Towards a comprehensive regularized continuum damage mechanics model for fatigue crack growth

Efficient crack propagation models are required to ensure that critical parts of an aircraft continue to perform their function even in the presence of cracks. One of the main difficulties is to model the different types of damage that can occur in the same formalism given the variety of loading paths during in-service conditions that these parts are subjected to. To meet this challenge, incremental models are of particular interest, especially those based on Continuum Damage Mechanics (CDM). However, in order to obtain mesh-independent results, these models require the use of regularization methods. In addition, to check the ability of the model to catch the crack growth kinetics when the model is intrinsically continuous, a damage-to-crack transition is required in order to monitor the crack size. All these issues need to be addressed before any comparison can be made between numerical results from finite element calculations and those from a conventional Linear Elastic Fracture Mechanics (LEFM) approach. This paper proposes a possible solution by combining a regularized time-incremental model that takes into account two different types of damage (here fatigue and ductile damage) with a continuous–discontinuous strategy to insert an easy-to-track discrete crack. The procedure is investigated on a nickel-based superalloy subjected to fatigue loading conditions at elevated temperatures. The high-temperature cyclic behavior of the material is modeled within the framework of unified elastic–viscoplasticity. The discrete crack, within structural calculations, is represented by successive crack-path tracking and remeshing steps. Finite element calculations illustrate the ability of the proposed strategy to recover features identical to those obtained with conventional LEFM methods.