Reevaluating Fracture Forming Limits in Bulk Forming Under Non-Monotonic Strain Loading Paths

This paper examines the applicability of the fracture forming limits (FFLs) derived from conventional monotonic upset compression tests for assessing the formability of non-monotonic strain loading paths. The work uses a simple test specimen subjected to various non-monotonic deformation histories, and combines experimental force measurements, digital image correlation, finite element analysis, and scanning electron microscopy (SEM) to characterize strain loading paths and crack opening mechanisms under varying testing parameters. Results demonstrate that non-monotonic strain loading paths can result in fracture strains that differ from those obtained through conventional monotonic bulk formability tests in the effective strain versus stress triaxiality space, depending on the considerations made in the transition between different loading stages. Consequently, reliance on monotonic test data may lead to inaccurate predictions of cracking in multi-stage industrial bulk forming processes.