Crack Arrest Toughness Research Articles

A physics based model to simulate brittle crack arrest in steel plates incorporating experimental and numerical evidences

This study proposes a physics based model to simulate brittle crack arrest in steel plates, based on the local fracture stress criterion by incorporating experimental and numerical evidences of actual phenomena of brittle crack propagation/arrest behaviors. To accurately simulate actual phenomena of brittle crack propagation behavior, this study modelled the local tensile stress in the vicinity of a propagating crack tip, closure stress of unbroken shear lip, and shear lip thickness based on a series of finite element analyses and crack propagation experiments. The proposed model was applied to simulate temperature gradient crack arrest tests, and it successfully predicted the temperature dependency of brittle crack arrest toughness and crack arrest lengths.

A simplified method for evaluation of brittle crack arrest toughness of steels in scaled-down bending tests

The wide-plate crack arrest test under tensile load for characterizing the brittle crack arrest toughness of high strength shipbuilding steel requires high economical cost and long lead-time, so there is still a substantial industry need for the simplified evaluation using the scaled-down specimen. The alternative method using a single edge-notched bending specimen, which assumes the complete load redistribution during the crack propagation, was recently proposed. However, it was confirmed that the results obtained using this method exhibited a low correlation with the arrest toughness obtained using the wide-plate tests. A new scaled-down version of the crack arrest test under bending load and a series of simplified evaluation based on the dynamic elasto-plastic FEA were introduced to characterize the crack arrest performance of the two types of steel plates in this investigation. The specimen is a single edge-notched tapered plate subjected to a three-point bending load in an isothermal environment. The geometry was selected so that the dynamic crack driving force calculated using FEA decreases monotonically with the crack propagation. For adopting a simplistic analytical approach considering the dynamic effect, the dynamic FEA simulation assumed the constant crack velocity and flat crack front and output the opening stress distributed ahead of the growing crack. The experimentally obtained values of the arrested crack length and the analytically obtained transition curves of the dynamic SIF were employed to characterize the crack arrest performance of the two steels. The results obtained using the simplified method developed in this investigation exhibited a high correlation with the arrest toughness obtained using the wide-plate crack arrest test.

Welding Residual Stress Effect in Fracture Toughness.

The thickness of steel plates used for large structures has been increasing with the rapid increase in the size of welding structures. The growing capacity of large-scale ships, such as container ships, has led to an increase in the thickness and strength of steel plates for shipbuilding. The steel plate toughness and resistance to brittle fractures tend to decrease for thick plates because of the so-called thickness effect. This study uses 80-mm-thick steel plates and two welding processes (i.e., flux cored arc welding process and electron gas welding process) to produce full-thickness weld joints. The welding residual stress in both welded joints is measured to evaluate the brittle crack propagation path. This study aims to investigate the effect of welding variables on the crack arrest toughness and crack propagation path of thick steel-plate welds. The thick steel plate has a high possibility in brittle fracture. A quantitative analysis is conducted through a temperature-gradient ESSO test to clarify the effect of the welding variables on the flux cored arc welding and electron gas welding process joints of steel plates with 50 and 80 mm thicknesses. The welding residual stress is also measured to evaluate the welding residual stress effect in both welding processes on the brittle crack propagation path using a neutron science analysis.

Effect of Impact Energy in ESSO Test (Part 3: Experimental Validation for New Validity Criteria of Impact Condition Giving Constant Evaluation in ESSO Test)

