High Strain Fatigue In Pipes

Abstract

If a component is subjected to a cyclic plastic strain superimposed upon a constant load in an orthogonal direction there is an irreversible plastic ratchet strain, in the direction of the constant load, for each cycle of plastic strain. A typical example of this behaviour is a cylinder subjected to cyclic plastic axial loading superimposed upon an applied hoop stress. The effect of ratchetting for successive cycles is cumulative and may cause a component to fail, and is of concern in the design of pressure vessels and piping runs within the power generation industry. The Design Code [1] specifies basic stress intensity limits based upon static collapse, rather than incremental collapse resulting from high-strain low-cycle fatigue conditions. It has been suggested [2] that the code is over conservative leading to excessive piping supports and higher thermal stresses. As part of a programme of work into high-strain low-cycle fatigue conditions the plastic deformation of thin walled tubes has been experimentally determined under ±1% cyclic axial strain with constant hoop stresses of approximately 0, 1/4, 1/2 and 3/4 the uniaxial yield stress [3,4]. Buckling of the tubes under these biaxial stress conditions often occurs after a few cycles of plastic load and renders further data collection unreliable. Validated finite element models may predict ratchet strain data for a variety of loading conditions and material properties without the buckling problem. Initial work for 304Sll austenitic stainless steel is promising. This work has been carried out using the ANSYS® finite element code and its associated material non-linearity routines. Wide ranging sensitivity studies have previously been carried out on the validated model into the effect of material parameters on ratchet strain limits [5,6]. The effect of magnitude of plastic cyclic load and material shakedown upon ratchetting limits are reported here together with a comparison of the bilinear and multilinear kinematic hardening models which are available within the code.