ESSO Test Research Articles

Crack arrest simulation in steel in account of competing ductile and cleavage fracture

Full-scale crack arrest ESSO tests are again popular for thick shipbuilding plate. Now researchers are seeking for correlations with small-size tests in order to decrease the cost but reliably predict the crack arrest fracture toughness of steel in full thickness, especially for low-temperature service projects. A lot of tests are widely used for ductile-brittle transition temperatures evaluation but the correlation of their results is still uncertain or qualitative. A fracture-mechanics based concept has been suggested by the authors allowing to get quantitative solutions. The present paper holds a discussion on applicability of simple criteria for the simulation of competing cleavage and ductile fracture.

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.

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 the Esso Test (Part 1: Basic Experiments and Validation of FEM Analysis)

Abstract To enhance the integrity of manufactured steel plate structures, the arrest toughness is an important parameter and is actually required in several structural applications, e.g., shipbuilding, tankage, and power plants, where brittle cracking occurs. Currently, the Esso test is the most popular evaluation method for arrest toughness. In the Esso test, a notch located at the top of the specimen is forced open by impact energy. Generally, it is expected that the arrest toughness may be affected by impact. However, the effect of the impact has not been considered in the evaluation methods so far. Furthermore, the impact energy is selected arbitrarily and differs depending on the test organization. Considering the current situation, it is quite important to evaluate the effect of impact on the obtained results. In the present work, in order to clarify the effect of impact on the arrest toughness evaluation, changes in the driving force at the running crack tip are examined by finite element method (FEM) analyses including impact. In particular, the fundamental behavior of the specimen due to impact and the validation of FEM analysis are investigated.

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 toughness distribution in the thickness direction on long brittle crack propagation/arrest behaviour of heavy gauge shipbuilding steel

ABSTRACTBrittle crack arrestability of the heavy gauge steel plates for shipbuilding is now an important issue for the recent mega container ships. In the present work, the brittle crack arrestability of the steel plate with different toughness distributions in thickness is examined in ultra-wide duplex ESSO tests. It is examined whether a running long brittle crack arrests or not in flat temperature condition in ultra-wide duplex ESSO test that are harder mechanical conditions similar to an actual ship hull condition. Test temperatures are selected at which arrest toughness, Kca, evaluated by temperature gradient type standard ESSO test is the same for two test plates. The steel plate with higher toughness in mid-thickness (t/2) than that in quarter thickness (t/4) could arrest a running long brittle crack although the plate with lower toughness in mid-thickness than that in quarter thickness could not arrest it. The typical split-nail shape appeared at the arrested crack front in the plate with higher toughness in mid-thickness than that in quarter thickness. The numerical analyses also demonstrate that the local stress intensity factor at the arrested crack tip is changing sensitively to the crack front shape. It suggests that the higher brittle crack arrestability appears due to the split-nail shape of the arrested crack front enhanced by the inhomogeneous toughness in thickness.

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

Observation of micro-cracks beneath fracture surface during dynamic crack propagation

In this study, micro-cracks generated during dynamic crack propagation were investigated for an ESSO specimen and V-notched Charpy specimen. The ESSO test with temperature gradient and the Charpy impact test were conducted for steel for shipbuilding. The macroscopic roughness of the fracture surface increases with increasing temperature. The section beneath the fracture surface generated during dynamic brittle crack propagation was observed using a scanning electron microscope, after cutting with a focused ion beam or grinding. Many micro-cracks can be found beneath the brittle fracture surface in the V-notched Charpy specimen. In the ESSO specimen, which had a macroscopic flat surface, micro-cracks can be detected beneath the main crack propagated in the temperature range from −140°C to −100°C. On the other hand, there were a few micro-cracks beneath the main crack propagated at relatively high temperature, at which the fracture surface was macroscopically rough. The number of micro-cracks in the Charpy specimen is fewer than those in the ESSO specimen at the same temperature. The observations indicated that the correlations of Charpy impact properties with crack arrest performance for the ESSO specimen was inconvincible in terms of the differences in the micro-scale fracture behavior as well as lack of theoretical basis.

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.

