Evaluation of Residual Stress Effects on Brittle Fracture Strength Based on Weibull Stress Criterion

A constraint loss correction procedure using the Weibull stress criterion is specified in ISO 27306. However, this standard is applicable only to structural steel components with defects, not to welded joints. Therefore, we propose a method for estimating the brittle fracture limit of a weld with a notch in the heat-affected zone (HAZ) and residual stress based on the Weibull stress criterion. Three-point bending (3PB) tests and wide-plate (WP) tension tests of HAZ-notched welds made of 780-MPa class high-strength steel were conducted at −40 °C. The minimum critical crack tip opening displacement (CTOD) of the WP specimen fracturing at the coarse-grained region of the HAZ (CGHAZ) was approximately four times that of the 3PB specimen. Then, the effects of specimen geometry, residual stress, crack-front shape, and HAZ microstructure classification on the Weibull stress were investigated by using a finite element analysis (FEA). The results of these analyses showed that the specimen geometry, the welding residual stress, and HAZ microstructure affect the Weibull stress of HAZ-notched welds as crack driving force. Based on above results, the CTOD–Weibull stress curves for 3PB and WP specimens fracturing at CGHAZ were calculated by using an FEA. It was confirmed that the brittle fracture limit of an HAZ-notched weld with residual stress could be predicted from the Weibull stress criterion because predicted critical CTOD of WP specimens obtained by Weibull stress included experimental critical CTOD of WP specimens.

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Application of Weibull Stress Criterion to Brittle Fracture Assessment of HAZ-Notched Welds With Residual Stress

Volume 3: Materials and Joining; Risk and Reliability ◽

10.1115/ipc2014-33731 ◽

2014 ◽

Author(s):

Yusuke Seko ◽

Yasuhito Imai ◽

Masaki Mitsuya ◽

Noritake Oguchi ◽

Fumiyoshi Minami

Keyword(s):

Residual Stress ◽

Finite Element ◽

Brittle Fracture ◽

Crack Front ◽

Welding Residual Stress ◽

Specimen Geometry ◽

Stress Criterion ◽

Front Shape ◽

The Difference ◽

Weibull Stress

This paper presents a method for estimating the brittle fracture limit of a weld with a notch in the heat-affected zone (HAZ) and residual stress based on the Weibull stress criterion. A constraint loss correction procedure using the Weibull stress criterion is specified in ISO 27306. However, this standard is applicable only to structural steel components with defects, not to welded joints. Therefore, we conducted fracture tests and finite element analyses to propose a new evaluation method for welded structural components. In this study, three-point bending (3PB) tests and wide-plate (WP) tension tests of HAZ-notched welds made of 780-MPa-class high-strength steel were conducted at −40°C. Brittle fractures occurred in the HAZ regions of all the specimens, and the critical crack tip opening displacement (CTOD) values obtained in the 3PB and WP tests were approximately 0.02–0.07 mm and 0.08–0.11 mm, respectively. The minimum critical CTOD of the WP specimen fracturing at the coarse-grained region of the HAZ (CGHAZ) was approximately four times that of the 3PB specimen. These results confirmed that the difference of specimen geometry affects the brittle fracture resistance of a HAZ-notched weld with residual stress. Hence, the assessment of the brittle fracture limit of a welded structural component with a defect obtained by the fracture toughness of a 3PB specimen would be excessively conservative. The effects of specimen geometry, residual stress, crack-front shape and HAZ microstructure classification on the Weibull stress were investigated to clarify the difference of experimental critical CTOD for 3PB and WP by using a finite element analysis. The results of this analysis showed that the Weibull stress of WP specimen was larger than one of 3PB specimen in all CTOD region due to difference of geometry. The welding residual stress increased the Weibull stress only for WP. Compressive residual stress and crack front shape for 3PB specimen did not affect the Weibull stress. The difference of HAZ microstructure distribution for same welded joint affects the Weibull stress for 3PB and WP specimens. Finally, it was confirmed that the brittle fracture limit of a HAZ-notched weld with residual stress could be predicted from the Weibull stress criterion because critical CTOD of WP specimens predicted by critical CTOD of 3PB specimens fracturing at the CGHAZ included critical CTOD of WP specimens obtained by experiments.

