Strain-based Failure Assessment Research Articles

Strain-based fracture analysis for internal surface cracks of X80 pipe girth welds

The girth weld is the weakest region of the entire pipeline. When the pipeline crosses areas with complex terrain or where natural disasters are frequent, the pipeline may be subjected to large displacement effects and large plastic deformations. In these scenarios, the traditional stress-based assessment methods will no longer be applicable. Therefore, it is of great importance to develop strain-based assessment methods for pipelines. This paper investigates the effects of crack size, strength mismatch, heat affected zone (HAZ) width and HAZ softening/hardening coefficient on the crack driving force (CDF) of weld center crack (WCC) and HAZ center crack (HAZCC) on the inner surface of X80 pipes using finite element method. The results show that the CDF is significantly affected by crack size, crack location, strength mismatch, HAZ softening coefficient but slightly affected by HAZ width when HAZ width is greater than 2 mm. The CDF of WCC is greater than that of HAZCC under large strain conditions with HAZ width of 2 mm. The distribution of the J-integral at the crack tip is then investigated and the results show that the maximum value of the J-integral does not necessarily occur at the deepest point of the crack tip. Finally, the factors influencing the failure assessment curve (FAC) of WCC are investigated and the lower bound strain-based failure assessment diagram (SB-FAD) considering strength mismatch and HAZ softening coefficient is developed with varying crack depths. This study can provide a reference for the safety assessment of crack-like defects on the inner surface of X80 pipeline girth welds.

Strain-Based Failure Assessment Based on a Reference Strain Method for Welded Pipelines

Abstract Engineering critical assessment (ECA) is an evaluation procedure for structures with flaws and has been widely applied for assessing pipeline integrity. The standards for structural integrity assessment, including BS 7910, involve stress-based ECA, and they are known to produce overly conservative results. Therefore, strain-based ECA has been recently developed as an alternative approach. One of the effective methods for improving the accuracy of strain-based ECA is the reference strain method. However, only a limited number of studies have applied this method to welded pipelines. Therefore, a numerical analysis based on strain-based ECA was performed for girth-welded joints with a circumferentially oriented internal surface crack. Particular attention was given to the strength mismatch effects. The equivalent stress–strain curve in BS7910 was used to reflect the strength mismatch effects in the reference strain. The results of the proposed method were validated with the results of a finite element analysis (FEA) in terms of the J-integral. Previous methods and the proposed method exhibit a reasonable correlation of the J-integral in the case of over-matching (OM). In the under-matching (UM) cases, while the previous procedures tended to underestimate or excessively overestimate the elastic-plastic energy release rate in comparison with the FEA, the proposed method evaluated the J-integral of pipelines with sufficient accuracy.

Developments of strain-based failure assessment diagram applications: measuring reference strain by displacement and a modified assessment method

A modified strain-based failure assessment diagram (FAD) and a method of using the displacement of the outer boundary of the load (DOBL) to measure the reference strain in strain-based FAD (SB-FAD) application are established in this paper. Measuring reference strain by DOBL is a new method of introducing displacement to characterize reference strain and has a great potential of promoting the application of strain-based FAD. A modification parameter with a physical meaning of reflecting defect size, component geometry and material properties, is introduced to improve the SB-FAD Option 2 (one important SB-FAD assessing curve) and makes the assessment method conservative and safe. For some typical 2D and 3D crack structures in engineering, the modification parameter query tables are presented for the application convenience. The modified SB-FAD Option 2 is conservative and can be well combined with the method of using DOBL to measure reference strain in SB-FAD application. The two aspects of the developments of SB-FAD are of general practicality, and are illustrated in the paper to assess the structural integrity of 3D surface cracks in reel pipeline projects.

