Hiroyuki Motohashi

Effect of Strength Matching and Strain Hardening Capacity on Fracture Performance of X80 Line Pipe Girth Welded Joint Subjected to Uniaxial Tensile Loading

By conducting curved wide plate tensile tests for girth welded joints of X80 line pipe containing a surface notch in the weld metal, the effects of strength matching on fracture performance were evaluated. Parametric studies were also conducted using a finite element method simulating the experiments to clarify the effects of strain hardening capacity of the base metal, softening in the heat affected zone, and groove configuration on fracture performance. A strain at failure significantly decreased with the decreasing strength matching. This was expected to be due to a difference in local straining behavior at the notch tip caused by the shielding effect. The analytical studies revealed that the strain hardening capacity of the base metal, the softening in the heat affected zone, and the groove configuration affected the allowable strain for a given toughness level in the case of overmatching. However, these factors hardly affected the allowable strain in the case of undermatching.

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

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]

Effect of Strength Matching on Ductile Fracture Behavior of X80 Linepipe Girth Welds Subjected to Tensile Loading

This paper describes the effects of yield stress matching on the ductile fracture behavior of X80 linepipe girth welds. Three girth welded joints were made on an X80 linepipe using several consumables to obtain about a 20% yield stress overmatched, evenmatched and about a 20% undermatched weld metal. For these three welded joints, curved wide plate tensile tests were then conducted with a surface notch in the weld metal and the heat affected zone. The yield stress matching significantly affected fracture behavior and the ductile crack initiation, that is, the overmatched welded joint had a higher resistance to ductile fracture than that of the undermatched welded joint for both the weld metal and HAZ notch. The ductile crack initiation was successfully detected using a direct-current electric potential method. The allowable strength matching level could be determined using the relationship between the strength matching and the gauge length strain from the viewpoint of preventing the ductile crack initiation.