Impact Fracture of a Tough Ductile Steel

Abstract

The impact fracture of a tough ductile material is examined by comparing the results of static and dynamic viscoplastic finite element computations with experimental observations of the crack tip opening displacement (CTOD) during static and dynamic (impact) fracture. Experiments measured the relative displacement of two small indents 100 μm behind a fatigue crack tip in a three-point bend specimen to determine the CTOD as a function of load (in static tests) or as a function of time (in dynamic tests). Static computations show that a two-dimensional simulation can capture both the overall structural response, as measured by boundary quantities (such as the applied load and the crack mouth opening displacement), and the near-tip CTOD when crack closure is accounted for. Dynamic finite element simulations also use a two-dimensional model of the three-point bend specimen, but must include the projectile and loading tup to account for the complex stress wave interactions generated during impact loading in order to simulate the CTOD versus time response. Since the finite element model correlates with the observed CTOD history, it can be used to determine relevant field quantities, such as the J-integral, as well as stresses and strains of interest. The finite element model shows that it is essential to include the effects of viscoplasticity in order to capture the correct CTOD response in the dynamic experiments. These calculations show that the rate of change of the stress intensity factor is K ∼ 5 × 106 MPa √m/s in the impact fracture tests. The fracture initiation time has not been unambiguously determined; therefore a unique fracture toughness has not been established.