Rapid Assessment of the Role of Microstructural Variability in the Fatigue Behavior of Structural Alloys using Ultrasonic Fatigue
Abstract
Ultrasonic fatigue (20kHz) has been used to investigate the role of microstructural variability on fatigue life variability for very long fatigue lives, between 10(to the 7th power) and 10(to the 9th power) cycles, in structural aerospace materials. This AFOSR program has been augmented by support from DARPA (Dr. Leo Christodoulou) and a summary of the results from the combined support is described in this report. Four distinctly different structural alloys have been studied: (1) a particle reinforced aluminum alloy, 2009Al/15SiCp, where clustering of SiC particles and the presence of processing-related inclusions is expected to control fatigue life, (2) an alpha/beta titanium disk alloy, Ti-6Al-2Sn-4Zr-6Mo, that has been processed to have a fine, homogeneous, two-phase microstructure that is essentially free of inclusions or other defects; (3) a polycrystalline nickel-base disk alloy, Rene 88DT which has a small but finite number of large grains that influence fatigue crack initiation; and a single crystal nickel-base superalloy for blade applications, PWA 1484. In this program, an advanced ultrasonic fatigue system capable of fatigue testing aerospace materials at 20 kHz and at temperatures form ambient to 1000 degrees C has been developed. For the polycrystalline materials, the magnitude of fatigue life variability was observed to be strongly dependent on the microstructure-dependent crack initiation processes, which, in turn, were different for each material. For 2009Al/15SiCp, crack initiation occurred in almost all cases at 20-30 micrometers diameter inclusions. Less than half an order of magnitude variation in fatigue life at 10(to the 9th power) cycles was observed and is explained by the absence of a significant crack initiation lifetime.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 23, 2007
- Accession Number
- ADA469738
Entities
People
- J. W. Jones
Organizations
- University of Michigan