Takahiro Kubo

Study of residual stress reduction in welded joints using phase transformation behaviour of welding material. studies on numerical simulation of temperature, microstructure, and thermal stress histories during welding and their application to welded structures (2nd report)

Abstract The residual stress generated in welded structures is well-known to exert a strong influence on features such as brittle fracture,1 buckling,2 stress corrosion cracking,3 fatigue,4 etc. It is therefore important to develop appropriate measures for evaluation of the residual stress distribution at structural locations requiring prevention of deformation or fracture and for suitable reduction of residual stress in response to the weld configuration being used.

Application of WES2808 to Brittle Fracture Assessment for High Strength Gas Pipelines

This paper presents the results of a preliminary study to establish an assessing method for the tensile strain limit against brittle fracture of pressurized gas pipelines subjected to axial tensile deformation. The basis of the assessment method is the Japan Welding Engineering Society standard WES2808–2003. WES2808 provides a procedure for evaluating the fracture limit using the CTOD design curve relating flaw size, applied strain and fracture mechanical parameter (CTOD). The main characteristics of the method are a consideration of the deterioration of the fracture toughness of material resulting from large cyclic and dynamic straining, a correction of CTOD fracture toughness for constraint loss in structural components in large scale yielding, and an estimation of critical CTOD value from Charpy test results. Modifications of the procedure to enable evaluation of the fracture properties of high strength gas pipelines under biaxial loading conditions are studied.Copyright

Ductile Cracking Evaluation of X80/X100 High Strength Linepipes

The ductile cracking behavior of girth weld joints, in X80 and X100 grade linepipe, was investigated using single edge notched (SENT) specimens, notched round bar (NRB) specimens and wide plate (WP) specimens. FE analyses were carried out to evaluate critical conditions for ductile cracking at the notch tip. The effect of Y/T ratio of base material on ductile cracking for welded joints was also studied. Ductile cracking from the notch tip in WP specimens can be estimated by using the critical equivalent plastic strain, which can be obtained from SENT or NRB specimens. In addition, a simplified prediction method for ductile cracking by using effective opening displacement was proposed and its validity demonstrated by comparison to the equivalent plastic strain at the notch tip. With respect to the influence of material properties on ductile cracking behavior, deformability of joint to ductile cracking was enhanced by reduction of Y/T ratio of base material. Based on the experimental results and FE analyses, pipe design to prevent ductile cracking from surface flaws under large deformation was discussed.Copyright

Crack Geometry Effect on Stress-Strain Fields for Crack Under Biaxial Loading

With increasing demand of high-strength and high-pressure pipelines in gas transmission industries, the fracture control design of pipelines has been a driving factor to ensure the integrity of the pipeline. This paper addresses the stress and strain fields for a crack in a wide plate component under biaxial loading, which simulates a large-diameter pipe subjected to inner pressure coupled with axial loading. Attention is focused on the initiation of brittle fracture (stress controlled type) as well as ductile fracture (strain controlled type). Three-dimensional finite element-analyses are conducted. It was found that biaxial loading has a significant effect on the stress fields of through-thickness crack; the near-crack-tip stress is elevated to a large extent by biaxial loading. By contrast, the stress field for a surface crack is not sensitive to biaxial loading, while the near-crack-tip stress at the crack corner is increased locally by biaxial loading. The Weibull stress criterion was applied to discuss the biaxial loading effect on the brittle fracture strength of the wide plate. Ductile crack initiation properties are also discussed with two-parameter (plastic strain and stress triaxiality) diagram. The ductile damage is increased by biaxial loading for a through-thickness crack, whereas a surface crack has little effect of biaxial loading on the ductile damage.

Strain capacity and material properties required for pipelines

Natural gas pipelines have a tendency both to become longer and to operate at higher pressure so there has been a growing demand for higher strength line pipes1. In Japan, a construction programme for high pressure gas pipelines using API 5L X80 (hereafter referred to as X80) has been in progress. Furthermore, earthquakeresistant design has become an important area for pipeline safety investigation in Japan and, as part of this, safety assessment for local buckling has become a critical issue. In this study, a fundamental concept for high pressure gas conduit earthquake-resistant design guidelines was outlined and the compressive and bending deformation capacity required for pipelines is presented. Furthermore, the relationship between steel pipe deformation capacity and material tensile properties is outlined and the importance of material design of steel pipe is demonstrated to improve pipeline deformation capacity.

