the metal or the ceramic is to have higher modulus of elasticity. Since 

 the compressive strength of the metal in the joint is invariably less than 

 that of the ceramic shell, it is imperative that the modulus of elasticity 

 of the metal never surpass that of ceramic. In this manner, the stresses 

 in the metallic joint held rigidly between the ceramic shells and under- 

 going the same strains as the flanges at the ends of the shell will be 

 equal or less than those in the ceramic flanges. Since the hoop stresses 

 in the ceramic shell flanges are generally more than 100,000 psi but less 

 than 200,000 psi, the metallic joint with matching modulus of elasticity 

 must be made of metallic alloy with at least 150,000 psi yield strength. 



The ceramics used in the construction of the experimental shell series 

 were Pyroceram #9606 and 99-percent alumina with 17.5 x 106 and 50 x 10 

 moduli of elasticity, respectively. In ideal terms, the use of titanium 

 would be the logical choice for joints in Pyroceram #9606 shells and beryl- 

 lium for joints in the 99-percent alumina shells. If the high cost, or 

 low strength of the ideally matching modulus joint materials is a deter- 

 rent to their application, then high strength aluminum alloys like 7075-T6, 

 7178-T6, or 7001-T6 can be substituted for the ideal joint material in 

 Pyroceram shells, while high strength steel alloys can be used in con- 

 junction with 99-percent alumina. As a matter of fact, 7075-T6 aluminum 

 alloy was the material from which the experimental joints were machined 

 for the Pyroceram #9606 material. Although different metals should be 

 used in the joints for different glasses and ceramics as explained above, 

 titanium will, in all probability, emerge as the sole joint material for 

 ceramic shells. It not only matches with its modulus of elasticity all 

 of the Coming's glass Pyroceram ceramic series, but also is quite com- 

 patible with the alumina ceramics whose moduli are higher than that of 

 titanium. When one adds to this the other mechanical and physical proper- 

 ties of titanium, like its high compressive and tensile strength and its 

 excellent resistance to corrosion, then its selection for joint appli- 

 cation becomes much more desirable. The only cylindrical shells that 

 would perform better with high- strength aluminum joints are glasses whose 

 moduli of elasticity generally match that of aluminum. Needless to say, 

 other factors, like absolute minimum of weight, and need for economical 

 fabrication of joints, may dictate the use of high- strength aluminum. 

 But, regardless of what considerations are employed in the selection of 

 materials for the mechanical joint, the field from which the alloy is 

 chosen is limited because of the magnitude of compressive stresses and 

 moduli of elasticity involved. Some tests have been performed with nylon 

 and teflon gaskets of 1/16 to 1/8 inch thickness on bearing surfaces and 

 so far have been generally found less desirable than bare metallic bearing 

 surfaces because of their tendency to extrude. The cold flow of these 

 plastics under even very moderate compressive stresses makes them defi- 

 nitely undesirable for applications when the hydrostatic pressure on the 



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