A broad spectrum of engineering applications subject metals to oligocyclic (or low-cycle) loading conditions. The Paris-Erdogan Law can be used to relate the crack speed and the amplitude of the stress intensity factor (SIF) within the realm of Linear Elastic Fracture Mechanics (LEFM). Although this law is reasonably well suited to describe fatigue crack propagation with moderate plastic yielding within the so-called small-scale yielding, Paris’ law may break down as the extent of bulk plasticity increases. Large-scale yielding corresponds to large plastic zone size (L_Y) with respect to the structural length scales. Many practical applications, such as for example combustion chambers in space shuttles, require metallic alloys to possess excellent thermal properties, which, however, exhibit large-scale yielding behavior. The Dowling equation is a generalization of the conventional Paris’ Law as it substitutes the SIF by the energy release rate to quantify the crack driving force in oligocyclic fatigue. Yet, in practice, the determination of this energy release rate is rather challenging, and very often nearly impossible. Here, we propose an alternative approach based on cohesive zone models of oligocyclic fatigue crack growth that circumvent this issue.

 We use copper chromium zirconium (CuCrZr) alloys to conduct our study and test the predictability of the proposed model. They give rise to a large plastic zone (L_Y ≈ 5 cm.) at the crack tip vicinity, allowing us to test the effect of large-scale plasticity on oligocyclic failure. We use double cantilevered beam (DCB) specimens of different sizes to study the mode-I fatigue fracture properties under different large-scale yielding conditions. The bulk elastoplastic properties are measured using KBr-type tensile specimens, while statistical fractography is used to measure the cohesive parameters (cohesive stress and critical crack opening) for monotonic and oligocyclic fracture. These properties are then used as input for the finite element (FE) cohesive zone model of crack growth in the DCB specimens of different sizes. The predicted crack speed as a function of the cyclic amplitude is compared with the results of our experimental fracture tests, showing the suitability of cohesive zone models to describe oligocyclic fatigue failure under large-scale yielding conditions. Finally, I will discuss the application of this new modeling strategy to the design and the predictive maintenance of aerospace components like combustion chambers.