Abstract In our second report, whether the impact had an important effect on the results or not depended on the critical condition that was proposed by calculating the ratio of the values of Kd both for where impact was considered and where it was not considered. This proposal was shown as a look-up figure where “effect” or “no effect” conditions were shown as regions illustrated on a graph, revealing the relationship between nondimensional impact energy and arrested crack length. In order to validate the criteria included in this proposal, some experiments under high impact energy conditions were carried out by using a steel plate with a known value of brittle crack arrest toughness under ordinary conditions. The experiments were conducted near the critical boundary that divides the area of “affect” and that of “not affect” on the evaluation. It was found that the experimentally evaluated plots closely followed the relationship of existing properties under low impact energy conditions of the same material on the Arrhenius plot. Hence, the proposed validity criteria can be interpreted as being made on the safe side. In addition, if the impact energy is larger than the specified condition, it may have an important effect on the test results. In this case, the results have to be treated as reference data in spite of being an expensive test. However, these criteria can only be determined after actual testing because the criteria include arrested crack length. Therefore, the authors also tried to make a simple recommendation for obtaining valid data that can be investigated before ESSO tests and without the effect of impact.

Correlation between steel initiation toughness and arrest toughness determined from small-scale mechanical testing

The ability of a material to arrest a fast-running brittle crack is vital in various industries such as offshore wind, oil and gas, and shipbuilding where cracks can initiate in regions of local stress and put lives at risk. Some modern steels show a high Charpy toughness, but low resistance to crack propagation – i.e. low crack arrest toughness. In this work, the relationship between initiation and arrest toughness is investigated in five different steels, including S355 structural steel, X65 pipeline steel and two high strength reactor pressure vessel (RPV) steels.Small scale mechanical testing was carried out to determine the material properties, which were correlated against the microstructural characteristics of the materials. The test program included instrumented Charpy, drop weight Pellini, fracture toughess, tensile testing, and microscopy. Nil ductility transition temperature (NDTT) is used as a measure of arrestibility. Initiation toughness showed the expected correlations with upper shelf Charpy and grain size measurements, however these did not correlate with the arrest toughness. The arrest toughness is better correlated against the T27J temperature – i.e. the onset of the lower shelf. This relationship is valid even for steels where the NDTT lies on the upper shelf of the Charpy curve.

A Study for Brittle Crack Prevention and Fatigue Life Improvement using a Welded Stiffener

Recently, thickness of steels used in ships is increasing along the increased size of vessels, e.g. container carriers in particular. In container vessels, thick plates are applied to hatch side coaming areas, and safety is also becoming important as steel plate thickness increases. Many studies are conducted on the brittle crack arrest characteristics to prevent catastrophic failure in those areas. A number of experiments have been conducted for investigating propagation characteristics of brittle crack from classification societies. Especially, ESSO test method is commonly employed to analyze the BCA (Brittle Crack Arrest) toughness. In order to prevent brittle crack, arrest holes, weld line shift and insert plates are suggested.Various researches have been carried out to arrest brittle crack in BCA steel. However, the actual structure thickness has to be applied to the experiment, and it is difficult to carry out experiments due to the large scale specimens. Thus, in this study, crack retardation method is researched considering the thickness, length, and location of a welded stiffener. Finite element analysis was conducted on the hatch side coaming to compare the allowable crack length according to the effect of stiffener and to compare fatigue life based on the characteristics of BCA steel. Key words: Brittle crack arrest steel, Brittle crack arrest toughness, ESSO test, Fatigue crack growth rate, Stiffener

Effect of Impact Energy in ESSO Test (Part 2: Proposal of Validity Criteria of Impact Condition in ESSO Test by FEM Crack Propagation Analysis

Abstract Based on the fundamental knowledge revealed in Part 1 of the present work, crack propagation analysis is carried out in Part 2 of the present work to the establish validity criteria for impact conditions. In cases where there is excessive impact energy in a brittle fracture test in the evaluation of crack arrest toughness, the impact energy gives conservative test results. In this research, numerical simulation was systematically conducted in order to clarify the effects of impact energy. Dynamic elastic finite element method Finite Element Method (FEM) analysis, which was validated by actual dynamic strain data in Part 1, was employed. The FEM analysis revealed that the impact effect reached its maximum immediately after impact and gradually decreased thereafter. However, the increase of the dynamic stress intensity factor Kd, with respect to the impact, remains significant in cases where applied stress is low and impact energy is high. The judgment of whether the impact had an effect on the results was made by calculating the ratio of the values of Kd both for cases where impact was considered and where it was not. The threshold condition from which the evaluated value should be corrected was set to 1.5. The specified range could be determined without considering the effect of impact by using the dimensionless ratio of the quantity impact energy Ei/(Es + Et).

