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Stress corrosion tests on precracked double-cantilever beam specimens from 2000- and 7000-series aluminum alloys have been in progress for up to seven years at a marine-atmosphere exposure site near Daytona Beach, Fla. One of the most significant results of these tests concerns the effects of specimen self-loading (because of exfoliation and corrosion-product wedging) on the relative crack growth resistance of different alloys. Self-loading is apparently affected by copper content but is not limited to copper-bearing alloys. Copper-free alloy 7039-T64, for example, showed self-loading effects after 3 to 4 years' exposure. Naturally aged 2XXX alloys and peak-aged, copper-containing 7XXX alloys were most sensitive to self-loading, usually showing the effects within six months. However, cracks in 7075-T73, a stress corrosion resistant alloy, also continued to propagate at rates that were not insignificant after several years' exposure, whereas alloy 2024-T851 appeared more resistant to corrosion-product wedging than 7075-T73.
Resistance to stress-corrosion cracking of 10 high-strength aluminum alloys in a variety of heat-treatment conditions was measured using precracked double cantilever beam (DCB) specimens. A new technique is described, and stress-corrosion crack growth rates for the alloys tested are presented as a function of the plane-strain stress intensity K(I). Double cantilever beam specimen data correlated with established trends from smooth specimens tested by alternate immersion in 3.5% NaCl solution. From the crack growth rate data and the speed and simplicity with which it is obtained, it is concluded that the DCB specimen will be highly useful for (1) comparing and rating alloys, (2) developing new alloys and heat treatments, (3) comparing the effects of environments, (4) achieving or ensuring product uniformity, and (5) studying mechanisms of cracking. (Author).
Propagation rates for stress-corrosion cracking (SCC) of high-strength aluminum alloys determined with mechanically precracked double-cantilever-beam (DCB) specimens provide a quantitative means of comparing the resistance to SCC of alloys and tempers. For relatively resistant materials however, the rates can be markedly influenced by test conditions and interpretation of the crack-growth data. This paper describes a relatively simple, accelerated testing method for determining SCC velocities that rank the SCC behavior of aluminum alloys in the same order as SCC propagation rates in a seacoast or industrial atmosphere. Practical difficulties associated with this test method, however, warrant further evaluation to justify this as a primary method of testing.
The stress corrosion cracking resistance of high strength, wrought aluminum alloys in a seacoast atmosphere was investigated and the results were compared with those obtained in laboratory tests. Round tensile specimens taken from the short transverse grain direction of aluminum plate and stressed up to 100 percent of their yield strengths were exposed to the seacoast and to alternate immersion in salt water and synthetic seawater. Maximum exposure periods of one year at the seacoast, 0.3 or 0.7 of a month for alternate immersion in salt water, and three months for synthetic seawater were indicated for aluminum alloys to avoid false indications of stress corrosion cracking failure resulting from pitting. Correlation of the results was very good among the three test media using the selected exposure periods. It is concluded that either of the laboratory test media is suitable for evaluating the stress corrosion cracking performance of aluminum alloys in seacoast atmosphere. Humphries, T. S. and Nelson, E. E. Marshall Space Flight Center NASA-TM-82393
Evaluates the usefulness of the current standards on exfoliation and corrosion testing of aluminum alloys and their applicability to new requirements and advanced alloys. The 13 papers, from an international symposium in San Francisco, May 1990, discuss whether the existing standards should be revis
The double torsion test specimen was evaluated to determine if it was suitable for use in subcritical crack growth studies of ductile engineering alloys. Several attractive features of the double torsion specimen were observed, including a simple, crack-length-independent expression for stress intensity and the apparent retention of plane-strain conditions in relatively thin plate specimens. Use of the double torsion specimen to determine V-K curves for high-strength aluminum alloys in a chloride environment produced conventional two-stage growth, plateau velocities being slightly higher than those observed in similar tests using conventional plan strain specimens. The double torsion specimen appears attractive for subcritical crack growth studies under plane-strain conditions, particularly in the Stage II regime where crack velocities are not strongly dependent on stress intensity.
An interlaboratory testing programme to evaluate the rising displacement test for stress corrosion crack (SCC) growth investigations was carried out within the framework of the "Measurements and Testing" programme of the European Community under BCR contract nr. MATI CT 930038. The contribution of the NLR to the project consisted of testing of precracked Double Cantilever Beam (DCB) Specimens made from the high strength aluminium alloy 7010-T651 under constant displacement and by continuous immersion in substitute ocean water. The results of these static tests were to be used as reference data for the dynamic rising displacement tests carried out by other participants in the project. Further, additional static tests were carried out by the NLR using alternate y. Duriimmersion in substitute ocean water. The results of these tests were compared with those of 7010-T651 plate from another batch of material tested previously at the NLR.