Observations on Mode I ductile tearing in sheet metals

Cracked ductile sheet metals, subject to Mode I tearing, have been observed to display a variety of fracture surface morphologies depending on the material properties, and a range of studies on the fracture surface appearance have been published in the literature. Whereas classical fractures such as cup-cone, cup–cup and slanting are widely observed, the phenomenon of a slanted crack which systematically “flips” back and forth in a roughly 45° orientation has only recently been reported. The present study aims to add details and understanding to this crack growth phenomenon – through experimental testing and comparison with published results. A series of crack propagation tests have been carried out, where cracks are driven many plate thicknesses under global Mode I loading. The current study employs both the edge crack specimen (ECS) loaded in combined in-plane bending and extension, and the double edge notched tension (DENT) specimen loaded in extension. Clear evidence of the alternating slant failure is observed for a normal strength steel and details on the phenomenon are brought out by grinding and polishing cross-sections in successive steps along the crack growth direction. Moreover, these results are compared to a soft aluminum (1050A) which predominantly shows cup–cup or slant failure. In general, both materials fail by the mechanism of void growth to coalescence (fractographs are included), and evidence points toward cup–cup being favored by materials that experience extensive amounts of void growth (thus severe thinning in the process zone), whereas slanting is typical in high strength materials that rapidly nucleate large populations of smaller voids.