Drop-weight Tear Test Specimens Research Articles

The Effect of Notch Depth on the Absorbed Energy of High-Toughness Steels in the DWTT

Abstract Because of the larger specimen dimensions and more significant ligament for crack growth, the drop-weight tear test (DWTT) has become more widely used than other impact tests. On the other hand, very little information is available from DWTT experiments on weld metal (WM). In the present study, in addition to the standard chevron notch (CN) DWTT specimen with a notch depth (ND) of 5.1 mm, API X65 specimens with 10- and 15-mm ND were tested on the WM and also the base metal (BM). Fracture energies versus the ND were plotted, based on which tests can be performed successfully with lower-capacity DWTT machines to predict the absorbed energy for each ND, by specimens with increased ND and reduced absorbed energy. The standard requires thickness reduction for specimens thicker than 19 mm. With the method proposed in the present study, high-grade specimens can be tested with their original thickness. In the DWTT, regardless of specimen materials (BM or WM), about 44 % of the absorbed energy is spent on crack initiation, and 56 % is spent on crack growth. In all specimens, the BM absorbed energy was more than that of the WM. Therefore, this study proposes that the API standard, which does not include DWTT on the WM, should require performing DWTT on WM specimens. Hence, the results of the current study can form the basis for further studies that can initiate the development of new standards on the WM.

Experimental measurement of critical crack tip opening angle in API X65 steel using drop weight tear test specimen

Experimental measurement of critical crack tip opening angle in API X65 steel using drop weight tear test specimen

Impact of crack propagation path and inclusion elements on fracture toughness and micro-surface characteristics of welded pipes in DWTT

Despite fundamental differences between the characteristics of base and weld metal and higher accuracies obtainable using original-size specimens in drop-weight tear test (DWTT), there still exists a research gap in fracture surface examinations of DWTT welded specimens. This study investigates the microscopic characteristics of the fracture surface of spirally welded API X65 steel with chevron notch (CN) and pressed notch (PN) DWTT specimens. Microstructures of different sub-zones were investigated, including, weld metal (WM), heat affected zone (HAZ), and base metal (BM). Scanning electron microscopy (SEM) investigations revealed the numerous inclusions in the WM that cause stress concentration. Micro-cracks are formed at the beginning of the fracture process when the energy level is high; nevertheless, in the shear fracture area, where energy is reduced, micro-cracks were not observed. Inverse fracture was located in the HAZ and BM in the PN and CN specimens, respectively. Comparison of the weight percentages of inclusion elements obtained by energy-dispersive x-ray spectroscopy (EDS) with API 5L standard target values showed that in most inclusions, Mn, Ti, and S were higher than standard values. After metallurgical and mechanical investigations, a framework for a new prospect has been introduced to conduct statistical analyses, based on which, the confidence intervals of the weight percentages of inclusion elements were carefully determined. Changes in the weight percentages of constituent inclusion elements in the WM are smaller compared to the BM and HAZ that would be reasons for better mechanical properties of the WM compared to the BM. Considering the impact of inclusions on weld-ability and toughness, industrial guidelines are presented to carefully control the elements within the confidence intervals of this research, during welding to minimize the formation of inclusions, in turn, reducing the formation and growth of micro-cracks, and significantly improving mechanical properties.

Automatic Brittle Fracture Ratio Estimation Using Convolutional Neural Network Regression Based on Classmap Regulation

A convolutional neural network (CNN) based regression is proposed for estimating the brittle fracture ratio (BFR) in a fracture image of a drop weight tear test (DWTT) specimen. Different with the previous complex semantic segmentation-based estimator, the method extracts the feature vector through global average pooling of feature map and calculates the BFR directly through the fully connected layer. By removing decoder network, the number of weights, training time, and required GPU memory dramatically reduced. To train the proposed CNN, a new loss function, which is the sum of L1-norm between class activation map and ground truth inspection image and L1-norm of BFR error, is also designed. To validate the present method, fracture images of 1532, 79, and 158 DWTT specimens obtained from real industrial site were used for training, validation, and test, respectively. The accuracy of the proposed method was evaluated based on the number of test samples with an error of 5% or less divided by the total number of test samples, which is the measure used in real industrial application. Despite having dramatically reduced the number of weights and inference time by 85.8% and 64.8%, respectively, the proposed method has a higher accuracy (96.2%) compared to that of the existing segmentation based BFR estimation method (94.9%).

