Multi-scale Modeling of Ductile Fracture under Normaland Shear Loading

QC251030

Abstract

Ensuring the long-term safety and performance of structural materials innuclear power plants is increasingly important as global energy demand risesand Sweden pursues its goal of a 100 % fossil-free electricity supply by 2040.Since existing reactors are approaching or exceeding their original design life,life-extension programs have become essential. This context underscores theneed for reliable constitutive models of ductile fracture under complex loadingconditions, as structural integrity must be ensured over extended serviceperiods.

Ductile failure is governed by the nucleation, growth, and coalescence of voids,and is strongly influenced by the prevailing stress state and intrinsic materiallength scales. To address these challenges, this thesis developed and appliednumerical and constitutive modeling techniques to characterize plastichardening beyond the peak load and to predict ductile fracture up to final failure.An Inverse Finite Interval (IFI) method based on finite element modeling wasproposed to identify the true stress-strain response in the post-peak regime. Bycombining a Voce-type extrapolation function with a controlled geometricdisturbance, the method overcomes the optical and measuring limitations of theBridgman correction method. Validation against both synthetic data andexperiments on multiple steel grades confirmed its accuracy, robustness, andbroad applicability.

A non-local Gurson-Tvergaard-Needleman (GTN) model was formulated tocapture fracture mechanisms across a wide range of triaxialities. The modelincorporated shear-enhanced damage evolution and introduced twocharacteristic length scales: one associated with dilatational void growth under high stress triaxiality, and another linked to shear-driven localization at lowtriaxiality. Damage evolution was regularized through a non-local integralapproach, ensuring mesh-independent finite element predictions. Numericalstudies and experimental calibration demonstrated that separating these lengthscales is essential for reproducing mixed-mode fracture processes, such as crackbranching and the transition from normal to shear-dominated failure.

To investigate size effects, round and square smooth bar specimens were testedin uniaxial tension. Standard specimens exhibited classical cup-cone fractures,whereas smaller specimens with dimensions approaching the size of the fractureprocess zone showed cup-cup and shear-dominated fractures, respectively. Thelatter resulted from constrained void growth, premature coalescence, and finalfailure characterized by abrupt edge rupture of thin walls or ligaments near thefree surface, thereby highlighting size-dependent fracture mechanisms.

In conclusion, this thesis contributes to a mechanistic understanding of materialbehavior when experiencing ductile failure and provides a foundation for futuredevelopments in ductile fracture modeling, especially in complex stress states.While the proposed IFI method offers a reliable framework for determiningplastic flow properties, the non-local GTN model establishes an initialfoundation for modeling complex fracture processes in structural materials.Limitations remain in simultaneously predicting global load-deformationresponses and fracture modes using physically motivated parameters, pointingto the need for further refinement through more sophisticated formulations.The results provide both methodological advances and physical insights withdirect relevance for safety assessments and life-extension strategies in nuclearpower plants, where aged materials are subjected to demanding serviceconditions.

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