ASSESSING LIFETIME OF POLYOLEFINS FOR WATER AND GAS
DISTRIBUTION SYSTEMS
James Atteck, Dr John Dear
Department of Mechanical Engineering
Imperial College London
SW7 2AZ
UK
james.atteck@imperial.ac.uk; j.dear@imperial.ac.uk
Dr Jean-Louis Costa, Dr Aurélien Carin and Jean-Pierre Michel
Ineos Polyolefins
B –1120 Brussels
Belgium
Jean-Louis.Costa@innovene.com; Aurelien.Carin@innovene.com;
Jean-Pierre.Michel@innovene.com
ABSTRACT
This study has the following research objectives:
•
To devise a method to achieve well-controlled brittle failures in polyethylene pipe materials that have very high stress
crack resistance. This is for repeat assessment of the method and its potential for subsequent use for quality control
of material manufacture.
•
To provide a method for researching a wide range of new and existing materials as to their brittle failure
characteristics. This is from the initial onset of brittle failure and its subsequent development.
•
To develop the method to include, for example, providing for researching the effects of varying the dimensions of
internal and external layers of multi-layered structures and the different properties of the materials in these layers.
Introduction
Continuing improvement in the properties of polyolefin materials is much contributing to the increased use of these materials in
different products. For pipes and other components of water and gas distribution networks there is much interest in more
rugged materials that can endure better these pipes’ demanding operational and environmental stresses. One reason for this
is that after installation, under busy streets of towns, cities and other thoroughfares, subsequent access to repair or replace
pipes is costly and can be disruptive to other services including transport. Improving the rugged strength of new pipe material
can require improved or new methods to evaluate the survival ability of the new or improved materials. One important factor, in
assessing polyethylene pipe materials is their resistance to brittle failure. This paper reports on research to evaluate a new
tougher generation of pipe material. This is to complement the use of the standard Full Notch Creep Test (FNCT) techniques
that were developed by Fleissner [1] and studied by other researchers [2].
The aim of this research study is to achieve more precise comparison of stress crack resistance data for the new and
existing polyethylene materials. This study includes researching the fracture surfaces of the different materials as they are
formed from the first stage of crack initiation. Important is that the forces applied to the material to propagate the crack can be
well defined. Also, of interest is the different propagation features generated by the crack in different materials. AFM and SEM
are two of the methods that will be used for these studies. The research outcome, of this study, could include improved
information identifying causes of early pipe failure for different operational and environmental stress conditions in water and
gas pipes.
Experimental Equipment
For the FNCT studies, the rigs are a constant stress pivot arm system with dead weights to apply appropriate loading. The arm
is a 4:1 ratio of length on either side of the pivot. Each arm holds a sample and there are three arms per rig. To be able to test
in an environment, the samples sit in a tank that is temperature controlled to within 0.5°C and a solution is circulated within the
tank. The bath is made up of a glass tank of approximately 12 litres placed inside a much larger PMMA tank. To provide good
insulation, expanding polyurethane foam is used to fill the space between the glass and PMMA tank. For continuous use of the
rig, individual broken samples can be removed from the tank easily whilst others are still running as the lid has six sections
allowing the tank to be partially opened. To accelerate the tests the solution in the bath used is a mixture of water and 2% by
weight Arkopal N100. This solution is heated and circulated at 80°C.
Timing switch
Pivot point
with bearing
Counter balance
Sliding balance
weight
Dead weights
Figure 1. Side view of pivot arm
There is a sliding weight on the long side of the arm used to balance the system prior to every experiment before the dead
weights are applied. For more accuracy a 0.25 kg weight is placed on the hangar and 1 kg placed on the short arm end.
Reset
Counter
Timing switch
Figure 2. Photo of timing device
Figure 2 shows the timing device used. It counts and stops when the switch is activated. The counter’s maximum resolution is
0.1 of an hour.
Thermometers
Cold Outlet
Hot inlet
Figure 3. Front view of rig
Universal Joints
Sample
Figure 4. Grips that hold the sample
Attached to the top and bottom of the sample are grips that hold the specimen to the arm and base of the rig. Between the grip
and the rig is a universal joint that prevents any twisting or bending of the sample (Figure 4) and ensures no shear stresses
are applied. The samples are 10 mm x 10 mm cross-section and 100 mm long.
Notching methods
There are two methods that have been investigated thus far with varying notch depth, which are a four-sided square notch and
a circular notch. The square notch is prepared by forcing a razor blade into the sample at a rate of 0.25 mm/min using an
Instron machine with custom fixtures (Figure 5). The specimen sits in a base that has a stop on the bottom end of the sample
and a push screw at the top. This ensures that the notch is always at the right height. A new blade is used after 4 notches.
Razor blade
Sample
Figure 5. Instron fixtures
The Circular notch is made in two stages. First a V-groove with an angle of 85° is cut into the sample on a lathe that is rotating
at 300 rpm. At a very slow feed rate, the cutter is fed into the sample. The sample is removed from the lathe and by hand a
razor is dragged across the bottom of the groove creating a notch ca. 0.2 mm deep. This is being replaced by a special
attachment for the lathe to control the slicing depth more precisely.
Notching Geometries
To date, two types of notching geometry have been investigated, square (Figure 6(a)) and circular with a groove (Figure 7(b)).
