applied fracture mechanics

300Applied Fracture Mechanics600500400J = 4 R fs (∆ a)R fs = 353 MPaJ = 278.21 (∆ a) 0.525J (N/mm)300200J m = 258,1 N/mm100J in = 46,2 N/mmJ 0,2 = 149,4 N/mm00 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2 2,4Figure 12. R-curve for curved CT specimensdelta a (mm)By accounting the scatter of the results in the form of the J - ∆a points, caused both by anatural process of subcritical crack growth and by inaccuracies in determining the J-integraland, in particular, the crack extension during monotonic loading of a specimen, it can bestated with a high level of reliability that the fracture toughness of a pipe materialdetermined on straightened CT specimens is practically the same as the fracture toughnessdetermined on curved CT specimens.4.2. The effect of stress corrosion(Gajdoš et al., 2011) investigated the stress corrosion fracture toughness of gas pipelinematerial, and compared it with fatigue fracture toughness. The material used for theinvestigation was a low-C steel according to CSN 411353 (equivalent to ASTM A519),containing 0.17% C, 0.035% P, 0.035% S. The test CT specimens were manufactured from areal pipe section cut out from a DN 150 gas pipeline 4.5 mm in wall thickness while it wasbeing repaired after 20 years of operation. Before the CT specimens were manufactured, thepipe section was press straightened. Owing to the small thickness of the specimens (a lowconstraint), the fracture toughness values cannot be qualified to represent the real fracturetoughness values. However, they can be used as a comparative measure of fracturetoughness, thus enabling quantification of the effect of stress corrosion cracks on theapparent fracture toughness.The CT specimens were first cyclically loaded by a routine procedure used in determiningfracture toughness; the only difference was that the cycling was stopped when the growth ofthe fatigue crack reached approximately the magnitude ∆aFA ≈ 1.5 mm. After that, the CT