Elastic-plastic defect interaction in (a)symmetrical double edge notched tension specimens

Interaction of defects tends to intensify their crack driving force response compared to the situation where these defects act independently. The interaction between multiple defects is addressed in engineering critical assessment standards like BS7910 and ASME B&PV Section XI. Nonetheless, the accuracy of these rules is open to debate since all of them are based on re-characterization procedures which in essence introduce conservativeness. The authors have developed a fully parametric finite element (FE) model able to generate multiple notches in different topologies, in order to investigate their interaction effect. An experimental validation study is conducted to verify the FE model in terms of CTOD response and surface strain distribution. To that end, symmetrically and asymmetrically double edge notched tension specimens are tensile tested and their deformation monitored by means of 3D digital image correlation. In this study the CTOD is opted as a local criterion to evaluate the interaction between notches. These results are compared with an evaluation of strain patterns on a specimen’s surface, as a global interaction evaluation. Through this comparison a deeper understanding is gained to allow us to develop a novel approach to address flaw interaction. Moreover, the validation of the FE model allows future studies of interaction between other defect types (e.g., semi-elliptical, surface breaking) in plate-like geometries

Introduction to Critical Strain and a New Method for the Assessment of Mechanical Damage in Steel Line Pipe

The pipeline industry has conducted a vast amount of research on the subject of mechanical damage. Mechanical damage makes up a large portion of the total amount of pipeline failures that occur each year. The current methods rely on engineering judgment and experience rather than scientific theory. The method for the assessment of mechanical damage introduced in this study uses a material property called critical strain to predict the onset of cracking within the pipe wall. The critical strain is compared to the strain within a dent using a ductile failure damage indicator (DFDI). To investigate the use of the DFDI to indicate the onset of cracking within a dent, the study attempted to accomplish three tasks. The first was to investigate the use of various techniques to locate the critical strain from the stress-strain curve. Five samples taken from the pipe material was used to generate both engineering and true stress-strain curves. A sensitivity analysis was conducted to show the effects of different variables on the critical strain value. The DFDI compares the critical strain value to the calculated strain at the deepest depth location within a dent. The strain calculations use the curvature of the dent and thus require a dent profile. A high resolution laser scanner was used to extract dent profiles from a pipe. The second task of the study was to investigate the reliability of the laser scanner equipment used for this study. The results from the investigation showed that the laser scanner could be used to scan the inside of the pipe despite its design for external scanning. The results also showed that the scans should be 1 mm in length along the axis of the pipe at a resolution of 0.5 mm and 360 degrees around the pipe. The final task was to conduct the denting test. The test used a spherical indenter to dent the pipe at increments of 3% of the outside diameter. The results from the test showed that a visible crack did not form on the inside pipe surface as expected from the DFDI method. This does not mean a crack did not form. During the denting test distinct popping sounds were observed possibly indicating cracks forming within the pipe wall

Finite Element-based Methods for Dent Assessment on Pipelines

As a well-developed form for energy transportation over wide ranges and long distances, pipelines always encounter various threats in service. In recent years, with the strong demand of clean energy, hydrogen transport in existing pipelines induces new challenges to the pipelines. Dent is a common mechanical defect present on pipelines, compromising structural integrity and causing pipeline failures. To date, there have been limited methods available to assess dent, and a dent combined with other types of defects such as corrosion. In this work, novel methods and criteria were developed for assessment of pipeline dent, corrosion in dent and hydrogen distribution at the dent using finite element (FE) modeling. Denting and spring-back processes were modeled and plain dents were created on the pipeline. A new criterion based on ductile damage failure indicator analysis was proposed. Pressure-bearing capacity was assessed on corroded pipelines containing a dent, where the mutual interaction between corrosion and the dent were determined. In addition, a method was developed to assess the corrosion in dent by considering both mechanical and electrochemical forces. For dented pipelines repurposed for transporting hydrogen gas, a FE-based model was developed to determine the stress/strain and H atom concentrations at the dent, where denting, spring-back and cyclic loading processes were modeled. Furthermore, the hydrogen-induced crack initiation on the pipeline subject to denting process was investigated using the phase field method

Structural Behaviour of Dented Pipelines

A dent is a defect in the pipe wall in the form of localized inward plastic deformation. Dents are a matter of serious concern for pipeline operators because they may cause a rupture or a leak in the pipeline. Hence, a reliable strain-based criterion for the assessment of dents is very important. An understanding of the local strain distributions in the dent is very important for the development of a strain-based dent evaluation criterion. Therefore, this study was undertaken using full-scale tests and a parametric study to assess the influence of various parameters on the strain distributions in a dent. Additionally, the ASME strain-based dent evaluation criterion was reviewed. It was shown that strain distributions and strain values in a dent are significantly influenced by the dent depth, internal pressure, and dent shape. The study also noted that upgrading is required for the ASME criterion

Behaviour of Wrinkled Energy Pipes Subjected to Axial Cyclic Induced Fatigue Failure

The energy industry has since been faced with the issue of development of wrinkle defect on buried pipelines located in vicious environment. Although, there have been different literatures on how to assess the integrity of pipelines with structural defects such as; corrosion, dents, buckles and welds, limited research data or guideline(s) are available on how to assess the severity of small wrinkle defects. Failure to assess the severity of these wrinkle defects, especially in buried pipelines located in the regions prone to geotechnical movement and extreme seasonal temperature variation, may lead to shutdown of the pipeline operations, and as result lead to loss of revenue. Therefore, this current study is focused on investigating the failure and behaviour of wrinkled pipes with varying wrinkle geometry, when they are subjected to axial cyclic loads representative of extreme seasonal temperature variation and cyclic freeze-thaw. This research program was conducted using both experimental method and finite element analysis (FEA) based approach. This study shows that a pipe with a wrinkle defect subjected to displacement-controlled axial cyclic loading, may fail by fatigue and result to fracture at the crest of the wrinkle due to localized stress and strain concentration. Additionally, the fatigue life of a wrinkled pipe is dependent on the magnitude of internal pressure applied and the type of cyclic loading applied. A methodology based on the strain life approach used in this study, was considered sufficient in analysing the remaining life of wrinkled pipes subjected to displacement-controlled axial cyclic loading

Dent behaviour of steel pipes under pressure load

Formation of dent defects in steel pipelines is not uncommon. A dent is a plastic deformation causing strains in the pipe wall which can be a threat to the structural integrity of the pipeline. This study investigated the effect of dent shapes, dent depths, and internal pressures on the strain distribution of the pipe. The work was completed using full-scale tests and numerical method. The study found that as the D/t ratio and the pressure increases so does the maximum strain around the dent. The study found that the location of the maximum strain value does not change with D/t ratio or internal pressure for rectangular dents. The maximum strain occurs at 125 mm away from the dent centre and at the dent centre for the longitudinal and circumferential axes, respectively. For spherical dent the location of the maximum strain in the longitudinal and circumferential axes differs for different pressures