Towards validation of crack nucleation criteria from V-notches in quasi-brittle metallic alloys: Energetics or strength?

We discuss crack nucleation observations from a series of experiments that have proven to be particularly challenging for model validation. In particular, we focus attention on crack nucleation from a series of V-notched specimens of an AISI 4340 steel alloy subjected to four-point bending. Details of the specimen geometry, loading, and experimentally-measured forces are provided for V-notches with opening angles of 30, 60, and 90 degrees. Despite the relatively simple geometry and loading, and the fact that the specimens exhibit small scale yielding, model-based simulations that can accurately reproduce the critical forces at crack nucleation across the full set of specimens have proven to be elusive. Along these lines, we present model-based simulation results from finite-element discretizations of phase-field models for fracture. We provide results using both a well-established phase-field model for brittle materials as well as a recently-developed phase-field model for ductile materials. Importantly, both models tie crack nucleation to critical values of energy, and both are shown to be lacking in their accuracy to varying degrees. We also use standard finite-element calculations of a calibrated plasticity model to demonstrate that much greater accuracy in the calculated critical forces can be obtained with the use of a particular strength envelope.