Effects of the Crack Tip Constraint on the Fracture Assessment of an Al 5083-O Weldment for Low Temperature Applications.

Dong Moon,Myung Kim,Jeong Park

doi:10.3390/ma10070815

Abstract

The constraint effect is the key issue in structural integrity assessments based on two parameter fracture mechanics (TPFM) to make a precise prediction of the load-bearing capacity of cracked structural components. In this study, a constraint-based failure assessment diagram (FAD) was used to assess the fracture behavior of an Al 5083-O weldment with various flaws at cryogenic temperature. The results were compared with those of BS 7910 Option 1 FAD, in terms of the maximum allowable stress. A series of fracture toughness tests were conducted with compact tension (CT) specimens at room and cryogenic temperatures. The parameter for the Al 5083-O weldment was evaluated to quantify the constraint level, which is the difference between the actual stress, and the Hutchinson-Rice-Rosengren (HRR) stress field near the crack tip. Nonlinear 3D finite element analysis was carried out to calculate the parameter at cryogenic temperature. Based on the experimental and numerical results, the influence of the constraint level correction on the allowable applied stress was investigated using a FAD methodology. The results showed that the constraint-based FAD procedure is essential to avoid an overly conservative allowable stress prediction in an Al 5083-O weldment with flaws.

Highlights

Liquefied natural gas (LNG) storage, transportation, and supply systems, such as LNG carriers (LNGC), floating storage regasification units (FSRU), and LNG fuel gas supply systems (FGSS), should be designed to ensure structural integrity under a wide range of loading conditions at cryogenic temperature through the appropriate selection of low temperature materials
At least five specimens were tested at each material and temperature, and the average values were obtained from the were tested at each material and temperature, and the average values were obtained from the corresponding crack tip opening displacement (CTOD)
Remarks crack configurations at cryogenic temperature obtained from Option 1 and constraint-based failure assessment diagram (FAD)

Summary

Introduction

Liquefied natural gas (LNG) storage, transportation, and supply systems, such as LNG carriers (LNGC), floating storage regasification units (FSRU), and LNG fuel gas supply systems (FGSS), should be designed to ensure structural integrity under a wide range of loading conditions at cryogenic temperature through the appropriate selection of low temperature materials. Typical low temperature materials applied to LNG cargo tanks include aluminum alloys, nickel alloy steels, and stainless steels. These materials exhibit excellent mechanical properties at cryogenic temperature. Most cargo tanks for LNGC are exposed to severe loading conditions, i.e., fatigue, sloshing impact, and thermal loading. These loading conditions can often cause the failure of structural components at welded joints with flaws. Fracture assessment procedures for welded joints with flaws have been published, such as BS 7910 [1], R6 [2], and API 579 [3] These standard procedures are based on the failure assessment diagram (FAD). The fracture assessment of welded joints with a flaw is based

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