ISSN: 1945-7553
CODEN: JTEVAB
Page Count: 10

Plastic Strain Arising from Fatigue Crack Initiation

Coulon, A
Research EngineerMember of ASTM, Steam Turbine Department, Alsthom-Atlantique, Belfort,

Knosp, B
Metallurgistmember of ASTM, Materials Department, Laboratoire de Marcoussis, Marcoussis,

Saisse, H
Metallurgistmember of ASTM, Materials Department, Laboratoire de Marcoussis, Marcoussis,

Abstract

Of the methods used for characterizing 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 monotonic 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.

A comparison of experimental results and theoretical models also shown that: (1) where the nominal stress (σ_nom) is less than the yield stress (R_e), Molski's model provides an accurate description of the behavior of the material in the notch root under monotonic loading, whereas Neuber's model overestimates the strain; (2) when the ratio R_e/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 specimen thickness; and (3) the size of the plastic zone around the point of fissure (with length on the order of 2 mm) is proportional to [K_max/R_0.2%]²; this conforms to linear elastic fracture mechanics.

Keywords:
fatigue, crack initiation, plastic strain, steel for blading (12% chrome steel), Neuber's model, Molski's model, notch root

Paper ID: JTE11246J
DOI: 10.1520/JTE11246J
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