Volume 27, Issue 3 (May 1999)
Further Evaluation of Automated Stress Corrosion Ring (ASCOR) Testing of Aluminum Alloys
In 1991 a simple automated stress corrosion testing method called the ASCOR (automated stress corrosion ring) test was developed to test aluminum alloys according to ASTM Practice for Evaluating Stress Corrosion Cracking Resistance of Metals and Alloys by Alternate Immersion in 3.5% Sodium Chloride Solution (G 44) and using direct stressing as in ASTM Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens (G 49). The present investigation concentrated on evaluating two aspects of the test method: • The SCC initiation criterion (2% load decrease). • The possibility of defining a threshold stress for micro SCC growth in addition to that for macro SCC growth.
It was found that substantial SCC growth had occurred at 2% load decrease. We therefore propose reducing the load decrease criterion to 0.5% to limit the contribution of SCC growth to the SCC initiation life.
Fracture strength ratios (fracture strength of loaded exposed specimens divided by fracture strength of unloaded exposed specimens) less than 100% were found at exposure stresses below the threshold stress for macro SCC growth. Fractographic investigation showed SCC on the fracture surfaces of specimens tested at these exposure stresses. A threshold stress for micro SCC growth was determined and is defined as the exposure stress at and below which the fracture strength ratio is 100%.