Masaki Mitsuya

Application of Weibull Stress Criterion to Brittle Fracture Assessment of Heat-Affected Zone-Notched Welds With Residual Stress

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.

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

Beam-Mode Buckling of Buried Pipeline Subjected to Seismic Ground Motion

During seismic events, buried pipelines are subjected to deformation by seismic ground motion. In such cases, it is important to ensure the integrity of the pipeline. Both beam-mode and shell-mode buckling may occur in the event of compressive loading induced by seismic ground motion. In this study, the beam-mode buckling of a buried pipeline that occurred after the 2007 Niigataken Chuetsu-oki earthquake in Japan is investigated. A simple formula for estimating the critical strain, which is the strain at the peak load, is derived, and the formula is validated by finite-element analysis. In the formula, the critical strain increases with the pipeline diameter and hardness of the surrounding soil. By comparing the critical strain derived in this study for beam-mode buckling with the critical strain derived in a past study for shell-mode buckling, the formula facilitates the selection of the mode to be considered for evaluating the earthquake resistance of a pipeline. In addition to the critical strain, a method to estimate the deformation caused by seismic ground motion is proposed; the method can be used to evaluate the earthquake resistance of buried pipelines. This method uses finite-element analyses, and the soil–pipe interaction is considered. This method is used to reproduce the actual beam-mode buckling observed after the Niigataken Chuetsu-oki earthquake, and the earthquake resistance of a buried pipeline with general properties is evaluated as an example.Copyright

Full Gas Burst Test for HFW Linepipe at Low Temperature

A full gas burst test at low temperature below −40°C was performed using a high frequency welded (HFW) linepipe with high-quality weld seam, “MightySeam®,” [1–4] in order to verify the applicability of the Drop Weight Tear Test (DWTT). Residual stress exists in the pipe body of HFW linepipe because the manufacturing method includes a sizing process. Therefore, it is necessary to clarify the difference between the arrestability in the DWTT without residual stress in the specimen and that in the full gas burst test with residual stress in the pipe body.The full gas burst test is performed using a test pipe specimen in which a notch is introduced into the base material by an explosive cutter. In addition, a test pipe specimen with a notch introduced into the weld seam was used in this study because the developed HFW linepipe, “MightySeam®,” has excellent low-temperature toughness as a result of control of the morphology and distribution of oxides generated in the welding process by temperature and deformation distribution control. The Charpy transition temperature of “Mighty Seam®” was much lower than −45 °C.Ductile cracks were initiated from the initial explosive notch, and these cracks were arrested after ductile crack propagation of about 1 m in base material on both sides. The fracture behavior was similar in appearance in the DWTT without residual stress and the full gas burst test with residual stress.Copyright

Cyclic Deformation Behavior and Buckling of Pipeline With Local Metal Loss in Response to Axial Seismic Loading

Buried pipelines may be corroded, despite the use of corrosion control measures such as protective coatings and cathodic protection, and buried pipelines may be deformed due to earthquakes. Therefore, it is necessary to ensure the integrity of such corroded pipelines against earthquakes. This study has developed a method to evaluate earthquake resistance of corroded pipelines subjected to seismic motions. Pipes were subjected to artificial local metal loss and axial cyclic loading tests to clarify their cyclic deformation behavior until buckling occurred under seismic motion. As the cyclic loading progressed, displacement shifted to the compression side due to the formation of a bulge. The pipe buckled after several cycles. To evaluate the earthquake resistance of different pipelines with varying degrees of local metal loss, a finite-element analysis method was developed that simulates cyclic deformation behavior. A combination of kinematic and isotropic hardening was used to model the material properties. The associated material parameters were obtained by small specimen tests that consisted of a monotonic tensile test and a low-cycle fatigue test under a specific strain amplitude. This method enabled the successful prediction of cyclic deformation behavior, including the number of cycles required for the buckling of pipes with varying degrees of metal loss. [DOI: 10.1115/1.4024451]

Cyclic Deformation and Buckling Behavior of Pipe With Local Metal Loss Subjected to Seismic Ground Motion

Buried pipelines may be deformed due to earthquakes and also corrode despite corrosion control measures such as protective coatings and cathodic protection. In such cases, it is necessary to ensure the integrity of the corroded pipelines against earthquakes. This study developed a method to evaluate the earthquake resistance of corroded pipelines subjected to seismic ground motions. Axial cyclic loading experiments were carried out on line pipes subjected to seismic motion to clarify the cyclic deformation behavior until buckling occurs. The test pipes were machined so that each one would have a different degree of local metal loss. As the cyclic loading progressed, displacement shifted to the compression side due to the formation of a bulge. The pipe buckled after several cycles. To evaluate the earthquake resistance of different pipelines, with varying degrees of local metal loss, a finite-element analysis method was developed that simulates the cyclic deformation behavior. A combination of kinematic and isotropic hardening components was used to model the material properties. These components were obtained from small specimen tests that consisted of a monotonic tensile test and a low cycle fatigue test under a specific strain amplitude. This method enabled the successful prediction of the cyclic deformation behavior, including the number of cycles required for the buckling of pipes with varying degrees of metal loss. In addition, the effect of each dimension (depth, longitudinal length and circumferential width) of local metal loss on the cyclic buckling was studied. Furthermore, the kinematic hardening component was investigated for the different materials by the low cycle fatigue tests. The kinematic hardening components could be regarded as the same for all the materials when using this component as the material property for the finite-element analyses simulating the cyclic deformation behavior. This indicates that the cyclic deformation behavior of various line pipes can be evaluated only based on their respective tensile properties and common kinematic hardening component.Copyright