Satoshi Kanno

Fracture behavior of carbon steel pipe with local wall thinning subjected to bending load

To evaluate the structural integrity of power plant piping, monotonic bending tests are conducted on 4- and 3.5-in. diameter full-scale carbon steel pipe specimens with local wall thinning. The local wall thinning is simulated as erosion/corrosion metal loss. The eroded area of the wall thinning is subjected to tensile or compressive stress by applied bending moment. The deformations or fracture behaviors at maximum moments are found to be classified into three types. When the eroded area is subjected to tensile stress, ovalization or crack initiation/growth occurs at the maximum moment. When an eroded area is subjected to compressive stress, ovalization or local buckling occurs. The occurrence of ovalization, crack initiation/growth, or local buckling depends on the initial size of local wall thinning. From the relationships among ovalization, crack growth and local buckling, allowable sizes for local wall thinning are proposed.

Fracture and general yield for carbon steel pipes with local wall thinning

Abstract Monotonic bending tests without internal pressure are conducted on 4 and 3.5 in. diameter full-scale carbon steel pipe specimens with local wall thinning in order to evaluate the structural integrity of power plant piping. The local wall thinning is simulated as erosion/corrosion metal loss. The eroded area of the wall thinning is subjected to tensile or compressive stress by applied bending moment. The maximum moments obtained from the tests are compared with the plastic collapse moments based on the net-section stress approach. The net-section stress approach based on flow stress σ f gives a conservative estimation, sometimes overly conservative. Although the net-section approach based on ultimate tensile strength σ u gives a slightly non-conservative estimation for some cases, the calculated values are close to the experimental data. Using the net-section stress approach based on σ u , the eroded depth and the angle at which a pipe undergoes general yielding were obtained.

Low cycle fatigue and ductile fracture for Japanese carbon steel piping under dynamic loadings

Abstract Dynamic fracture behavior of circumferentially cracked pipe is important to evaluate the structural integrity of nuclear piping from the viewpoint of the LBB concept under seismic conditions. Fracture tests have been conducted for Japanese carbon steel (STS410) circumferentially through-wall cracked pipes that are subjected to monotonic or cyclic bending loads at room temperature. In the monotonic-loading tests, the maximum load to failure increases slightly with increasing loading rate. The failure cycles can be expressed simply by ratio of the load amplitude to the plastic collapse load. Fracture analysis has been also conducted to model the pipe tests. A new equation for calculating ΔJ for a circumferentially through-wall cracked pipe subjected to bending has been proposed. The failure cycles under cyclic loads are satisfactorily evaluated using an elastic-plastic fracture mechanics parameter ΔJ .

Allowable sizes of axial flaws in pressurized pipes made of moderate toughness materials

Failure stresses for axially part-through flawed pipes made of moderately tough materials are predicted by several fracture mechanics. However, allowable flaw sizes using these fracture mechanics cannot be simply described because there are many effective parameters such as pipe diameter, wall thickness, material properties, etc. To establish codes and standards to evaluate flaws for piping of light water reactors, we determine unified allowable sizes for axial flaws in pipes subjected to internal pressure from J-integral based fracture mechanics. The allowable sizes are simply tabulated using a single parameter which consists of pipe geometry and material properties.

A research program for dynamic fracture evaluation of Japanese carbon steel pipes

A research program was developed to investigate the dynamic load effect on fracture behavior of Japanese carbon steel STS410 pipe. The program comprises material tests, pipe fracture tests and development of estimation scheme. Material property tests showed that the flow stress was nearly constant or slightly increased with strain rate. Pipe tests showed that fracture load was nearly predicted by the net-section collapse criterion for both quasi-static and dynamic loading. Significant dynamic effect was not observed for STS410 carbon steel piping. Crack growth was well formulated by using J-integral parameter for low cycle fatigue with large scale yielding. Combining the crack growth behavior and unstable fracture criterion, an estimation scheme was newly developed and validated for constant amplitude cyclic loading conditions.

Diagnostics of Nuclear Power Plant Components Due to Thickness Measurement by Using Digital Radiography

Technical Guide for Diagnostics of Power Plant Components Technique due to Thickness Measurement by Digital Radiography, JEAG 4224–2009, was issued from Nuclear Standard Committee of Japan Electric Association on June 2009. Two types of digital radiography are applied to thickness measurement; imaging plate (IP) and color image intensifier (Color I. I. ). Organization and detailed contents of the technical guide is introduced in this paper.© 2010 ASME

SCC in Shroud Support Cylinder Vertical Weld Lines (Tokai-2)

SCC was found outer surface of shroud support cylinder vertical weld lines (V8) made of Nickel based alloy, alloy 182, for Tokai-2 (BWR-5) operated since 1978. Three SCC among 4 weld lines of V8 were observed. The material of Alloy182 was known to have SCC potential in BWR environment. Based on the result of finite element method analysis, it was estimated that tensile circumferential stress generated at the upper corner of cylinder corresponding crack location when H7 welded. The integrity assessment against seismic load in design was performed using shell model in finite element method analysis. It assumed conservatively that four vertical through wall cracks along whole length of V8 weld lines and four horizontal through wall partial cracks along H7 weld line at intersections of V8 and H7. The collapse load was estimated by twice slope method, a kind of limit load analysis. As the result of the integrity assessment, a critical horizontal through wall crack length along H7 weld line was about 1400 mm per one crack (about 70% of all circumferences). SCC growth was evaluated to reach the critical length after about 20 years, where maximum crack growth rate, 63mm/year was assumed. It is not judged that the immediate repair of vertical SCC is necessary.Copyright

J-integral Analysis Based on Failure Assessment Diagram and Its Application to Evaluation of Crack Growth Due to Cyclic Overload in Carbon Steel Pipes.

R 6 method proposed by U. K. CEGB (Central Electricity Generating Board, present Nuclear Electric plc) was applied to fracture evaluation using a failure assessment diagram (FAD). Since FAD has been developed based on J-integral criterion, J-integral can be inversely estimated using FAD. Because the fatigue crack growth rate is determined by J-integral value, it is suggested that R 6 method can be applied not only to fracture assessment but also to low-cycle fatigue crack growth assessment. Carbon steel pipes with a defect were monotonically and cyclically fractured to confirm the applicability of the crack growth assessment method using FAD. In the monotonic bending tests, J-resistant curve obtained by R6 method agreed with that obtained by η-Factor approach. In the cyclic bending tests, fatigue lives could be successfully estimated by the analysis using R 6 method.

Elastic-plastic fracture analysis of carbon steel piping using the latest CEGB R6 approach

Abstract The elastic-plastic fracture of carbon steel piping having various pipe diameter and circumferential crack angle subjected to a bending moment is analyzed using the latest CEGB R6 approach. The elastic-plastic fracture criterion must be applied instead of the plastic collapse criterion with increase of the pipe diameter and the crack angle. A simplified elastic-plastic fracture analysis procedure based on the R6 approach is proposed.

Elastic-Plastic Failure Analysis for a Rotor Disk with a Surface Crack

In order to evaluate a structural integrity of rotating machinary, a roter disk model specimen with an inner surface crack was used for rotating test and J-integral was calculated by the finite element method (FEM). This method can predict that stable crack growth occurs at a rotating speed in which the maximum value of the J-integral along the front of the surface crack becomes greater than the fracture toughness JIC of the disk material. In addition, Two-criteria method was applied to the assessment of rotor disk integrity. It was recognized that the critical speed from Two-criteria assessment agreed well with that from the JIC-criterion.