Effect of Transformation Temperature of Weld Metal on Welding Distortion

Welding distortion during welding is an inevitable outcome of weld metal shrinkage due to non-uniform heating and cooling in welded joints. Several studies have investigated methods of estimating and controlling welding distortion in order to construct steel structures correctly, easily and efficiently. In the present study, the effect of transformation expansion of the weld metal on welding distortion was investigated in order to develop a welding wire that reduces welding distortion. Concretely, experiments and numerical simulations were performed on the effect of the transformation temperature of the weld metal on angular distortion of a horizontal fillet welded joint. The results showed that angular distortion of the welded joint decreases when the martensitic transformation start temperature of the weld metal decreases in the range from 350°C to 450°C. Based on this result, a welding wire that is effective in reducing welding angular distortion was developed considering usability, weldability and the mechanical properties of welded joints.Copyright

Fracture Assessment of Welded Joint With Geometrical Discontinuity Using the 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.© 2006 ASME

Ductile Fracture Evaluation of Welded Joints With a Surface Flaw Under Large Deformation

Tensile tests using welded joint specimens with a surface flaw were conducted in order to study the large deformation behavior and ductile fracture of welded structural component with a flaw. Two types of 490MPa class steel plates with different yield strength are prepared for this study. The surface flaw was introduced at the center of welded metal of welded joint specimen. Strain distribution during loading and the maximum strain at ductile fracture of specimen were measured in order to investigate the influence of stress-strain properties on ductile fracture behavior of the welded joint. Three dimensional elastoplastic finite element analyses were also carried out by using the welded joint specimen models in order to calculate the detailed stress and strain distributions around notch tip. In addition, a simplified analytical model to estimate fracture strain and critical flaw size of the welded joint was proposed using the stress-strain relationship combination between base material and welded metal by Swift’s equation. The results from the analytical model show fairly good agreement with experimental results.Copyright

Residual Stress Redcution by Using Weld Metal With Property of Low-Temperature Phase Transformation

Residual stress in welded joints by using a new weld metal with the property of low-temperature phase transformation is numerically analyzed. 10 % Nickel and 10 % Chromium are involved in the developed welding material for producing the property of martensitic phase transformation at a low temperature and for generating compressive residual stress. A fillet-welded joint between plate and stiffener is used for the numerical simulation of the thermal elastic-plastic finite-element analysis with coupling phase transformation effect. Moving heat source is considered by using the gradual deposition of the finite-element during welding. Distribution of the computed residual stress mostly agrees with the measuring values by strain gauge. Compressive residual stress distributes in the weld metal for both longitudinal and transverse directions with weld line. The effects of the material of weld metal and welding pass sequence on residual stress are investigated. Residual stresses on the weld toe and root are improved lower by using the low-temperature transformation weld wire than the conventional one, regardless of the sequence of welding deposition. It is found that the weld metal with property of low-temperature phase transformation is effective to reduce residual stress near weld metal.Copyright

Study on Unstable Brittle Crack Arrest Toughness of Extremely-Low Carbon Bainitic Steel Plates

The extremely-low carbon bainitic steel (ELCB steel) is a high strength steel with about 0.02 mass% or less carbon. In this research, unstable brittle crack arrest toughness of ELCB steel plates was investigated by temperature-gradient ESSO tests, compared with that of conventional TMCP steel plates. Both of ELCB and TMCP steel plates without pre-straining had sufficient crack-arrest toughness at 0°C. After 10% prestraining, the TMCP steel plate had not sufficient crack-arrest toughness at 0 °C . The ELCB steel plates, however, maintained high crack arrest toughness at 0°C. even after 10% pre-straining. ELCB steel were also different from TMCP steels in the correlation between transition temperature of crack arrest toughness and fracture appearance transition temperature (vTrs) obtained by Charpy impact test. When the vTrs of an ELCB steel and that of a TMCP steel were the same value, crack arrest toughness of an ELCB steel was higher than that of a TMCP steel. In the cross section of the ESSO test piece of the ELCB steels, many sub-cracks and micro-crack branching were observed. However, in the cross section of the ESSO test piece of the conventional TMCP steels, there were few subcracks and branching. Initiation of sub-cracks and branching around the main crack tip reduces the stress intensity factor of the main crack. It was considered that the above features of the ELCB steel were caused by initiation of sub-cracks and branching at the tip of the main brittle crack.Copyright