applied fracture mechanics

298Applied Fracture Mechanics600500400J = 4R fs .∆aR fs = 353 MPaJ = 327,05(∆a) 0,6406J (N/mm)300200J m = 261 N/mmJ 0,2 = 154 N/mm100J in = 16,5 N/mm00 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2 2,4delta a (mm)Figure 9. R curve for CT specimens manufactured from a press-straightened semi-productEight specimens were used for fracture-mechanical tests of curved CT specimens. Crackswere cycled up at a frequency of 4 Hz, using the testing rig, Fig. 8. The stress state in theinner side of a specimen was bigger than in the outer side, because of the bending momentinduced by the out-of-axis action of the vertical component of the tangential force withregard to the intersection of the middle cylindrical area of the specimen with the symmetryplane of the specimen. For this reason, the growth rate of the fatigue crack was higher in theinner side than in the outer side.This resulted in uneven length of the fatigue crack on the two sides of a specimen afterfinishing the cycling up. On one half of the specimens, a slant front of the starting notch wastherefore made in such a way that the notch was 1 mm deeper on the outer side. By thisoperation, a much more even front of the fatigue crack was obtained. This is clearlydemonstrated in Figs. 10 and 11, which show the fracture surfaces of specimens with astraight front and a slant front of the starting notch. In the two photographs, we can observeareas corresponding to the notch, fatigue, static crack extension and final break after thespecimens were cooled down in liquid nitrogen.On the basis of the finite element analysis and the compliance measurements made byEvans (Evans et al., 1995), it was concluded that the use of standard expressions fordetermining K factor will not cause error greater than 4% for curved CT specimens. Byproceeding in the same way as in standard J – ∆a testing, an R-curve was obtained forcurved CT specimens. It is described by a power function (21), and is presented in Fig. 12.J 278.21 a 0.525(21)