  Two-dimensional digital image correlation (2D-DIC) is a widely used optical
technique to measure displacement and strain during asphalt concrete (AC)
testing. An accurate 2-D DIC measurement can only be achieved when the camera's
principal axis is perpendicular to the planar specimen surface. However, this
requirement may not be met during testing due to device constraints. This paper
proposes a simple and reliable method to correct errors induced by
non-perpendicularity. The method is based on image feature matching and
rectification. No additional equipment is needed. A theoretical error analysis
was conducted to quantify the effect of a non-perpendicular camera alignment on
measurement accuracy. The proposed method was validated numerically using
synthetic images and experimentally in an AC fracture test. It achieved
relatively high accuracy, even under considerable camera rotation angle and
large deformation. As a pre-processing technique, the proposed method showed
promising performance in assisting the recently developed CrackPropNet for
automated crack propagation measurement under a non-perpendicular camera
alignment.