Elastic-plastic fracture analyses for misaligned clad pipeline containing a canoe shape surface crack subjected to large plastic deformation

Offshore pipelines are subjected to large plastic deformation during the reeling stage or in-service operation. These pipelines are joined together by a welding process, and defects and weld misalignment are frequently introduced into the pipelines, posing tremendous challenges to the integrity of these pipelines. In this study, the fracture responses of misaligned clad pipeline containing a canoe shape surface crack located in weld centre line (WCL) and fusion line (FL) were investigated using 3-D elastic-plastic finite element (FE) analyses. The influences of crack depth ratio, crack length to perimeter diameter ratio and centerline offset ratio on the crack tip opening displacement (CTOD) were analyzed in detail. The relationship between the CTOD and the global strain (εg) was built up, and a linear relationship was observed for εg ranging from 0.6% to 2%. The fracture assessment equation of British Standard (BS) 7910 predicted an over-conservative result in comparison with that obtained by FE analyses. Therefore, a new strain-based failure assessment curve was developed to assess the fracture behaviour of misaligned clad pipeline subjected to large plastic deformation.

Fracture Response of Girth-Welded Pipeline With Canoe Shape Embedded Crack Subjected to Large Plastic Deformation

Long-distance offshore pipelines always suffer large plastic deformation during installation and operation. Accompanied by high internal pressure, potential flaws are found to initiate from the girth welds, and this brings a significant challenge to the structural integrity of the pipelines. The currently used procedures for fracture assessment of pipelines are usually stress based, which are unsuitable for application to cracked pipeline subjected to large plastic deformation. Therefore, the aim of this paper is to investigate the fracture assessment of pipeline subjected to large plastic deformation and identify and understand the critical parameters influencing the fracture responses under actual loading conditions. The evolution of crack tip opening displacement (CTOD) of a pipeline segment with an embedded canoe shape crack located in the middle of the girth weld is investigated under pure bending and biaxial loading through 3D elastic–plastic finite-element simulations. The effects of crack width, crack length, pipeline thickness, material hardening, and internal pressure on fracture response are discussed. Finally, a strain-based failure assessment diagram (FAD) is developed, and comparison between fracture assessment by BS7910:2013 and finite-element simulations concludes that the former produces conservative predictions for deep crack.

Modification Strain-Based Failure Assessment Diagram for High Strength Pipeline Steel

The strain-based failure assessment diagram (SB-FAD) has been developed to predict failure due to high plastic strains. This paper validates the SB-FAD by finite element results for high strength pipeline steel (X80, X80HD, and X90) with four representative specimens (CT, CCP, DECP, and SCEP) of different crack sizes, respectively. The influence of material properties, geometries and crack sizes on failure assessment curves were compared and analyzed. Meanwhile, the modified Option-1 curve of SB-FAD is given in this paper. The results showed that the modified Option-1 curve of SB-FAD is more accurate when the value of abscissais Dr small and more conservative when the value of abscissa Dr is large.

Application of the strain-based FAD to failure assessment of surface cracked components

Purpose – The purpose of this paper is to validate the strain-based failure assessment diagram (SB-FAD) approach for surface cracks in components subjected to displacement controlled boundary conditions. Design/methodology/approach – Numerical analyses are performed for several crack geometries and materials representative for aerospace applications. The performance of the SB-FAD is judged by comparing numerically calculated J-integrals to respective analytical estimates, using both Options 1 and 2 approximations. Findings – In the most cases, both Options 1 and 2 SB-FAD method results in reasonably conservative J-estimates. Exceptions are for surface cracks in a pressurized vessel made of a material with low-strain hardening, for which Option 2 assessment produces non-conservative results. In contrast, Option 1 assessment is conservative for all geometries considered. In general, Option 1 results in a considerable overestimation of the crack driving force, whereas Option 2 produces rather accurate results in many cases. Originality/value – The results demonstrate both the potential of the SB-FAD method and needs for its further improvements.

A modified reference strain method for engineering critical assessment of reeled pipelines

Reel-lay is a highly efficient method employed to install small or medium diameter offshore pipelines that play a key role in transportation of crude oil and natural gas. However, pipelines are subjected to 1–3% plastic straining during the process of reel-lay; thus engineering critical assessment (ECA) must be carried out to obtain the crack-like defect acceptance criteria. In our work, the effects of pipe geometry, crack geometries, and material properties on the reference strain are investigated. Furthermore, an empirical formula is proposed to estimate the reference strain. By incorporating the reference strain, the application range of the strain-based failure assessment diagram method is extended to ECA of severe circumferential surface cracks in reeled pipes.

