Drop Weight Tear Test Research Articles

Automated Brittle Fracture Rate Estimator for Steel Property Evaluation Using Deep Learning After Drop-Weight Tear Test

This study proposes an automated brittle fracture rate (BFR) estimator using deep learning. As the demand for line-pipes increases in various industries, the need for BFR estimation through drop-weight tear test (DWTT) increases to evaluate steel's property. Conventional BFR or ductile fracture rate (DFR) estimation methods require an expensive 3D scanner. Alternatively, a rule-based approach is used with a single charge-coupled device (CCD) camera. However, it is sensitive to the hyper-parameter. To solve these problems, we propose an approach based on deep learning that has recently been successful in the fields of computer vision and image processing. The method proposed in this study is the first to use deep learning approach for BFR estimation. The proposed method consists of a VGG-based U-Net (VU-Net) which is inspired by U-Net and fully convolutional network (FCN). VU-Net includes a deep encoder and a decoder. The encoder is adopted from VGG19 and transferred with a pre-trained model with ImageNet. In addition, the structure of the decoder is the same as that of the encoder, and the decoder uses the feature maps of the encoder through concatenation operation to compensate for the reduced spatial information. To analyze the proposed VU-Net, we experimented with different depths of networks and various transfer learning approaches. In terms of accuracy used in real industrial application, we compared the proposed VU-Net with U-Net and FCN to evaluate the performance. The experiments showed that VU-Net was the accuracy of approximately 94.9 %, and was better than the other two, which had the accuracies of about 91.8 % and 93.7 %, respectively.

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.

Quantification and microstructural origin of the anisotropic nature of the sensitivity to brittle cleavage fracture propagation for hot-rolled pipeline steels

This work proposes a quantitative relationship between the resistance of hot-rolled steels to brittle cleavage fracture and typical microstructural features, such as microtexture. More specifically, two hot-rolled ferritic pipeline steels were studied using impact toughness and specific quasistatic tensile tests. In drop weight tear tests, both steels exhibited brittle out-of-plane fracture by delamination and by so-called “abnormal” slant fracture, here denoted as “brittle tilted fracture” (BTF). Their sensitivity to cleavage cracking was thoroughly determined in the fully brittle temperature range using round notched bars, according to the local approach to fracture, taking anisotropic plastic flow into account. Despite limited anisotropy in global texture and grain morphology, a strong anisotropy in critical cleavage fracture stress was evidenced for the two steels, and related through a Griffith-inspired approach to the size distribution of clusters of unfavorably oriented ferrite grains (so-called “potential cleavage facets”). It was quantitatively demonstrated that the occurrence of BTF, as well as the sensitivity to delamination by cleavage fracture, is primarily related to an intrinsically high sensitivity of the corresponding planes to cleavage crack propagation across potential cleavage facets.

Temperature Effects on Fracture Toughness Parameters for Pipeline Steels

The present article showcases a temperature dependent cohesive zone model (CZM)-based finite element simulation of drop weight tear test (DWTT), to analyse fracture behavior of pipeline steel (PS) at different temperatures. By co-relating the key CZM parameters with known mechanical properties of PS at varying temperature, a temperature dependent CZM for PS is proposed. A modified form of Johnson and Cook model has been used for the true stress–strain behavior of PS. The numerical model, using Abaqus/CAE 6.13, has been validated by comparing the predicted results with load–displacement curves obtained from test data. During steady-state crack propagation, toughness parameters (such as CTOA and CTOD) were found to remain fairly constant at a given temperature. These toughness parameters, however, show an exponential increase with increase in temperature. The present paper offers a plausible approach to numerically analyze fracture behavior of PS at varying temperature using a temperature dependent CZM.

Normalization of the Cold Shortness of Plate Steel: II. Short-Brittleness Thresholds in Pipe Tests

The following serial curves are obtained for a pipe steel of strength class K65 (Kh80): strength and plasticity curves during tension in the temperature range from–80 to +20°C, fracture energy from drop weight tear test (DWTT) diagrams, and impact toughness KCV curves (from–180 to +20°C). The ductile–brittle transition temperature range determined from the DWTT energy is shown to be higher than that determined from a KCV diagram by 80 K. The KCV–40 criterion is concluded to be more reliable than the DWTT–20 criterion. KCV and DWTT tests are shown to reproduce the crack starting conditions in a pipe, and the resistance of a high-strength pipe steel to extended fracture should be estimated using other criteria.

