Relation of Structure to Fatigue Properties in Aluminum-Base Alloys.
Abstract
Initiation of fatigue cracks was directly observed at magnifications up to 800 times in aluminum alloys 2024 and 2124. The latter is of higher purity and coarser grained. At low stresses the cracks in 2024 initiated in the matrix near constituent particles. This occurred less in 2124 giving a higher fatigue resistance. The probability that a constituent particle initiates a fatigue crack falls off very rapidly as the particle size decreases below 7 microns. Growth of microcracks is impeded by grain boundaries. The plastic work required for a unit area of fatigue crack propagation was measured by cementing tiny foil strain gages ahead of propagating fatigue cracks and recording the stress-strain curves. The data for six aluminum alloys, three steels and one nickel alloy fit an empirical equation for the fatigue crack propagation rate showing it is inversely proportional to the plastic work, yield strength squared and the shear modulus. The fatigue crack propagation rate of aluminum is substantially lower at 77 than at 298 degrees Kelvin. The origin of the decrease is a large increase in plastic work. Dislocation cells result from cyclic straining at 298 degrees Kelvin, tangles at 77 degrees Kelvin. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 06, 1978
- Accession Number
- ADA062698
Entities
People
- Morris E. Fine
Organizations
- Northwestern University