The objective structure of the present study is LPG storage tank and numerous bending COD tests and tensile tests with through thickness notches have been conducted on the practical low-temperature steel weldments. Statistical approach has been applied for considering the scattering features of critical COD in the practical joints and correlation of critical COD obtained from both tests.
The bending critical COD scatters widely even in the practical welded joint due to the existence of local brittle zone. The bending critical COD (δBc) and the tensile critical COD (δTc) are influenced by the plastic constraint dependent on specimen geometry. In case the fracture stress is lower than yield stress (σy), δTc≅δBc. Nevertheless, when the fracture stress is higher than σy, that is fracture occurs in full yielding and general yielding. δTc becomes considerably larger than δBc due to the decrease of the constraint.
According to the above experimental results, consideration has been carried out on the procedure of the assessment of fracture toughness by using bending COD test. Accordingly, the following two cases should become acceptable in bending COD test for the welded joint of LPG storage tank.
(a)Critical COD values of three bending COD test specimens are satisfied required value; e.g. 0.25 mm according to API 1104.
(b)When one of three bending COD values is not satisfied the required value and min. value is more than critical COD of general yielding fracture in the practical welds; e.g. δc=0.00028σy (σy in MPa, δc≅0.1 mm in HT50), a second set of three test specimens may be performed. Five of the six tests must exceed the min. required critical COD. Furthermore, in that case, average value of δBc is more than 0.5 mm.
