Small-Size Fracture Toughness Specimen with High Plastic Constraint Generated by Mechanical Heterogeneity

Abstract

This work develops a new method for plane-strain fracture toughness test to get valid-K lc results according to ASTM E 399 standard. The idea is based on using the mechanical heterogeneity effect on plastic constraint in crack-tip region. By inserting two hard zones very near to the crack plane of a very small-size fracture toughness test specimen, the crack-tip plastic constraint was elevated and reached the level specified by ASTM E 399 standard. In the first report by authors, this idea has been introduced and shown to be very successful. In this paper, a parametric survey was done to quantitatively determine the geometrical and mechanical parameters controlling the plastic constraint at crack-tip region. The distance from the crack tip to the hard zone, the hard zone width and the specimen thickness were the main geometrical parameters considered in this study. The mismatching difference between the base metal and the hard zone was the mechanical parameter investigated. The considered parameters were analyzed using three-dimensional elastic-plastic finite element analysis for series of single notch bend specimens, SNB. Local approach, using the Weibull stress distribution, was applied to determine the required condition to achieve the plane-strain plastic constraint needed to get valid -K lc results. A valid zone of Weibull stress-CTOD relation was achieved and accordingly, the controlling parameters of the plastic constraint were quantified. In the light of the results a small-size fracture toughness specimen for the material used was proposed. The requirements for this type of specimens to achieve valid-K lc results can be summarized as follow: 1. Hard zone strength will be double of that for the base metal as a minimum ratio, 2. Distance from hard zone to crack plane should be not greater than 8%of the thickness used and 3. Hard zone width can be as large as B/2 or more.