Experimental and Analytical Investigations on Plane Strain Toughness for 7085 Aluminum Alloy

Data are presented on plane strain fracture toughness, yield strength, and strain hardening for three orientations of samples from quarter-thickness (t/4) and midthickness (t/2) locations of alloy 7085 plates with different gages aged past peak strength with different 2nd step aging times (T7X). These data are fit to an expression adapted from Hahn and Rosenfield (1968), in which toughness is proportional to strain hardening, the square root of yield strength, and the square root of a critical strain ε _c . Strain-hardening exponent n is replaced by an alternative measure, since the stress-strain data do not follow a power law. With increased overaging, the increase of strain hardening dominates the decrease of strength, such that toughness increases. The critical strain, which represents the influence of the microstructure on toughness, has no trend with overaging time. Constituents and grain boundary precipitates, thought to be the microstructural elements most differentiating alloy 7085 from alloy 7050, are quantified at t/4 and at t/2 on one plate. From this the greater critical strain at t/2 than at t/4 is mainly attributed to greater effective spacing of constituents. Critical strain is also greater with longitudinal loading and crack propagating in the long transverse direction, but definite understanding of this will require better anisotropic fracture mechanics and further microstructural characterization.