Effect of dispersed retained γ-Fe on brittle crack arrest toughness in 9% Ni steel in cryogenic temperatures

In structural steels, high crack arrest toughness is required to stop brittle cracks from propagating to prevent eventual fracture of the structure. It is known that Ni can reduce the risk of cleavage fracture of α-Fe, as it exists in a solid solution as a substitutional alloying element. Additionally, Ni can enhance the stability of γ-Fe between the martensite-laths, leading to improved toughness. Although there are several interpretations of the effect of retained γ on the fracture toughness, especially regarding dynamic crack propagation, there is little knowledge on the role of retained γ. In this study, the relationship between the amount of transformation and crack arrest toughness is evaluated by an experimental approach. As a result, it is revealed that the γ-α transformation at the propagating crack tip enhances the crack arrest properties. Furthermore, compared to static loading conditions, γ-Fe seems reluctant to transform to martensite as a result of an adiabatic temperature increase due to plastic work. Stress relaxation effects due to deformation-induced transformation of γ-Fe are the proposed mechanism of enhancement of the crack arrest properties.

Estimation of the Effective Grain Size Controlling Brittle Crack Arrest Toughness of High-strength Steel

Estimation of the Effective Grain Size Controlling Brittle Crack Arrest Toughness of High-strength Steel

Influences of Effective Grain Size and Ni Content on Brittle Crack Arrest Toughness of High Strength Steel Plates

Influences of Effective Grain Size and Ni Content on Brittle Crack Arrest Toughness of High Strength Steel Plates

Development of simplified evaluation method of brittle crack arrest toughness on small-scale bending test in steels

The priority items in the safety evaluation of steel structural components generally include an ability to arrest crack propagation, which is necessary to prevent a catastrophic failure even if a brittle fracture occurs. The ‘double integrity’ concept of brittle crack initiation control and arrest has been considered to be an effective and rational methodology for several decades. The wide plate tensile test such as ESSO test is one of the major methods for evaluating brittle crack arrest toughness of steel plates. Although ESSO test makes it possible to accurately evaluate arrest toughness which indicates the Arrhenius type temperature dependence, it is not suitable for quality assurance test at mass production of steel plates due to its high economical cost and long lead-time. Thus, a number of studies has attempted to establish simplified evaluation method of brittle crack arrest toughness for many years. Generally, bending test is certainly one of the most hopeful methods since it does not require a relatively high test load. However, the phenomenon that brittle crack propagates at extremely high speed in bending condition becomes highly complicated. When brittle crack propagates at almost the same speed as stress wave in bending condition, stress distribution is the middle of the initial state and fully reallocated state by the static equilibrium. It is not easy to make out the detail only by theoretical consideration. In this study, by performing the dynamic elasto-plastic FEA in various test designs based on SEN(B) test, the authors calculated the stress distribution at the crack tip and developed a new test design suitable for evaluating arrest toughness. Moreover, the authors investigated the correlation between the result of ESSO tests and that of the developed tests and presented its applicability.

