Digital Repository, Giornata IGF Forni di Sopra (UD) 2011

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Ratcheting strain as a crack driving force for crack growth
J. Tong, L.-G. Zhao, B. Lin, C. Cornet

Last modified: 2011-02-25


Ratcheting deformation has significant implications on material damage and fatigue life of
components under service loading conditions. In this work, we explore the concept of ratcheting strain as a
crack driving force in controlling crack growth, both time-independent and time-dependent, utilizing elastoplastic,
visco-plastic and crystal-plasticity constitutive models. The viscoplastic and the crystal-plasticity models
were implemented in the finite element software ABAQUS via user-defined material subroutines. Both stresscontrolled
and strain-controlled experiments were carried out to obtain the material parameters necessary to
calibrate the models. Characteristics of crack tip deformation were examined at room and elevated temperature
for both stationary and growing cracks using the finite element method. Whilst the strain range and the stress
range remained stable throughout the cycles, distinctive strain ratcheting behaviour near the crack tip was
captured in all cases, leading to progressive accumulation of tensile strain normal to the crack growth plane. It is
possible that this tensile strain, or ratcheting strain, may be responsible for material separation leading to crack
growth. Results show also that low frequencies and superimposed hold periods at peak load significantly
enhanced strain accumulation near the crack tip. Application of a criterion based on ratcheting strain in the
prediction of crack growth at elevated temperature was also attempted.

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