The findings in this report are not to be construed as an official Departnent of the Amy position unless so designated by other authorised documents. .

An investigation was performed to determine the engineering properties and plane strain fracture toughness characteristics at cryogenic temperatures for ¼, ½, and 1-in.-thick plates of 5Al-2.5Sn ELI (extra low interstitial) and commercial grade titanium alloys. These results were then translated into design information data applicable to liquid-fueled rocket booster tanks for service at -320 and -423 F. The tensile properties of these materials were determined at room temperature, -110 F, -320 F, and -423 F using small round tension specimens. The plane strain fracture toughness was determined using an instrumented precracked bend specimen. The dimensions of these specimens were such that the criterion (crack length and specimen thickness are equal to 2.5 (KIc/σys)2) was met at testing temperatures of -320 and -423 F. These data may be considered valid values of plane strain fracture toughness. The commercial 5Al-2.5Sn titanium alloy gave values of KIc of approximately 25,000 and 24,000 psi √ in. at testing temperatures of -320 and -423 F, respectively. On the other hand, the 5Al-2.5Sn ELI titanium alloy gave values of KIc of 65,000 and 55,000 psi √in. at these testing temperatures, respectively. These data show a superiority of approximately 1.5 in terms of critical crack size for the ELI grade. The relative level of texture hardening in this material was estimated by measuring the R values of these plates. The predicted biaxial yield strength of tanks fabricated from this material was compared with experimental data.

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The effect of unidirectional solidification on the mechanical properties of high-strength steels was investigated by means of split heats of AISI 4340 and 25 per cent nickel maraging steel. One half of these heats were unidirectionally solidified and the other half of these melts were allowed to solidify to produce an equiaxed structure. These ingots were forged and rolled to a light gage sheet 0.040 to 0.060 in. thick. The materials were evaluated by means of standard tension tests and fracture toughness studies. By means of compliance measurements, it is possible to determine the crack resistance as a function of absolute crack extension. This investigative technique was used to study the AISI 4340 steel. Unidirectional solidification did not alter the crack resistance properties of this steel. However, a 10 per cent improvement in plane-strain fracture toughness as determined by the pop-in technique was observed for the unidirectionally solidified material. In order to obtain very high strength levels, the total content of hardener elements was increased in the 25 per cent nickel maraging steels. Consequently these materials were quite brittle and the results not definitive. However, it was demonstrated that a homogeneous structure is necessary for high values of fracture toughness in this material. The data were interpreted in terms of critical crack size for instability at yield stress. Both through-cracks and part-through-cracks were considered in this analysis.

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