A Basic Study on Brittle Crack Propagation Path with Ultra Large Steel Plate Weld Joints

As the recession in shipbuilding industry became longer, domestic shipbuilders focused on the need for high value added and high technology, and strived to build safer vessels. There are LNG carriers and container ships that require representative technologies. Both ship types are rapidly increasing ship size in the 2000s. Recently, there have been the increase of ship size and the development of oil and gas in arctic region. These trends have led to the requirements such as high strength, good toughness at low temperature and good weldability for prevent of brittle fracture at service temperature. Studies on unstable fracture have mainly been conducted to evaluate the brittle crack stopping properties of the high strength steel welded joints through large scale fracture tests, and studies on the difficulty of stopping brittle cracks in welded joints. It is known that BCA (Brittle Crack Arrest) steel has recently been developed to prevent unstable fracture if cracks propagate toward the BCA steel even if brittle cracks occur. Therefore, it is considered that the ship can be dried more easily if a technology capable of inducing the brittle crack to the BCA steel is developed. In this study, we describe a technique for securing unsafe fracture safety by inducing cracks toward the BCA steel, which has excellent brittle crack stopping ability, regardless of the welding process when brittle cracks occur in the superstructure of super large container ships. Key words: Brittl crak arrestability, Crack arrest toughness, Thick steel plate, FCAW, ESSO test, Welding residual stress

Determining critical CTOA from energy-load curves with DWTT specimen

Running ductile fracture is one of the most catastrophic accidents of pipelines for natural gas transportation. Crack arrest toughness is important for preventing crack extension to a long distance along pipeline. Critical crack tip opening angle (CTOAC) is a promising parameter for characterizing crack arrest toughness. In this study, a new method is proposed to calculate the CTOAC from the energy-load curves in an instrumented drop weight tearing test (DWTT). Numerical analyses with the Gurson damage model have been conducted to verify the proposed method. It shows that results calculated from the energy-load curves are slightly lower than the results measured from specimen surface. Results calculated from the proposed method also agree well with experimental results reported in the literature.

Crack Arrest Toughness of High Grade Gas Pipeline

Pipeline transportation is the most economical and reasonable way to transport natural gas. However, there are still some technical problems in high grade gas pipeline, in which fracture control is one of the key problems. The ductile fracture for long-range expansion is the most destructive failure mode of high pressure gas pipeline. In this study, the determination and prediction methods of crack arrest toughness of high grade (X80 and above) pipe line steels were introduced. Their application range, advantages and disadvantages were analyzed. The results showed that the toughness of X80 pipe line steel can meet the needs of the pipeline crack arrest requirements. It is difficult for the ultra-high-grade (X90 and above) pipe line steel to arrest crack by their own toughness. Therefore, the crack arrester should be installed. This paper introduced the crack arrest principle, advantages and disadvantages of various kinds of crack arresters.

Simplified Evaluation of Brittle Crack Arrest Toughness in Heavy-thick Plate by Combined Small-scale Tests

Simplified Evaluation of Brittle Crack Arrest Toughness in Heavy-thick Plate by Combined Small-scale Tests

Effect of specimen size, applied stress and temperature gradient on brittle crack arrest toughness test

Temperature gradient type ESSO test is one of the most popular test methods for evaluating the brittle crack arrest toughness, K_{ca}. However, test conditions which are specimen shape, tab plate shape, applied stress and temperature gradient affect K_{ca}. This document reports effects of specimen geometries that are specimen width, tab plate length, tab plate thickness and tab plate width on K_{ca} evaluation. In addition, effects of applied stress and temperature gradient have also been investigated. Temperature gradient type ESSO tests are conducted at three different steel mills in Japan. Then, test conditions were varied and test results were compared. In the result, influence range of effect specimen width, tab plate thickness, applied stress and temperature gradient were demonstrated. The applicable range of specimen geometry, applied stress and temperature gradient were clarified and implemented to the brittle crack arrest standard.

