The crack tip strain field of AISI 4340 - Part II - Experimental measurements

Crack tip strain maps have been measured for AISI 4340 high strength steel. No significant creep was observed. The measured values of CTOD were greate r than expected from the HRR model. Crack tip branching was observed in eve ry experiment. The direction of crack branching was in the same direction a s a major "ridge'' of epsilon(yy) strain, which in turn was in the same dir ection as predicted by the HRR model. Furthermore, the measured magnitudes of the epsilon(y)y strain in this same direction were in general greater th an the values predicted by the HRR model. This indicates more plasticity in the crack tip region than expected from the HRR model. This greater plasti city could be related to the larger than expected CTOD values. The followin g discrepancies between the measured strain fields for AISI 4340 and the HR R predictions are noteworthy: (1) The crack branching. (2) Values of CTOD s ignificantly higher than predicted by HRR. (3) The major "ridge'' of epsilo n(yy) strain an angle of about 60 degrees with the direction of overall pro pagation of the fatigue precrack, in which the measured magnitudes of the e psilon(yy) strain were greater than the values predicted by the HRR model. (4) Asymmetric shape of the plastic zone as measured by the epsilon(yy) str ain. (5) Values of shear strain gamma(xy) significantly higher than predict ed by the HRR model. (C) 1999 Kluwer Academic Publishers.