造船用厚肉鋼板の長大脆性亀裂伝播停止挙動に及ぼす板厚方向靭性分布の影響

Brittle crack arrestability of the heavy gauge steel plates for shipbuilding is now an important issue for the recent mega container ships. In the present work, the brittle crack arrestability of the steel plate with different toughness distributions in thickness is examined in ultra-wide duplex ESSO tests. It is examined whether a running long brittle crack arrests or not in flat temperature condition in ultra-wide duplex ESSO test that are harder mechanical conditions similar to an actual ship hull condition. Test temperatures are selected at which arrest toughness, Kca evaluated by temperature gradient type standard ESSO test are the same for two test plates. The steel plate with higher toughness in mid-thickness (t/2) than that in quarter thickness (t/4) could arrest a running long brittle crack although the plate with lower toughness in mid-thickness than that in quarter thickness could not arrest it. The typical split nail shape appeared at the arrested crack front in the plate with higher toughness in mid-thickness than that in quarter thickness. The numerical analyses also demonstrate that the local stress intensity factor at the arrested crack tip is changing sensitively to the crack front shape. It suggests that the higher brittle crack arrestability appears due to the split nail shape of the arrested crack front enhanced by the inhomogeneous toughness in thickness.

Brittle crack propagation/arrest behavior in T-joint structure of heavy gauge steel plate

Brittle crack arrestability is extremely important in welded joints of heavy gauge steel plates used in large container ships. Recently, much attention has been focused on potential crack propagation along welds fabricated by large heat input welding. This paper examines the application of a T-joint to the strength deck structure of container ships to enhance crack arrestability. The unwelded face and the crack arrest toughness, K ca, for crack arrest were varied. In the case of a T-joint with an unwelded face, the ESSO test of the T-joint components showed that a brittle crack arrested at the T-joint embedding the unwelded face. In the case of a full penetration T-joint, the brittle crack was arrested if the steel plate used for the flange had a high K ca value in the range from 4900 to 7300 N/mm3/2. FEM analysis of the stress intensity factor K indicated that brittle crack propagation was arrested under the condition that the K value at the running crack tip was less than the K ca of the material. In the T-joint, it was noted that the K value around the deepest point of the crack decreased and was finally less than the K ca of the flange plate when the brittle crack penetrated into the flange plate. This phenomenon shows the advantage of using a T-joint for brittle crack arrest in the flange plates of strength deck structures. In order to verify the long brittle crack arrestability of the T-joint structure, ultra-large-scale tests were carried out under the recommended conditions obtained from a dynamic FEM analysis. The test results showed that long brittle cracks could arrest at the aforementioned T-joint structures.

조선 및 해양플랜트 구조물의 불안전 파괴방지 설계기술

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. There has been the key issue of crack arrestability in large size structure such as container ship. In this report for the first time, crack arrest toughness of thick steel plate welds was evaluated by large scale ESSO test for estimate of brittle crack arrestability in thick steel plate. For large structures using thick steel plates, fracture toughness of welded joint is an important factor to obtain structural integrity. In general, there are two kinds of design concepts based on fracture toughness: crack initiation and crack arrest. So far, when steel structures such as buildings, bridges and ships were manufactured using thick steel plates (max. 80~100mm in thickness), they had to be designed in order to avoid crack initiation, especially in welded joint. However, crack arrest design has been considered as a second line of defense and applied to limited industries like pipelines and nuclear power plants. Although welded joint is the weakest part to brittle fracture, there are few results to investigate crack arrest toughness of welded joint. In this study, brittle crack arrest designs were developed for hatch side coaming of large container ships using arrest weld, hole, and insert technology.

鋼溶接継手における残留応力，靭性分布および溶接開先形状を考慮した三次元脆性き裂伝播経路予測モデル

In the present study, three-dimensional cleavage crack path prediction model is proposed. The model is based on the local fracture stress criterion in which three candidates for a cleavage plane in a computational element is assumed and the one which has maximum value of normal stress divided by local fracture stress is selected. In this model, normal stress acting on each plane is obtained by local stress intensity factor which is calculated by combination of approximate solutions where crack front non-straightness, crack surface irregularity and crack closing force acting at ridges between crack planes are considered. The model also incorporates multiaxial stress field and non-uniform toughness distribution. Therefore, this model is applied to welded joint of ESSO test where welding residual stress and non-uniform toughness distribution exist, and the predicted results of crack paths show a general agreement with the test results. Parametric study is also conducted in order to discuss the effects of applied stress, welding residual stress, toughness distribution and type of welding groove on brittle crack path behavior.