CTOA Testing of Thick DWTT Specimens: Experiment and FEA Simulation for Development of an Annex for ASTM E3039

Abstract ASTM E3039-16, Standard Test Method for Determination of Crack-Tip-Opening Angle of Pipe Steels Using DWTT Specimens (superseded), is limited to drop-weight tear test (DWTT) specimens with thickness B in the range 6–20 mm. There is a trend in the pipeline industry toward heavier wall pipe in the range of 25-mm thickness; hence, there is a need to test thicker DWTT specimens for crack propagation resistance. To address this need, proportionally larger DWTT geometry is proposed to allow a larger steady-state region with the same analysis procedure as in ASTM E3039. To support the development of an E3039 Annex, tests of an X70 pipe steel with B = 26.8 mm were performed at quasistatic and intermediate rates. The critical crack-tip-opening angle (CTOAB/2) of 15.0° ± 1.7°, estimated according to ASTM E3039, was close to the CTOA of 14.7° measured at midthickness sections. The effect on CTOA of varying the loading rate between 5 × 10−5 m/s and 0.1 m/s was found to be negligible. Fracture surfaces of specimens interrupted during testing showed severe crack tunneling. Finite element analysis simulations of crack propagation using a damage-mechanics model were performed on standard and proportionally large DWTT specimens to derive and compare CTOAs. These results show that ASTM E3039 can be applied successfully with proportionally scaled specimens to measure CTOA for steels of thicknesses significantly larger than 20 mm.

Brittle Crack Arrestability and Inverse Fracture in DWTT

Abstract The drop-weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle crack propagation, and the shear area fraction SA% in the DWTT has been adopted in the requirement for the linepipe steels. However, recent studies have pointed out the issue of ambiguity in evaluation of the DWTT when a ductile crack initiates from the notch and then transits to a brittle crack during ductile crack propagation. This fracture behavior is termed “inverse fracture.” According to the API Recommended Practice 5L3 (API RP 5L3), a test is considered invalid when a DWTT specimen shows inverse fracture. In this case, it is difficult to examine the acceptance criterion (85% shear area transition temperature) for linepipe steels. Because the purpose of the DWTT is to evaluate the brittle crack arrestability of the steels in a pressurized linepipe, the DWTT results should be examined with a propagating brittle crack arrest test. A large-scale brittle crack arrest test called the West Jefferson test is generally conducted to reproduce the crack propagation and arrest behavior in actual linepipes. However, it is somewhat difficult to control the lower test temperature and to initiate brittle crack in recent high-toughness steels in this burst test. Although the test stress conditions of the uniaxial tension in the plate tension brittle crack arrest test and the biaxial tension in a pressurized pipe are different, the plate tension brittle crack arrest test has the advantages of accurate control of the test temperature, test stress, and brittle crack initiation in comparison with the actual pipe burst test. Therefore, in this study, the brittle crack arrestability of linepipe steel which showed inverse fracture in the DWTT was investigated by conducting plate tension brittle crack arrest tests under an isothermal condition (crack arrest temperature test (CAT test)), which simulates the condition of the actual pipelines in service. This study also investigated the local shear lip thickness fraction in the CAT tests together with the shear area fraction SA% measured in DWTTs. Based on the results, the effect of brittle crack arrestability on inverse fracture appearance in the DWTTs was discussed in comparison with the brittle crack arrest behavior in the CAT tests.

