Extending image processing strain measurement system to evaluate fracture behavior of wall-thinned pipes

In this paper, an image processing strain measurement system was extended so that fracture behavior of wall-thinned pipes, such as out of plane and large plastic deformation observed in bulging, can be evaluated by the system. Regular grids with nominal size of 10 mm × 10 mm were marked on 100A carbon steel pipes and the images taken with 6 CCD cameras of 15 million pixels were correlated to achieve resolution of 0.3% strain. Strain of the cylinder's outer surface was evaluated by (1) modeling the grids as a cylindrical shell, (2) measuring deformation of the grid on a projected plane, and (3) applying an updated Lagrangian method. The validity of the system was confirmed by applying the proposed system to the burst tests of a wall-thinned cylinder. In addition, the system measured the non-uniform strain distribution that explained the unexpected cracking location.

► An image processing strain measurement system was proposed.
► The system can measure (1) out-of-plane deformation and (2) large strain.
► System was applied to wall-thinned pipe (WTP) burst test, which bulging is observed.
► System was effective for understanding the unexpected burst location in a WTP.