applied fracture mechanics

296Applied Fracture Mechanicsmagnitude of the J integral for a stable subcritical crack extension; J0.2 is the J magnitudecorresponding to the real crack extension ∆a = 0.2 mm, and Jm is the magnitude of the Jintegral corresponding to attaining the maximum force at the “force – force pointdisplacement” curve. We should point here to two aspects of fracture toughness that can beencountered when dealing with pressure pipelines. One of them is the effect of pipe bandstraightening, and the other is the effect of stress corrosion cracks on fracture toughness.4.1. The effect of straighteningFracture toughness tests are carried out with fracture mechanical specimens, e.g. single edgenotched bend (SENB) specimens or compact tension (CT) specimens. Both types are planespecimens. When investigating the integrity of thin-walled pressure pipelines, we face theproblem of ensuring the planeness of the semiproducts for manufacturing the fracturemechanical specimens. The only way is press straightening of pipe bands taken from thepipe that is under investigation. As a consequence of the plastic deformation that the semiproductundergoes during straightening, internal stresses are induced not only in the semiproductbut also in the final specimens. Therefore there are still some doubts about thereliability of the fracture toughness characteristics obtained with straightened specimens. Inorder to verify this matter, Gajdoš and Šperl (Gajdoš & Šperl, 2012) carried out anexperimental investigation of fracture toughness, as determined using press straightened CTspecimens and curved CT specimens, manufactured directly from a pipe band, i.e. ensuringthat their natural curvature and wall thickness were preserved.The so-called curved CT specimens (see Fig. 7) to some extent simulate the stress conditionsin the pipe wall upon loading by internal pressure. In order to apply a circumferential forceon these specimens, we used a special testing rig, similar to that developed by Evans (Evanset al., 1995). The rig is shown in Fig. 8.Figure 7. The shape and dimensions of the curved CT specimens