CDM approach applied to fatigue crack propagation on airframe structural alloys

AbstractCharacterization of materials requires a large number of experimental tests with various loading modes in order to ensure that it meets the design requirements. The aim of his work is to develop a numerical model to predict the behaviour of cracked panels, in place of expensive and complex experiments to generate data for designers. In particular, the model has to simulate toughness test and fatigue crack propagation at high level of ΔK for a wide variety of metallic alloys including aluminium alloys, titanium alloys and steels.The proposed model is based on a thermodynamic approach within the framework of Continuum Damage Mechanics which provides numerical robustness as well as a precise description of the physical micromechanism of fracture. An extension of a classical CDM approach, named Lemaitre model [1], is presented in this paper. Lemaitre model [1], which has been shown to be particularly well suited to describe ductile damage under monotonic loading, has been only applied separately to low cycle fatigue and high cycle fatigue. The extension has to simulate fatigue crack propagation. Experimental results have evidenced that the role of two distinct mechanisms in the high fatigue crack growth rate regime. In order to account for the different mechanisms involved in fatigue crack propagation test two damage variables are introduced in the Lemaitre model [1]. The first variable, called static damage, represents the damage variable associated with ductile fracture modes. While the static damage evolution law is consistent with the classical Lemaitre model [1], the second variable is assumed as a consequence of cumulated plastic strain and does not influence plasticity. It is named cyclic damage.Then, to correctly predict fatigue crack propagation rates, an internal length has to be defined. An energetic equivalence criterion has been inserted to model crack propagation and to reduce mesh dependence. This method, originally developed by J.Mazars and G. Pijaudier-Cabot [2], fills the gap between damage and fracture and avoids the introduction of additional boundary conditions or complex code developments associated with non-locality. The damage law coefficients have been calculated by taking into account the size of the elements. This model has been implemented in the commercial finite element code ABAQUS/CAE. Explicit computations have been performed to highlight drawbacks and advantages of the method