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Fracture Assessment of Structural Component With Residual Stress on the Basis of the Weibull Stress Criterion

Volume 1: Codes and Standards ◽

10.1115/pvp2008-61536 ◽

2008 ◽

Author(s):

Yoichi Yamashita ◽

Fumiyoshi Minami

Keyword(s):

Fracture Toughness ◽

Residual Stress ◽

Structural Component ◽

Welding Residual Stress ◽

Fracture Toughness Test ◽

Stress Criterion ◽

Conventional Procedure ◽

Calculation Results ◽

Fracture Assessment ◽

Weibull Stress

This paper studies the method for estimating the residual stress effects on brittle fracture of structural component based on the Weibull stress criterion. Experiments show that the critical CTOD and the critical load of wide plate with welding residual stress are apparently smaller than those of wide plate without residual stress. It has been found that the critical CTODs of wide plate with and without residual stress can be predicted from the 3PB fracture toughness test results based on the Weibull stress criterion. Constraint loss effects on CTOD of wide plate with residual stress can be assessed by the equivalent CTOD ratio. The equivalent CTOD ratio β is defined as the ratio, β = δ/δWP, where δ and δWP, are CTODs of the standard fracture toughness specimen and wide plate, respectively, at the same level of the Weibull stress. Calculation results of beta are also shown for various residual stress levels and crack lengh based on the Weibull stress criterion. Fracture assessment results using β are shown within the context of CTOD design curve. An excessive conservatism observed in the conventional procedure is reasonably reduced by applying the equivalent CTOD ratio, β.

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Fracture Assessment of Welded Joint With Geometrical Discontinuity Using the Weibull Stress

Volume 7: Operations, Applications, and Components ◽

10.1115/pvp2006-icpvt-11-94047 ◽

2006 ◽

Cited By ~ 1

Author(s):

Satoshi Igi ◽

Takahiro Kubo ◽

Masayoshi Kurihara ◽

Fumiyoshi Minami

Keyword(s):

Fracture Toughness ◽

Brittle Fracture ◽

Welded Joint ◽

Fracture Toughness Test ◽

Geometrical Condition ◽

Stress Criterion ◽

Toughness Test ◽

Fracture Assessment ◽

Geometrical Discontinuity ◽

Weibull Stress

Recently the Weibull stress is used as a fracture driving force parameter in fracture assessment. The Weibull stress is derived from a statistical analysis of local instability of micro cracks leading to brittle fracture initiation. The critical Weibull stress is expected to be a critical parameter independent of the geometrical condition of specimens. Fracture toughness test using 3-point bending and tensile tests of welded joint specimens with geometrical discontinuity were conducted in order to study the applicability of fracture assessment procedure based on Weibull stress criterion. Steel plates prepared for this study had tensile strength of 490 MPa for structural use. Two kinds of welded joint specimens, “one-bead welded joint” and “multi-pass welded joint” were prepared for fracture toughness test by using gas metal are welding. In tensile test specimen, corner flaws were introduced at the geometrical discontinuity part at where stress concentration is existed. Three dimensional elastoplastic finite element analyses were also carried out using the welded joint specimen models in order to calculate the Weibull stress. The critical loads for brittle fracture predicted by the Weibull stress criterion from CTOD test results of one-bead and multi-pass welded joint specimens show fairly good agreement with experimental results of welded joint specimens with geometrical discontinuity.

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

Materials Science Forum ◽

10.4028/www.scientific.net/msf.638-642.3931 ◽

2010 ◽

Vol 638-642 ◽

pp. 3931-3936 ◽

Cited By ~ 1

Author(s):

Yoichi Yamashita ◽

Fumiyoshi Minami

Keyword(s):

Fracture Toughness ◽

Residual Stress ◽

Plastic Zone Size ◽

Assessment Method ◽

Welding Residual Stress ◽

Residual Stress Field ◽

Stress Criterion ◽

Stress Effects ◽

Ductile Brittle Transition Temperature ◽

Size Criterion

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.

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