Finite Element Analysis to Investigate the Effect of Loading Modes on the CTOA of DWTT Specimens

To replace the traditional Charpy-test-based fracture arrest methodology, the crack tip opening angle (CTOA) has been proposed as a fracture propagation parameter for prediction of fast ductile fracture propagation and arrest of axial cracks in pipelines. Transferability of CTOA from the DWTT specimens determined according to ASTM E3039 to other specimens and especially pipes is critical to the applications of CTOA. As part of the research effort to investigate the transferability of crack tip opening angle (CTOA) from the drop weight tear test (DWTT) specimens to pipelines, this work focuses on the effect of loading modes (bending vs. tension) of DWTT specimens on CTOA based on finite element analysis. The damage-mechanics model (Modified Mohr-Coulomb Model) and element deletion technique to simulate crack initiation and propagation is found to be accurate and agrees well with test data. This model accounted for variations in stress triaxiality and Lode angle at the crack front. For the DWTT model under bending, the surface CTOA is considerably larger than the interior CTOA consistent with the experimental observations. However, for the DWTT specimen under tension, the CTOA on surface and interior are almost the same. The interior CTOA is found to be very similar in all cases. The CTOA distribution as a function of distance behind the crack tip is found to be very consistent up to one half-thickness behind the crack tip, where CTOA starts to increase gradually. For both the bending and tension DWTT simulations, steady-state crack growth was achieved at a reasonably low crack extension. These results support the transferability of CTOA from standard DWTT tests according to ASTM E3039 to other loading cases.

Application of the modified Bai-Wierzbicki model for the prediction of ductile fracture in pipelines

The complex mechanical and corrosive loads of modern pipeline systems transporting oil, natural gas and CO2 impose steadily increasing requirements on material properties. The majority of current design standards still limit the application of modern high toughness linepipe steels due to the simple specification of material requirements in terms of energy levels from Charpy impact or Battelle Drop-Weight-Tear (BDWT) tests. In consequence, research activities have been conducted recently aiming at developing modified or novel experimental methods for the characterization of the ductile fracture behavior. To quantify the effects of various parameters on fracture behavior and derive suitable correlations, it is necessary to accompany these activities by numerical simulations with appropriate ductile damage models. In this paper, the MBW model is applied to study the structural behavior of pipelines in ductile fracture regime. Due to its precise incorporation of the underlying load conditions, the damage model is successfully used to simulate the slant fracture behavior in Battelle Drop weight tear test specimens and pipe sections. In comparison to ductile damage models applied in former studies, namely the Gurson-Tvergaard-Needleman and Cohesive Zone model, the presented numerical methodology allows for a more detailed investigation of loading, material and geometry effects on fracture and crack arrest behavior of pipelines.

Weld metal fracture characterization of API X65 steel using drop weight tear test

Measurement of fracture energy using drop weight tear test (DWTT) specimen with large dimensions (as an alternative to small Charpy test sample) gives more realistic results. Although large amounts of test data are available from DWTT experiments on different pipeline steels (base metal), apparently no information has been reported for fracture energy of welded seam of these pipe steels. In the current study, the fracture energy of spirally welded steel pipe was measured using DWTT specimens with two different notches (i.e. pressed and with chevron notch) and compared with base metal (BM) fracture energy. The test samples were machined from an actual gas pipe with a spiral seam having 1219 mm outside diameter and 14.3 mm wall thickness. The DWT tests were conducted on an impact machine with 30 kJ capacity equipped with Wheatstone load-cell. The measured total fracture energy in pressed-notch (PN) specimen (6211 J) was around 5% higher than that of chevron-notch (CN) specimen (5890 J). Based on the measurements, in both specimens a correction factor of 1.8 was suggested for welded seam (the fracture initiation energy was around 43% of the total fracture energy). Since the test samples were taken from a real pipe with original thickness, this result was more realistic than those previously reported for conventional Charpy fracture test specimens for similar steel material. The fracture energies for weld metal (WM) specifications were less than that of the base metal (7085 J). Therefore, immediate attention must be paid to DWTTs of WM in pipelines, which had not been investigated before, and the results indicate the importance of urgent improvement of the toughness of welded zone in manufacturing processes, which requires further research. Since, there is no standard for WM in the DWTT, the results of this research can lead to the development of new standards based on WM DWTT.

