Abstract
The effect of strength mismatch (ratio between the yield stress of weld metal and base metal, My) on the ductile crack growth resistance of welding pipe was numerically analyzed. The ductile fracture behavior of welding pipe was determined while using the single edge notched bending (SENB) and single edge notched tension (SENT) specimens, as well as axisymmetric models of circumferentially cracked pipes for comparison. Crack growth resistance curves (as denoted by crack tip opening displacement-resistance (CTOD-R curve) have been computed using the complete Gurson model. A so-called CTOD-Q-M formulation was proposed to calculate the weld mismatch constraint M. It has been shown that the fracture resistance curves significantly increase with the increase of the mismatch ratio. As for SENT and pipe, the larger My causes the lower mismatch constraint M, which leads to the higher fracture toughness and crack growth resistance curves. When compared with the standard SENB, the SENT specimen and the cracked pipe have a more similar fracture resistance behavior. The results present grounds for justification of usage of SENT specimens in fracture assessment of welding cracked pipes as an alternative to the traditional conservative SENB specimens.
Highlights
The original idea of application of conventional fracture mechanics was that a single parameter, such as crack tip opening displacement (CTOD) and J-integral, could well characterize the crack-tip stress filed
There is often a mismatch between the base metal and the weld metal for the nature of materials. For such a mismatched weldment, the crack tip stress field is influenced by the geometric constraint and the strength mismatch that is induced by the inhomogeneous material properties [14,15]
Weld strength mismatch ratio shows a strong effect on the fracture toughness and ductile crack growth resistance curves for welding pipes
Summary
The original idea of application of conventional fracture mechanics was that a single parameter, such as crack tip opening displacement (CTOD) and J-integral, could well characterize the crack-tip stress filed. The unique fracture resistance curve, J-R or CTOD-R, was enough to reflect the material behavior The shortages of such idealized one-parameter theory, become increasingly clear by studying the diversified crack-tip fields [1,2,3]. Weld strength mismatch plays a crucial role in structural integrity assessment For this known reason, researches focused on characterizing the local stress fields of weldment and heat affected zone (HAZ) [11,12,13] were carried out in recent decades. Qiang Bin et al studied ductile crack growth behaviors at different locations of a weld joint for an X80 pipeline steel by experimental and numerical methods, it demonstrated that the Gurson-Tvergaard-Needleman (GTN) model can differentiate the constraint conditions and predict the J-R curves at different joint locations for SENB and SENT specimens accurately [23]. The tri-material and interface crack problem will be discussed in another manuscript
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