applied fracture mechanics

Evaluating the Integrity of Pressure Pipelines by Fracture Mechanics 299Figure 10. Fracture surface – straight front of the notchFigure 11. Fracture surface – slant front of the notchA comparison of the two R-curves shows that the decline of the R-curve obtained with thecurved CT specimens is less than the decline of the R-curve obtained with plane, i.e.straightened, CT specimens. The higher decline of the R-curve with the straightened CTspecimens is most probably connected with work hardening of a semiproduct duringstraightening. In the mathematical description of the R-curve of the curved CT specimens,not only the exponent but also the constant is less than for the standard R-curve. This meansthat the standard R-curve is situated above the R-curve of the curved CT specimens.However, the lower position of the R- curve for the curved CT specimens does not meansignificantly lower magnitudes of the fracture toughness characteristics. For example, the Jmvalue is lower by 1.1%, the J0.2 is lower by less than 3%, and the magnitude Jin is even higherthan the respective characteristics for plane (straightened) CT specimens. In absolute units,the difference is 2.9 N/mm for Jm and 4.6 N/mm for J0.2. There is a significant difference in Jin ,namely 29.7 N/mm in favour of the curved CT specimens.