Flow Behaviour and Ductile Fracture Toughness of a High Toughness Steel

2004 ◽  
Author(s):  
S. Xu ◽  
R. Bouchard ◽  
W. R. Tyson
Keyword(s):  
Flow Stress ◽  
Ductile Fracture ◽  
Tensile Tests ◽  
Test Method ◽  
Opening Angle ◽  
Absorbed Energy ◽  
Crack Propagation Resistance ◽  
High Toughness ◽  
Drop Weight ◽  
Drop Weight Tear Test

This paper reports results of tests on flow and ductile fracture of a very high toughness steel with Charpy V-notch absorbed energy (CVN energy) at room temperature of 471 J. The microstructure of the steel is bainite/ferrite and its strength is equivalent to X80 grade. The flow stress was determined using tensile tests at temperatures between 150°C and −147°C and strain rates of 0.00075, 0.02 and 1 s−1, and was fitted to a proposed constitutive equation. Charpy tests were carried out at an initial impact velocity of 5.1 ms−1 using drop-weight machines (maximum capacity of 842 J and 4029 J). The samples were not broken during the test, i.e. they passed through the anvils after significant bending deformation with only limited crack growth. Most of the absorbed energy was due to deformation. There was little effect of excess energy on absorbed energy up to 80% of machine capacity (i.e. the validity limit of ASTM E 23). As an alternative to the CVN energy, the crack tip opening angle (CTOA) measured using the drop-weight tear test (DWTT) has been proposed as a material parameter to characterize crack propagation resistance. Preliminary work on evaluating CTOA using the two-specimen CTOA test method is presented. The initiation energy is eliminated by using statically precracked test specimens. Account is taken of the geometry change of the specimens (e.g. thickening under the hammer) on the rotation factor and of the effect of strain rate on flow stress.

Effect of Separation on Ductile Crack Propagation Behavior During Drop Weight Tear Test

2010 ◽  
Author(s):  
Takuya Hara ◽  
Taishi Fujishiro
Keyword(s):  
Crack Propagation ◽  
Ductile Fracture ◽  
Fracture Resistance ◽  
High Strength ◽  
Opening Angle ◽  
Ductile Crack ◽  
Line Pipe ◽  
Propagation Behavior ◽  
Drop Weight ◽  
Drop Weight Tear Test

The demand for natural gas using LNG and pipelines to supply the world gas markets is increasing. The use of high-strength line pipe provides a reduction in the cost of gas transmission pipelines by enabling high-pressure transmission of large volumes of gas. Under the large demand of high-strength line pipe, crack arrestability of running ductile fracture behavior is one of the most important properties. The CVN (Charpy V-notched) test and the DWTT (Drop Weight Tear Test) are major test methods to evaluate the crack arrestability of running ductile fractures. Separation, which is defined as a fracture parallel to the rolling plane, can be characteristic of the fracture in both full-scale burst tests and DWTTs. It is reported that separations deteriorate the crack arrestability of running ductile fracture, and also that small amounts of separation do not affect the running ductile fracture resistance. This paper describes the effect of separation on ductile propagation behavior. We utilized a high-speed camera to investigate the CTOA (Crack Tip Opening Angle) during the DWTT. We show that some separations deteriorate ductile crack propagation resistance and that some separations do not affect the running ductile fracture resistance.

CTOA Test Method Using Drop-Weight Tear Test (DWTT): Background and Development of Standard

2016 ◽  
Author(s):  
S. Xu ◽  
W. R. Tyson ◽  
C. H. M. Simha ◽  
M. Gesing ◽  
J. Liang
Keyword(s):  
Crack Propagation ◽  
Standard Test ◽  
Test Method ◽  
Charpy Specimen ◽  
Pipe Steels ◽  
Absorbed Energy ◽  
Drop Weight ◽  
Drop Weight Tear Test ◽  
Charpy Absorbed Energy ◽  
Fracture Arrest

Arrest of fast ductile fracture in the design of gas pipelines has traditionally been assured by specifying Charpy absorbed energy (Cv) of pipe steel based on the Battelle two-curve method. However, the Charpy test has been shown to be inadequate to characterize crack propagation in modern high-strength, high-toughness pipe steels. For steels with Cv more than approximately 100 J, fracture arrest methodologies based on Cv can lead to non-conservative predictions. The problem is that the Charpy specimen is too small to characterize full-scale fracture, and for tough steels the ductility can be so high that the Charpy specimen bends without fracturing completely. To overcome these limitations, the use of a larger full-thickness specimen, the “Drop-Weight Tear Test” (DWTT) specimen, has been proposed. The test is instrumented to measure the force on and displacement of the impactor during crack propagation. The data is interpreted to yield the “crack-tip opening angle” (CTOA), which is constant during steady-state crack growth and characterizes the propagation resistance. The CTOA has been proposed for some time as a suitable property to assess fracture propagation and arrest in high-pressure gas pipelines, but up to now a standard test method for measurement of the CTOA has not been available. To remedy this situation, a draft standard has been developed by the authors and is being balloted by ASTM E081. In this paper, the CTOA parameter and CTOA-based fracture arrest methodology will be introduced briefly. The background and development of the draft ASTM standard test method for determination of CTOA using the drop-weight tear test (DWTT) specimen will be reviewed including the procedure and the results of an international round robin. In the CTOA test method, the only adjustable parameter is the rotation factor (rp). Using a modified Xue-Wierzbicki damage mechanics model and a statistical analysis, rp has been determined to be a weak function of yield strength, Charpy absorbed energy and specimen thickness. Although no physical model has been developed to explain the interplay of these factors, they are all related to the extent and distribution of plastic deformation ahead of the crack. The technical background and quantification of rp will be described in this paper. It is intended to apply the CTOA test method to a broad range of steels, including thin (less than 6 mm) and thick (larger than 20 mm) pipe steels.

