The current paper characterizes constraint in fracture: J-modified resistance (JMR) 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 JMR curves exhibited an inflection point after some crack extension, and the data were excluded beyond the inflection. Qualified JMR curves for the two materials showed similar behavior, but R-curves were identical for equal ligament length-to-thickness ratio, RL, for the aluminum alloy, with increasing slope for increasing RL, 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 JMR 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.