Chihiro Narazaki

Evaluation of Fracture Characteristics of Ni-Base Weld Metal for BWR Components

Fracture behavior of Ni-base weld metals used for boiling water reactor (BWR) was investigated. The elastic-plastic fracture toughness (JIc ) tests were conducted for Alloys 182 and 82 in room temperature (R.T.) and 300°C air using 2TCT specimen. It was found that the ductile crack growth resistance of Alloy 82 is higher than that of Alloy 182, and also the ductile crack growth resistances at 300°C are higher than those at R.T. for both Alloys 182 and 82. The fracture loads of CT specimen were predicted using existing limit load equations. The estimated limit load Pc using flow stress which is defined as the average of 0.2% proof stress and tensile strength coincided well with the experimental maximum load Pmax . It was confirmed that the conservative limit load estimation is possible when using 2.7Sm as the effective yield stress in the limit load equation.Copyright

Stability of residual stress induced by laser peening under cyclic mechanical loading

Purpose – The purpose of this paper is to investigate the behavior of compressive residual stress induced by laser peening under external loading on an age‐hardened high‐strength aluminum alloy A2024‐T3, a low‐carbon austenitic stainless steel SUS316L (Type 316L) and a nickel‐based alloy NCF600 (Alloy 600).Design/methodology/approach – The surface residual stress was measured intermittently by X‐ray diffraction during cyclic uniaxial loading.Findings – The compressive residual stress due to laser peening significantly decreased during the first few cycles at stress ratio of 0.1 with the maximum loading stress exceeding the 0.2 per cent yield stress. No remarkable decrease was observed afterward until the end of the loading cycles.Originality/value – Under symmetric loading at the stress ratio of −1 to A2024‐T3, a major decrease took place in the compression side of the first loading cycle. The surface residual stresses remained in compression within all the extent of the present experiments, even if the m...

Re-Evaluation of Fatigue Crack Growth Curve for Austenitic Stainless Steels in BWR Environment

The Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers (JSME Code) has the reference fatigue crack growth curve for austenitic stainless steels in BWR environment. This reference curve was determined as the upper bound of crack growth data excluding the outlier data. However, the other reference curves for fatigue crack growth rate such as austenitic stainless steels and ferritic steels in air environment and ferritic steels in water environment in the ASME Boiler and Pressure Vessel Code, Section XI and the JSME Code, austenitic stainless steels in PWR environment in the JSME Code and Ni-base alloys in PWR environment in the JSME Code Case are determined based on the 95% upper confidential limit by statistic data treatment. In the present study, the fatigue crack growth data of austenitic stainless steels in BWR environment were re-evaluated statistically. It was found that the current reference curve almost coincides with 95% upper confidential limit of fatigue crack growth data in the Paris region. Consequently, the current reference fatigue crack growth curve for austenitic stainless steels in BWR environment in the JSME Code can be regarded to stand on the same technical bases with other reference fatigue crack growth curves. Furthermore, the authors proposed to extend applicable upper bound of load rising time tr from 1000 s to 32000 s.Copyright

Fracture Assessment of Pipes Having Multiple Flaws Based on Ramberg–Osgood-Type Stress–Strain Relationships

This paper describes a fracture assessment method for a pipe having multiple circumferential flaws. According to Fitness-for-Service (FFS) codes for nuclear facilities published by the Japanese Society of Mechanical Engineers (JSME), the fracture strength of a high-ductility pipe having a circumferential flaw is evaluated using the limit load assessment method assuming the elastic–perfectly-plastic stress–strain relationship. In this assessment, flow stress is used as a proportional stress. However, previous experimental results [1, 2, 3] show that a crack penetrates before the entire flawed pipe section reaches the flow stress. Therefore, stress concentration at a flaw was evaluated on the basis of the Dugdale model [4], and the fracture strength of the crack-ligament was evaluated. This model can predict test results with high accuracy when the ligament fracture strength is assumed to be tensile strength. Based on this examination, a fracture assessment method for pipes having multiple flaws was developed considering the stress concentration in the crack-ligament by using the realistic stress–strain relationship (Ramberg–Osgood-type stress–strain curve). The fracture strength of a multiple-flawed pipe estimated by the developed method was compared with previous experimental results. When the stress concentration in the crack-ligament was taken into consideration, the fracture strength estimated using the Ramberg–Osgood-type stress–strain curve was in good agreement with experimental results, confirming the validity of the proposed method.© 2011 ASME

Fracture Assessment of Austenitic Stainless Steel Piping With Multiple Flaws in Heat-Affected Zone

Stress corrosion cracking (SCC) has been observed as circumferential multiple flaws in the weld heat-affected zone of primary loop recirculation system piping and core shrouds made of low carbon stainless steel. In the Japan Society of Mechanical Engineers code, Rules on Fitness-for-Service for Nuclear Power Plants, there is no fracture assessment of piping with multiple flaws which are not subject to flaw combination rule criteria. Through fracture testing of piping with two circumferential flaws in the weld heat-affected zone, the limit load estimation method was used for fracture assessment of stainless steel piping.Copyright

Fracture Assessment of Austenitic Stainless Steel Piping With Twin Flaws in TIG Weld

Fracture behavior of austenitic stainless steel piping for boiling water reactor (BWR) internals with circumferential through wall twin flaws at the weld was investigated. A 150A Sch.40 piping of type 316L stainless steel which has an outer diameter of 165.2 mm and a thickness of 7.1 mm was butt welded by tungsten inert gas (TIG) weld and single or twin through wall slits were introduced by an electro discharge machining (EDM) on the weld bead. Four point bending tests were conducted and failure stress was evaluated by currently proposed limit load equation for a piping with multiple flaws. The fracture loads obtained by the test were higher than the limit load based on the recently proposed equation for pipe with multiple flaws using 2.7Sm . It was concluded that the limit load criterion is able to be applied to the fracture assessment of austenitic stainless steel piping with twin flaws in the TIG weld. Through the pipe fracture test, it was found that the crack tends to grow in base metal rather than weld metal.© 2009 ASME

