Kunihiro Iida

Simplified analysis and design for elevated temperature components of Monju

Abstract The Japanese Prototype Fast Breeder Reactor Monju is now at a stage of submitting applications to the government for getting Construction Permission. Detailed structural design of components is now in progress. As well understood, a number of components of Monju have to be subjected to elevated temperature design. For this need, Power Reactor and Nuclear Fuel Development Corp. has developed “Elevated Temperature Structural Design Guide for Class 1 Components of Prototype Fast Breeder Reactor” (ETSDG). It is now incorporated into “Construction Code for Prototype Breeder Reactor”. The basic concept of ETSDG is originated from ASME Boiler and Pressure Vessel Code Section III, Case Interpretation N-47, ETSDG prepares some extended simplified inelastic analysis methods in addition to the basic concept of N-47. This extension is necessary to make easy the complicated inelastic analyses. The simplified methods have been developed through many case studies with detailed inelastic analyses, elastic analyses and experimental verifications.

Construction codes developed for prototype FBR Monju

Abstract The paper describes the regulation system for LWRs in Japan and discusses some typical points which are attained in the newly developed CCPBR (Construction Code for Prototype Breeder Reactor). Revised welding standards are briefly introduced for the Monju construction.

A review of fatigue failures in LWR plants in Japan

Abstract A review was made of fatigue failures of nuclear power plant components in Japan, which were experienced in service and during periodical inspection. No case has been recently reported of a service fatigue failure of a reactor pressure vessel itself, excluding nozzle corner cracks, that occurred many years ago. But, service fatigue failures have been occasionally experienced in piping systems, pumps, and valves, on which fatigue design seems to have been inadequately applied. The causes of fatigue failures can be divided into two categories: mechanical-vibration-induced fatigue and thermal-fluctuation-induced fatigue. Vibration-induced fatigue failure occurs more frequently than is generally thought. The lesson gleaned from the present survey is a recognition that a service fatigue failure may occur due to any one or a combination of the following factors: (1) lack of communication between designers and fabrication engineers, (2) lack of knowledge about a possibility of fatigue failure and poor consideration about the effects of residual stresses, (3) lack of consideration on possible vibration in the design and fabrication stages, and (4) lack of fusion or poor penetration in a welded joint.

Effects of strain rate change on fatigue life of carbon steel in high-temperature water

In high temperature waters that contain dissolved oxygen (DO) to certain content, the fatigue life of carbon steel is strongly affected by strain rate. A formula has been advanced to quantify this effect when the strain rate is held constant. However, the strain rate changes continuously in most of transients of actual plant operation. There is no way currently to assess the effects of strain rate when the strain rate is varied as in the actual plant transients. To find a solution to this problem, a series of strain controlled fatigue tests have been conducted with the strain rate changed stepwise or continuously. It is shown that a method, in which the product of the environmental effect and the strain increment within a unit time interval in a transient period is integrated from the minimum strain to the maximum, evaluates the environmental effect with satisfactorily high accuracy. This method is called the modified rate approach method. It is shown also that the procedure of taking the strain rate as averaged over the minimum to peak of the strain change as giving rise to more conservative evaluations than the ones the modified rate approach method produces.

Experimental Study on Fatigue Strength of Small-Diameter Socket-Welded Pipe Joints

The authors conducted fully reversed four-point bending fatigue tests on socket-welded joints 20 to 50 mm in nominal diameter, and rotating bending fatigue tests on socket-welded joints 20 mm in nominal diameter. S-N curves for 33 series of different types of specimens were obtained. Examination was made of the effects of various parameters listed in the forthcoming on fatigue strength such as steel types (carbon and stainless steels), diameter, pipe thickness (Sch), fillet shape, slip-on gap, and root defects. Bending fatigue test results indicated fatigue strength for socket-welded joints to be less for longer life regions than reported in the literature by Markl and George (1950). Fatigue strength for socket joints of 50 mm nominal diameter at 10 7 cycles of fatigue life was 46 MPa for carbon steel and 60 MPa for stainless steel with nominal bending stress on the pipe surface. Cracks generally originated from the toe when stress amplitude was high with shorter fatigue life and from the root when amplitude was small with longer life. Fatigue strength was greater for smaller diameter, larger Sch (thicker pipe wall), final welding pass on the toe of pipe side, and in the absence of a slip-on gap. From fatigue test results of socket joints with weld defects at the roots, an empirical equation for the relation of defect size with decrease in fatigue strength was established. Fatigue strength was found to decrease to 60 percent the original level for defect size 25 percent of leg length.

