Strengthening of thin metallic cylindrical shells using fibre reinforced polymers
Abstract
Steel silos are widely used as long-term or short-term containers for the storage
of granular solids, of which a huge range are stored, from flour to iron ore pellets,
coals, cement, crushed rocks, plastic pellets, chemical materials, sand, and concrete
aggregates. The radius to thickness ratio for silos is in the range of 200 to 3000, so
they fall into the category of thin shells, for which failure by buckling is the main
concern and requires special attention in design. The primary aim of this thesis is to
investigate the possible application of Fibre Reinforced Polymer (FRP) as a new
repair and strengthening technique to increase the buckling capacity of thin metallic
cylindrical shells. Extensive research has been conducted on the use of fibre
reinforced polymer (FRP) composites to strengthen concrete, masonry and timber
structures as well as metallic beams. However, all these studies were concerned with
failure of the structure by material breakdown, rather than stability. As a result, this
thesis marks a major departure in the potential exploitation of FRP in civil
engineering structures.
Many analyses of cylindrical shells are presented in the thesis. These are all
focussed on strengthening the shell against different failure modes. Two loading
conditions were explored: uniform internal pressure accompanied by axial load near
a base boundary, and axial loads with geometric imperfections. For the latter, local
imperfections are usually critical, and two categories of imperfection were studied in
detail: an inward axisymmetric imperfection and a local dent imperfection.
For the first loading condition, which leads to elephant’s foot buckling, an
analytical method was used to derive general equations governing the linear elastic
behaviour of a cylindrical shell that has been strengthened with FRP subject to
internal pressure and axial compression. It was used to identify optimal application
of the FRP. All the later studies were conducted using nonlinear finite element
analysis (using the ABAQUS program) to obtain extensive predictions of many
conditions causing shell buckling and the strengthening effect of well-placed FRP.
In all the cases studied in this thesis, it was shown that a small quantity of FRP
composite, applied within a small zone, can provide a significant enhancement of the
resistance to buckling failure of a thin metal cylinder. These calculations demonstrate
that this new technique is of considerable practical value. However, it is clear that
not all the relevant questions have been fully answered, so the author poses
appropriate questions and makes suggestions for future work.