Anthony J. Horn

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

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.

Development of Test Guidance for Single Edge Notch Bend Fracture Toughness Specimens Containing Notches Instead of Fatigue Pre-Cracks

Structural integrity assessment codes such as R6 and BS7910 provide guidance on the assessment of flaws that are assumed to be infinitely sharp. In many cases, such as fatigue cracks, this assumption is appropriate, however it can be pessimistic for flaws that do not have sharp tips such as lack of fusion, porosity or mechanical damage. Several methods have been proposed in the literature to quantify the additional margins that may be present for non-sharp defects compared to the margins that would be calculated if the defect were assumed to be a sharp crack. A common feature of these methods is the need to understand how the effective toughness, characterised using the J-integral for a notch, varies with notch acuity. No comprehensive guidance currently exists for obtaining J experimentally from specimens containing notches, hence the typical approach is to use equations intended for pre-cracked specimens to calculate J for notched specimens.This paper presents a comprehensive set of test guidance for calculating J from Single Edge Notch Bend (SENB) fracture toughness specimens containing notches instead of fatigue pre-cracks. This has been achieved using 3D Finite Element Analyses to quantify the accuracy of formulae intended for pre-cracked specimens in fracture toughness testing standards ASTM E1820, BS7448-1 and ESIS P2-92 when applied to specimens containing notches. The paper quantifies the accuracy of these equations for notched SENB specimens and identifies the conditions under which the equations can lead to inaccurate measurement of J for notched specimens.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

Framework for Key Influences on Tensile Strain Capacity of Flawed Girth Welds

A key influence factor in the strain-based assessment of pipeline girth weld flaws is weld strength mismatch. Recent research has led to a framework for tensile strain capacity as a function of weld flow stress (FS) overmatch. This framework is built around three parameters: the strain capacity of an evenmatching weldment, the sensitivity of strain capacity to weld FS overmatch, and the strain capacity at gross section collapse (GSC). A parametric finite element study of curved wide plate (CWP) tests has been performed to identify the influence of various characteristics on each of these three parameters. This paper focuses on flaw depth, tearing resistance of the weld, stress-strain behavior of the base metal, and weld geometry. Influences of these characteristics are mostly found to be limited to one or two of the three framework parameters. A preliminary structure is proposed for equations that further develop the strain capacity framework.

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

A Comparison of Notch Failure Assessment Diagram Methods for Assessing the Fracture Resistance of Structures Containing Non-Sharp Defects

This paper analyses and compares a range of Notch Failure Assessment Diagram (NFAD) methods for assessing the fracture resistance of structures and components that contain defects with non-sharp tips. As micromechanistic failure criteria for predicting fracture from notch tips have developed, several forms of NFADs have been proposed over the last 20 years with notable developments having been made in the last 10 years. This paper quantifies the differences between four different types of NFAD approach and uses test results from test specimens containing notches of varying acuities to evaluate each approach. The results highlight significant differences in fracture predictions between the different NFAD approaches due to differences in the definition of the NFAD axes, the failure loci, the assumed failure mechanism and the corresponding micromechanistic failure criteria employed by each method.Copyright

A Numerical and Experimental Study of the Factors Influencing the Differences in Cleavage Fracture Behaviour Between CVN and SENB Specimens

The cleavage fracture behaviour of Charpy V-notch (CVN) and Single Edge Notch Bend (SENB) specimens differ due to the effects of notch root radius, notch depth, notch flank angle and loading rate. This paper presents an experimental and numerical study of the cleavage fracture behaviour of CVN, SENB and a range of non-standard intermediate specimen geometries. The intermediate geometries were selected to provide a basis for quantifying the differences in cleavage fracture behaviour between the CVN and SENB specimens by isolating the individual effects of notch-root radius, notch flank angle and notch depth. Finite element analyses were used to quantify crack and notch-tip stress fields. The study also includes a comprehensive experimental programme covering the range of notch geometries modelled using finite element analysis. The overall aim of the project is to establish links between Wallin’s Master Curve for fracture toughness and the common temperature dependence of Charpy energy observed by EricksonKirk et al.Copyright

Observations Arising From Exponential Fitting Methods to a Charpy V-Notch Energy Database From Tata Steel

Charpy testing across a range of temperatures is a cost effective way to characterise the ductile-to-brittle transition region. It is often convenient to fit a curve to Charpy data through the transition region: a commonly used method is to use a continuous tan-h fit, a single mathematical expression that links lower shelf, transition region and ductile upper shelf behaviour in one continuous curve. Using this method, the temperature dependence of Charpy energy is a unique feature of each individual steel with some steels exhibiting steep transition curves and some shallow curves. In contrast to Charpy data, fracture toughness data are usually analysed by partitioning upper shelf and transition region data. The transition region data is generally accepted to fit a universal temperature dependence, the Master Curve, as proposed by Wallin [1] and standardised in ASTM E1921 [2]. Recent research on nuclear pressure vessel steels [3, 4] has indicated that when Charpy data is assessed using a similar method to that used for fracture toughness data, a common exponential temperature dependence is observed. This paper presents the current results from an on-going investigation aimed at assessing the effect of exponential curve fitting methods on a large dataset of Charpy V-notch energy data from Tata Steel. The Tata Steel data cover a wide range of parent plate steels. The results are compared to the recent studies on nuclear pressure vessel steels and a similar exponential temperature dependence is observed.Copyright