Recent experiments with a magnetostriction apparatus show that cavitation-damage rate is time dependent. This is confirmed by an analysis of the experimental data obtained in various earlier investigations. There are four zones of damage rate with respect to testing time; namely, (a) incubation, (b) accumulation, (c) attenuation, and (d) steady state. In the fourth, or steady-state zone, the damage rate varies as the square of the amplitude of oscillation within the range tested for water at 80 F. The damage rate increases with frequency and then decreases with increasing frequency. The depth of liquid in the beaker, the beaker diameter and the depth of immersion of the specimen do not seem to affect the damage rate substantially. The average depth of erosion is independent of the diameter of the specimen. Based on these experimental results, certain recommendations are made for testing materials for cavitation damage resistance.
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-1 V 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.
In addition. he is u member of SNAME, the Nutiorial Associatioii of Corrosiori Engineers, arid the Electrochemical Society. Mr. Preiser has written extensively in the techriical literature and holds 25 U.S. Patents 011 various corrosion arid fouling control methods arid devices. Mr. Donald R. Laster received his B.S. degree in Physics f r o m the Uiiiversity of South Carolilia in 1960 arid his M.S. degree iri Physics from the Drexel Institute of Technology in 1968. He has beeri employed at the David W. Taylor Naval Ship R & D Center since 1960 working in sensitig systems applications and techt~ology such us tionacoustic A S W. ship performance measurement. arid fuel use ariulysis. A member of ASNE sime 1980. he ulso is a member of IEEE arid Sigma Pi Sigma. ABSTRACTThe rapidly increasing cost and the uncertain supply of oil provide strong impetus to find ways to conserve ships' fuel and to optimize its use. Underwater mechanical removal of marine fouling from U.S. Navy ship hulls and propellers has been shown to result in immediate and rather substantial fuel savings. The long-term savings from underwater cleaning is not nearly so certain because of the interactions between the cleaning techniques, paint performance, and regrowth of the fouling. Of particular interest is the sensitivity of the fuel savings as a function of cleaning frequency.The following three questions are addressed: Does underwater cleaning lengthen Anti-Fouling (AF)Does underwater cleaning really save fuel? Is underwater cleaning cost effective?The answers to these questions obviously depend strongly upon the use of the ships and the manner in which they are operated. For U.S. Navy ships, underwater hull cleaning will save fuel provided adequate attention is given to the scheduling of the cleaning in terms of drydock cycle and deployment. Additional efforts toward optimization of underwater cleaning also are discussed. paint service life?
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