Michiya Sakai

Damping and response characteristics of passive type MR damper

The characteristics of a passive type MR damper were evaluated experimentally. The damper has variable damping characteristics as well as an existing semi-active damper, although it is fully passive device without any sensor, power supply and power controller. A prototype of the damper was manufactured in order to verify the damping and response characteristics. The excitation tests with sinusoidal displacement demonstrate that the damping force changes in response to the sign of the product of the velocity and displacement, which is analogous to one of semi-active control. The base excitation tests wi th a single-degree-of-freedom system show that the passive type MR damper has excellent damping effect in a wide range of frequency. In particular, the advantage is noticeable at re sonance frequency. It is also shown that the base isolation system with the present damper is quite effective in a reduction of the re sponse to earthquake excitation.

Experimental Investigation of Failure Estimation Method for Circumferentially Cracked Pipes Subjected to Combined Bending and Torsion Moments

When a crack is detected in a nuclear piping system during in-service inspections, failure estimation method provided in codes such as ASME Boiler and Pressure Vessel Code Section XI or JSME Rules on Fitness-for-Service for Nuclear Power Plants can be applied to evaluate the structural integrity of the cracked pipe. In the current codes, the failure estimation method for circumferentially cracked pipes is applicable for both bending moment and axial force due to pressure. Torsion moment is not considered. Recently, two failure estimation methods for circumferentially cracked pipes subjected to combined bending and torsion moments were proposed based on analytical investigations on the limit load for cracked pipes. In this study, experimental investigation was conducted to confirm the applicability of the failure estimation method for cracked pipes subjected to bending and torsion moments. Experiments were carried out on 8-in. diameter Schedule 80 stainless steel pipes containing a circumferential surface crack. Based on the experimental results, the proposed failure estimation methods were confirmed to be applicable to cracked pipes subjected to combined bending and torsion moments.

Seismic Evaluation Method for Piping Systems With Local Wall Thinning Caused by LDI

The earthquake-proof safety of piping systems with local wall thinning due to liquid droplet impingement erosion (LDI) was evaluated using a hybrid experiment which has been incorporated a numerical analysis of the whole system with a static loading test of elbow pipe model. Seismic performance effects of wall thinned elbow were clarified by comparing three cases of different thickness elbow models such as no defect, 50% defect and 75% defect. No damage was observed for in-plane and out-of-plane bending of elbows in the 75% condition under a seismic load equal to five times the design basis earthquake required to reach allowable stress level. In addition, torsion buckling occurred and through wall crack penetrated by cyclic loading under eight times large amplitude the above mentioned seismic motion.Copyright

Hybrid Seismic Response Evaluation Method for Wall Thinning Piping System

Pipe wall thinning is a one of the major degradation mechanisms in aged nuclear power plants (NPPs). In Japan, the seismic safety of wall thinning piping system during earthquake must be evaluated in aged NPPs. Seismic safety of piping systems with wall thinning had been investigated by other researchers using shaking table tests of reduced scale and numerical analyses. However, there exist the limitations such as the scale effect of pipe model for shaking table tests and the limit of the evaluation for numerical analysis concerning the criteria of pipe integrity. By the way, elbow can be one of the most important elements to evaluate the seismic safety of piping system. So, in order to evaluate seismic safety of piping systems with wall thinning elbow, hybrid tests have been conducted, in which the seismic response of the whole piping system is treated as a numerical model, and the real piping is used only for the element on which the transformation and damage locally concentrated. The through-wall crack only occurred in the case of a uniform thinning model although cracks didn’t penetrate in the non thinning model and the local thinning model. In the experimental condition, the failure mode of wall thinning elbow under seismic loadings had been low cycle fatigue, and effectiveness of this evaluation method has been demonstrated.© 2013 ASME

Experimental Investigation on Failure Estimation Method for Circumferentially Cracked Pipes Subjected to Combined Bending and Torsion Loads