ICONE19-43979 The Effect of Mechanical Loading on Residual Stress Induced by Laser Peening

The stress corrosion cracking(SCC) of austenitic stainless steel and nickel-based alloy is a primary concern for stable operation in Boiling Water Reactors(BWRs) and Pressurized Water Reactors (PWRs). Laser peening is one of the promising technologies for prevention of SCC initiation to induce compressive residual stress near the surface region. However, the compressive residual stress by laser peening is likely affected due to applied stress. In this study, the effect of mechanical loading on the compressive residual stress by laser peening was investigated. The specimens of low carbon austenitic stainless steel (Type316L), nickel-based alloy (Alloy600) and their weldments were prepared, and several levels of cyclic loading were applied to their specimens. The surface residual stress at surface was measured by X-ray diffraction method before and after loading. The relaxation behaviour of the compressive surface residual stress by laser peening was evaluated at the base metal and heat affected zone (HAZ) of the weldment. It was confirmed that compressive surface residual stress remained after stress loading which corresponds to 0.2% proof stress, and that the relaxation of the surface residual stress remarkably observed after initial loading during cyclic loading test. These relaxation behaviour were similar in Type316L, Alloy600 and their HAZ.

Technical Basis of Fatigue Crack Growth Rate Curve for Ni-Base Alloy Weld Metal in Air Environment

Recently, incidents of SCC in Ni-base alloy weld metal of BWR components have been reported. When the defects are detected by inspection, structural integrity assessment should be performed for the technical judgment on continuous service based on the Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code). The structural integrity assessment includes fatigue crack growth analysis. However, fatigue crack growth analysis for Ni-base alloy is impossible since the fatigue crack growth rate curves in air and the BWR environment are not prescribed yet in the JSME FFS Code. The curve in air environment is needed for the structural integrity assessment of the flaw when the embedded flaw repair which is one of the repair techniques to isolate the defect from water environment by seal welding is applied. In this study, fatigue crack growth tests in air environment were performed for Ni-base alloy weld metal. Based on the test data, fatigue crack growth rate curves with ΔKth of Ni-base alloy weld metal were investigated. It is found that fatigue crack growth data in the Paris region hardly depend on the test temperature and the stress ratio, whereas data around ΔKth are dependent on them. Hence, the curve in the Paris region was regarded as the same curve despite the difference of the test temperature and the stress ratio. The minimum ΔK of final crack growth data in the ΔK decreasing test was adopted as ΔKth of the curve.Copyright

Fracture Assessment for Butt Welded Plate of Ni-Based Alloy

The objective of this study is to establish the fracture assessment method for nuclear power components made of Ni-based alloy weld metal. Fracture tests of butt welded plates of alloys 182 and 82 with a surface flaw were conducted in tension at 300°C. The dimensions and shapes of weld groove of the test plates were intended to simulate the weld joint of BWR shroud support. Semi-elliptical surface flaws were introduced by EDM. Fatigue pre-crack was introduced from an EDM notch for one test plate of alloy 182 to investigate the effect of crack tip shape on the fracture. It was found that the fracture load is conservatively estimated by using plastic collapse load with 2.7Sm for flow stress. It was also understood that the plastic collapse load using flow stress of actual data of tested materials gives good estimation for the experimental fracture load. The net section stresses at the maximum load of both fatigue pre-cracked and EDM notched alloy 182 test plates were almost the same. Consequently, the fracture mode of butt welded joint of alloys 182 and 82 at 300°C is characterized to be plastic collapse.Copyright

Evaluation of Stress Intensity Factor for Transverse Cracks in Weld Joint of BWR Shroud Support and Its Application to the Evaluation of Trapezoidal Crack

Since the weld joints of BWR shroud support are made of Ni-base weld metal that has susceptibility to SCC in the BWR water, the SCC crack prefers to grow in weld metal rather than base metal of Ni-base alloys. It is important to prepare the evaluation method for SCC crack initiated in these alloys. To develop the flaw evaluation method for shroud support, stress intensity factor of postulated axial cracks in H10 weld of shroud support in BWR were analyzed by FEM. Results were compared with simplified SIF solutions in codes and standards to confirm the applicability of these simplified methods. It was understood that these simplified methods were available to evaluate the SIFs of axial crack in H10 weld although the thickness of H10 weld is not constant and is tapered. In the present study, SIF for trapezoidal crack was also analyzed because the front shape of stress corrosion crack in the weld metal of Ni-base alloy is not always semi-elliptical and is thought to have complex shape. It was found that the trapezoidal crack could be substituted by elliptical crack for conservative evaluation when the crack is deep and the ratio of upper base 2d to lower base 2c is small. Based on this result, the authors analyzed SIF for more number of trapezoidal cracks in a plate and compared with SIF of semi-elliptical crack which has the same depth a and surface length 2c of trapezoidal crack to investigate the effective range of substitution by an elliptical crack. It was found that the SIF of deepest point KA of semi-elliptical crack becomes larger than that of trapezoidal crack when d/c becomes smaller than about 0.37. These results show the substitution of trapezoidal crack with small d/c by semi-elliptical crack makes conservative SIF evaluation and it is very useful because the SIF solutions for semi-elliptical crack provided in many codes and handbooks can be applied.Copyright