SMALL-SIZE SPECIMEN WITH HARD ZONES NEAR CRACK TIP FOR FRACTURE TOUGHNESS EVALUATION OF PRESSURE VESSEL IN SERVICE (PART II: NUMERICAL DISCUSSION ON CONTROLLING FACTORS OF PLASTIC CONSTRAINT)

摘要

A series of study, Parts I and II, demonstrated that small-size specimen with a couple of hard zone near the crack tip was useful for fracture toughness evaluation of pressure vessels in service. Fracture toughness obtained with the small-size specimen was compatible with the toughness of full-thick specimen. This was attributed to the plastic constraint caused by the hard zone. Major factors controlling the crack tip constraint were the yield strength ratio σ_Y~H/σ_Y~B between the hard zone and base metal and the distance e_H from the hard zone to crack tip. The significance of toughness results for the small-size specimen was discussed in terms of the Weibull stress σ_W. The critical Weibull stress σ_(W,cr) at fracture initiation in the small-size specimen was consistent with σ_(W,cr) for the full-thick specimen. On the basis of the Weibull stress fracture criterion, the use of the small-size specimen for plane-strain fracture toughness evaluation was studied. The K_(IC) toughness could be obtained with a 5mm thick specimen, provided that the yield strength ratio σ_Y~H/σ_Y~B and the distance e_H from the hard zone to crack tip are 2.8<σ_Y~H/σ_Y~B and e_H<0.4mm, respectively.