A comparison of mechanical properties and residual stresses of line pipes sized by expansion and compression process

We have investigated and compared the mechanical properties and residual stress distributions in API-X70 pipes sized by expansion and compression processes with different sizing ratios. Various testing methods were applied to examine the target properties (i.e., hardness, yield-to-tensile ratio, drop weight tear test (DWTT) and crack tip opening displacement (CTOD)) which are critical design parameters in line pipe industries. Residual stresses were determined using a conventional hole drilling technique. The results show similar tensile strength and hardness distributions in both processes, whereas an opposite trend in the yield-to-tensile (Y/T) ratio was found where, in compressed pipes, the ratio was lowered. Fracture behaviour showed higher crack arresting and better fracture characteristic in the compression. Compared to the original pipe, residual stresses were considerably reduced in both processes, and in particular there were little or no residual stresses at a compression ratio of 0.3%. The effectiveness of the compression process over the expansion process is elucidated and discussed with respect to mechanical, economic and environmental aspects.

Effect of Slab Centerline Segregation on DWTT Properties of 22mm Thick X80M Hot Rolled Steel Strip

The replacement of 22mm thick X80M hot rolled strip for the traditional heavy plate production technology has remarkable advantages in improving the yield, reducing the cost of manufacturing and improving the outer diameter adjustment of pipeline. Low temperature anti Drop Weight Tear Test (for short DWTT) performs one of the key research problems of 2250mm hot rolling line production of this specification. Aiming at the problem of the substandard performance of rolled DWTT development process, the slab fracture morphology of different proportion and slab thickness direction component segregation degree of system were analyzed through the electron microscope. The results showed that the thickness of steel strip was serious segregation and is the main factor affecting the performance of DWTT. By optimizing the caster arc alignment and secondary cooling water, then improving the central segregation effect, the performance of DWTT can be effectively improved, and a stable development and production of 22mm thickness X80M pipeline steel hot rolledstrip can be achieved.

Dynamic measurement of CTOA of unstable ductile fracture of high pressure gas pipelines

Demand for natural gas is increasing worldwide and pressure and diameter of gas pipelines are increasing. In such high-pressure gas pipelines, prevention of unstable ductile fracture is an important issue. Crack tip opening angle (CTOA) is widely used for fracture criterion of unstable ductile fracture, and measured by Drop Weight Tear Test (DWTT). But, measuring CTOA during dynamic crack propagation in pipe burst is very limited. Therefore, in this study, we measured CTOA in a pipe burst test. The crack propagation was photographed by high-speed camera. The CTOA values by the DWTT and the burst tests were found somewhat different. A care should be taken when using CTOA value obtained by DWTT for analyzing dynamic crack propagation behavior in pipelines.

Cleavage fracture assessment of cold charged steel slabs using experimental and numerical approaches

Drop Weight Tear Test (DWTT) set-up has been used to investigate cleavage fracture of steel slabs at different elevated temperatures up to 500°C. Fracture surfaces of broken samples have been analyzed using Scanning Electron Microscope (SEM) to understand ductile and brittle fracture appearances at the elevated temperatures. The eXtended Finite Element Method (XFEM)-based cohesive segment technique has been used to simulate the dynamic crack propagation of the DWTT at all tested temperatures. The obtained numerical results were in good agreement with the observed experiments. Eventually, a hybrid experimental-numerical method is proposed to define a set of critical cleavage stresses for cold/hot charged steel slabs.

An Experimental and Numerical Investigation of the Anisotropic Plasticity and Fracture Properties of High Strength Steels from Laboratory to Component Scales

Experimental and numerical investigations on the plasticity, damage and fracture behavior of a pipeline steel were conducted in this study. Anisotropy effects on the plastic deformation and fracture properties of the material were taken into consideration by performing tests along different loading directions with respect to the rolling direction. Fracture behavior of the steel was characterized through conducting a comprehensive experimental program covering a wide range of stress states by using fracture specimens of various geometries along different loading angles. Besides the fracture experiments in the laboratory scale, the drop weight tear tests of full plate thickness along different loading directions were performed as well. The recently proposed evolving non-associated Hill48 model was adopted to describe the anisotropic hardening behavior of the investigated material. Finite element analysis on the fracture behavior of this material was performed by using a coupled damage mechanics model with triaxiality and Lode angle dependence. Through the hybrid experimental and numerical investigations, the influences of anisotropy on the plastic deformation and fracture behavior of the pipeline steel are analyzed.

