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How the bands push out surface ridges and start cracks has not been derived from dislocation physics.","posed_since":"","precise":"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. 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