Equivalent CTOD Research Articles

Fracture assessment procedure developed in Japan for steel structures under seismic conditions

The Engineering Standard, WES 2808, has been developed in Japan for assessing the brittle fracture of steel-framed components at the earthquake. This paper describes the key contents of WES 2808 and demonstrates the application to beam-to-column connections. WES 2808 includes two unique ideas: (1) a reference temperature concept for the evaluation of the material fracture toughness under cyclic and dynamic loading, and (2) an equivalent CTOD concept for the correction of CTOD toughness for constraint loss in structural components. A skeleton strain is employed to define the pre-strain prior to fracture during cyclic loading. The revision of WES 2808 has been done in 2017 to expand the range of use and to improve the accuracy of CTOD toughness correction on the basis of ISO 27306-2016. It is shown that fracture strains of beam-to-column subassemblies predicted by WES 2808 are in a good agreement with those measured.

Revision of ISO 27306 for CTOD Toughness Correction for Constraint Loss

As the result of the international standardization work in Japanese IST project, ISO 27306 were published in 2009 for correction of CTOD fracture toughness for constraint loss in steel components. ISO 27306 employs an equivalent CTOD ratio based on the Weibull stress criterion, which leads to more accurate fracture assessment than the conventional fracture mechanics assessment. On the occasion of the 1st periodical review, the revision of ISO 27306 has been proposed from Japan. This paper describes the key contents of the new ISO 27306. A case study is included on the fracture assessment of a wide plate component according to FAD (failure assessment diagram) approach specified in BS 7910:2013.

WES 2808 for Brittle Fracture Assessment of Steel Components under Seismic Conditions – Part V: Equivalent CTOD ratio for correction of constraint loss in beam-to-column connections

Crack-tip plastic constraint in beam-to-column connections with a surface crack 1) at the bottom of a conventional type of weld access hole and 2) at weld start/end points of butt welds to connect beam flange and diaphragm subjected to bending moment were analyzed by FEM. The equivalent CTOD ratio β used for engineering toughness correction for constraint loss for the beam-to-column connections are compared with those for the wide plate components with the same size of a crack subjected to tension load. Then, the analyses of β by means of the wide plate component, CSCP and ESCP, are conducted, and the crack depth effect on β is formulated with focusing on crack depth ratio a/t.

WES 2808 for Brittle Fracture Assessment of Steel Components under Seismic Conditions – Part VI: Application of WES 2808 to Beam-to-column Connections

This paper describes application of revised fracture assessment method specified in WES 2808 to full-scale models of beam-to-column connections subjected to cyclic and dynamic loading. Fracture tests of full-scale models of beam-to-column connections were carried out (Morita, 1999, and Yoshimura, 2006). Cyclic loading tests were conducted, which included dynamic loading tests. Brittle fracture occurred from weld defects (lack of fusion or weld crack) or artificial defects in beam-to-column connections. In accordance with WES 2808, the fracture strain at the beam end in the connection was evaluated. Fracture toughness of a local area including fracture initiation site was replaced by the static fracture toughness at reference temperature defined by WES 2808. The equivalent CTOD ratio β was employed to correct the constraint loss in structural components on the basis of the Weibull stress criterion. The fracture performance of full-scale models of beam-to-column connections evaluated by the procedure specified in WES 2808 was almost consistent with the experimental results.

WES 2808 for Brittle Fracture Assessment of Steel Components under Seismic Conditions – Part I: Fracture Assessment Procedure

The Welding Engineering Standard, WES 2808, has been developed in the Japan Welding Engineering Society (JWES) for assessing the brittle fracture in steel components under seismic conditions. WES 2808 includes two unique ideas: 1) a reference temperature concept for the evaluation of the material fracture toughness under cyclic and dynamic loading, and 2) an equivalent CTOD concept for the correction of CTOD toughness for constraint loss in structural components. The CTOD design curve is employed for the assessment of the crack driving force of components. The revision of WES 2808 is in progress in JWES to expand the range of use and to improve the fracture assessment procedure. This paper describes the key contents of WES 2808.