完全溶込みＴ継手における脆性き裂伝播停止挙動

Arrestability of brittle cracks is extremely important in the structural integrity of welded joints of heavy gauge steel plates used in large container ships. Recently, a great attention has been focused on potential crack propagation along welds made with large heat input.In this study, a T-joint is applied to the strength deck structure of container ships to enhance crack arrestability. Material crack arrest toughnesses, Kca, of some steel plates are varied. The ESSO tests of T-joint components showed that a brittle crack is arrested at the T-joint if the steel plate used in the flange has a high Kca value in the range from 4900 to 7300 N/mm3/2.FE-analyses of the stress intensity factor K indicated that brittle crack propagation is arrested under the condition that the K-value at the running crack tip is less than the Kca of the material. It is noted in the T-joint that the K-value at the deepest point of the crack decreases and is finally below the Kca of the flange plate when a brittle crack rapidly penetrates the flange plate to a depth of 30-40mm. These phenomena show the advantage of the T-joint in brittle crack arrestability in the flange plate of the strength deck structure.

Effect of distance of loading points on long brittle crack propagation/arrest behaviour

The effect of the distance between loading points on a long brittle crack arrest behaviour was investigated in the ultrawide duplex ESSO tests. The effect of loading conditions was also analysed. Arrest of a long brittle crack was more facilitated in the tests with a distance between loading points of 5 m than in those with 10 m. A load drop was larger in the tests with a loading point distance of 5 m than in those with 10 m. If the specimen length is 6·8 m or more, or the distance between loading points are 10 m or more in the ultrawide duplex ESSO test, the effect of the unloading stress wave on the dynamic stress intensity factor of a crack penetrating into the test plate becomes negligible, within the range of brittle crack propagation rate between 500 and 800 m s−1.

Brittle Crack Propagation/Arrest Behavior in T-Joint Structure of Heavy Gauge Steel Plates

Brittle crack arrestability is extremely important in welded joints of heavy gauge steel plates used in large container ships. Recently, much attention has been focused on potential crack propagation along welds using large heat input. This paper examines the application of a T-joint to the strength deck structure of container ships to enhance crack arrestability. The crack arrest toughness, Kca, for crack arrest was varied. The ESSO test of T-joint components showed that brittle crack was arrested at the T-joint if the steel plate used for the flange had a high Kca value in the range from 4900 to 7300N/mm3/2. FE-analysis of the stress intensity factor K indicated that brittle crack propagation was arrested under the condition that the K-value at the running crack tip was less than the Kca of the material. In the T-joint, it was noted that the K-value around the area of the deepest point of the crack decreased and was finally less than the Kca of the flange plate when the brittle crack penetrated suddenly into the flange plate to a 10mm depth. This phenomenon shows the advantage of using a T-joint for brittle crack arrest in the flange plates of strength deck structures.

集合組織による脆性き裂伝播停止靭性向上メカニズムに関する一考察

Fracture surface morphologies in ESSO test specimens were examined to investigate the enhancement of brittle crack arrest toughness due to texture. In the steels with high crack arrest toughness due to texture, many sub-cracks were detected on the fracture surface during fast crack propagation. The cross sectional observations of the fracture surfaces revealed that these sub-cracks on fracture surface resulted from crack-micro-branching. The mechanism of crack-micro-branching was also investigated from the viewpoint of the crystallographic orientation. FEM analyses were also conducted to examine the effect of crack-micro-branching on the stress intensity factor. The dominant effect of crack-micro-branching to reduce the fracture driving force during brittle crack propagation was discussed, comparing with the effect of macroscopic zigzag crack propagation.

Correlation between Charpy Transition Temperature and Brittle Crack Arrest Temperature Considering Texture

The correlation between ESSO test results and Charpy impact properties has been empirically proposed to qualify brittle crack arrest toughness, Kca of structural steels. The brittle crack arrest temperature TK6000 at Kca=6000N/mm3/2, conventionally used as a measure of crack arrest property, is interrelated generally with the Charpy transition temperature vTrs. However, ESSO test results occasionally deviates from the TK6000-vTrs correlation in some TMCP steels. Considering a thickness effect in ESSO test results, TK6000 was converted to a reference temperature 25TK6000 for 25mm thickness. Nevertheless, a scatter is found in 25TK6000-vTrs correlation.This study focuses on the effect of texture intensity on crack arrest properties. It is found that steel plates with texture show lower 25TK6000 than other steels without texture, even if the transition temperatures vTrs of these steels are on the same level. The role of the (211) texture is more significant than the (100) texture. A new parameter, vTrs (°C) -12X(100) -22X(211), is proposed in this study to correlate the crack arrest temperature 25TK6000 with the material properties of structural steels, where X(100) : pole density of (100) parallel to surface of steel plate, X(211): pole density of (211) parallel to surface of steel plate.