PSI - Issue 5

Yoichi Kayamori et al. / Procedia Structural Integrity 5 (2017) 286–293 Yoichi Kayamori et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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Table 1. Mechanical properties and stress-strain curve parameters used for FEA.

Swift parameters

Y / T

Marks

 ys (MPa)

 uts (MPa)

N



YR60 YR90

312 468

520 520

0.6 0.9

9.09×10 -3 1.82×10 -2

2.27×10 -1 1.10×10 -1

Fig.3 Contour maps of the equivalent plastic strain on the mid-thickness plane.

3.2. Analytical results Four contour maps of the equivalent plastic strain on the mid-thickness plane are exemplified in Fig.3, where a contour band is shown in logarithmic classification. An almost zero p  area was recognized in the ligament, and rotational deformation was implied in each C(T) specimen model. The distribution of the plastic strain increment in the crack opening direction,  p,yy , along the ligament on the mid thickness plane is shown in Fig.4, where x is the distance from the crack tip. There was a region of  p,yy = 0 in each relation between  p,yy and x/ ( W-a 0 ), and the plastic rotational center was unclear. In addition, the size of the zero  p,yy region was reduced with increasing  FEM . For reference, the distribution of the total strain increment in the crack opening direction,  yy , along the ligament on the mid-thickness plane is also shown in the figure, where the total strain means the sum of the elastic and plastic strain components. It is straightforward to find the point of  yy = 0 in the zero  p,yy region. In theory, r p should be derived from plastic behavior only. However, in this study, r p was calculated from the following equation, where x 0 is the distance from the crack tip to the point of  yy = 0 in the zero  p,yy region:

0 W a x 

r



(7)

p

0

Changes in r p with increasing loading up to  FEM = 0.2 mm are shown in Fig.5, where the range of  FEM is in the vicinity of a required CTOD fracture toughness for offshore structures in DNVGL-OS-C401 (2015). Two curves of low and high Y/T were closely located, and their changes in r p were small during loading. The average value of r p resulted in 0.519 for Y/T =0.6 and 0.524 for Y/T =0.9.