О ПРОБЛЕМАХ ПЕРЕХОДА К КОЛИЧЕСТВЕННЫМ ОЦЕНКАМ ЭНЕРГОЕМКОСТИ РАЗРУШЕНИЯ ПРИ ИСПЫТАНИЯХ ПАДАЮЩИМ ГРУЗОМ ОБРАЗЦОВ НАТУРНОЙ ТОЛЩИНЫ

О ПРОБЛЕМАХ ПЕРЕХОДА К КОЛИЧЕСТВЕННЫМ ОЦЕНКАМ ЭНЕРГОЕМКОСТИ РАЗРУШЕНИЯ ПРИ ИСПЫТАНИЯХ ПАДАЮЩИМ ГРУЗОМ ОБРАЗЦОВ НАТУРНОЙ ТОЛЩИНЫ

Shape effects of the traction–separation law on the global response of the dynamic fracture for pipeline steels

A recently reported four-parameter cohesive zone model (CZM) has applications in dynamic fracture such as the drop-weight tear test simulation. In this paper, the CZM method is utilized to analyze the global response of the dynamic fracture problems. And the two key CZM parameters, maximum traction and cohesive energy, are determined from the nominal stress–strain (SS) curve by considering the damage scalar and a simulation of the tensile test, respectively. The other two parameters, $$S_{0}$$ and $$S_\mathrm{m}$$ , are related to the shapes of the traction–separation law (TSL), which characterize its stiffness and softening shape, respectively. The influences of TSL’s shape on the global response such as the nominal SS curves and the load–displacement curves are analyzed by the finite element method. Evident (TSL’s) shape effects are found in the case of high-speed dynamic fracture of pipeline steels, which is completely different from quasi-static models.

Preliminary Study of Testing Specimen Thickness Effect on Fracture Toughness for X70 Steel

The influences of testing specimen thickness (TST) on fracture toughness, including drop weight tear test shear area (DWTT) and crack tip opening distance (CTOD), are studied by experimental method for X70 steel. Results show that DWTT and CTOD reduce gradually with the specimen thickness increasing, showing an apparent TST effect. DWTT results also show that the TST effect is more significant at the middle transition temperature range. Parameters including J-integral, constraint parameters (T33&Tz) and plastic volume ratio (Vr) of SENB specimens with different thickness are calculated by FEA method. Then the TST effect correlation of |T33|, Tz and Vr are compared. All parameters show a strong thickness dependent. Crack tip constraint parameters, |T33| and Tz, have a similar variation tendency, but different from that of J. Contrary, Vr has a more similar variation tendency with J. Therefore, plastic deformation behavior cannot be ignored during studying the TST effect of X70 steel. Vr can be treated as an alternate parameter to explain TST effect.

Forces and strains during drop weight tear test

As follows from the results of drop weight tear tests (DWTT), modern strength class K65 steels for gas-main pipelines exhibit an almost 100% fraction of ductile component in a fracture surface. The results of ground tests demonstrate that the lengths of extended fracture of a pipe made of such steels from different suppliers are substantially different. The possibility of extracting other characteristics from DWTT tests of full-thickness notched specimens and the information content in them are estimated. High-speed detection of the strain and crack growth is performed during static and dynamic loading. As follows from an analysis of fracture kinematics and force and energy balance, fracture is a multistage process, where mechanisms change during crack growth.

