Fatigue Microcrack Behavior under the Influence of Surface Residual Stresses.
Abstract
Direct observations of the surface crack opening displacement (SCOD) of surface microcracks (2c to approximately 400 microns) have been made on samples of 4140 and HY-130 steels under both cyclic and static loading conditions. Preliminary observations indicate that small cracks which reside entirely within the zone of shot-peen-induced compressive residual stress remain completely closed, even at applied loads approaching the bulk yield stress in HY-130. Results of SCOD measurements on HY-130 specimens which were cycled in an aqueous 3.5% NaCl solution show a large reduction in SCOD (effective delta K) due to a wedging open of the crack by oxides on the crack faces. As far as we know, the results of these near-threshold, low R-ratio tests represent the first direct observations of oxide wedging effects in microcracks and are consistent with independent data for large crack behavior. Predictions using the analytical approach developed earlier in this program give excellent correlation with experimental observations in 4140 steel in the absence of surface residual stresses. Similar analyses are currently being conducted for HY-130. In addition, a boundary-integral program for the direct calculation of crack face displacements is being used to determine complete crack opening profiles in the presence of a surface residual stress gradient. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 30, 1981
- Accession Number
- ADA108691
Entities
People
- Gerald R. Leverant
- John E. Hack
Organizations
- Southwest Research Institute