Constraint Effects Observed in Crack Initiation Stretch

The current paper characterizes constraint in fracture: J-modified resistance (J_MR) curves were developed for two tough structural materials, 6061-T651 (aluminum) and IN718-STA1 (nickel-base superalloy). A wide variety of configurations was tested to consider load configurations from bending to tension including three specimen types (compact tension, center-crack tension, and single-edge notched tension), and a range of ligament lengths and thicknesses, as well as side-grooved and smooth-sided ligaments.

The J_MR curves exhibited an inflection point after some crack extension, and the data were excluded beyond the inflection. Qualified J_MR curves for the two materials showed similar behavior, but R-curves were identical for equal ligament length-to-thickness ratio, R_L, for the aluminum alloy, with increasing slope for increasing R_L, while for the nickel, the resistance curves aligned for equal ligament thickness, B, and the slope increased for decreasing B.

Displacements at the original crack tip (δ₅) were recorded throughout the test for several specimens. δ₅-versus-crack extension curves were developed, and data were excluded beyond the inflection point (as with the J_MR curves). The data collapsed into two distinct curves, thought to represent the surface, plane stress effect and the central, plane strain effect. This was observed for both materials.

A technique called profiling is presented for the aluminum alloy only, where the crack face displacements are recorded at the final point of the test as a function of the position throughout the crack cavity, along with an effort to extract the observations in a usable form. Displacements were consistent throughout the cross-section at and behind the original crack tip. In the region where the crack grew, this displacement was developed by a combination of stretch and crack growth. The stretch required to initiate crack extension was a function of the depth beneath the surface into the cross-section.