Crack propagation and arrest simulation of X90 gas pipe

To determine whether X90 steel pipe has enough crack arrest toughness or not, a damage model was suggested as crack arrest criterion with material parameters of plastic uniform percentage elongation and damage strain energy per volume. Fracture characteristic length which characterizes fracture zone size was suggested to be the largest mesh size on expected cracking path. Plastic uniform percentage elongation, damage strain energy per volume and fracture characteristic length of X90 were obtained by five kinds of tensile tests. Based on this criterion, a length of 24 m, Φ1219 × 16.3 mm pipe segment model with 12 MPa internal gas pressure was built and computed with fluid-structure coupling method in ABAQUS. Ideal gas state equation was used to describe lean gas behavior. Euler grid was used to mesh gas zone inside the pipe while Lagrangian shell element was used to mesh pipe. Crack propagation speed and gas decompression speed were got after computation. The result shows that, when plastic uniform percentage elongation is equal to 0.054 and damage strain energy per volume is equal to 0.64 J/mm3, crack propagation speed is less than gas decompression speed, which means the simulated X90 gas pipe with 12 MPa internal pressure can arrest cracking itself.

Technical specifications for X80 OD 1422 mm line pipes and corresponding products

The line pipes with large diameter, thick wall and high steel grade are preferred to meet the requirements of extremely high gas transmission rate (380 × 108 m3/a) in the Sino–Russian eastern route project. In this paper, the technical standards of line pipes at home and abroad were comparatively analyzed after the research and formulation process of technical specifications of X80 line pipes (OD 1422 mm) used in the above-mentioned project was introduced. The key technical indexes such as chemical components, crack–arrest-toughness values, etc. and its formulation process were discussed. In addition, the development process and product performance of the X80 line pipes (OD 1422 mm) were described. It is proved by trial production and product tests that using X80 line pipes (OD 1422 mm) in this gas project is a rational and effective solution to the following technical issues such as chemical composition control, fracture control and welding stability maintenance. This type of line pipes is proved not only to meet project requirements, but to suit for production cases. As a result, the safety of Sino–Russian eastern route gas pipeline will be guaranteed essentially. The research results in this paper provide a strong technical support for the application of X80 line pipes (OD 1422 mm) in the Sino–Russian eastern route project, and play a guiding role in drawing up the technical specifications of other gas pipeline projects.

Experimental and numerical investigation on relationship between grain size and arrest toughness in steels

Today, steel plates of container carriers become thicker because larger ships are needed for carrying more baggage and reducing the transportation cost. Thus, even though brittle fracture occurs, arresting the crack is essential as “double integrity” for structures. It is directly effective to improve the arrest toughness, which is the material resistance against the brittle crack propagation. However, there was not established theory to explain the dependence of the microstructure on arrest toughness. In particular, although it is empirically known that of the finer grain size makes higher crack arrest toughness, the quantitative relationship was not clarified.Recently, Shibanuma et al. proposed a multiscale model to simulate the brittle crack propagation/arrest behaviour qualitatively. However, the microscopic energy absorbing mechanism in the model is too simple to reveal quantitative prediction of arrest toughness. According to the above background, we conduct an experiment to quantitative clarify the relationship between arrest toughness and grain size, and then modify the above multiscale model based on the experimental results. Consequently, the results obviously shows the relationship between arrest toughness and grain size of steels. The experimental result was contrary to the result of microscopic model. Thus, it is necessary to correct a numerical expression of the microscopic energy absorbing mechanism in the model to accurately simulate the actual brittle crack propagation and arrest behavior in steel of arbitrary grain size.

Numerical Simulation of Brittle Crack Propagation and Arrest in Steels Considering Shear-Lip Formation

A model for simulating brittle crack propagation and arrest in steel plates is presented. The model considers the influence of shear lips formed on a fracture surface acting as a crack closure force and the influence of the loss of the plane-strain condition at a propagating crack tip with an increasing dynamic stress intensity factor. The shear lips are assumed not to fracture during the test, so they will yield and provide a crack closure force. The crack closure effect depends on not only the stress intensity factor but also the crack velocity. The model was applied to temperature-gradient crack-arrest tests and ultrawide duplex crack-arrest tests. The model successfully explained anomalous crack-arrest toughness under excessively high applied stress in the temperature-gradient tests and crack arrest at a stress intensity factor far above the cleavage crack-arrest toughness in the ultrawide duplex tests.