In an effort to reduce the time to failure further, other geometries will be investigated including those in Figures 6 and 7.
a)
c)
b)
d)
e)
Figure 6. Cross-section views of different notching geometries: a) Four equal depth notches - square ligament, b) Circular
notch - centrally located, c) One deep notch and two shallow notches – three-sided rectangular ligament similar to PENT test
[3], d) Two equal depth notches - rectangular ligament, e) Two equal depth notches - square ligament located in one corner.
a)
c)
b)
d)
Figure 7. Cross-section views: (a,b) and side views: (c,d) of notching geometries with grooves: a) Square ligament with square
groove, b) Circular ligament with circular groove, c) U-groove made with end mill for square groove, d) V-groove made on lathe
for circular notch and circular saw milling for square ligament.
Results
This research explores factors that can affect the evaluation of brittle failure life-time of polymers studied. These include the
latest materials that have been developed to achieve an improved resistance to brittle failure of pipe material. One variable
studied relates to the preparation of material specimens for brittle crack evaluation. For the square notch (Figure 6(a)) in the
latest toughened material the depth of the inserted razor was varied from 1.6 mm to 4 mm in one study. Increasing the depth
of the sharp crack interestingly increased the time to brittle crack failure of the specimen. A possiblity is that the action of
pressing the razor blade into the tough material sample had the undesired effect of disturbing the crack tip material in a way to
blunt the induced crack tip. Also it has been shown that there can be different densities of compacted fibrils remaining at the
crack tip formed by a razor blade in this material[4]. For the circular notching method (Figure 7(b)) the razor crack was
produced by a corner of the sharp edge of the blade as the specimen was rotated. The indications are this is producing much
less compacting at the induced crack tip as there was not increasing time of crack failure with increase of crack depth. This is
now subject to more detailed study.
Figure 8 shows the time to failure of different ligament cross sectional areas for square and circular geometries as found at this
time. This is for constant ligament stress conditions. To be explained by further research is the scatter of results and more
precisely the change of indicated time to brittle failure relative to depth of induced razor blade crack. Early indications are that
the circular notching method promotes a shorter failure time. The intention is to explore this and other notching methods.
70.0
Ineos square notch
IC square Notch
60.0
IC Circular Notch
Predicted Line
Ligamnent area / mm2
50.0
Trend Line
40.0
30.0
20.0
10.0
0.0
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
450.00
500.00
Falure time / hrs
Figure 8. Time to failure versus Ligament area for different geometries for constant ligament stress conditions
Fracture Surfaces
Figures 9 and 10 show the FNCT loading geometry with an example of a fracture surface formed. Figure 9 is a square notch
as described in Figure 6(a) and Figure 10 is a circular notch as described in Figure 7(b) and 7(d). Both have failed in a brittle
manner as seen in the figures.
Figure 9. Square notch 1.6mm deep on all sides
Figure 10. Circular notch groove 1.6mm deep
Load Traces from Notching
Developing the use of an Instron testing machine for notching of specimens usefully provides traces of load versus depth of
insertion of the razor blade. This provides an indication of residual stresses in the polymer created when the samples were
moulded and cut to shape. These residual stresses may well affect the crazing process and therefore the time to failure. Sides
1 and 2 are the top and bottom of the plaque that were in contact with the moulding surfaces and sides 3 and 4 were milled
away to make the specimen. Figure 11 shows that more force was required to notch faces 1 and 2, than 3 and 4, implying that
there are residual stresses remaining from the moulding process that may affect the time to failure.
Figure 11. Load versus extension for a square notched specimen (1-top, 2-bottom, 3-left, 4-right faces of the specimen)
Conclusions
Worldwide, there is demand for polymeric material for gas and water distribution pipes that are highly resistant to failure by
brittle crack propagation and other failure processes. Many improvements in the properties of polymeric pipe materials are now
arriving on the market and there is a need to changes in assessment procedures to provide failure data for these new
materials. The changes to the evaluation methods need to preserve those currently being used for existing pipe materials still
widely in use. This research to date has identified some of the problems to be overcome to achieve smaller scatter of results of
fracture data for the same material and pre-crack conditions. To also be achieved is an understanding of fracture surface
morphology generated by different fracture conditions in laboratory experiments and how these relate and can be helpful in
researching failure processes in pipes installed in distribution networks. It is very important that consistent results can be
obtained. This is to relate to the inherent properties of existing and new pipe materials as to their fracture characteristics and
the conditions identified as to the risk of the pipes failing in service in different installation conditions. Assessment data is
needed for manufacturing quality control of material and for this purpose assessments needs to performed quickly so that
remedial action, if needed, can be taken before a great deal of material has been produced. With this in mind a 500hr or less
assessment time is the aim.
References
1.
Fleissner, M. “Experience With a Full Notch Creep Test in Determining the Stress Crack Performance of Polyethylenes”,
Polymer Engineering and Science, vol. 42, 330-340, 1998.
2.
Ting, S.K.M., Williams, J.G. and Ivankovic, A. “Characterization of the Fracture Behaviour of Polyethylene using Measured
Cohesive Curves. I: Effects of Constraint and Rate”, Polymer Engineering and Science, vol. 46, 763-777, 2006.
3.
Lu, X. & Brown, N., "A Test for Slow Crack-Growth Failure in Polyethylene Under A Constant Load", Polymer Testing, vol.
11, no. 4, pp. 309-319, 1992.
4.
Williams, J.G., Private Communication, 2006.