A strain based criterion to assess ductile failure initiation in dented pipelines under tensile loading

Abstract

This work addresses the problem of describing ductile fracture behavior and predicting ductile failure initiation in dented pipelines under tensile loading based upon a 3-D computational cell approach coupled with a stress-modified, critical strain (SMCS) criterion for void coalescence. A series of tension tests conducted on notched tensile specimens with different notch radius for a carbon steel pipe provides the stress–strain response of the tested structural steel from which the SMCS criterion is calibrated. Full scale cyclic bend tests also performed on a 165 mm O.D tubular specimen with 11 mm wall thickness enable verification of the proposed approach in assessing ductile cracking behavior in damaged pipelines. These exploratory analyses predict the tensile failure load for the pipe specimen associated with ductile crack initiation in the highly damaged area inside the denting and buckling zone which are in good agreement with experimental measurements.

Assessment of the Von Mises Stresses and Stress Triaxiality in Notches Using Modified Tensile Specimens

“…The values of α and β found by Ruggieri 20 are 1.77 and 0.5, respectively, close to the values obtained in Figure 5(c). On the other hand, Plata Uribe et al 21 found α = 4.2 and β = 1.7, which are closer to the values shown in Figure 5(b). Considering only the notched specimen, the values change.…”

“…The problem is to determine the instant at which the crack is about to start propagating. Normally, the point of maximum stress on the true stress x true strain curve is considered (instant 3) 18,20,21 . However, the stress in the elements starts to decrease before this instant.…”

“…Studies show that it is possible to obtain the critical plastic fracture strain ε p,crit relative to stress triaxiality, Equation 1 18,[20][21][22][23][24] . The parameters α and β, Equation 1, are constants of the material and can be calibrated using results of tensile tests with different notches 20 .…”

“…Alternatively, Ruggieri 20 considered the elements with porosity f = 0.1, analyzing JIS STPT370 steel, a material also applied to pipelines. Using the same material, Plata Uribe et al 21 applied a different methodology to discover α and β. The point of the crack propagation onset was obtained experimentally and used as the correspondent point in the simulation.…”

“…The point of the crack propagation onset was obtained experimentally and used as the correspondent point in the simulation. These are the values used in the SMCS (Stress-Modified Critical Strain) damage model 21,24 . , exp…”