The marine steel structure and the oversized tonnage hull currently use thick plate structure. Due to its large plate thickness, numerous welded joints and complicated joint form, it is more and more important to have good fracture toughness. The toughness of the material is the combination of material strength and plasticity. Performance, related to the material itself, heat treatment and processing. The fracture toughness is based on the concept of fracture mechanics. It is assumed that there is a defect-crack in the structure, and its bearing capacity is studied. When the load is applied, the stress at the crack tip is concentrated, plastic deformation will occur, and the crack tip will be passivated. Open it. CTOD (Crack Tip Opening Displacement) is the displacement δ of the crack tip surface. δ can be used as a measure of the stress field strength at the crack tip. The larger the δ value, the better the crack resistance of the crack tip. Conversely, the toughness is worse.
According to the EEMUA 158:1994 specification, post-weld heat treatment (PWHT) or CTOD tests are required for steel plates with a thickness of 50 mm or more. Norwegian Norske Standard M101 stipulates that welded joints with a steel plate thickness exceeding 50 mm shall be subjected to CTOD test; if the yield strength exceeds 500 MPa, the welded joint shall be subjected to CTOD test when the thickness of the steel plate exceeds 30 mm.
Both the marine structure and the hull structure are welded by thick plates. PWHT is usually used to eliminate residual stress after welding and improve the fracture toughness of welded joints. However, PWHT has high cost and long construction period, so it is necessary to adopt a more cost-effective alternative method. In the DNV-OS-F101 standard, “When the fracture toughness reaches a certain required value, the post-weld heat treatment can be dispensed with”, and the fracture toughness test can be performed by the CTOD test. According to the relevant regulations of classification society and steel structure, the manufacturing industry in Europe and the United States, in the welding thickness of ≥50mm steel plate, in order to avoid post-weld heat treatment, CTOD test is widely used as a test method.
Tiantai CTOD experiment uses Instron8802 dynamic test system, the minimum experimental temperature can reach minus 30 °C, and the relevant CTOD test can be carried out on high strength steel plates and welds of 60~90mm thick EH36/EH47.
The CTOD results of some products are as follows:
TF-210+Ni1K fracture morphology
TF-210+Ni1K F-V curve