Effect of prestrain on ductility and toughness in a high-strength line pipe steel

Fracture properties of a mother plate for API grade X100 line pipe after pre-straining up to 6% are investigated using tensile notched bars and CT pre-cracked specimens. The material has an anisotropic plastic and damage behavior due to the thermo-mechanical control rolling process. Experiments evidence a decrease in both ductility and toughness for both rolling and long transverse direction with increasing prestrains. This effect is however more pronounced at low prestrain levels ( $$0\rightarrow 2\%$$ ) than at higher levels ( $$2\rightarrow 4\rightarrow 6\%$$ ). The modified GTN model proposed by Shinohara et al. (Int J Fract 197:127–145, 2016) is used to represent the database. A good agreement is obtained provided some damage model parameters are modified so as to obtain a slightly higher damage rate for the prestrained materials. This represents the fact that void growth tends to be faster for materials with a lower work hardening rate as evidenced by unit cell calculations. In addition, stress/strain distributions in test specimens are modified for reduced hardening so that stress triaxiality is increased at failure initiation points. This further lowers measured mechanical properties.