▎ 摘　　要

The amount of rippling in graphene sheets is related to the interactions with the substrate or with the suspending structure. Here, we report on an irreversibility in the response to forces that act on suspended graphene sheets. This may explain why one always observes a ripple structure on suspended graphene. We show that a compression-relaxation mechanism produces static ripples on graphene sheets and determine a peculiar temperature T-c, such that for T < T-c the free-energy of the rippled graphene is smaller than that of roughened graphene. We also show that T-c depends on the structural parameters and increases with increasing sample size.