An Image-Based Approach for Measuring Dynamic Fracture Toughness

In order to model the dynamic failure of engineering structures it is necessary to have a thorough understanding of dynamic fracture processes. Dynamic fracture toughness has been experimentally analysed by fitting the K-dominant solution to the displacement field measured with a local or full-field technique, such as caustics, photoelasticity or digital image correlation. For highly dynamic crack propagation the stress state at the crack tip is influenced by stress waves. A dynamic propagating crack emits stress waves which can be reflected and/or scattered away. These waves are felt by the evolving crack front, as well as through the sample configuration, and hence material inertia may lead to effects more subtle (yet still present) than those associated with load transfer. The K-dominant solution only indirectly accounts for these inertial effects by including the crack velocity as an input or by using higher order terms in the series expansion. The aim of this work is to develop a new image-based method for measuring dynamic fracture toughness. This method uses full-field measurements to perform an energy balance on a fracture specimen and calculate the energy consumed by crack growth. Using full-field data the impact energy, strain energy and kinetic energy can be measured. When the material cracks the fracture energy is the difference between the impact energy and the sum of the strain and kinetic energy. Explicit dynamics simulations using cohesive elements were used to validate the methodology. The finite element data was used for simulated image deformation experiments. These virtual experiments were used to analyse measurement error propagation from camera spatial and temporal resolution. Future work will include additional image deformation simulations and a first experimental validation of the test method.