Abstract
M.Ing. (Civil Engineering)
Pipe material behaviour plays a major role in the crack opening of pressurised water
distribution systems. Several studies have shown that the amount of water that leaks
from a crack can be much more sensitive to pressure than theory suggests. The aim
of this investigation is to understand the structural behaviour of longitudinal cracks in
pipes under pressure. This is achieved by subjecting several plates with different
length of cracks to tension and monitoring the opening of the cracks. A theoretical
model for a longitudinal crack opening is derived using the orifice equation, as a
function of the pressure, pipe material properties, pipe geometry and fluid properties
for uni-axial stress state. Subsequently an equation describing the increase of the
leakage flow rate as a function of the increase of the crack area in uni-axial stress
state is determined. Results show that the material around the longitudinal crack
exhibited elastic expansion behaviour due to hoop stresses induced by internal
pressure in the pipe. Therefore, the crack opening area increases until the material
exceeds its yield strength, causing a bulging of crack faces thus resulting in a
significant increase of the leakage flow rate. Conclusions are made in relation to the
influence of longitudinal crack expansion to an increase flow rate through longitudinal
crack opening in pressurised pipes. Results include the effects of geometrical and
material variables on the expansion of the longitudinal crack. A better explanation of
the increased flow rate through longitudinal crack, demonstrated by experimental data
is presented in this study as a theoretical model. The leakage exponents resulting from
the derived flow rate equation show a linear relationship with pressure a low stress
and an exponential relationship at high stress. Further investigations are
recommended to derive a model of flow rate under varying internal pressure for an
increase of crack length combined with an increase of crack propagation. A coefficient
as a corrector factor should be determined in order to greatly improve the theoretical
derived equation of the maximum deflection of rectangular plates with a central crack
under uniaxial tensile stresses.