Drop Weight Tear Testing of High Toughness Pipeline Material

2004 ◽  
Author(s):  
Kjell Olav Halsen ◽  
Espen Heier
Keyword(s):  
Fracture Initiation ◽  
Test Method ◽  
Side Impact ◽  
Battelle Memorial Institute ◽  
Line Pipe ◽  
Pipeline Steels ◽  
High Toughness ◽  
Drop Weight ◽  
Drop Weight Tear Test ◽  
Impact Side

Drop Weight Tear Testing is a common test method for determining a material’s ability to arrest a propagating crack. This testing method was developed by Battelle Memorial Institute, and is conducted in accordance with standards as API RP 5L3 ‘Recommended Practice for Conducting Drop Weight Tear testing on Line Pipe’ and EN 10274 ‘Metallic Materials - Drop Weight Tear Test’. One problem that has been encountered when performing Drop Weight Tear Testing of high toughness TMCP materials is that the pre-deformed material in the pressed notch is not sufficiently embrittled to ensure initiation of a brittle fracture. According to prevailing standards a brittle initiation is necessary for a valid test result. The material opposite the notched side (impact side) will deform quite considerably and is due to strain hardening expected to loose toughness prior to the actual fracture initiation takes place. Consequently high toughness material may give poor test results. In that respect, DNV initiated a Joint Industry Project called ‘Drop Weight Tear Testing of High Toughness Pipeline Material’, where the main objective was to obtain a better understanding on how the results from the DWTT should be interpreted for high toughness pipeline steels. During the project an extensive amount of Drop Weight Tear tests (DWTT) were performed on relevant modern pipeline steels. The resulting shear ratios were determined according to conventional fracture surface evaluation methods as well as newly developed methods as presented in the literature. The appearance of energy curves for both regular DWTT specimens and specimens with varying back gouge depths was also considered in the investigation and the consistency between the estimated shear ratios and the corresponding measured absorbed energies were thoroughly evaluated. This paper summarizes the results and recommendations obtained in the performed investigations.

New Computational Procedures for Evaluation of Fracture Surfaces of the Drop Weight Tear Test of the Piping Steel

2011 ◽  
Author(s):  
Pavel Zˇidli´k ◽  
Petr Ferfecki ◽  
Bohumi´r Strnadel
Keyword(s):  
Fracture Surface ◽  
Ductile Fracture ◽  
Unstable Fracture ◽  
Fracture Surfaces ◽  
Drop Weight ◽  
Highly Sensitive ◽  
Drop Weight Tear Test ◽  
Computational Procedures ◽  
New Procedures

Drop weight tear test (DWTT) is one of the standard methods for evaluation of the ductility of large-dimensional structural components, such as pipelines used for gas and/or oil transportation. In general, the pipelines are even used in places with temperatures close to −40 °C, and in such environments, it is necessary to guarantee the resistance of the material used for pipeline against the initiation of unstable fracture. Currently, the percentage portion of the ductile fracture of the DWTT specimen is determined by an expert evaluator. The objective of this paper is to introduce new procedures working on the principle of deterministic, statistical and fractal description of the fracture surface. For the proposed computational procedures, the fracture surface of the test specimen is scanned at the macroscopic level using the 3D-Cam scanner. The newly investigated procedures show highly sensitive to the determination of the percentage portion of the ductile fracture on the tested DWTT specimens. The developed procedures to assess the fracture surfaces of the DWTT specimens contributes to making the results of this test more correct, objective and also increases the reliability and safety of the manufactured pipelines.

scholarly journals Evaluation and instrumentation of the drop weight tear test

2012 ◽  
Vol 3 (1) ◽  
pp. 52-58
Author(s):  
S. Vervaet ◽  
W. De Waele
Keyword(s):  
High Strength ◽  
Test Method ◽  
Opening Angle ◽  
Safety Issues ◽  
Natural Gas Pipelines ◽  
Drop Weight Tear Test ◽  
Fracture Appearance ◽  
Oil And Natural Gas ◽  
Linepipe Steels ◽  
Crack Tip Opening

With the use of high strength and high toughness steels in the pipeline industry it has become necessary tobetter understand the factors which influence the reliability and integrity of oil and natural gas pipelines. TheDrop-Weight Tear Test (DWTT) is a common test method to determine the fracture appearance andfracture ductility of steel. Its fundamental purpose is to determine the appearance of propagating fracturesin steels over the temperature range where the fracture mode changes from brittle to ductile. But there arestill many subjects of discussion concerning which results must be obtained, in which manner they shouldbe obtained and how they should be interpreted. Is it still possible to deduce a shear appearance fromsamples which have such an abnormal fracture that they used to be discarded as invalid ? Could resultsfrom the DWTT be correlated with the Crack Tip Opening Angle (CTOA), which is particularly important forfinite element modelling ? What to think about methods such as the two specimen CTOA and the simplifiedsingle specimen method ? How severe is the effect of tunnelling in contemporary linepipe steels and howcan this be dealt with ? Many questions still remain and many aspects are still vague despite the correlatingecological, economical and safety issues. Therefore, there is a major necessity for further investigations.