Effects of Water Flow Rate on Fatigue Life of Carbon Steel in Simulated LWR Environment Under Low Strain Rate Conditions

The flow rate of water flowing on a steel surface is considered to be one of the important factors strongly influencing the fatigue life of the steel, because the water flow produces difference in the local environmental conditions. The effect of the water flow rate on the fatigue life of a carbon steel was thus investigated experimentally. Fatigue testing of the carbon steel was performed at 289°C for various dissolved oxygen contents (DO) of less than 0.01 and 0.05, 0.2, and 1 ppm, and at various water flow rates. Three different strain rates of 0.4, 0.01. and 0.001 %/s were used in the fatigue tests. At the strain rate of 0.4 %/s, no significant difference in fatigue life was observed under the various flow rate conditions. On the other hand, at 0.01 %/s, the fatigue life increased with increasing water flow rate under all DO conditions, such that the fatigue life at a 7 m/s flow rate was about three times longer than that at a 0.3 m/s flow rate. This increase in fatigue life is attributed to increases in the crack initiation life and small-crack propagation life. The major mechanism producing these increases is considered to be the flushing effect on locally corrosive environments at the surface of the metal and in the cracks. At the strain rate of 0.001 %/s, the environmental effect seems to be diminished at flow rates higher than 0.1 m/s. This behavior does not seem to be explained by the flushing effect alone. Based on this experimental evidence, it was concluded that the existing fatigue data obtained for carbon steel under stagnant or relatively low flow rate conditions may provide a conservative basis for fatigue life evaluation. This approach seems useful for characterizing fatigue life evaluation by expressing increasing fatigue life in terms of increasing water flow rate.

Corrosion fatigue behavior and life prediction method under changing temperature condition

Axially strain controlled low cycle fatigue tests of a carbon steel in oxygenated high temperature water were carried out under changing temperature conditions. Two patterns of triangular wave were selected for temperature cycling. One was in-phase pattern synchronizing with strain cycling and the other was an out-of-phase pattern in which temperature was changed in anti-phase to the strain cycling. The fatigue life under changing temperature condition was in the range of the fatigue life under various constant temperature within the range of the changing temperature. The fatigue life of in-phase pattern was equivalent to that of out-of-phase pattern. The corrosion fatigue life prediction method was proposed for changing temperature condition, and was based on the assumption that the fatigue damage increased in linear proportion to increment of strain during cycling. The fatigue life predicted by this method was in good agreement with the test results.

The bending fatigue strength of welded steel pipe with uniform mismatch

Abstract Uniform bending fatigue tests were carried out on butt-welded pipes of an STS-38 (low carbon) steel and of a SUS-304 TP stainless steel. Experimental factors investigated were the amount of uniform mismatch, the effect of the existence of a weld reinforcement, the effect of grinding off the reinforcement toe roundly and the influence of solution heat treatment of the stainless steel specimen. The specimen was subjected to pulsating bending load, the cycling rate being 250 to 400 cycles/min. The main conclusions are: (1) the bending fatigue strength decreases with the increasing amount of uniform mismatch; (2) the notch formed by the finishing end of the machining inner surface provides a more detrimental effect on fatigue strength than does either the effect of uniform mismatch or of a weld toe; (3) solution heat treatment on the stainless steel specimen results in a pronounced decrease in the fatigue strength.

Effects of Temperature and Dissolved Oxygen Contents on Fatigue Lives of Carbon and Low Alloy Steels in LWR Water Environments

In order to modify the fatigue design method for pressure vessel components of light water reactors (LWRs), strain controlled fatigue tests were carried out under various environmental conditions, and the effects of temperature and dissolved oxygen (DO) contents on fatigue lives of carbon steel in LWR-simulated water environments were evaluated and analyzed. The results of the present study are summarized as follows: (1) the decrease in fatigue life of carbon steel STS410 in BWR-simulated water environments occurs at temperatures above 200 C and with DO higher than 0.1 ppm, the rate of decrease is greater at higher temperatures and higher DO concentrations; (2) to offset differences in test condition such as temperature, DO, strain amplitude etc., a new index, Rp, which represents a ratio of the environmental index P-values previously proposed, was introduced, also a new fatigue life equation based on Rp was proposed; (3) by the new equation, the fatigue life of carbon steel under a certain environmental condition of temperature and DO can be predicted better than by the previously proposed method.

Very low cycle fatigue life influenced by tensile or compressive prestrain

The effects of excessive prestrain in tension or in compression upon very low-cycle fatigue life were investigated for two mild steels and two high strength steels as an aid in the analysis of a ship failure in service, in which the bow structure was broken off due to local buckling and by low-cycle fatigue in extremely heavy sea conditions. The test series in the present investigation was: (1) uni-directional tension test after prestraining, (2) completely reversed strain cycling tests of non-prestrained material with several strain ranges, (3) tensile or compressive straining directly followed by completely reversed strain cycling, and (4) completely reversed strain cycling tests of specimens which were machined out of previously prestrained large diameter specimens. The residual static fracture ductility decreases as a function of the amount of prestrain, showing much more loss of ductility in the case of compressive prestrain. Fairly good agreements on strain range and fatigue life were found between experimental data and estimated values calculated after Mansons and Iidas formulae. The failure life of a specimen subjected to prestrain showed a remarkable decrease from the failure life of the original material depending on three parameters: amount of prestrain, direction of prestrain, and specimen surface conditions. The worst case in the range of the present tests is that of tensile prestraining directly followed by strain cycling. In this case the low-cycle fatigue failure life of a prestrained specimen is, as an example, reduced by 93% of the failure life of the original material, when the amount of the tensile prestrain is 60% of the original static fracture ductility based on area reduction to failure.