Crack closure behavior of surface cracks in four point bend specimens subjected to bending loading is investigated by using an unloading elastic compliance technique. The plane strain closure response at the maximum depth point of a surface crack is monitored by an extensometer spanning the surface crack at the midpoint of its length. The plane stress closure at the surface intersection point, i.e., the near-tip plane stress closure is measured by multiple strain gages placed at appropriated intervals at the front of crack tip. The closure behavior of surface cracks under bending loading is compared with that of through-thickness cracks in single-edge notch (SEN) specimens under bending loading and that of surface cracks under axial loading. It is observed that the crack opening point of surface cracks at the maximum depth point is much lower than that of through-thickness cracks in SEN specimens, while the crack opening point at the surface intersection point is almost equal to that of through-thickness cracks in SEN. Growth rates of surface cracks can be well described by the effective stress intensity factor range under bending loading. The closure behavior under bending loading is different from that under axial loading.
