Prediction, Analysis, and Measurement of Ductile Fracture of Metals.

Abstract

A new method of predicting ductile fracture initiation is presented based on comparison of the energy dissipation rates of the bulk continuum system to the fractured medium. A fracture criterion is posited for plastic materials with no pre-existing cracks as a critical state being reached when the energy release rate of the bulk system is balanced by the energy release rate associated with the fractured medium. The energy dissipation of the continuum system includes that of plastic work while that of the fractured system includes the surface energy of the crack formation, plastic work, and frictional losses (if any) at the instant of crack initiation. Two fracture modes are considered which are commonly addressed in fracture mechanics: Mode I crack opening perpendicular to the fracture plane and Mode II shear rupture tangential to the fracture plane. The theory introduces a length scale and a new material constant which we call toughness stress. The toughness stress is defined as the surface energy release rate divided by the micro-structural characteristic length of the material. A study of the use of the criterion for a plastic material with power-law hardening is examined and compared with published experimental data for aluminum. Furthermore, a series of uniaxial tests and cylindrical indenter experiments on AH32 steel, a mild steel, were conducted to investigate fracture initiation states and the scale/mesh size effect. Strains at fracture are obtained using digital image correlation (DIC) analysis, and the corresponding states of stress are obtained via the constitutive relationships and the stress-strain relationships using the measured strains. The strain fields are calculated to the point of fracture initiation where we define the fracture initiation as the condition when the first visible crack appears in the digital image of the test specimen. The toughness stress of AH32 steel for the two fracture modes are calculated using the current experimental results. It is shown that the surface energy release rates for both the aluminum alloy and the AH32 steel calculated from the new theory for ductile fracture compare well with the surface energy release rates used in traditional fracture mechanics.