Implementation of Weibull Stress to Fracture Assessment Standards

This paper describes the fracture assessment standards developed in Japan. The outline of ISO 27306 is given, where an equivalent CTOD concept is standardized at the same Weibull stress level for the fracture toughness specimen and tension components. The equivalent CTOD is implemented into WES 2808 for assessing the fracture performance of beam-to-column connections subjected to large dynamic and cyclic strain during earthquake.

Equivalent CTOD Ratio β for Engineering Assessment of CTOD Correction for Constraint Loss

The critical CTOD δWP for structural components associated with plastic constraint loss in case of the brittle fracture over small-scale yielding condition can be corrected from CTOD fracture toughness δ by means of the “equivalent CTOD ratio β” defined as δ/δWP, which is based on the Weibull stress criterion. In this study, taking the case of specific wide plate components subjected to uni-axial tensile load, the effect on β is analyzed taking account of Weibull shape parameter m, loading level, constraint effect that can be influenced by material work-hardening and crack type/size in structural components, etc., and volumetric effect. It is found that the β-value is almost constant beyond the applied CTOD level that is lower than CTOD of small-scale yielding limit (SSY-limit) for 25 mm thick toughness specimen. From an engineering point of view, the β-value at the CTOD level of 0.01 mm is used in the whole loading range beyond SSY-limit CTOD, which provides to some extent conservative measure of fracture toughness of structural components. The defined β is found to decrease with increasing Weibull shape parameter m and yield-to-tensile ratio YR of steel for all type of wide plates concerned. The crack length effect on β is quasi-theoretically formulated, which can convert the β for the wide plate with reference crack size to β for target crack size. These β and quasi-theoretical equations for the correction of crack size effect can be utilized for estimating the CTOD for wide plate in consideration of constraint loss effect without numerical calculation of the Weibull stress.

Equivalent CTOD Concept for Correction of CTOD Toughness for Constraint Loss in Steel Weld Components

This paper presents a new fracture assessment method, IST method developed as ISO 27306. The IST method implements an equivalent CTOD ratio, β, for the CTOD toughness correction for constraint loss in structural components. Using β, the standard fracture toughness specimen and structural components are linked at the same level of the Weibull stress. This paper extends the equivalent CTOD concept to weld components. Effects of the weld strength mismatch and residual stress on β are discussed. It is shown on the failure assessment diagram (FAD) that the CTOD toughness correction with β leads to accurate fracture assessments of weld panels, whereas the conventional procedure gives much conservative results.

Application of Equivalent CTOD Ratio to Fracture Assessment of Structural Components

Plastic constraint correction using the equivalent CTOD concept has been studied in the International Standardization of Fracture Toughness Evaluation Procedure for Fracture Assessment of Steel Structures project. This project was carried out over a 3-year period with the foundation of the Japanese Ministry of Economy, Trade, and Industry, and the results were summarized in a draft standard “Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel component.” Equivalent CTOD ratio β is proposed in terms of the constraint loss correction between the fracture performance of steel structures and fracture toughness tests using the Weibull stress as the driving force of the brittle fracture. This paper provides the application procedure of equivalent CTOD ratio to the fracture assessment of structural components. Equivalent CTOD ratio was taken in the failure assessment diagram, and discussed the applicability by comparison with large scale test results of structural component such as edge surface crack panel, center through-wall crack panel, and other geometries.

Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress – Part II: Application of toughness correction methodology

Abstract This part II paper presents the verified results of the toughness correction methodology for welded joints of wide plates. The equivalent CTOD ratio, βr, is applied to the fracture data of the welded joints from lower to upper ductile–brittle transition temperature region. In the part I paper, βr is defined as the ratio of CTOD in the standard fracture toughness specimen to CTOD in the wide plate with welding residual stress at the same level of the Weibull stress. In this part II paper, the equivalent CTOD ratio, βr, under the welding residual stress field has been verified for assessment of constraint loss effects on CTOD fracture toughness of wide plate. Fracture assessments have been conducted by applying the methodology for “After Weld Notch” and “Before Weld Notch” type welded joints. It has been found that an excessive conservatism observed in the conventional procedure is reasonably reduced by applying the proposed methodology.

