Plane Strain Fracture Toughness and Mechanical Properties of 5Al-2.5Sn ELI and Commerical Titanium Alloys at Room and Cryogenic Temperatures

Abstract

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.

A Review of Cryogenic Fracture Toughness Behavior

“…Plane strain fracture toughness data for various tempers of 2219 aluminum alloy have been reported by Boeing [10.11] and Carman [12]. These data are summarized in Table IV.…”