Estimating the plastic collapse of pressure vessels using plasticity contours

Abstract

This investigation concerns the use of non-linear finite element analysis (FEA) to predict the plastic collapse of pressure vessels with and without defects. Computational non-linear plastic analysis of pressure vessels to estimate the collapse pressure is an iterative process. As a consequence, if the models are large and complex, considerable computer resources and time are required to generate solutions. As normal materials tend not to be ideally elastic–plastic (no sudden plastic collapse), a number of techniques are used to estimate the collapse pressure based on the FEA solution. These techniques often rely on plotting the pressure–strain curve; however, other techniques are also possible and are considered here. The aim of this paper is to assess the different methodologies in conjunction with FEA. Firstly, to consider their accuracy and secondly, to ascertain whether a particular technique would result in a reduction in the analysis time necessary to recover the plastic collapse pressure. Of the methods analysed, one based on the measurement (as a percentage) of the leading edge of the effective plastic strain (EPS) contour band showed promise. Under increasing pressures, regardless of geometry or material yield curve, the %EPS contour area coverage displays linear behaviour before through-wall plasticity is achieved. Based on this it is possible to extrapolate from low levels of plastic deformation up to plastic collapse pressures without requiring additional non-linear analyses, resulting in significant cost reductions in computational analysis times.