where
Wo: Angular distortion as span 2l (1000mm)
6] Temperature: 0℃, which is design temperature for pressurized LPG tank
2) Method of calculation for required COD
According to WES 2805, required COD is calculated by following formula.
δ = 3.5e・a
where
a: characteristic value of defect
e: strain
e = e1 + e2 + e3
e1: strain caused by boundary stress
αb: Coefficient defined by position of defect: 0.75 at tensile stress side
e2: strain caused by residual stress after welded
e2 = αreY
ey: strain caused by yield stress
αr: Coefficient defined by kind of welding and defect and relative position.
Defect a: Butt welding and surface defect cross to joint at right angle: 0.6
Defect b: Butt welding and surface defect runs parallel to welding: 0.2
e3: strain caused by unstable shape of joint
e3 =(k1 - 1) e1
kt = 1 + 3(w + h)/t
kt: Coefficient of strain
In case of defect on dented side of angular distorted butt weld joint, kt, is shown as follow.
kt = 1 + 3(w + h)/t
where
w: Amount of angular distortion (mm)
h: Aligmnent of butt weld (mm)
t: Thickness (mm)
Result of calculation for required COD for each plate thickness by above method is shown in Table. 7.
Relation between critical COD and test temperature at plate thickness 50mm by test result is shown in Fig. 10. Relation between critical COD and plate thickness at test temperature -10℃ is shown in Fig. 11. It was confirmed that critical COD is decreased as test temperature decreased and plate thickness increased. And it was not so different between the result with PWHT carried out and not.
All of critical COD calculated by test result were larger enough than required COD. Accordingly, It was confirmed as safety enough from a viewpoint of occurrence of brittle fracture under the foregoing assessment.
Table 7. Result of calculation for required COD δ
