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The objectives of the investigation described were to find the variation of fracture toughness with thickness for 7075-T6 and 7075-T73 materials and to provide data upon which a theory could be established for the variation of fracture toughness with thickness. A test program was conducted on 60 centrally cracked specimens varying in thickness from 0.05 to 0.75 in.
Analysis of the results showed that the two materials displayed substantially different characteristics. This is attributed to the fact that, owing to the low proportional limit of the T73 material, the net section stress was in the plastic range for most specimens, whereas for the T6 material the net section stress was always in the elastic range.
It was noted that during slow crack growth, the crack grew faster in the center than at the surface of the material. A generally parabolic shaped crack front, therefore, existed at failure. It was hypothesized that the crack shape corresponded to the development of plane stress at its boundaries and that fracture toughness based on final crack length would be constant. This appeared to be true for the T6 material, within the range tested, but could not be proved for the T73 material.
A lumped parameter, redundant force analysis of three plates of varying thickness was made. The results show the development of plane strain conditions with increased thickness. Although correlation with the test results was not established, extension of the work may provide explanation of the phenomena observed in tests.
An appendix provides theoretical load-deflection relationships for centrally cracked plates which include the effects of plasticity.
failure, fractures (materials), cracking (fracturing), crack propagation, toughness, damage, tolerance (mechanics), aircraft, plates (structural members), metal sheets, aluminum alloys, fracture tests, stress analysis
Assistant technical manager, Douglas Aircraft Co., McDonnell Douglas Corp., Long Beach, Calif.