Use of Internal Damping Measurements in the Study of Environmental Cracking

The problem of environmental cracking of metals and their alloys has become very important and warrants a concentrated study with new ideas for its solution. The major objectives of such a study are, (1) detection of susceptibility, (2) quantitative measurement of susceptibility, and (3) understanding the mechanism of susceptibility. An approach utilizing measurements of the internal damping characteristics of materials prone to environmental cracking is presented to show the feasibility of detecting significant changes occurring in the materials long before any macroscopic evidence of such changes becomes noticeable. Tests on 2024-T4 aluminum, 2024-T3 aluminum, Ti-6Al-4V and Ti-8Al-1Mo-1V are reported wherein considerable changes in the internal damping of these materials were observed following their exposure in a stressed state to environments in which they are known to be susceptible to cracking. (Corroborative tests on 1020 mild steel and electrolytic copper stressed and exposed to a corroding environment of air saturated synthetic sea water produced no changes in internal damping. These materials are normally immune to stress corrosion cracking under the above conditions of exposure.) Theoretical ideas with supporting experimental evidence are presented in an attempt to arrive at a basic understanding of the phenomenon of environmental cracking by correlating the laws of diffusional mechanics of materials to those pertaining to changes in their internal damping.