2 research outputs found
Non-associative plasticity for structural instability of cylindrical shells in the inelastic range
Thick-walled cylindrical metal shells, commonly used in tubular structures and pipelines,
during their lifetime may be subjected to considerable compressive loads, which can lead
to local buckling. In modelling their structural behaviour, the use of standard J2 flow
plasticity is known to produce unrealistically high buckling load estimates. Alternative
constitutive models, which consider the formation of yield surface ‘corners’, can provide
more accurate predictions, but they have been used scarcely, due to the limitations and
complexities they introduce.
The present work develops an efficient and versatile plasticity model to simulate the
structural response of metal shells under compressive loads. It combines the simplicity of
a Von Mises yield surface, with a non-associative flow rule, mimicking the effect of a
yield surface corner. The model allows for tracing the equilibrium path of the shells and
identifying structural instability in a consistent manner. A robust backward-Euler
integration scheme is developed, suitable for three-dimensional (solid) and shell finite
elements. The corresponding algorithmic moduli are obtained for nonlinear isotropic
hardening materials. The nonlinear dependence of plastic strain increments on the
direction of total strain increments is accounted for rigorously.
The constitutive model is implemented in Abaqus as a user material subroutine (UMAT).
Simulations of thick-walled metal cylinders under uniform compression show good
agreement with experimental data in predicting the buckling and post-buckling
performance of shells. The influence of geometric imperfection is considered, and
comparisons are made with models employing the J2 flow plasticity. The reliability of the
developed approach is further demonstrated by investigating more demanding problems
of bending and pressure in inelastic cylinders, taking into account ovalization, bifurcation
instabilities, imperfection. These problems involve non-trivial prebuckling equilibrium
paths, non-uniform loading and significant non-proportionality, before instability onsets,
which activate the model’s particular features, and illustrate their role in the evolution of
buckling. Analyses showcase the model’s capabilities, producing accurate instability
estimates, ultimate load and deformation predictions in line with experiments and clarify
aspects of the buckling of inelastic shells. Extending a traditional practice, a simple
method is presented for estimating the instability of inelastic cylinders under bending and
pressure loads, drawing on similarities in their buckling with that of cylinders under
compression