Inverse Fracture in DWTT and Brittle Crack Behavior in Large-Scale Brittle Crack Arrest Test

The drop weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle fracture propagation. Although there is an ambiguity in the evaluation of DWTT results if inverse fracture appears on the fracture surfaces, the cause of inverse fracture is not yet fully understood. In the present work, DWTTs were performed with X65, X70, and X80 steel linepipes. In addition to the conventional DWTT specimen with a pressed notch (PN), PN specimens with a back slot (BS) and specimens with a chevron notch (CN) or static precrack (SPC) were also examined, and the fracture appearances in different strengths and different initial notch types were compared. Although the frequency of inverse fracture in these DWTTs was different with each material and each specimen type, there was no material or specimen type that was entirely free from inverse fracture. The purpose of the DWTT is to evaluate the brittle crack arrestability of the material in a pressurized linepipe. Therefore, the DWTT results should be examined with a running brittle crack arrest (BCA) test. A large-scale BCA test with temperature gradient was also performed with the X65 mother plate, and the shear area fraction measured in the DWTT fracture surface was compared with the local shear lip thickness fraction in the BCA test. Based on the results, the count of inverse fracture in the DWTT was discussed in comparison with the long BCA behavior in the BCA test.

Evaluation of Pre-strain Effect on Abnormal Fracture Occurrence in Drop-Weight Tear Test for Linepipe Steel with High Charpy Energy

Brittle fracture control is one of the most important subjects in natural gas transmission pipeline in order to maintain structural integrity over several decades. The Drop Weight Tear Test (DWTT) is widely used as the test method to evaluate the resistance against the brittle fracture for linepipe steels. However, an abnormal fracture frequently occurs during the DWTT in recent high toughness line pipe steels. The abnormal fracture is also known as inverse fracture. The abnormal fracture is defined as the cleavage fracture is observed at the hammer side in DWTT specimen although the ductile fracture firstly initiates from the notch tip side. Many studies for abnormal fracture appearance/behavior have been carried out in order to clarify the mechanism of the abnormal fracture occurrence and to ensure the prevention of long brittle fracture propagation for pipelines.In this study, a compressive pre-straining at the impact hammer side in the DWTT specimen was evaluated under quasi-static load conditions. The specimen’s surfaces were electrolytically-etched to print circle patterns with 5 mm in diameter in order to measure plastic strain. Charpy impact specimens were taken from the quasi-static loaded and unloaded DWTT specimen to measure the possible influence of pre-straining on toughness. The impact test results show that more than 2 % of the compressive pre-strain gave 7 to 10 % decrease of the Charpy upper-shelf energy. The effect of pre-straining on tensile property was also evaluated. These present experiments indicate that the occurrence of abnormal fracture near the hammer side can be attributed to the compressive pre-straining. Furthermore, the chevron-notched and the pre-cracked DWTTs and the partial gas burst test were conducted in order to compare the brittle-to-ductile transition temperature. Based on these experiments, the effect of notch configuration on the brittle-to-ductile transition temperature and the correlation between DWTTs and pipe test were discussed. In addition, the relationship between the pre-straining and the abnormal fracture appearance was considered.

Non-local phenomenological damage-mechanics-based modeling of the Drop-Weight Tear Test

We modify the Xue–Wierzbicki damage mechanics model to include strain rate effects, implement it in an explicit finite element code and use it to model the dynamic response and failure of X70 pipe steel in the Drop-Weight Tear Test (DWTT). The damage evolution depends on Lode angle, hydrostatic pressure, and strain rate. Strength of the steel is assumed to depend on strain hardening, strain rate and thermal softening. To mitigate the expected mesh dependence of results, we use the non-local integral formulation due to Bazant and Pijaudier-Cabot. A highlight of the computational results is the ability to model the fracture mode transition between impact and static loading: slant fracture for impact loading and flat fracture for static loading of the DWTT specimen. Furthermore, the measured experimental force–displacement curves are in excellent agreement with the computed curves. We estimate the Crack Tip Opening Angle and compare with experimental measurements. Our results also provide insight into the influence of the Lode-angle-dependent damage evolution on modeling slant fracture.

