The Effect of Fabrication Processes on the Governing Mechanical Failure Criteria for Alloy 22, Ti-Grade 2/7, and Ti-Grad
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The Effect of Fabrication Processes on the Governing Mechanical Failure Criteria for Alloy 22, Ti-Grade 2/7, and Ti-Grade 5/24 Alloys Aladar A. Csontos1, Darrell S. Dunn2, Yi-Ming Pan2, and Gustavo A. Cragnolino2 1 U.S. Nuclear Regulatory Commission, Mailstop: T7-F3, Washington, DC 20555-0001 USA 2 Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166, USA ABSTRACT The effect of fabrication processes on the governing mechanical failure criteria for Alloy 22, TiGrade 2/7, and Ti-Grade 5/24 has been evaluated through the development of material specific failure assessment diagrams (FAD). The Barsom and Rolfe Charpy V-Notch (CVN) energy to KIC empirical relationship was shown to be conservative when compared to experimental KJC data for Alloy 600 and 690, which indicates that the KIC derivation is most likely conservative for Alloy 22 as well. In contrast, the CVN to KIC relationship was found to be non-conservative for Ti-Grade 2/7 and Ti-Grade 5/24. The derived KIC value for Alloy 22 was then used to construct a K-based FAD, which indicated that the governing mechanical failure theory was the Tresca criterion for plastic collapse. Both the derived KIC and experimental KQ values were used in the development of the Ti-Grade 2/7 and Ti-Grade 5/24 FADs. The Ti-Grade 2/7 FAD indicated that the governing mechanical failure theory is also plastic collapse while the Ti-Grade 5/24 FAD suggested mixed mode failure processes where neither linear-elastic fracture mechanics (LEFM) nor Tresca can accurately predict. INTRODUCTION Alloy 22 (UNS N06022) is a Ni-Cr-Mo-W alloy that is currently the material that the U.S. Department of Energy (DOE) intends to use for the outer barrier of the waste package for the potential high-level nuclear waste repository at Yucca Mountain, Nevada. Ti-Grade 7 is a CP Ti alloy (Ti-Grade 2) with the addition of ~0.2wt% Pd and Grade 24 is a traditional Ti-6Al-4V alloy (Ti-Grade 5) with ~0.06wt% Pd. These small Pd additions are designed to further enhance the stress corrosion cracking, pitting, and crevice corrosion resistance of these Ti alloys which are currently the materials that DOE intends to use for the drip shield design in the potential Yucca Mountain repository. All three alloys (Alloy 22, Ti-Grade 7, and Ti-Grade 24) were selected due to their resistance to corrosion and stress corrosion cracking over a variety of environmental conditions. Nevertheless, fabrication processes such as welding can deleteriously alter the corrosion and mechanical properties and possibly the governing failure theory and acceptable flaw sizes for these alloys. In other words, a material or weld defect may weaken a component so that it fails by fracture at stresses below that determined by the Tresca plastic collapse model for yielding. Therefore, this investigation examines the role of fabrication and flaws on the governing mechanical failure criteria, i.e. LEFM, plastic collapse, or mixed mode failure method. This objective is achieved thr
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