Akira Maekawa

The objective to extend the usable area of pipe supports up to plastic region is below; one is to enlarge the loading capacity, and the other is to add damping to piping systems using the absorption effect of seismic energy due to the elastic-plastic hysteresis property. To generate the energy absorption effect based on the elastic-plastic behavior of all pipe supports during the earthquake events, the small pipe supports are required in the small-bore piping due to the small seismic reaction. However, the structures to fix the piping on the wall are relative larger in fact. Taking into account the requirement and the specifications, a new hybrid type of elastic-plastic pipe support composed of the framework type component which is large and strong and the cantilever type component which is small and flexible is proposed in this study. The cantilever type component adds the damping by the plasticity and the framework type component protects the excessive deformation of the cantilever type component caused by the unanticipated plastic accumulation. This structure of the proposed supports is also useful for the practical design from the viewpoint of the safety design.

When using an elastic-plastic seismic response analysis, the relationships between load and displacement of the support structures frequently are approximated by bilinear curves. However, the analysis results depend on how to approximate them. In this study, the seismic response results by the elastic-plastic analysis using typical four approximation methods are compared and discussed. The approximation is assumed within twice the elastic displacement. The four methods chosen for this study are as follow; The first one is to use the last slope of the load-displacement relationship, the second one is defined by the load-displacement relationship and the terminal point, the third one is based on the mechanical energy balance, and the fourth one is to use the elastic limitation as yield point. When the damping ratios are calculated using the absorption energy and the strain energy obtained from the hysteresis of the cyclic loading-displacement relationship, they are nearly equal with the exception of the fourth method. The similar result is shown for the comparison of the seismic response analysis using one mass models with the rigidity estimated by all methods.

This study presents the effects of the elastic-plastic behavior of elastic-plastic pipe supports on the seismic response of piping systems. The effects were evaluated by time-history seismic response analysis using two kinds of two-span models which have the simplest piping geometry. One is a symmetric model, in which the length of spans is same, and the other is an asymmetric model, in which the total span length is the same as that of the symmetric model but the length of the two spans is different. For the symmetric model, the maximum response reduced by the effect of the elastic-plastic pipe support. However, for the asymmetric model, the maximum response did not change comparing to the elastic response. Finally, the effect of the elastic-plastic pipe support was different between the two models. This indicates the maximum response does not always decrease due to the relationship between the piping configuration and the input waves even if the elastic-plastic behavior occurs in the elastic-plastic pipe support.

This paper presents the current status of the seismic safety enhancement for the pressurized water reactor (PWR) nuclear power plants of the Kansai Electric Power Co. (KEPCO). KEPCO made significant upgrades to seven units, including incorporating new measures to meet new safety regulations in the wake of the Fukushima accident. For assuring the seismic safety, various measures were taken such as the exhaustive evaluation to prove the sufficient seismic resistance and the reinforcement works to improve the seismic performance. As a representative example, the seismic safety measures used for Mihama unit 3 are reviewed and summarized in this paper. Also, the future expectations for the development of the seismic design methods and the seismic retrofitting measures are described briefly.

Two kinds of functions are required for the elastic-plastic pipe supports which can be used within plastic region; One is to enhance the design bearing capacity and the other is to provide damping using the seismic energy absorption due to elastic-plastic hysteresis. The effectiveness evaluation for the elastic-plastic design of the piping systems with elastic-plastic pipe supports was done within the limit of no plastic failure by comparing with the allowable load at the Level-D. The comparison results revealed that the traditional elastic-plastic pipe supports could increase the design bearing capacity, but they could not always provide the larger damping effect because of the dependence of hysteresis shapes on the support geometries. These results were thought to be due to the combination of the two function requirements in the same steel member. Therefore, the design idea of the elastic-plastic pipe supports with independent design of the two functions was proposed. The effectiveness was proven by the seismic response analysis.

In piping systems equipped with elastic-plastic pipe supports as seismic response reduction design, the neighboring pipe supports are suspected to fail one by one when the elastic-plastic pipe supports are deformed extremely by seismic motions beyond design basis, which is called a zipping phenomenon. This is why excessive loading is added to the neighboring pipe supports due to change of the loading distribution of the whole piping systems by the extreme deformation of the supports. To judge the failure possibility of the piping systems in the severe event beyond design basis seismic motions, a simple screening method was proposed based on the idea of Allowable Support Span Method.

The aim of this study is to reveal the effects of strain velocity caused by dynamic strain under earthquake for the material strength of nuclear equipment and structures quantitatively. From the obtained results, it can be clarified how degree the strain velocity contributes to seismic safety margin in the equipment and structures. This paper addresses the fracture modes of stainless steel and the effect of strain velocity caused by the cyclic impact loading such as seismic motions. The ductile type and brittle type of fracture modes appeared in excess large loading and relative large loading, respectively. The two fracture modes took turns without the transition state rapidly. Compared based on the strain amplitude, the numbers of cycles to fracture in the different stain velocities were the same. Within the extent of the experiment, it was found the fracture behavior of stainless steel did not depend on the strain velocity.

