Influence and evaluation of constraint on fracture toughness in pipeline research

Accessing nowadays fossil fuel reserves requires a strain-based design approach. Within such design, the ductile tearing resistance is a key parameter in assessing the defect tolerance. To determine this tearing resistance, full scale (pressurized) tests can be performed. However, such approach would be costly and time consuming. Consequently, effort is made to select appropriate small scale test specimens. Most research has focused so far on the single edge notch bend (SENB) and tensile (SENT) specimen. To evaluate the suitability of these test specimens, the crack tip stress fields can be examined or the resistance curves compared with full scale structures. This paper aims at comparing the trends observed using these techniques. Furthermore, the suitability of the small scale test specimens is evaluated. It is concluded that sufficiently long (length-to-width ratio equal to ten) clamped SENT specimens have the potential to predict the tearing resistance of full scale pipes. In addition, the internal pressure does not significantly affect the fracture toughness. These conclusions are stated by both experimental results and finite element simulations

Design of a (mini) wide plate specimen for strain-based weld integrity assessment

Wide plate tension tests are commonly executed to investigate the integrity of defective welds under a uniaxial load. The specimen can be flat or curved, depending on the geometry from which it has been extracted (plate or pipe). Despite its usefulness, the design of the (curved) wide plate test is still not standardized up-to-date. This paper compares two specimen designs with a different length-to-width ratio through finite element analysis, using a design-of-experiments approach to account for different influential factors. The results reveal significant differences between the interpretation of tests with net section collapse and gross section collapse, promoted by weld strength overmatch. Further, both investigated designs tend to provide similar estimates of failure mode, strain capacity and crack driving force. Hence, the shorter specimen is considered an acceptable alternative to the slightly more representative longer specimen

Investigation of strain measurements in (curved) wide plate specimens using digital image correlation and finite element analysis

Some pipelines face global plastic straining due to the nature of their installation process or harsh environmental conditions during operation. The ability of the girth welds to withstand these plastic strains is often evaluated on the basis of wide plate tests. Key for the validity of these tests is a representative measurement of remote strain, mostly obtained by linear variable differential transformers and/or strain gauges. The outcome of the remote strain measurement depends on the specimen geometry and the position of these sensors. In an attempt to investigate a specific geometric design of wide plate specimens and to find appropriate remote strain sensor positions, the authors have performed a series of tension tests on medium-sized wide plate specimens, supported by digital image correlation strain measurements. In addition, finite element simulations have been performed to evaluate whether the experimental observations can be extrapolated to a wider range of conditions. The results indicate that the strain distribution is mostly influenced by the weld strength mismatch, which governs the lateral restraint. For all experiments and simulations, nevertheless, the strain field was highly uniform in an identified zone, resulting in simple guidelines regarding specimen geometry and sensor positioning

The writings of Belgian engineer Arthur Vierendeel (1852-1940): homo universalis or contemporary propagandist?

During the last decade of the nineteenth century, Belgian engineer and professor Arthur Vierendeel - mostly known for the Vierendeel, a frame without diagonal rigidifying elements - published a series of books in which he expounded his views on the use of steel in architecture and engineering. Vierendeel described the structural possibilities of constructing in iron, and also theorized how this ‘new’ material should capture its own architectural style. Structural aesthetics is derived from rivets, proportions, tie rods, columns and covering strips whereas auxiliary aesthetics can be created through adding ceramics, other metals and decorative painting. Formal issues had to surpass structural considerations, or in Vierendeel’s words: “Pour les constructions métalliques les dimensions doivent être déterminées à priori par des considérations esthétiques et qu’après seulement il y a lieu de recourir à la formule mathématique.

