Study on a Brittle Fracture Initiation from a Weld Defect (First Report)

Fracture from a Weld Crack

Abstract

Brittle fracture from the weld crack of SM50 steel was investigated and compared with the fractureof base metal with machined notch. Bending COD tests and tensile tests were performed at the temperature where the structure is usually in service. The results are summarized as follows; 1) Bending COD tests show that the base metal with machined notch have a critical COD δ_c>1 mm attemperature above -50°C, and that the weld joint with crack have δ_c≤0.2 mm at temperature below -30°C. The notch acuity of weld crack is as same as fatigue pre-crack on the same weld metal.
2) For the base metal, stable crack growth precedes unstable fracture, at which the stress in the remaining uncracked section is greater than yield stress of the material. In this case, the fracture can be estimated by the relation of half length of equivalent through thickness crack, a and overall strain at fracture, e₁.
3) For the weld joint, unstable brittle fracture is occured without fibrous crack. In this case, the fracture can be predicted on the safe side by the fracture limiting lines expressed as follows.through thickness crack δ_c/2πe_ya=0.56e₁/e_y surface crack δ_c/2πe_ya=e₁/e_y where δ_c is COD obtained from bending tests and e_y is yield strain of the material.