# Effects of High Toughness and Welding Residual Stress for Unstable Fracture Prevention
## 1 Introduction  
Preventing unstable fracture in weld joints is considered as a crucial concept in structural design, especially for large-scale constructions, e.g., ships and building structures. Both Liquefied natural gas (LNG) carriers and large container ships were characterized by a rapid increase in size in the 2000 s and typically require advanced technology for construction. In particular, a high-strength thick steel plate has been applied to the upper structure of container ships due to their structural characteristics. As the structure is enlarged in this way, the range of applied steel types has increased, along with the demand for thick steel plates. For the construction of such a large container ship, the strength and thickness of the steel are expected to increase naturally; thus, the thickest and strongest steel among those applied to the entire ship is used for hatch side coaming (H/C) of a 24,000 TEU container ship. Based on the container ship class, steel thicker than 95 mm, with a yield strength of over 460 MPa, is applied in these cases [1]. Since 2006, the International Association of Classification Societies (IACS) [2], together with other counterparts [3, 4] in each country, has established various rules to improve the safety against instability in the application of high-strength, extremely thick materials to vessels. The large structure is constructed using strong and thick steel plates, which exhibit poor fracture toughness compared to thin ones [5]. Generally, the welding residual stress distribution exhibits differences between thick and thin steel weld joints. Previous research [6, 7, 8] confirmed that the brittle crack propagation path depends on the welding residual stress distribution in the crack front. Although residual stresses are self-balanced inside the component, tensile residual stresses can be detrimental to the structural integrity, as they accelerate susceptibility to brittle fracture, fatigue, creep, and stress corrosion cracking [9, 10]. Specifically, welding residual stresses are inherently generated during heating and cooling due to thermal-mechanical misfit among different parts within the joining bodies [11, 12]. With regard to crack propagation, compressive welding residual stress is useful, as it can suppress thegrowth of brittle cracks. However, tensile residual stress can accelerate unstable fractures. Therefore, to prevent unstable fractures, welding residual stress distribution needs to be controlled. To date, some studies have investigated the possibility of using low transformation temperature (LTT) welding to decrease the tensile stress in weld joints, strengthening their mechanical properties and reducing their stress regarding distortion, cracking, and fatigue to improve relevant performance [13; 14; 15; 16; 17; 18; 19; 20; 21].  
In this study, we present: (1) an examination of the effect of welding residual stress along the thickness of thick steel plate weld joints on brittle fracture; (2) the prevention of brittle fracture of thick steel plates through the application of rainbow welding techniques with two (or more) different types of large toughness welding materials; (3) the application of LTT welding materials to the weld to create residual compressive stress to prevent unstable fractures.