The relationship between microstructure, crystallographic orientation, and fracture behavior in a high strength ferrous alloy

The study described here focuses on toughening and crack-arrest mechanism in a 560 MPa microalloyed pipeline strip steel processed by combination of thermo-mechanical controlled processing and ultra-fast cooling (TMCP-UFC) and compared with the strip processed at low cooling rate and high cooling interrupt temperature (TMCP-LC). Furthermore, delamination mechanisms involved in drop weight tear test (DWTT) were also studied from the perspective of microstructure, crystallographic orientation, and stress field ahead of the crack. The TMCP-UFC processed strip was primarily composed of acicular ferrite (AF), bainitic ferrite (BF), and small-sized martensite/austenite (M/A) constituent, and the microstructure was homogeneous across the thickness. While the TMCP-LC processed strip mainly consisted of quasi-polygonal ferrite (QPF), degenerate pearlite (DP), and small fraction of BF. The TMCP-UFC processed strip exhibited excellent low-temperature toughness with upper shelf energy (USE) of ∼302 J and transition temperature (TT) of ∼−75 °C in relation to TMCP-LC processed strip with USE of ∼273 J and TT of ∼−30 °C. Significant consumption of crack-propagation energy during ductile fracture and smaller effective grain size of ∼2.4 μm were responsible for higher USE and lower TT of TMCP-UFC processed strip compared to TMCP-LC processed strip with effective grain size of ∼4.0 μm. The smaller effective grain size with respect to TMCP-UFC processed strip contributed to excellent crack-arrest property by deflecting the propagated crack. In the TMPC-UFC processed strip, delamination occurred along the crystallographic cleavage plane of {001} under plane strain condition at mid-thickness. In contrast, besides delamination mechanism along the cleavage plane, delamination also occurred along the macro-segregation band under plane strain condition at mid-thickness in the TMCP-LC processed strip, which led to severe delamination.

Effect of microstructure on the crack propagation behavior of microalloyed 560 MPa (X80) strip during ultra-fast cooling

A novel thermo-mechanical controlled processing (TMCP) involving ultra-fast cooling (UFC) was used to process microalloyed 560MPa (X80) strip with critical thickness of 22mm. The microstructure and mechanical properties including tensile, Charpy v-notch impact toughness, and drop weight tear test (DWTT) properties were studied, with particular focus on the effect of microstructure on fracture and crack propagation behavior during DWTT test. The study underscores that the processed strip comprising of acicular ferrite (AF), bainitic ferrite (BF), together with finely distributed martensite/austenite (M/A) constituent provided excellent combination of strength, toughness and crack arrest property. The yield strength (618MPa), tensile strength (752MPa), and low-temperature toughness (upper shelf energy of 302J, transition temperature of −74°C) as well as DWTT shear area (100%) met the requirements of API SPEC 5L. The predominantly AF microstructure with small-size M/A constituent improved the low-temperature toughness by increasing the critical fracture stress and increased the ability to hinder crack propagation during Charpy test. The AF and finely distributed M/A constituent effectively deflected the crack propagation during DWTT indicating excellent crack arresting property of pipeline strip. It is the cooling schedule of UFC that was responsible for decreasing the effective grain size and increasing the mechanical properties.

Analysis of energy absorptions in drop-weight tear tests of pipeline steel

The energy absorptions of drop-weight tear tests (DWTT) are analyzed using a finite element model based on the cohesive zone model. The parameters in the traction–separation law for the cohesive zone are validated by matching the simulation results to test data of standard specimens of DWTT. Based on the simulations the fracture energy (defined as the adhesion energy plus the plastic energy surrounding the crack tip) is quantified from the total energy absorption in the standard specimen. A ratio of the fracture energy to the total energy absorption for the steady-state crack propagation in the standard specimen is obtained which could be used to estimate the energy-based toughness of pipeline steel based on the measurable total energy absorption in DWTT.

Numerical simulation of dynamic brittle fracture of pipeline steel subjected to DWTT using XFEM-based cohesive segment technique

In the past several numerical studies have addressed the ductile mode of fracture propagation. However, the brittle mode of pipeline failure has not received as much attention yet. The main objective of this study is to predict brittle fracture behaviour of API X70 pipeline steel by means of a numerical approach. To this end, the eXtended Finite Element Method (XFEM)-based cohesive segment technique is used to model Drop Weight Tear Test (DWTT) of X70 pipeline steel at -100°C. In this model the dynamic stress intensity factor and crack velocity are calculated at the crack tip at each step of crack propagation.

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.