Development of the SENT Test for Strain-Based Design of Welded Pipelines

Abstract

Strain-based design (SBD) pipelines are being considered to develop hydrocarbon resources in severe environments. As part of a research program to develop a SBD methodology, work was conducted to develop a suitable fracture mechanics test that can be used as part of a strain capacity prediction technique. The single edge notched tensile (SENT) specimen geometry has been chosen due to the similarity in crack-tip constraint conditions with that of defects in pipeline girth welds. This paper describes a single-specimen compliance method suitable for measuring ductile fracture resistance in terms of crack tip opening displacement resistance (CTOD-R) curves. The development work included investigation of the following items: specimen geometry, crack geometry and orientation (including crack depth effects), direct measurement of CTOD. The results demonstrate that toughness measurements obtained using a B = W configuration (B = specimen thickness, W = specimen width) with side grooves are similar to those using a B = 2W configuration without side grooves; however, specimens with side grooves and B = W geometry facilitates even crack growth. Studies of crack depth have shown that ductile fracture resistance decreases with increasing ratio of the initial crack depth to specimen width, a0/W. Studies of notch location and orientation (outer diameter (OD) and inner diameter (ID) surface notches and through-thickness notches) have shown an effect of this variable on the CTOD-R curves. This has been partly attributed to crack progression (tearing direction) with respect to weld geometry and this effect is consistent with damage modeling predictions. However the experimentally observed difference of CTOD-R curves between ID and OD notches is believed to be primarily due to the material variability through the pipe thickness.

J - CTOD relationship for clamped SE(T) specimens based on three-dimensional finite element analyses

“…(3) is approximately equivalent to Eq. (1) with m (1) = m SSY = 2 for low loading levels (d pl /d el % 0) and m (1) = m (3) for high loadings levels (d el /d pl % 0). Over the last decade, the use of the non-standard single-edge tension (SE(T)) specimen to determine the R-curve has gained much attention in the energy pipeline industry largely as a result of the development of SBD [3,4,12].…”

“…(1) with m (1) = m SSY = 2 for low loading levels (d pl /d el % 0) and m (1) = m (3) for high loadings levels (d el /d pl % 0). Over the last decade, the use of the non-standard single-edge tension (SE(T)) specimen to determine the R-curve has gained much attention in the energy pipeline industry largely as a result of the development of SBD [3,4,12]. The cracktip stress and strain fields of the SE(T) specimen are more relevant to the full-scale pipe containing surface cracks under internal pressure and/or longitudinal tension than the conventional standard SE(B) and C(T) specimens [12][13][14].…”

Single-specimen evaluation of tearing resistance in SENT testing

“…A commonly applied method for the evaluation of the CTOD in SENT testing, is the double clip gauge method [25][26][27]. The method considered in this publication essentially relies on Rice's 90° degrees intercept method [28], starting from the original crack tip.…”

Comparison of girth weld tearing resistance obtained from Curved Wide Plate and Single Edge Notch Tensile testing

“…Subsequently, the measured potential drop across the crack is related to the crack size, through the use of either analytical equations or finite element simulations. For the evaluation of the crack tip opening displacement (CTOD), the double clip gauge method has been applied for both the medium and small scale tests [12,13]. Different parameters are known to impact the tearing resistance.…”