Strain based design considerations for spiral welded pipelines

Pipelines are constructed in hostile environments where the occurrence of imposed plastic deformations can necessitate a strain based design approach. Under such conditions not only the strength and toughness properties have to be considered; also the strain capacity of pipe and weld metal become crucial. Considering the use of spirally welded linepipe sections, the helical seam weld and anisotropic material properties pose real challenges to pipeline designers. In our work, the tensile strain capacity and defect tolerance of high strength, high toughness spiral pipes will be investigated. This paper briefly discusses the different steps in the spiral pipe manufacturing process and their influence on the mechanical properties of the pipe. The forming angle is a key parameter as it determines (a) the anisotropy in strength and toughness of the pipe steel, and (b) the orientation of possible seam weld defects. Each mechanical operation (forming, expansion) and each thermal operation (welding, coating) will affect local or global strength, toughness and ductility properties of the pipe metal. A thorough material characterization at each process step is needed for a qualitative and quantitative understanding of these effects

Considerations in selecting laboratory scale test specimens for evaluation of fracture toughness

The assessment of defects in large steel structures requires a trustworthy evaluation of the material’s toughness. This toughness is not only a material property but is also influenced by the loading conditions and geometry; the so-called constraint. The resulting representative value is referred to as the apparent toughness. The evaluation of apparent fracture toughness in a flawed structure is preferentially performed through laboratory scale testing, as full scale tests are both expensive and often challenging to perform. Several laboratory scale test specimens are available, among which a Single Edge Notch Bending specimen, Single Edge Notch Tensile specimen, Double Edge Notch Tensile specimen and Centre Cracked Tensile specimen. Each of these specimens has its own specific constraint. Therefore, the selection of an appropriate test specimen is of primary importance for limiting the conservatism and avoiding potential unconservatism with respect to full scale behaviour. This paper provides a general framework to select an appropriate test specimen based on detailed finite element simulations of both the full scale structure and the laboratory scale test specimens. These finite element calculations allow for a characterization of the crack tip stress fields in both situations. Different theoretical frameworks are available for this characterization; the Q -parameter is considered in this paper. To demonstrate the applicability of this procedure, an example case is presented for circumferentially oriented defects in pressurized pipelines under longitudinal tension. It is concluded that the presented framework allows for efficiently selecting a laboratory scale test specimen, which enables to evaluate the apparent fracture toughness for a given large scale structure. The obtained toughness can thus be incorporated in analytical flaw assessment procedures, reducing the degree of conservatism. This in turn allows an economically effective design

Combined numerical-experimental framework for strain based design and flaw assessment of girth welds

An increasing number of transmission pipelines have to be installed and operated in harsh conditions, due to the diminishing reserves of exploited fossil fuel sources. Under certain environment related circumstances, longitudinal plastic strains are imposed. When tensile, these may induce failure in girth welds in conjunction with the inevitable presence of weld defects. A large number of factors related to environment, material, geometry and operating conditions influence the tensile strain capacity and, hence, the acceptability of girth weld flaws. Therefore, a project specific development of guidelines in this strain based context is recommended. This paper provides systematic frameworks on (a) how to design pipelines under strain based conditions, and (b) how to assess girth weld flaws that were detected using non destructive testing. Attention is given to several technical and economical aspects related to the selection and qualification of pipe and weld metals, and to the evaluation of flaw acceptability. Both frameworks comprise a combined experimental-numerical approach, collecting project development reports from literature with research carried out at Soete Laboratory. Concretely, four in-house realizations are adopted: the UGent equation for strain capacity, the curved wide plate tension test, the UGent stress-strain equation and a finite element model of full scale pressurized pipe tension testing. The proposed frameworks aim to facilitate in performing thorough and economically justifiable strain based design and assessment processes

Resterende sterkte van gecorrodeerde omtreklassen uit pijpleidingen

Laboratorium Soete heeft, op vraag van Fluxys NV, de aanvaardbaarheid van corrosie in omtreklassen van pijpleidingen onderzocht. Hiertoe werden 33 middenschalige trekproeven op lassen uitgevoerd, waarbij metaalverlies via frezen werd gesimuleerd. De resultaten hebben aanleiding gegeven tot een beoordelingscriterium en voorwaarden m.b.t. lassterkte en –taaiheid om dit criterium te kunnen toepassen

Validation of a wide plate finite element model using digital image correlation

To investigate the influence of global plastic deformations on girth weld defect tolerance in pipelines, a parametric finite element model has been developed. This paper provides an experimental validation of the model. It describes the test setup and instrumentation used for the evaluation of plastic strain fields around a notch in a tension loaded non-welded X65 mini wide plate. LVDT measurements and digital image correlation (DIC) results are compared to each other and to the results of finite element simulations. Whereas some deviation is observed owing to unavoidable experimental uncertainties and limitations of finite element modelling, the overall correspondence is more than satisfying