The Strain-Based Failure Assessment Diagram for Commercially Pure Titanium TA2

Strain-based failure assessment methods have been usually used in recent years to propose the fitness-for-service flaws. In this paper, the stress-based and strain-based failure assessment diagram (SB-FAD) approaches were reviewed firstly. Then, SB-FAD approach was used to the material commercially pure titanium TA2 with high strain-hardening coefficient. Several failure assessment curves obtained from strain-based method for four typical specimens, i.e., CCP (center-cracked plate), DECP (double edge cracked plate), SECP (single edge cracked plate) and CT (compact tension) were proposed. It was found that the geometries and crack sizes had obvious effects on the failure assessment curves. By comparing with the stress-based failure assessment curves, the traditional strain-based failure assessment curves could lead to more conservatism in the stage after yielding. However, the SB-FAD modified by Ainsworth can be used.

Failure Prediction of Curved Wide Plates Using the Strain-Based Failure Assessment Diagram With Correction for Constraint and Notch Radius

The strain-based failure assessment diagram (SB-FAD) has been developed for predicting failure from flaws in components subjected to high plastic strains. In this paper, a combined numerical and experimental approach is used to apply the SB-FAD to predict failure from a series of API 5L grades X80 and X100 curved wide plate (CWP) specimens with shallow notches machined into the pipe girth weld. For the CWP specimens tested in this work, the SB-FAD in its unmodified form resulted in over-conservative predictions of failure. This is attributed to the SB-FAD assuming high constraint conditions and the presence of a sharp fatigue crack, whereas the CWP specimens tested in this work were low constraint and contained a shallow machined notch without fatigue cracks. A modification of the SB-FAD is then proposed to account for nonsharp defects loaded to high plastic strains under conditions of low constraint. The resulting predictions of the modified SB-FAD show significantly reduced conservatism compared to the unmodified SB-FAD.

The treatment of secondary strains within a strain-based failure assessment diagram

In a companion paper, proposals for the shape of a strain-based failure assessment diagram (FAD) have been made. The developments in that paper and in related works on strain-based fracture assessment have largely been for remotely applied strains, such as those produced by welding. This paper re-writes existing methods for treating secondary stresses within stress-based FADs in the framework of the proposed strain-based FAD. It is shown that residual strains must be included fully in the elastic regime but at large applied strains their contribution to fracture can be relaxed considerably. Practical methods for inclusion of secondary strains within the strain-based FAD are then developed and shown to be conservative relative to some detailed finite element calculations.

The shape of a strain-based failure assessment diagram

There have been a number of recent developments of strain-based fracture assessment approaches, including proposals by Budden [Engng Frac Mech 2006;73:537–52] for a strain-based failure assessment diagram (FAD) related to the conventional stress-based FAD. However, recent comparisons with finite element (FE) data have shown that this proposed strain-based FAD can be non-conservative in some cases, particularly for deeper cracks and materials with little strain-hardening capacity. Therefore, this paper re-examines the shape of the strain-based FAD, guided by these FE analyses and some theoretical analysis. On this basis, modified proposals for the shape of the strain-based FAD are given, including simplified and more detailed options in line with the options available for stress-based FADs in existing fitness-for-service procedures. The proposals are then illustrated by a worked example and by comparison with FE data, which demonstrate that the new proposals are generally conservative.

Failure assessment diagram methods for strain-based fracture

Stress-based fracture assessment procedures contain conservatism when assessing strain- or displacement-controlled loads in excess of yield. Alternative strain-based methods are discussed in this paper and compared with an existing approach in the R6 defect assessment procedure that allows reduction in conservatism for the case of fixed displacement or rotation loading. Published finite element results for semi-elliptical cracks in plates are re-interpreted as values of J against remote strain and used to assess recent strain-based estimates of J. An assessment procedure based on a strain-based failure assessment diagram is outlined.