A. Bannister

Prediction of SENB Fracture Toughness From Charpy Data Using the Beremin Model in the Lower Transition Region

This work focuses on the application of a mechanistic local approach model to describe the statistical distribution of experimental Charpy (CVN) impact test data obtained at several temperatures in the ductile to brittle transition temperature range. The current objective is to develop a correlation in the lower transition regime between quasi-static CVN absorbed energy (CVE) and the J-integral fracture toughness (Jc) obtained from deeply pre-cracked Charpy (PCCVN) specimens tested quasi-statically to laboratory test standards. The Beremin model for cleavage fracture has been applied to a ferritic steel which has been comprehensively tested using standard CVN, shallow U-notched and PCCVN specimen types in the lower ductile to brittle transition. This has enabled a prediction to be made of the absorbed CVE at cleavage fracture initiation for a Charpy specimen tested quasi-statically in the lower part of the CVN transition curve. By applying the Beremin model to PCCVN single edge notch bend specimens at quasi-static rates it was possible to use the Weibull stress, to achieve a reliable correlation between CVE and Jc in the lower ductile to brittle transition region. The results from this work indicate that the Beremin model can provide a theoretically based correlation for CVE to Jc fracture toughness for a ferritic steel under quasi-static loading conditions. The overall objective of the project remains to predict dynamic CVN absorbed energy using micromechanical modelling and which is valid for all ferritic steels.Copyright

HAZ Toughness: Realistic Testing for Pipeline Integrity

Fracture toughness testing of the heat-affected zone (HAZ) of linepipe seam welds is a requirement for most pipeline projects. Occasionally, low individual values can be measured in the HAZ and these have been attributed to, among other factors, the statistical nature of the HAZ and the associated probability of encountering local brittle zones. The structural significance of these outliers has remained a subject of debate between linepipe users and manufacturers [1], especially as their low significance can be demonstrated via large-scale structurally-representative tests [2–3]. To circumvent the higher cost of such large-scale testing, constraint-corrected fracture toughness testing can be used such that the conditions in the small-scale test more closely reflect those in service. However, there is little consistency between the many test and application codes in terms of how such tests should be carried out, and what steps are required to demonstrate that the measured toughness is structurally representative. Furthermore, the level of benefit to be obtained cannot be easily predicted. In the current study, a range of fracture mechanics tests was conducted on the HAZ of the longitudinal seam weld of a grade X65 U-O-E SAW pipe. Varying degrees of constraint, scale and loading mode were evaluated to establish the characteristic toughness of the HAZ in a statistical manner, with over fifty specimens tested in total. The specimens tested included notched bend (SENB) and tension (SENT) designs as well as surface notched tension (SNT), all with varying crack depth. The range of specimen and loading types, when compared with the requirements of the various relevant standards, highlighted the contradictory nature of current standards. The toughness established for each set of specific test conditions was used in a theoretical Engineering Critical Assessment (ECA) assuming various levels of applied stress, residual stress and flaw size inputs. The wide range of conclusions that would have been reached based on the small-scale toughness tests carried out under varying levels of constraint was easily demonstrated via the ECA. The conflicting requirements of several testing and application standards for longitudinal welds should be addressed, and their consistency with current approaches for girth welds improved. The study also shows that a single-parameter fracture criterion is an insufficient indicator of real HAZ toughness and constraint (metallurgical and geometrical) level must also be considered. The use of standard deeply-notched CTOD specimens, representing high constraint, gives a highly pessimistic view of seam weld integrity, especially when subsequently combined with an ECA.Copyright

Constraint Based Fracture Assessment of Seam-Welded Pipes Containing Axial Flaws

An Engineering Critical Assessment (ECA) of a pipeline containing an axial defect is usually conservative if standard fracture test pieces are used for the fracture toughness testing. Conventional fracture toughness testing standards employ specimens containing deep cracks in order to guarantee conditions leading to high stress triaxiality and crack-tip constraint. In the current work, single edge notch bend (SENB) and single edge notch tension (SENT) test specimens of two different a/W (crack depth/specimen width) ratios (0.15 and 0.6) were used to obtain HAZ fracture toughness of a seam weld. The influence of specimen geometry and a/W ratio on fracture toughness was investigated. The Master Curve methodology was employed to characterise HAZ fracture toughness of the seam weld in the ductile-to-brittle transition region. The reference temperature T0 was estimated using the test results obtained on specimens of different geometries and constraint levels. A series of ECAs of the pipe containing a surface axial flaw was performed and the benefits of a constraint based fracture mechanics analysis were demonstrated.Copyright

A Method to Derive the JC Value of a 1T SE(B) Using Charpy Impact Energy in the Lower Ductile to Brittle Transition

This paper describes a method by which the elastic-plastic crack driving force, J, of a 1T SE(B) may be calculated using Charpy V-notch absorbed impact energy, Uel+pl,LLD. The method is applicable in the lower ductile-to-brittle transition regime of fracture behaviour and permits the calculation of equivalent critical J-integral values, Jc, using Uel+pl,LLD data. The method is demonstrated using a ferritic steel study material. Comparisons are made between the predictions of a Uel+pl,LLD scaling approach, which was derived using a Weibull stress model, and experimental test data for a ferritic steel. The approach was evaluated using experimental test data composed of instrumented Charpy impact test results and SE(B) fracture toughness test results.The probabilistic predictions were found to be in good agreement with experimental values. Extension of the methodology is recommended to include other material flow properties. Further work is required to ascertain the accuracy of the approach at higher temperatures in the ductile-to-brittle transition temperature range. A practical method for applying the methodology in practice to allow description of the behaviour of a wide range of ferritic steel materials has been outlined.Copyright

An Engineering Procedure for Calculating Cleavage Fracture Toughness From Charpy Specimen Data Using a Mechanistic Approach for Ferritic Steels

This research develops an engineering approach which permits the treatment of Charpy specimen absorbed energy data in the lower transition of Charpy specimen fracture behaviour. The procedure has been shown to be applicable to a ferritic steel study material. The calculation method comprises several steps to correct the input Charpy data to the equivalent material fracture toughness of a ferritic steel under consideration. The engineering procedure develops existing methods for constraint and notch correction to data [Sherry et al, EFM 2005] [Horn and Sherry, IJPVP 2012].Micromechanical modeling of cleavage fracture behaviour has been applied in conjunction with sequential experimental testing. This work addresses the important geometric differences between a single edge notch bend, SEN(B), fracture toughness specimen and the standard Charpy V-notch specimen. The engineering approach is demonstrated using a suitable study ferritic steel material and by undertaking an experimental laboratory testing programme comprising standard fracture toughness specimens and non-standard U-notch and V-notch Charpy sized specimens with a range of notch geometries.It has been found that constraint and notch assessment methodologies premised upon micro-mechanical modeling of cleavage fracture offer an accurate probabilistic description of fracture behaviour in these specimen geometries. Refinement of a notch angle correction is necessary within the procedure. These findings permit the extension of the approach to develop a material specific guidance to practitioners undertaking structural integrity assessments. The final extension of the research to Charpy impact data requires the measurement of ferritic steel material flow behaviour under dynamic conditions and represents further research.© 2014 ASME