UWE Bristol Engineering showcase 2015

James Pickup
Meng Mechanical Engineering
Project Supervisor
Laura Maybury, in association with MBDA
Bonding in pressure vessels
An investigation in to the plausibly of permanently bond together a pressure vessel with epoxy adhesive.
The existing pressure vessel design is assembled
with screw-in bolts but according to MBDA’s
observations this tends to leak, losing it internal
pressure. It is thought that this is due to the
screws loosening over time, reducing the force
holding the lid down tight. MBDA wish to solve
this problem by finding a way of permanently
bonding the pressure vessel parts together,
ensuring that it does not leak or explode under the
internal pressure of 10 bars, as well as meeting all
necessary criteria.
The overall aim of this two year investigation is to
prove the plausibility of assembling a pressure
vessel with adhesive bonding, while reaching all
necessary performance requirements. This first
year focuses on the stress of the bond and show
whether the internal pressure will cause failure of
the pressure vessel.
The properties of the bonding method was
researched and the bonds tested through
analytical calculation, modelling in finite element
analysis and practical experimentation.
A practical element to this study is necessary to
allow for ‘real life’ tests to verify the results and
predictions from computer analysis. In order for
this to be possible there must be a set of results
that can be directly compared. The practical tests
will be design in a way that will be easily replicated
in the FEA modelling, by breaking down the
combinations of different stresses into their
individual components. The types of stress in the
bond will be found by studying the different ways
a joint can fail. It is therefore important for the
practical testing to include all aspects of joint
stress, as well as accurately portraying the real life
reactions for each type of stress so that the output
values can be compared.
The parts of the pressure vessel were built in
Abaqus and then assembles surface to surface and
meshed according to the tested methods of the
previous section, with the mesh becoming fine
closer to the bond, and a very fine mesh of three
elements thick for the layer of epoxy.
By removing the parts of the pressure vessel,
leaving only the epoxy the stress distribution is
clearly visible. The highest levels of stress are at
the inner surface, reducing at the radius increases
through the pipe wall as seen below.
Project summary
This investigation is based on the
improvement of an existing pressure vessel
design, built by MBDA. The existing design of
the pressure vessel fastens an lid to the main
container with bolts. Unfortunately, MBDA
have noticed a reduction in pressure, as the
vessels leaks. A possible solution to this
leaking is to redesign the vessel with a
permanent bonding method.
Project Objectives
Through a combination of analytical
calculations, FEA modelling and practical
calculations it is hoped to prove the
plausibility of using an epoxy resin to provide
a permanent bond to assemble the pressure
vessel.
The thick wall pressure vessel equations, shown
above, were used to give an initial estimate of the
levels of stress expected in the container. These
results were then validated in FEA Abaqus.
In order to prove that the pressure vessel will not
explode from the adhesive failing under the internal
pressure, it is necessary to build a complete vessel
model within the FEA software, Abaqus. This complete
model will include a solid part layer representing the
epoxy in the assembly. This is the only way to test the
whole pressure vessel, as it has not been possible to
obtain nor destructively test a fully sized, real pressure
vessel.
It is important to consider any possible error there
may be in the comparison results, but even with
the stress error doubling the current results the
epoxy would still hold the applied forces.
Critical results
Von Mises Stress: 8.63 x 1.2 = 10.356 MPa
Reserve factor: 30.1 x 10.356 = 2.90
Project Conclusion
The final figures of stresses expected in the
epoxy bond compared to the maximum
stresses of the epoxy show it is capable of
withstanding the internal pressure.