Fracture toughness of aluminum alloys in air and 115 MPa hydrogen gas and strength design of thick-walled cylinder

Abstract

The fracture toughness and toughening mechanism of six aluminum alloys, 5083-O, 6061-T6, 6066-T6, 7N01-T5, 7N01-T6 and 7075-T6, were examined in air and 115 MPa hydrogen gas. In both the environments, tentative fracture toughness, KQ, was about twice higher in 7N01-T5 and 7N01-T6 than in 5083-O, 6061-T6, 6066-T6 and 7075-T6. Stretch zone, dimples and inclusions at the dimple bottom were observed on the fracture surface by means of scanning electron microscopy. All the critical stretch zone width, SZWC, had a good correlation with tentative J integral, JQ, regardless of environments and materials. The dimple diameter was nearly equivalent in 6061-T6, 6066-T6, 7N01-T5 and 7N01-T6, although KQ was about twice higher in 7N01-T5 and 7N01-T6 than in the others. Among these four aluminum alloys, the existence probability of large inclusions (diameter ≥ 6 μm) at the dimple bottom was lower in 7N01-T5 and 7N01-T6, which had less impurity elements of Fe and Si. These results inferred that higher values of KQ were attributed to the delay in the initiation and growth of microvoids in 7N01-T5 and 7N01-T6. Taking into account the tentative fracture toughness in combination with slow strain rate tensile (SSRT), fatigue life and fatigue crack growth properties, a design by analysis of thick-walled cylinder was performed. The result suggested that 7N01-T5 and 7N01-T6 having higher KQ enabled the design of larger-sized cylinders with thicker wall.

Keywords

• high-pressure hydrogen gas
• six aluminum alloys
• fracture toughness
• stretch zone
• dimple
• inclusion
• design by analysis
• thick-walled cylinder