When a crack is detected in a nuclear piping system during in-service inspections, the failure estimation method provided in codes such as the ASME Boiler and Pressure Vessel Code Section XI or JSME Rules on Fitness-for-Service for Nuclear Power Plants can be applied to evaluate the structural integrity of the cracked pipe. In the current codes, the failure estimation method for circumferentially cracked pipes includes bending moment and axial force due to pressure. Torsion moment is not considered.The Working Group on Pipe Flaw Evaluation for the ASME Boiler and Pressure Vessel Code Section XI is developing guidance for combining torsion load within the existing solutions provided in Appendix C for bending and pressure loadings on a pipe. A failure estimation method for circumferentially cracked pipes subjected to general loading conditions including bending moment, internal pressure and torsion moment with general magnitude has been proposed based on analytical investigations on the limit load for cracked pipes. In this study, experimental investigation was conducted to confirm the applicability of the proposed failure estimation method. Experiments were carried out on 8-inch diameter Schedule 80 stainless steel pipes containing a circumferential surface crack. Based on the experimental results, the proposed failure estimation method was confirmed to be applicable to cracked pipes subjected to combined bending and torsion moments.Copyright

Evaluation of Local Thinning Shape by LDI for Seismic Safety Evaluation of Piping System

For seismic safety evaluation of piping system with local thinning surface by liquid droplet impingement erosion (LDI), hybrid seismic tests were conducted to the piping with a locally-thinned elbow. In this paper, a method for predicting the thinning shape by LDI on the elbow is developed. To determine the thinning shape by LDI, droplet behavior at the elbow is calculated for various flow conditions and geometries. With the calculation of the collision point and velocity for each droplet, collision frequency and average collision velocity on the elbow are estimated. Then, the thinning shape on the elbow is determined with the relationship between the flow conditions and thinning rate. Finally, the evaluated thinning shape is compared with an actual LDI case for the validation of the method.Copyright

Performance Evaluation on Jointed Pipes Used Underground for Fire Protection at a Nuclear Power Plant as a Response to the Effects of the 2007 Niigata-Ken Chuetsu-Oki Earthquake

This study discusses the strength of jointed pipes used underground for fire protection at a nuclear power plant and their capacity to accommodate ground displacement. For this purpose, bending tests, a finite element analysis and an elastic beam theory analysis were conducted. The bending tests were conducted for four types of joints previously subjected to actual use, including welded, flange, screw and coupling types. The welded and flange joints demonstrated significantly higher load capacity; five times that of the coupled joint. The bending test results for the flange, screw and coupling joints were applied to identify the bending moment and rotation angle at the moment the internal pressure reached the level of the atmospheric pressure due to leakage. The limit state of the welded joint was not obtained in the experiment but was estimated by nonlinear finite element analysis. The bending moment and rotation angle of the welded joint were identified at the moment the welded joint reached the limit state. Finally, the test results and finite element analysis were applied to estimate the capacity of each joint to accommodate ground displacement. The elastic beam theory was used by modeling the pipe as an elastic beam and idealizing the joint as an elastic rotational spring. Consequently, the ground displacement capacity of the welded joint exceeded that of the coupling and screw joints by more than 500 per cent.Copyright

Visual and Versatile Hybrid Seismic Testing System Incorporated With Non-Linear Finite Element Analysis

Hybrid simulation/testing systems have been developed incorporating a non-linear finite element method with a pseudo-dynamic test. In order to ensure stability and efficiency for time integration, the incremental formulation of the α-OS method has been implemented on this system. Visualization system has also been integrated to recognize both numerical simulation for whole systems and laboratory testing for local parts. Numerical hybrid examinations of the soil structure interaction problem have been conducted on this system. By these results, validity and effectiveness of this system has been demonstrated.Copyright

Verification Test for Hybrid Seismic Experimental Method Using Nonlinear Finite Element Method

An improved substructure hybrid seismic experimental method has been developed. This method consists of numerical computations using a general-purpose nonlinear finite element analysis tool and a pseudo-dynamic vibration test. Therefore, it enables seismic testing of large-scale structures that cannot be loaded onto a shaking table. The method also visualizes both data measured by sensors placed on the specimen and the results of the numerical analysis, and it helps us to understand the behavior of an entire structure consisting of a specimen and a numerical model. We performed verification tests for a piping system, in which we used a numerical model including supports, valves, and a branch pipe, and a specimen including two elbows. As results of tests, we conclude that the developed system has enough accuracy to be used as a seismic testing method.Copyright