the pinning centers. This corresponds to a transition from weak to strong pinning.

We now turn to the behavior of the dislocation line under applied stress. An increasing applied stress causes the dislocation to move between metastable configurations until, at a critical stress, metastability is lost and the dislocation moves indefinitely (depinning). In terms of the self affine roughness of the dislocation line, roughening extends to larger and larger scales until, at the critical stress, the dislocation exhibits a self affine shape on all scales with roughness exponent $\zeta \approx 1$. This roughening is illustrated in Figure \[fig:roughquenched\] for two different amplitudes $\hat{f}$ of the pinning field.

![Evolution of the structure function $R(d,\tau)$ under increasing applied shear stress in a stationary pinning field; top: amplitude of the pinning field $\hat{f}$ = 0.128 N/m, bottom: $\hat{f}$ = 0.004 N/m; the resolved shear stress is given as a fraction of the critical resolved shear stress (crss), see Figure \[fig:crss\], top, for the respective crss values.\[fig:roughquenched\]](roughquenched.pdf){width="3.3in"}

We define the critical resolved shear stress (crss) as the minimum stress required to move the dislocation once across the periodic simulation cell of width $2000b$ in the (mean) direction of dislocation motion. To compare with the 