  We propose a nonlocal strain measure for use with digital image correlation
(DIC). Whereas the traditional notion of compatibility (strain as the
derivative of the displacement field) is problematic when the displacement
field varies substantially either because of measurement noise or material
irregularity, the proposed measure remains robust, well-defined and invariant
under rigid body motion. Moreover, when the displacement field is smooth, the
classical and nonlocal strain are in agreement. We demonstrate, via several
numerical examples, the potential of this new strain measure for problems with
steep gradients. We also show how the nonlocal strain provides an intrinsic
mechanism for filtering high frequency content from the strain profile and so
has a high signal to noise ratio. This is a convenient feature considering
image noise and its impact on strain calculations.
