Plastification et déformation en fond d’entaille lors de l’amorçage d’une fissure de fatigue

Plastic strain arising from fatigue crack initiation

¹ Ingénieur métallurgiste, dépt. Recherche sur les turbines à vapeur, Alsthom, Belfort.
² Ingénieurs métallurgistes, Département Matériaux, Laboratoire de Marcoussis.

Abstract

Using measurements of notch bottom deformation taken from a 12% chromium steel for steam turbine blading, test specimens which have been subjected to bending, it can be seen that the elastic and plastic behaviour of the material closely agrees with the model recently proposed by Molski [1].

Of the methods used for characterising the plastic zone before and after crack initiation, the best results have been given by extensometry, which makes possible strain measurement in the notch root under monotone loading, and the observation of deformation bands, which enables the size of the plastic zones in the notch root and at the fissure bottom to be calculated.

The comparison of experimental results and theoretical models has also shown that:

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  When the nominal stress is less that the yield strength of the material, Molski’s model allows the ΔS = KT . σ_nom parameter to be simply related to the energy of deformation - It is therefore suggested that ΔS parameter can be retained as a fatigue cracking initiation criteria. Besides, Molski’s model provides an accurate description of the behaviour of the material in the notch root under monotone loading, whereas Neuber’s model overestimates the strain;

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  When the ratio yield strength/S_max is greater than 0.3, it was possible to calculate the size of the plastic zone in the notch root using an expression, in the plastic mode, of the stress gradient in the test-piece thickness;

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  The size of the plasticated zone around the point of fissure (with a length in the order of 2 mm) is proportional to (K_max/Re)² : this conforms to linear elastic fracture mechanics.