Simulation of ductile crack propagation and determination of CTOAs in pipeline steels using cohesive zone modelling

ABSTRACTThis paper presents recent results of numerical studies on stable crack extension of high toughness steels typical of those in modern gas pipelines using cohesive zone modelling (CZM). The main focus of the work is on the determination of crack‐tip opening angles (CTOAs) of these steels from CZM. Two sets of materials are modelled. The first material set models a typical structural steel, with variable toughness described by four traction–separation (TS) laws. The second set models an X70 pipe steel, with three different TS laws. For each TS law, there are three defining parameters: the maximum cohesive strength, the final separation and the work of separation. The specimens analysed include a crack in an infinite plate (small‐scale yielding, SSY) and a standard drop‐weight tear test (DWTT). Fracture propagation characteristics and values of CTOA are obtained from these two types of specimens. It is shown that cohesive zone models can be successfully used to simulate ductile crack propagation and to numerically measure CTOAs. The ductile crack propagation characteristics and CTOAs obtained from SSY and DWTT specimens are compared for each set of steels. In addition, the CTOA results from numerical CZM of DWTT specimens of X70 steel are compared with those from laboratory tests.

The Fracture During Drop-weight Tear Test of High Performance Pipeline Steel and its Abnormal Fracture Appearance

The dynamic cracking arrest capability is very important for pipeline in failure control. DWTT (Drop-Weight Tear Test) is widely applied as a standard test for evaluating crack arrest characteristics of pipelines. But for high performance pipeline steel, the abnormal fracture appearance is frequently observed on fracture surface of DWTT specimens and it make DWTT technique difficult to evaluate the dynamic toughness by the current test standard. In this study, various types of DWTT specimens of a commercial API-X70 pipeline steel were conducted at different temperature. The fracture appearances of specimens are observed. The intrinsic causes of abnormal fracture appearance are analyzed basing on the three-dimensional fracture theory. It is found that the dynamic propagation speed of crack in high performance pipeline steel is largely scattered. The propagation speed of fracture can change the three-dimensional stress constraint and lead to difference in the fracture appearance of specimen. Compress deformation, local thickness increase, toughness decrease and stress increase can make the crack dynamic propagation manner change from ductile to brittle and produce abnormal fracture appearance. The approaches to prevent or reduce the occurrence of abnormal fracture appearance for high performance pipeline steel are discussed in this paper.

Statistical characteristics of fracture surfaces in high-strength steel drop weight tear test specimens

Fracture surfaces of DWTT broken specimens are analyzed using statistical methods and fractal concepts. The dependence of fracture surface characteristics on the ductile fracture percentage is studied. The shape parameter of the Weibull distribution of fracture surface height coordinates grows with increasing ductile fracture percentage. The Hurst exponent of the investigated fracture surfaces ranges from a small length scale regime to a large length scale regime, which approximately amounts to the universal value of 0.8. The fractal dimension of the fracture surface as a decreasing function of the ductile fracture percentage is discussed from several points of view.

Crack Tunnelling and Crack Tip Opening Angle in Drop-Weight Tear Test Specimens

Interrupted drop-weight tear tests (DWTT) of typical low-strength and high-strength pipe steels were performed at quasi-static and impact loading rates, to quantitatively characterize crack tunnelling and crack-tip opening angle (CTOA). Crack front tunnelling was observed in all interrupted DWTT specimens, but the extent of tunnelling varied with the steel grade and the loading rate. CTOA values measured optically at the surface were higher than those measured at a cross-section close to mid-thickness. This is attributed to the effect of crack tunnelling, i.e. the effect of constraint, on CTOA. CTOAs derived from load vs. load-line displacement, using the simplified single-specimen method, are in good agreement with values measured at the mid-thickness.

