TECHNICAL: Exploratory research to determine the means to suppress the surface crack initiations in engineering materials, to increase the fatigue lives by orders of magnitude, will be performed. Recent research by the PI has shown that "duality" in S-N fatigue curves is seen in engineering materials when fatigue cracks initiating from surface compete to failure with the cracks initiating from the interior of the sample. The surface-crack fatigue lives are much lower due to aggressive environmental effect, thus there are often two S-N curves that are widely separated by orders of magnitude in fatigue life. Under this condition, there is some finite probability that the specimen would fail by surface cracks producing short-fatigue-lives. What this also means is that the true long fatigue life of a material, governed by internal cracks, cannot be fully realized always. This means failures can be unpredictable and engineering designs can be disastrous. PI suspects that the competition due to surface-initiated-cracks for fatigue failure is caused by extreme-sized particles that are distributed in the material. Some of these particles are often introduced intentionally, for the benefit of other mechanical properties. For example, Rene'95 superalloy, used in aircraft engines, contains oxides particles entrapped during powder processing and nitride/carbonitride particles for creep strengthening. PI hypothesizes that by providing finer hard particles with a narrower particle size-distribution, one can eliminate the extreme-sized particles and suppress the surface-crack-initiations that limit the fatigue life. In this research PI will focus on three principal tasks: (i) Identification of the extreme-sized hard particles that are responsible for surface crack initiations in a Rene'95Ni-base superalloy by postmortem analysis of fatigue samples. (ii) Making of a new Rene'95 alloy with a modified particle size distributions that eliminates the extreme-sized particles. (iii) Testing the new alloy for fatigue life improvement and determine the gain in life achieved by the suppression of surface crack initiations. NON-TECHNICAL: Competition for fatigue failure between the surface-initiated and the interior-initiated cracks and the consequent duality/variability in fatigue has been found to occur in actual application conditions in LCF tests conducted in GE (USA), SHINKANSEN steels (Japan), and SNECMA (France). The material and mechanics aspects that promote this unusual behavior seem to be connected to the spatial statistics of extreme-sized crack-initiating particles in the material. A clear resolution of this problem by identifying the extreme-sized particles that cause this behavior and an appropriate modification of the material will have a tremendous impact on engineering material design for fatigue critical applications. If successful, the work may as well represent a break-through in solving the "duality" fatigue problem that is common to many engineering materials. One graduate student and one undergraduate student (summer) will be employed to perform this research. Efforts will be made to employ a minority graduate student from an underrepresented group or a women candidate, to perform this research.
