This work develops a new method for plane-strain fracture toughness test to get valid-KIc 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 cracktip 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 -KIc 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-KIc 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.

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SummaryThe application of Laser welds close to the notch of toughness specimen was made to create a highconstrained condition near the notch tip. Laser welds enabled a very thin specimen (thickness, B = 2.5 mm) to promote brittle fracture at a lower CTOD level than the full-thickness (B = 20 mm) homogeneous specimen. This is due to the plastic constraint induced by the strength mis-matching.Controlling factors for the plastic constraint were addressed by 3 D FE-analysis.It was pointed out that the location of Laser welds (distance from Laser welds to the notch) and the shape of Laser welds give a marked effect on the plastic constraint around the notch. By contrast, the influence of the width of Laser welds was marginal. By using strength mis-matching effect, the thickness requirement for KICmeasurement could be reduced to a large extent. ExperimentsThree-point bend specimens were extracted from a long-term used 20 mm thick pressure vessel of a chemical plant. Material of the pressure vessel was a C-1/2 Mo steel of ASTM A 204 A class. The chemical composition and mechanical properties of the material extracted from the long-term used vessel are shown in Table 1. The yield strength and tensile strength met the value specified in the standard, while the impact energy showed a decrease by about 15 J from the standard value. This loss in the impact energy resulted from the embrittlement by long-term usage.The size of the test specimen extracted from the pressure vessel in service should be as small as possible to keep the structural integrity of the pressure vessel. In this study 2.5 and 5mm thick specimens were extracted. As a reference specimen, the full-thickness (B = 20 mm) specimen was also used. A couple of Laser welds were laid for thin specimens (B = 2.5, 5 mm) as shown in Fig. 1

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