Method of Constraint Loss Correction for CTOD Fracture Toughness under Welding Residual Stress Field

This paper studied the assessment method for welding residual stress effects and constraint loss effects on brittle fracture of structural component subjected to membrane stress. The methodology of CTOD fracture toughness correction for welded joints is proposed from lower to upper ductile-brittle transition temperature region. The methodology is based on the tensile plastic zone size criterion and the equivalent CTOD ratio derived from the Weibull stress criterion. It has been found that the proposed methodology has given the reasonable fracture assessment results.

Significance of Strength Mismatch in Fracture Performance Evaluation of Welded Joints — Characterization of Mismatch Effect by the Weibull Stress Criterion

The effect of strength mismatch in welds on the fracture driving force of the heat-affected zone (HAZ) is investigated by 3-dimensional FE-analysis. The Weibull stress is used for the evaluation of the fracture driving force. The CTOD ratio, γ = δMismatch /δMatch, is proposed to quantify the mismatch effect, where δMismatch and δMatch are CTODs of the mismatched joint and the matched joint, respectively, at the same level of the Weibull stress. The CTOD ratio γ is decreased by strength overmatching, which is due to the constraint effect of the overmatch WM on the stress fields in the HAZ. The equivalent CTOD ratio, β = δ/δWP, is employed to correct the constraint loss in the structural component, where δ and δWP are CTODs of the fracture toughness specimen and the wide plate component, respectively, at the same level of the Weibull stress. The analysis using β and γ indicates that strength overmatching leads to a severe requirement of the HAZ toughness, which cannot be directly derived from the CTOD — remote strain relationship for the wide plate joint. By contrast, the toughness requirement of the WM is relaxed by strength overmatching, as a result of the shielding effect of the overmatch WM.

延性脆性遷移温度領域における溶接残留応力場中のき裂の脆性破壊発生評価法

This paper presents the methodology of the CTOD fracture toughness correction for wide plates' welded joints from lower to upper ductile-brittle transition temperature region. Tensile plastic zone size criterion is adopted in lower transition region. The equivalent CTOD ratio, β r , is adopted in ductile-brittle transition and upper transition region. The equivalent CTOD ratio is defined as the ratio of CTOD in a standard fracture toughness specimen to CTOD in a wide plate with welding residual stress at the same level of the Weibull stress. Fracture assessments for After Weld Notch and Before Weld Notch type welded joints have been conducted by applying the toughness correction methodology. It has been found that an excessive conservatism observed in the conventional procedure is reasonably reduced by applying the proposed methodology.

溶接残留応力場中のき裂からの脆性破壊発生評価におけるワイブル応力仮説による塑性拘束補正方法

This paper studies the method for estimating the welding residual stress effect on brittle fracture of structural steel based on the Weibull stress criterion. Using wide plate (WP) specimens with and without welding residual stress, the brittle fracture tests have been conducted at -75°C. The material used is a high strength steel of 780MPa class. Welding residual stress significantly decreases the fracture force and the critical CTOD of the wide plate with welding residual stress at the onset of brittle fracture initiation. And it has been shown that the critical CTOD of the WP specimens can be predicted from fracture test results of the 3-point bend specimens without residual stress based on the Weibull stress criterion. The present paper proposes the assessment method of constraint loss effects on CTOD of wide plate with residual stress based on the equivalent CTOD ratio, βr under residual stress field. The equivalent CTOD ratio, βr is defined as the ratio, δ/δrWP,whereδandδrWPare CTODs of the standard fracture toughness specimen and wide plate with welding residual stress, respectively, at the same level of the Weibull stress.Fracture assessment results using βr are shown within the framework of failure assessment diagram (FAD). An excessive conservatism observed in the conventional procedure is reasonably reduced by applying the equivalent CTOD ratio, βr.

