Response and cyclic strain accumulation of pressurized piping elbows under dynamic out-of-plane bending

Abstract

In a previous publication (1),† an experimental programme, involving four carbon and four stainless steel pairs of 2 inch NPS, 90° welding elbows tested under conditions of steady internal pressure and dynamic inputs that induced resonant in-plane bending at frequencies typical of seismic excitations, was described and some results reported. In this contribution, a similar test programme using identical pressurized components under dynamic conditions that induced out-of-plane external moments at similar frequencies is considered. For each material, two thicknesses (extra strong and standard weight specifications) were chosen and for each thickness two bend radius geometries (long and short) were tested. The similarities and differences in instrumentation, response and behaviour between the two programmes are highlighted. While the dynamic responses and the levels of moment achieved were similar for both types of external loads, there was an important difference in strain behaviour. Firstly, the direction of maximum strain was found to be at about 45° between the hoop and axial directions. Secondly, while there was practically no strain accumulation in the axial direction, ratcheting in both hoop and maximum principal strain directions was initiated at about the same level of input. Thirdly, the direction of highest ratcheting was found to be along the hoop direction rather than the direction of maximum principal strain. The cyclic strain accumulation against response moment for each component is assessed and ratios of applied to limit moment at onset of ratcheting are given for each material.