supported by High-tech 863 Program under the project of “Development of a 4 500 m deep manned submersible” (Project No. 2009AA093303);Fourth Term of “333 Engineering” Program of Jiangsu Province under the project of “Fatigue and fracture performance of domestically produced high strength titanium alloys for the 4 500 m deep manned pressure hull” (Project No.BRA2011116)