Determination of critical angle of circumferential throughwall crack for structurally distorted 90° pipe bends under bending moment with and without internal pressure

Abstract

Throughwall circumferential cracks (TWC) in elbows can considerably minimize its collapse load when subjected to in-plane bending moment. The existing closed-form collapse moment equations do not adequately quantify critical crack angles for structurally distorted cracked pipe bends subjected to external loading. Therefore, the present study has been conducted to examine utilizing elastic-plastic finite element analysis, the influence of structural distortions on the variation of critical TWC of 90° pipe bends under in-plane closing bending moment without and with internal pressure. With a mean radius ( r) of 50 mm, cracked pipe bends were modeled for three different wall thickness, t (for pipe ratios of r/ t = 5,10,20), each with two different bend radius, R (for bend ratios of R/r = 2,3) and with varying degrees of ovality and thinning (0 to 20% with increments of 5%). Finite element analyses were performed for two loading cases namely pure in-plane closing moment and in-plane closing bending with internal pressure. Normalized internal pressures of 0.2, 0.4, and 0.6 were applied. Results indicate the modification in the critical crack angle due to the pronounced effect of ovality compared to thinning on the plastic loads of pipe bends. From the finite element results, improved closed-form equations are proposed to evaluate plastic collapse moment of throughwall circumferential cracked pipe bends under the two loading conditions.

Determination of bend angle and shape imperfection effect on collapse load of pipe bends under in-plane opening moment

“…28 The magnitude of shape distortion can be calculated through the contour of the pipe bend's cross-section captured by the first off-trail test using equations (1) to (3). 27,29–35 To incorporate shape imperfection at the mid-section of pipe bends for building the FE geometry model, has been speculated as shown below. The mid-portion (A–A 1 ) of the pipe bend is presumed to be a perfect elliptical shape if ovality is present.…”

“…To get the broad bend characteristics range ( λ ) for a specific diameter, different thicknesses and bend radii were chosen: where bend radius, nominal thickness and mean radius of the pipe bend are, respectively, specified by R , t , and r as shown in Supplemental Figure 1. The elastic-perfectly plastic (EPP) material 16,22,30,38 was assumed as the material model, and C3D20R (reduced integration 20 node brick element) was used to create the FE model. The precision of this element type remained unaffected even when the element shape and mesh were changed during deformation which is one of its main advantages (ABAQUS, 2014).…”

Determination of collapse moment of different angled pipe bends with initial geometric imperfection subjected to in-plane bending moments

“…The Static Riks method is implemented to capture decreasing applied load. 20,21 To include the large geometry change in the analysis, Nlgeom is toggled on while creating the Static Riks step. 22…”

Effect of shape imperfections on elastic stresses of U-bends subjected to in-plane moment

“…Christo et al (2011) compared the effect of hoop stress, induced in pipes with elliptical and semi-oval cross sections when subjected to pressure load. The reports on the crack analysis of pipes without ovality (Sevcik et al , 2012) and with ovality Sumesh et al , (2017) are also available. The consequences of ovality and the thinning on the plastic limit and collapse loads are also reported (Athanasopoulos et al , 2019; Buckshumiyan et al , 2013, 2014; Christo Michael et al , 2012a, 2012b, 2013) , whereas the reports on elastic analysis are limited.…”