Experimental and Analytical Investigations on Plane Strain Toughness for 7085 Aluminum Alloy
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ALLOY developers aim for improvements in properties such as elastic modulus, strength, ductility, toughness, fatigue, and corrosion resistance. Such improvements result from manipulation of chemical composition and material processing.[1] Correlations of property data with composition and processing parameters are important for managing production, but of diminishing value in finding new alloys. For that purpose, fundamental models relating properties to the internal structure of the material are desired. The present work investigates such models for the plane strain fracture toughness of aerospace aluminum alloys. Aluminum alloy 7085 was introduced a few years ago by Alcoa as the next generation of high strength thick plate alloy.[2] Relative to other alloys of the aluminum 7xxx series, 7085 offers very high strength and toughness along with very low sensitivity to gage. Table I gives the registered composition range of 7085 and of alloy 7050, the incumbent aerospace thick-plate alloy. A recent article compares properties of 7085 and 7050 R.T. SHUEY, Senior Technical Consultant, and M.E. KARABIN, Senior Staff Engineer, are with the Alcoa Technical Center, Alloy Technology and Materials Research Division, Pittsburgh, PA 150690001. F. BARLAT, Professor, formerly with the Alcoa Technical Center, Alloy Technology and Materials Research Division, is with the Materials Mechanics Laboratory, Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea. Contact e-mail: [email protected] D.J. CHAKRABARTI, formerly with the Alcoa Technical Center, Alloy Technology and Materials Research Division, is retired in Woodside, NY 11377. Manuscript submitted March 14, 2008. Article published online January 7, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
and relates differences in properties to differences in composition.[3] In the course of establishing alloy 7085 for various applications, Alcoa has generated abundant data systematically relating fabrication process, internal structure, and mechanical properties. The present article takes advantage of this data to improve fundamental understanding of how the internal structure determines plane strain fracture toughness. As context there are numerous studies involving alloy 7050, which have been published over three decades. Section II reviews relations between three macroscopic mechanical properties: strength, toughness, and strain hardening. Section III describes a designed study at full scale in which these three mechanical properties are measured. Section IV reduces the data to a critical strain, which is found to vary systematically with orientation and location. Some quantitative metallography is added to help relate this pattern to internal structure. Section V develops a qualitative interpretation in the context of published models relating microstructure to toughness.
II.
FRACTURE MECHANICS
A. Mechanical Definition of Toughness Toughness quantifies the resistance of an existing crack
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