Effects of Specimen Thickness and Notch Shape on Fracture Modes in the Drop Weight Tear Test of API X70 and X80 Linepipe Steels

This study is concerned with effects of specimen thickness and notch shape on drop weight tear test (DWTT) properties and fracture modes of API X70 and API X80 low-carbon microalloyed linepipe steels. Detailed fractographic analysis of broken DWTT specimens showed that the fracture initiated in an initial cleavage mode near the specimen notch and that some delaminations occurred at the center of the fracture surface. The chevron notch (CN) DWTT specimens had broader initial cleavage areas than the pressed notch (PN) DWTT specimens. The larger inverse fracture areas (i.e., cleavage areas close the hammer impact side) appeared in the PN DWTT specimens, because their higher fracture initiation energy at the notch allowed a higher strain hardening in the hammer-impacted region. The number and length of delaminations were larger in the CN DWTT specimens than in the PN DWTT specimens, and increased with increasing specimen thickness due to the plane strain condition effect. As the test temperature decreased, the tendency of delaminations increased, but delaminations were not found when the cleavage fracture prevailed at very low temperatures. The DWTT test results such as upper shelf energy (USE) and energy transition temperature (ETT) were discussed with relation to microstructures and fracture modes including initial cleavage fracture, ductile fracture, inverse fracture, and delaminations.

Abnormal fracture appearance in drop-weight tear test specimens of pipeline steel

The abnormal fracture appearance (AFA), which is frequently observed on the drop-weight tear test (DWTT) fracture surface of high performance pipeline steels, makes the DWTT technique difficult to utilize. Different DWTT specimens are tested at different temperatures. The cleavage regions of different AFA result from different phenomena during specimen fracture. All of the specimens exhibiting AFA should not be discarded as invalid specimens. Some new approaches are proposed to prevent or reduce the occurrence of AFA. The evaluation of DWTT specimens exhibiting AFA is re-examined considering the coupled effects of deformation, material behavior and fracture on the fracture appearance of high performance pipeline steel.

Fracture Appearance Evaluation of High Performance Pipeline Steel DWTT Specimen with Delamination Cracks

The delamination cracks and its effects on the fracture of pipeline steel are investigated experimentally by using of Drop-Weight Tear Test (DWTT). The delamination cracks are produced by the stress perpendicular to the weak interfaces before main crack beginning or accelerating, no new delamination crack is produced during the stabile propagation of fracture. The quantity, splay degree of delamination crack and the space between two delamination cracks are influenced by the stress state of the crack tip at beginning or accelerating point of main crack and the length of delamination crack is influenced by the stress state of the crack tip during the propagation of fracture. The surface of delamination crack is cleavage fracture appearance with large cleavage facet. There is no delamination crack on the brittle fracture surface below the brittle-to-ductile temperature or on the brittle fracture region of mix-mode fracture surface with ductile and brittle region. The part of fracture surfaces with delamination crack ought to be evaluated as the shear area because the delamination cracks are produced only on the ductile fracture surface or on the ductile part of fracture surface.

Effects of microstructure on inverse fracture occurring during drop-weight tear testing of high-toughness X70 pipeline steels

The effects of microstructure on inverse fracture occurring in the hammer-impacted region were analyzed after conducting a drop-weight tear test (DWTT) on high-toughness pipeline steels. Three kinds of steels were fabricated by varying the alloying elements, and their microstructures were varied by the rolling conditions. The pressed-notch (PN) or chevron-notch (CN) DWTT and Charpy V-notch (CVN) impact tests were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from CVN tests of prestrained specimens. In the hammer-impacted region of the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and the inverse fracture area correlated well with the upper-shelf energy (USE) obtained from the CVN test and with the grain size. The steel specimens having a higher USE or having coarse polygonal ferrite tended to have a larger inverse fracture area than those having a lower USE or having fine acicular ferrite. This was because steels having a higher impact absorption energy required higher energy for fracture initiation and propagation during the DWTT. These results were confirmed by the CVN data of prestrained steel specimens.