Ductile fracture of metallic materials through micromechanics based cohesive zone elements

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2020-9
Tandoğan, İzzet Tarık
Gaining popularity after its coupling with the finite element method, cohesive zone modelling has been used extensively to model fracture, especially in delamination problems. Its constitutive relations, i.e. traction-separation laws, are mostly derived phenomenologically without considering the physical mechanisms of crack initiation and propagation. The approach could also be used for ductile fracture where the micromechanics of the phenomenon is explained by nucleation, growth and coalescence of pores. In this context, the objective of the current thesis is to develop and implement a cohesive zone modelling framework for ductile fracture in metallic materials. In order to accomplish this, a micromechanics based traction-separation relation which considers the growth of a physical pore is developed based on the previous works in [1–3]. Tractions are directly represented as a function of pore fraction, and its evolution is driven by separations. The model is implemented as an intrinsic cohesive zone model in a two-dimensional (2D) setting. Implementation steps and methodology including the finite element framework are presented in detail for mode-I, mode-II and mixed-mode fracture cases. The derivation of the mixed-mode case leads to a yield function representation of tractions and separations, instead of an explicit expression. Hence, an incremental implicit elasto-plastic numerical integration scheme is developed to solve mixed-mode system of equations. Implementation is validated by running tests with a single cohesive element. In addition, the framework is implemented as a user element subroutine in Abaqus (UEL) and the numerical simulations are conducted with compact tension (CT) and single edge notch (SEN) specimens to show the capability of the model and the influence of the micromechanical parameters such as pore size and shape on the ductile crack initiation and propagation. The work is concluded by presenting an outlook for the usage of the model in micron sized specimens where the developed micromechanical model presents a great potential in explaining certain deformation mechanisms in high strength aerospace alloys.

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Citation Formats
İ. T. Tandoğan, “Ductile fracture of metallic materials through micromechanics based cohesive zone elements,” M.S. - Master of Science, Middle East Technical University, 2020.