Modeling Of Material Multiaxial Ratcheting And Ratcheting Prediction Of Pressure Piping

Ratcheting of pressurized straight pipes and elbows made of S20C wereexperimentally studied with multiaxial fatigue testing system and aquasi-three-point bending apparatus for straight pipe and in-plane bendingapparatus for elbow were designed. Ratcheting strains were acquired bymulti-channel strain processor with strain gauges and the radial deformation ofthe pipe was measured by self-designed radial displacement extensometers. Forstraight pipes, it is found that ratcheting initiates firstly in hoop direction and thatin axial direction follows with the increase of loading but less in magnitude. Thecircular cross section turns into ellipse as the ratcheting strain accumulates. It wasfound that, for 90o elbows, the maximum ratcheting strain occurs at the flank inhoop direction. For 45o elbows, ratcheting strain at 45o is larger than that at flanks.Ratcheting strain rate grows with the increase of reversed bending load or internalpressure for both different specimen with different loadings and same specimenwith multi-step loadings. In multi-step loading, ratcheting rate suffers from theratcheting history, especially for that with ratcheting history at higher levelloading. Ratcheting boundary for straight pipe is determined with the aid of thequasi-three-point-bending apparatus. Analysis of published rate-independent models for cyclic plasticity revealsthat model suitable for all materials and all loading paths is not available. Butsome models definitely satisfy a kind of material for most loading paths.Moreover, Ohno-Wang model and its modified models bring lights to theprediction of multiaxial ratcheting both for material and components.Taking advatanges of the User Programmable Features of ANSYS,elasto-plastic analysis for components was accomplished with user ANSYS inwhich Ohno-Wang model and its modified models were programmed.Ratcheting strains were predicted with the user ANSYS for pressurizedpiping under reversed bending with a number of models. It is found thatmodified Jiang-Setitoglu model predicts well for straights pipe and elbowsexcept for that with long radius which may be well predicted by Chen-Jiao-Kimmodel.Ratcheting boundary determination methods available in the current codeand literature were discussed for pressurized straight pipe under reversedbending. It is found that ratcheting boundary determined by EPFEA(Elasto-Plastic Finite Element Analysis) with Modified Jiang-Sehitoglu modelby ratcheting rate control method suggested by C-TDF divides the shakedownregion well. Ratcheting boundaries were determined for E90L, E90S and E45L.Based on the equal-strength of pressurized elbow, the theoretical equalstrength thickness distribution was deduced and an ideal thickness distributionwas suggested with the consideration of elbow processing characteristics.Ratcheting analysis of the elbows with this kind of suggested thicknessdistribution shows great resistance to ratcheting compared with equal thicknesselbows.