# Fracture Assessment of Weld Joints of High-Strength Steel in Pre-Strained Condition
## 1 Introduction  
In general, 600-780 MPa-class high-strength structural steels are developed by a thermomechanical controlled process (TMCP), in steel mills [1]. Recently, the demand for large steel structures has been increasing, and in order to build a large steel structure, the strength must be increased. The application of high-strength steels, such as the 600 MPa-class steel, can be found in recent structures. These steels are used to increase the design stress and reduce transportation cost, owing to their light weight. Steel structure designs generally employ high-strength steel as the structural element. For the safe application of these steels in steel-framed structures, safety during earthquakes (dynamic loading and pre-strain occurrence) and toughness of the weld joint, especially in the heat-affected zone (HAZ), should be ensured. In general, the toughness decreases in the HAZ as the steel undergoes cycles of heat and strain, during multi-pass welding. In multilayer welding, the texture and toughness of the HAZ due to multithermal cycles show very complex distributions. As a result of measuring the toughness change by the crack tip opening displacement (CTOD) test for the multilayer welded HAZ, the decrease in toughness in the HAZ was remarkable [2; 3; 4]. The HAZ is characterized by unequal growth of theaustenite formed in the dual phase, where C and Mn are not evenly distributed and concentrated on both sides, which thus, remains unstable in the low temperature region, without being decomposed into ferrite at high temperatures. The austenite instead transforms into martensite. This is defined as the M-A (Martensite-Austenite) constituent, which is a very poor phase. From a metallographic viewpoint, the reason why the HAZ is the most vulnerable phase is due to the M-A.  
The promotion of a brittle fracture is a very dangerous problem that occurs, owing to dynamic and pre-strain effects. The fracture driving force increased, owing to the dynamic loading effect, and the fracture toughness decreased, owing to the pre-strain effect.  
Unstable fractures of high-toughness steel structures in weld joints tend to occur after ductile crack initiation or propagation. During the great earthquake, as reported in [2; 5; 6], damage to steel structures occurred in weld joints after large strain deformation. The pre-strain in the cyclic loading, during the earthquake, occurred at the strain concentration area, like the weld toe in the weld structure. The great earthquake occurred in 1995. In most collapsed steel structures, a maximum of 15% pre-strain was recorded at the steel structure connections [3; 4]. An almost-unstable fracture was observed in the beam-to-column connection areas where geometrical discontinuity exists. In the collapsed structures, unstable failures occurred after large-scale plastic deformation. This earthquake was characterized by the brittle fracture of structural steel under high-speed ground motion (104 kines (cm/s)) and large ground displacement (27 cm).  
As a complement to the classical, unstable crack-initiation/propagation failure toughness, brittle crack-arrest failure toughness has become an important mechanical property to address material fracture and failure mechanisms, to avoid unstable fractures in structures [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17].  
The objectives of this study are as follows: (i) investigation of the change in the strength and toughness of the TMCP steel welds, with pre-strain and dynamic loading; (ii) development of a fracture assessment procedure for TMCP high-strength steel welds, under seismic conditions; and (iii) verification of the fracture assessment procedure by a large-scale component test. These can establish the procedure for the fracture performance assessment of TMCP high-strength steel welds, under seismic conditions. Attention was focused on the impact of the HAZ softening on structural integrity after pre-straining. The applicability of WES 2808 [18] to 780 MPa TMCP steel and its welds were examined.