Abstract:
A leak-before-break (LBB) concept was developed for composite pressure
vessels (CPVs) to achieve a safe, predictable and controllable way of failure
preventing the consequences of a catastrophic rupture. Artificial defects were
introduced in the structure in prearranged patterns, acting as weak areas and
enforcing failure initiation and propagation from these locations. A continuum
damage constitutive model was developed through testing and simulation of
tensile and compression specimens at [0°]8, specimens under cyclic in-plane
shear at [±45°]2s, [+45°]₈ and [±67.5°]₂s as well as out-of-plane specimens at [0°]₁₀
and [0°]₁₂. A methodology was established for the introduction of artificial defects
in the composite material for its failure control considering fibre cuts and
interfacial defects. The LBB concept was investigated through the simulation of
the behaviour of CPVs including defects under internal pressurisation. The
assessment of the LBB behaviour was based on the ability to discern between
the occurrence of two leakages; the first associated to the leakage phenomenon
for pressure relief in the case of over-pressurisation and the second
corresponding to ultimate failure. The influence of size and degree of damage
induced through the defects was investigated, as well as the use of local
reinforcing patches to enhance the LBB behaviour. The most suitable design for
the optimal function of the LBB behaviour involves a circular fibre cut defective
area of 87.5% fibre cut damage which results in a clear separation between leak
and damage by a pressure difference of about 280 bar. The selected case was
used for the manufacturing of a closed-end loaded composite pipe to validate the
concept. The results of the testing showed that leakage did not occur from the
introduced weak points due to manufacturing defects; however, the prediction of
the damage initiation from the introduced defects was accurately identified with a
difference of 2% compared to the simulation results.
