Environmentally Assisted Cracking Measurements in Structural Aluminum Alloys Under Accelerated Test Conditions

Environmentally assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is an important maintenance and safety issue for U.S. Department of Defense assets. EAC initiation and propagation of cracks is influenced by the complex interactions of load, environment, and alloy properties. Traditional environmental fracture testing conducted under immersion or constant humidity conditions may produce results that are different than measurements collected under thin electrolyte layers or droplets formed during atmospheric exposure. In addition, most standard methods do not provide instantaneous measures of crack velocity that can be used to identify specific environmental conditions that promote cracking. Improved assessment of EAC susceptibility and the conditions that promote cracking of aluminum alloys has been accomplished with an autonomous, in situ measurement system that can be used in accelerated corrosion test chambers and outdoor exposure sites. Continuous measurements of crack length throughout a corrosion study can be obtained using a tensile loaded notched specimen, compact load frame, and a force sensor to track load shedding with crack propagation. These measurements can be used to compare alloy performance, determine environmental conditions that promote EAC, and evaluate the effectiveness of corrosion control coatings and methods. Aluminum alloy testing with varying environmental and mechanical parameters (e.g., relative humidity, salt composition, degree of sensitization, and stress intensity) has demonstrated a strong dependence of crack velocity on cyclic relative humidity (RH). Specifically, in a number of tests, crack velocity increased to a maximum during drying (decreasing RH) at intermediate humidity. This result may be important to understanding the processes that promote EAC and indicates that high humidity and salt loading may not always be the most aggressive conditions for evaluating EAC susceptibility. Results of AA5083 alloy testing in cyclic accelerated corrosion tests, including ASTM B117, ASTM G85 A5, and GM9540P are reported. A subset of results for AA7075-T651 is also presented to demonstrate applicability of the test method for a different alloy that was not as highly sensitized to EAC.