How persistent slip bands initiate fatigue cracks
In plain words
Under repeated small loads, metals form thin bands of concentrated slip with a ladder-like dislocation structure, and cracks begin where these bands meet the surface. How the bands push out surface ridges and start cracks has not been derived from dislocation physics.
Precise statement
In cyclically loaded Cu single crystals, persistent slip bands with dislocation ladder walls spaced about $10^{-4}$ cm carry localized plastic strain and produce surface extrusions and intrusions where cracks nucleate. Determine the mechanism producing extrusions (vacancy generation by annihilation of edge dislocation dipoles, as in the Essmann-Gosele-Mughrabi model, or irreversible slip) and predict the number of cycles to crack initiation versus plastic strain amplitude. An answer is a model validated against measured extrusion growth rates and initiation lives.
What would settle it
In situ measurement of extrusion growth and point-defect content in persistent slip bands, compared with a dislocation-based model predicting initiation life.
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