Investigation Into the Use of a Single Specimen for the Determination of Dynamic Steady State Propagation Resistance in High Toughness Line-Pipe Steels

2002 ◽  
Author(s):  
David L. Rudland ◽  
Gery Wilkowski ◽  
Yong-Yi Wang ◽  
David Horsley ◽  
Brian Rothwell ◽  
...  
Keyword(s):  
Steady State ◽  
Fracture Energy ◽  
Ductile Fracture ◽  
Opening Angle ◽  
Single Specimen ◽  
Pipe Steels ◽  
Line Pipe ◽  
Propagation Resistance ◽  
High Toughness ◽  
Dynamic Ductile Fracture

This paper summarizes efforts funded by TransCanada PipeLine Limited on improving the methodology for predicting a true measure of the dynamic steady-state fracture toughness of line-pipe steels using a single mill test specimen. In the past, ductile fracture methodologies generally involved using the Charpy V-notch test to empirically quantify the material dynamic ductile fracture propagation resistance. However, due to its geometry, the use of the Charpy test has proven to be unreliable for high-toughness materials, for materials that have rising-shelf energies, and for higher-grade steels (relative to those for which correlations were originally established). An improved methodology for characterizing the dynamic ductile fracture resistance is to utilize the energy from a full-thickness impact specimen, of which the Drop-Weight Tear Test (DWTT) specimen is the most frequently used type. It has been demonstrated that the total energy from a DWTT-type specimen includes; (1) the energy associated with initiation of the crack (including indentation energy and yielding of the specimen), (2) the energy for transient crack growth from initiation to reaching steady-state fracture, (3) steady-state fracture energy, and (4) a non-steady-state fracture energy region at the end of the test. During the steady-state fracture region it was observed that both the crack velocity and constant crack-tip-opening angle (CTOA) remained constant. This paper presents the results of an investigation aimed at identifying a single specimen that will capture only the steady-state fracture energy present in standard DWTT specimens. Detailed experiments and three-dimensional finite element analyses were used to verify various procedures for eliminating the initiation energy and the residual energy at the end of the tests. A non-instrumented modified specimen, the back-slotted, static-precracked DWTT (BS-SPC-DWTT) specimen, has been developed from the results of these analyses. Energy results from this specimen, for a variety of line-pipe steels, are presented. A correlation between these energies and the propagation energy from standard DWTT specimen is presented. This correlation will aid in the methodology for predicting axial crack arrest in line-pipe steels having higher toughness, a rising upper shelf, or a higher grade.

Effects of Notch Type and Loading Rate on CTOA of Modern X65 and X70 Pipe Steels

2014 ◽  
Author(s):  
S. Xu ◽  
J. Sollen ◽  
J. Liang ◽  
R. Zavadil ◽  
W. R. Tyson
Keyword(s):  
Loading Rate ◽  
Large Diameter ◽  
Test Method ◽  
Opening Angle ◽  
Pipe Steels ◽  
Drop Weight Tear Test ◽  
X65 Steel ◽  
Rotation Factor ◽  
Crack Tip Opening

The crack-tip opening angle (CTOA) has been investigated as a fracture propagation resistance parameter for prevention of fast ductile fracture in gas pipelines. A CANMET simplified single specimen CTOA method has been proposed as a mill test and is being applied to characterize critical CTOA (CTOAc) of typical pipe steels to develop a toughness database and improve the test method. In this work, tests using standard machined V-notch and API pressed notch (PN) drop-weight tear test (DWTT) specimens at quasi-static and impact rates were performed on modern X65 and X70 pipe steels typical of those used for CO2 pipelines. The rotation factor of the X65 steel deduced from the deformed ligament geometry is equivalent to those of X70 to X100 steels. Pressed-notch DWTT specimens were successfully fractured in impact and yielded CTOAc values equivalent to those of V-notched specimens following the CANMET recommended practice for determination of CTOAc. The effect of loading rate on CTOAc between the quasi-static and impact rates (covering five orders of magnitude) is small or negligible, being within experimental scatter. This is in agreement with data in the CANMET database, except for a low-toughness X52 steel that showed an increase in CTOAc between quasi-static and impact loading rates. The effect of flattening on CTOAc was also investigated and is small or negligible for the large-diameter (at least 508 mm) pipes tested in this work. The results may be used to support and improve a proposed test method for determination of CTOAc being considered by an ASTM Task Group and currently being evaluated in a round-robin testing program.

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