Laser speckle photography and decorrelation techniques in elastic-plastic fracture studies

In elastic-plastic fracture, material behavior is often characterized by the J-integral or crack tip opening displacement (CTOD) parameters. In order to evaluate these parameters accurately, the location of the plastic hinge point, the shape and extent of plastic deformation ahead of the crack tip, and subsequently, the value of the plastic rotational factor r{dollar}sb{lcub}rm p{rcub}{dollar}, must be determined. Traditionally, hinge point location and r{dollar}sb{lcub}rm p{rcub}{dollar} have been inferred through crack opening displacement (COD) measurements. The plastic deformation has traditionally been delineated by macroetching techniques that reveal flow lines. However, this work indicates that laser speckle metrology can be used to analyze directly the hinging phenomenon and delineate the extent of plastic deformation around the crack tip. Using notched three point bend bars fabricated from a high strength low alloy steel, a full-field map of in-plane displacements was generated over the course of each test. The results indicate the existance of a hinge region, rather than an explicit hinge point. The hinge region appears to contain the computed hinge point location using the range of cited r{dollar}sb{lcub}rm p{rcub}{dollar} values. This indicates that it may be appropriate to use the centroid of the hinge region in subsequent CTOD and J-integral calculations. Laser speckle decorrelation was used to delineate the change in the plastic deformation for a small incremental loading. The results were compared with the flow lines generated by macroetching and indicate that decorrelation techniques may be more sensitive than flow lines for delineating plastic deformation caused by a small, incremental load.