For seismic studies of piping systems during strong seismic ground motion, there are only a few experiments and analyses to examine the dynamic behaviors of pipe support structures in the condition of their elasto-plastic deformation although many multifarious seismic research studies for pipes have been done. Therefore, the realistic seismic response of piping systems during strong motion should be investigated for the condition that the both pipes and pipe support structures simultaneously extend to elasto-plastic deformation. In this study, a three-dimensional piping system with frame-type supports was three-dimensionally excited using strong motion and the vibration response was researched. The elasto-plastic response of the piping system due to the plastic deformation of both pipe and support occurred during the maximum strong excitation and the response reduction caused by the plastic effect was observed.

Welding is the way of locally heating. Therefore, fatigue strength of member gets lower because of an occurrence of tensile residual stress around welded part. In this report, we investigated the effect of the reduction method of residual stress applying ultrasonic waves on material characteristics of SUS304. The reduction method of welding residual stress using ultrasonic waves does not give a bad influence on tensile strength of welding base metal, Churpy impact test value and curve of thermal deformation.

In welding, material is heating locally. Therefore, it generates tensile residual stress in the heat affected zone and changes the quality of material. Those will be causes problem of stress corrosion cracking in SUS304. One of the conventional prevention is to use material that hardly occur heat sensitization, but stress corrosion cracking is observed in that material. Therefore, it is desired to reduce or remove the tensile residual stress generated by welding. Accordingly, we have been proposed a method that adds ultrasonic vibration during welding. The method can reduce the tensile residual stress easily in a shorter time than conventional methods. Experimentally, it has ever been shown that this method can reduce the tensile residual stress on heated zone for thin plate of SUS304. In this study, it is examined that acceleration amplitude influences to reduction of the tensile residual stress for welding of thin plate of SUS304.

The present paper proposes an efficient contactless measurement method for vibration stress of piping systems, by which the measuring works are performed for an extreme short time and the measured stress can be evaluated immediately after the measurement. The proposed method uses only one LED-optical sensor to measure the vibration stress though multiple sensors must be used in the previous method developed by the authors. Therefore, the measuring system could be reduced in size and a light-weight and portable measurement instrument was developed. The measurement accuracy and reliability of the new method were verified by vibration experiments using a mock-up piping.

Dynamic effect of impact loading such as seismic motion was researched in the viewpoint of seismic margin of steel structures. Impact fatigue tests were conducted from extremely low cycle to low cycle using cycle-loading test apparatus of impact-tensile load and the influence of dynamic strain on material strength of carbon steels. As the test results, the dynamic fatigue lives depended on strain amplitude not strain rate, suggesting the fatigue failure by seismic motion was not affected by strain rate. The strength for impact load includes viscosity stress caused by strain rate. Therefore, the test results and discussion suggested the viscosity stress corresponded to implicit seismic margin in the current design on the basis of stress.

Welding is used to many structures and is important joining method in industry. Because metal is heating locally by welding, residual tensile stresses is generated in the heat affected zone. The tensile residual stress degrades fatigue strength of component, and will be one of causes of stress corrosion cracking in SUS304. Therefore it is important to decrease the tensile residual stresses. Accordingly, we have been proposed a method that adds ultrasonic vibration during welding. It can reduce the tensile residual stresses easily in a shorter time than conventional methods in SS400. In this study, reduction of the residual stress using ultrasonic vibration was studied in examination for build-up welding of thin plate in SUS304. SUS304 can keep strength in high temperature environment. Therefore, two ultrasonic vibrations that have different frequency were added to get plastic deformations that cause reduction of the residual tensile stresses by large vibrational amplitude. It is shown that using ultrasonic vibration can reduce the residual stresses in SUS304 which have high temperature property.

Accurate evaluation of occurrence location and amplitude of pressure pulsations in a piping system can lead to efficient plant maintenance to prevent fatigue failure of piping and components because the pulsations could be main causes of vibration fatigue and acoustic noise in piping. To measure pressure pulsation easily and directly, the method to combine strain measurement on outer surface of pipe with the formula for thick-walled cylinders was proposed. The experimental validation demonstrated the proposed method could measure amplitudes and behavior of pressure pulsations with a practical accuracy. Furthermore, the applicability of the formula for thin-walled cylinders was examined for various shaped pipes.

Welding is a joining method that is used in many structures. It is locally heating method. A tensile residual stress is generated in the heat affected zone. The residual stress degrades fatigue strength of component, and will be one of the causes of stress corrosion crack in SUS304. We have proposed a method that adds ultrasonic vibration during welding. It can reduce the tensile residual stress of SS400 easily in a shorter time than conventional methods. The method is applied to butt welding of thin plates made of SUS304. The effect of the method is confirmed experimentally.

Residual stress caused by welding process affects characteristics of strength and fracture of equipment and piping in nuclear power plants. Numerical analysis is powerful tool to evaluate weld residual stress in the actual plants. However, the three-dimensional precise analysis requires enormous computation time. In this study, the analysis code using iterative substructure method developed to speed up welding simulation was improved to simulate welding process of the plant equipment and piping. Furthermore, the improved analysis code was validated by measurements and other analysis results.