Fracture Mechanics Assessment of Beam-to-Column Joints Subjected to Cyclic and Dynamic Loading

With a lesson learned from the Kobe Great Earthquake, an engineering method, WES 2808, has been developed in Japan for evaluating the structural integrity under the seismic condition. This paper presents a method for fracture mechanics assessment of beam-to-column joints subjected to cyclic and dynamic loading on the basis of WES 2808. WES 2808 includes two key ideas. One is a reference temperature concept for fracture toughness evaluation. The material fracture toughness under the seismic condition is replaced by the static fracture toughness without pre-strain at a reference temperature of T−ΔT pd, where T and ΔT pd are the service temperature and the temperature shift of the fracture toughness by pre-strain and dynamic loading, respectively. ΔT pd is given as a function of the flow stress elevation in the seismic condition. Another idea is a correction of constraint loss in structural components. A large amount of plastic deformation causes a loss of plastic constraint in structural components. In order to account for the constraint loss in the fracture assessment of components, the equivalent CTOD ratio β = δ3p / δStruc. is implemented, where δ3p and δStruc. are CTODs of the fracture toughness specimen and the structural component, respectively, at the same level of the Weibull stress.

Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components

This paper presents a procedure for transferring the CTOD fracture toughness obtained from laboratory specimens to an equivalent CTOD for structural components, taking constraint loss into account. The Weibull stress criterion is applied to correct the CTOD for constraint loss, which leads to an equivalent CTOD ratio, β, defined as β = δ/ δ WP, where δ and δ WP are CTODs of the standard fracture toughness specimen and the structural component, respectively, at the same level of the Weibull stress. The CTOD ratio β is intended to apply to the fracture assessment of ferritic steel components to stress levels beyond small-scale yielding. Nomographs are given to determine the β-value as a function of the crack type and size in the component, the yield-to-tensile ratio of the material and the Weibull shape parameter m. Examples of the fracture assessment using β are shown within the context of a failure assessment diagram (FAD). An excessive conservatism observed in the conventional procedure is reduced reasonably by applying the equivalent CTOD ratio, β.

Constraint Correction of Fracture Toughness CTOD for Fracture Performance Evaluation of Structural Components

Abstract A correction of CTOD for constraint loss in large-scale yielding conditions is made on the basis of the Weibull stress fracture criterion that eliminates an excessive conservatism in the conventional fracture assessment and material fracture toughness requirement. A CTOD ratio β = δ3P / δWP (<1) is proposed, where δWP is the CTOD of a wide plate component and δ3P is an equivalent CTOD of the fracture toughness specimen at which the toughness specimen gives a compatible Weibull stress with the wide plate. The CTOD ratio β is decreased to a large extent after full yielding of the wide plate component, which is more significant for a high yield ratio YR and a deep surface crack in the wide plate. The Weibull modulus m exerts a marginal influence on β. A case study is presented on the application of the CTOD ratio β to the fracture performance assessment of full-scale column-to-beam connections subjected to cyclic and dynamic loading.

Equivalent CTOD Concept Based on the Local Approach and Its Application to Fracture Performance Evaluation of Welded Joints

In this study, an equivalent CTOD concept was proposed on the basis of the local approach. This concept was born to consider the transferability of 3-point bend CTOD results to fracture performance evaluation of structural components. The equivalent CTOD was defined as the CTOD at which the 3 -point bend specimen and the structural component provide the compatible Weibull stress. The fracture performance of a wide plate with a surface notch predicted by the equivalent CTOD concept was consistent with the experimental result. On the other hand, the analysis based on the conventional CTOD concept gave very conservative estimation of the fracture performance of the wide plate.A new procedure for fracture toughness requirement was presented based on the equivalent CTOD concept. It was pointed out that the equivalent bend CTODδR3P to meet the required deformability of the wide plate was much smaller than the required CTOD δRWPfor the wide plate. The equivalent CTOD concept was effective to quantify the strength mis-match effect between the base and weld metals on the required fracture toughness of the weld metals. The required weld metal CTOD δδRWP for the wide plate increased with decreasing the yield strength mis-match ratio σWY/σBY between the base and weld metals. By contrast, the required bend CTOD δR3Pof the weld metal was not always affected very much by the mis-match condition in welds.