Hiroe Kobayashi

Simulation of Excessive Deformation of Piping Due to Seismic and Weight Loads

The ASME Code for seismic design of piping system was revised in 1994 based on evaluation of pipe component test results submitted by the Piping and Fitting Dynamic Reliability Program (PFDRP). PFDRP indicated piping component failure to result in most cases from fatigue with ratchet. Excessive progressive deformation was noted by this program to ultimately occur in test #37, #39 and #40 piping models. This mode of failure was considered due to superposition of bending moment arising from vertical load produced by weight and horizontal seismic inertia force. To clarify the conditions leading to such failure, elastic-plastic dynamic analysis using the general-purpose FEM Code was carried out on the test#37 piping model of PFDRP. The analytical model and method were verified as effective means of study by comparison of analytical results with those obtained experimentally. The parameter determinations were made under condition of variation in dead weight stress and excitation and its dominant frequency. 1994 ASME seismic stress limits were shown effective for preventing excessive progressive deformation in tests #37, #39 and #40.Copyright

Strain Rate of Pipe Elbow at Seismic Event and Its Effect on Dynamic Strain Aging

NRC expressed their refusal of revised seismic stress criteria for piping systems of the ASME Section III, 2001 Edition at their proposed rule making by 10CFR50.55a. One of technical bases of their refusal is “Dynamic strain aging of carbon steel at high strain rate caused by earthquake and at high temperature condition of nuclear power operation”. This paper presents that the effect of dynamic strain aging at the allowable stress limit is practically negligible small even if the seismic allowable stress for nuclear piping increases up to 150% at ASME Section III 2001 and 2003 Edition from 3.0Sm (ASME Section III 1995 Edition). This seismic stress limit revision was achieved by using the B′2 that is equal to 2/3 B2 instead of B2 stress index in Equation of ASME Section III NB-3656 at seismic stress evaluation. At first, maximum elastic-plastic strain of pipe elbow was calculated at various stress level up to revised allowable stress limits by using the elastic-plastic FEM which was verified by the comparison with experimental results. Obtained strain was converted to strain rate by introducing the dominant frequency of piping system at seismic events. Secondary, data of relationship of strain rate and yield to tensile stress ratio for various type of carbon steel was collected. The yield to tensile stress ratio at very low strain rate was compared with that at strain rate of seismic event. The value of yield to tensile stress ratio at strain rate of seismic event was also evaluated. Finally, effect of maximum strain rate at seismic event under revised stress limit condition was discussed by evaluating the change of the yield to tensile stress ratio at low and seismic event condition and the value of the yield to tensile stress ratio at seismic event. From this discussion, dynamic strain aging would not occur in carbon steel piping at seismic event and high temperature condition under revised stress limit condition.Copyright