applied fracture mechanics

Evaluating the Integrity of Pressure Pipelines by Fracture Mechanics 293of the depth to the surface half-length of the examined cracks (a/c) was close to zero(a/c=0.053÷0.14), we can assume that the differences between the real values of the Qparameter and the values listed in Table 1 will be small.Figure 3. Schematic J-a dependence, (i) for a CT specimen, and (ii) for a pipe with an axial part-throughcrackThe figures shown in Table 1 provide an idea of the nature of the changes both in the plasticconstraint factor, C, and in the Q parameter brought about by changes in the relative cracklength, a/t. The diagrams shown in Figs. 4 and 5 can be obtained on the basis of the graphicrepresentation of the pairs C – a/t and Q – a/t.These diagrams clearly show the trends of the changes in the two parameters with a changein the relative crack depth, a/t. It follows that, in the range of relative depths examined here(a/t = 0.57 to 0.72), the plastic constraint factor, C, and the Q parameter are a growingfunction of the relative crack depth, a/t, the Q – a/t dependence being rather weak. Expressedsimply (i.e. linearly), the following relations are involved:C 2.56 at 0.53(17)Q 0.32at 0.83(18)The high scatter of the C and Q values is (i) due to differences in the cross sectiondimensions of the DN800 and DN1000 pipes and (ii) due to different values of the strainexponent n in the Ramberg-Osgood relation, because the pipes were made of three differentmaterials.