresses might cause buckling. tion on this effect. The collected data indi- cates the deleterious effect elevated tempera- Thermal buckling may lead to the develop- a material's physical properties ment of bulges on the surface of the body or which aftect the rigidity, deformation, ulti- control surfaces of the missile which coild 1-3.1. Effects on Material Properties. The deformation.
and not of the externally ap- mate strength, fatigue and buckling of struc- interfere seriously with the aerodynamic tural members. The data also shows that for performance. In particular, the accuracy of the........... ........ , -, For these 4he figl path m a, be ..... ....... every structural material there exists a maxi- the miigit su r d reasons the missile structural designer and mum temperature beyond which the load- analyst must be conversant with the calcu- carrying capacity is reduced to such an lation c' thermal stresses and with the extent that the material cannot be advan- A I tageously utilized.- lormuis predicting thermal buzkling. Buckling.
The second ;mportant effect of important effect of elevated temperatures, 1-3.2. Thermal Stresses and Thermal 1-3.3. Croep and Creep Buckling. A third aerodynamic heating is the development of comparatively little known in conventional internal stresses in the structure. Even in structures and machinery, is creep. This phe- S iterasteesi teeracture. risen in ahetdrdisujcetoaosantnil hecase of uniform eometuericnomenon can be observed, for instance, when structural element, externaltureeric cn- a heated rod is subjected to a constant tensile strauctsr a mnaxirna eomdieetiateon- load.
Under this load the rod elongates con- straints or a combination of different mate- iusywthie.lhogladfrex - rials will induce it ternal stresses. T'hese tiiuously with time. Although lead, for exam- stresses are sometimes called temperature pi., exhibits creep at room temperature, with s!resses. In almost every practical problem, commonly employed structural materials the however, uniform temperatures in a heated p enomenon is practically non-e-istent at structural element are rather unlikely to room temperature. Creep becomes important, strutura eleent re rtherunliely o wever, aial ma- even lead to the elimnina.- occ,.r.
Two principal reasons are that the I of a ic ma tera to e in aerodynamic heing itself is non-uniform tion of a rticular material from ie in over the exterior surface of the structure, structures, when the temperature is raised and that the heat capacity of the structure sufficiently. v e fThe first structural effect of creep is the varies from point to point. In consequence of deformation created. It is important that the the non-uniform temperature distribution,and analyst be the material would expand non-uniformly msiesrcua einradaaytb weeth ia ossibl geoetrcally.Asuthe y able to predict the time when the deforma- were this possible geometrically.
As the nat- tions will become so large that they substan- ural 4eformations of the material are incom- patible, internal stresses are set up in the tially change the aerodynamic shape of the non -uniformly heated st.uctural elements to missile or that they interfere with the proper re-establish the continuity of the deforma- functioning of the eemeable parts of the tions. These stresses are known properly as system. thermal stresses. However, in this section the A second effect of creep is the occurrence term thermal stresses shall be 'used for both of two entirely new types of structural insta- the temperature stresses of uniformly heated bility. The first is instability in tension. When 3