Gunnar Bergström, Mathias Flansbjer, Linda Karlsson,
Sven-Erik Sällberg och Kristian Thörnblom
Building Technology and Mechanics SP Report 2009:21E
SP Technical Research Institute of Sweden
Acceptance criteria for scratches and
indentations in plastic pipes
Gunnar Bergström, Mathias Flansbjer, Linda Karlsson,
Sven-Erik Sällberg och Kristian Thörnblom
Summary
Development of materials and laying techniques have arisen the question how scratches and indentations in plastics pipes affect the strength and technical lifetime of pipelines. Scratches may occur both in the manufacturing of the pipes, in connection with the installation and subsequent maintenance. The problems in assessing the failure risk from scratches and indentations are similar for pipes used in gas and water distribution and in district heating applications. Therefore, a broad effort to identify the risks in relation to current pipe materials is technically and economically justified.
The project has aimed to evaluate the effect of scratches and indentations on the technical lifetime of plastics pipes and to present criteria for maximum allowable depth of scratches and indentations.
The study on pressure pipes focused on the conditions for scratched polyethylene pipes to achieve a lifetime of 50 years. It is noted that a scratch damage can not be judged solely on the basis of its depth and sharpness. To assess the impact of the scratch on the
serviceability of the pipe, consideration must also be taken to the material from which the pipe was made and the safety factor used in the design. Extensive pressure tests show that pipes made of modern materials can withstand surface scratches to a higher degree than pipes of older materials. For pipes made of modern bimodal PE80 and PE100 materials, scratches up to 10% depth may be accepted without reduction of rated pressure. However, for pipes of older material, a reduction in pressure may be required already at smaller scratches. The study also indicates that for the same relative scratch depth, a greater reduction in pressure is required with increasing pipe dimension.
The study on the nonpressure pipes shows that the studied polypropylene pipes resist both deep scratches combined with ovalisation and large indentations without any cracks penetrating the pipe wall. However, the extent and development of crazing and surface cracking vary with scratch depth, deformation level and material. The test pipes were subjected to extreme conditions very rarely or never occurring in practice. This suggests that small scratches at moderate ovalisation and realistic indentations in temperatures around room temperature do not affect the lifetime of the pipes.
One objective of the project was to develop a method for the evaluation of the scratch resistance of a pipe. In the proposed test method the force needed to produce a specified scratch is measured. This force is used as a relative measure of the scratch resistance of a pipe material when compared to other materials.
In order to assess to what extent a scratch affects the lifetime of the pipe, the depth of the scratch must be estimated with reasonable accuracy. Since the scratch in many cases is found on existing pipelines the method must be suited for field use. To achieve that a simple instrument for scratch depth measurements was made and evaluated.
Key words: Plastics pipes, PE100, pressure pipes, scratches, indentations, life time, crazing
SP Sveriges Tekniska Forskningsinstitut SP Technical Research Institute of Sweden SP Report 2009:21E
ISBN 978-91-86319-08-3 ISSN 0284-5172
Contents
Summary ContentsSummary 3
Contents 4
The authors’ preface
6
1
Introduction 7
1.1 Background 7
1.2 Purpose, objectives and limitations 7
1.3 The structure of this report 8
2
Scratches and indentation in plastic pipes
9
2.1 Introduction 9
2.2 Failure mechanisms in polyethylene 9
2.3 How scratches and indentations occur 10
2.4 Effects of scratches and indentations 13
2.4.1 Pressurised pipes 13
2.4.2 Non pressurised pipes 13
2.4.3 Casings on district heating pipes 14
3
Pipes for pressurised applications
16
3.1 Introduction 16
3.2 Pipe materials 17
3.3 Pipe dimensions, type of scratches and depth of scratches 17
3.4 Test method and procedure 18
3.5 Results 20
3.6 Discussion 30
4
Pipes for nonpressurised applications
32
4.1 Introduction 32
4.2 Pipe materials 32
4.3 Test method and test procedure 33
4.3.1 General 33 4.3.2 Ovalisation 33 4.3.3 Spherical indentation 34 4.4 Results 36 4.4.1 Ovalisation 36 4.4.2 Ball indentation 41 4.5 Discussion 44
5
Method for assessing scratch resistance
46
5.1 Introduction 46
5.2 Pipe materials 46
5.3 Test method and test procedure 47
5.4 Results 49
5.4.1 Evaluation of the test method 49
5.4.2 Evaluation of the scratch cutting tools 55
5.4.3 Investigation of multilayer pipes 56
6
Scratch depth measurement in the field
59
6.1 Equipment and use 59
6.2 Verification of the scratch depth gauge 60
6.2.1 Probe tip geometry 60
6.2.2 Contact between the probe tip and the material 61
6.2.3 The geometry of the scratch 61
6.2.4 Measuring scratch depths with the two different probe tips 62
6.3 Summary 64
7
Conclusions 65
8
References 67
Appendix 1
Results for pressurised pipes
Appendix 2
Results for nonpressurised pipes
Appendix 3
Results from scratch testing
Appendix 4
Using the depth gauge
The authors’ preface
The parties in the plastic pipe market have sought acceptance criteria for scratches and indentations in different types of pipes and for different types of pipe duties. A combined effort, to investigate and identify the risks of mechanical damage to pipes made of the materials concerned, has therefore been regarded as both technically and economically justified.
The work has been financed by the Swedish Gas Centre / Swedish Energy Agency, the Swedish Water and Waste Water Association, Borealis, pipe manufacturers through the Nordic Plastic Pipe Association, the Swedish District Heating Association and SP Technical Research Institute of Sweden. This report SP Report 2009:21E is a translation of SP Report 2009:21. In the event of any differences between the reports, it is the Swedish version that shall take precedence.
Our grateful thanks are due to the project’s reference group, which has put forward valuable views during the progress of the work. The group consisted of Bo Andersson, Ingemar Björklund, Hans Bäckman, Staffan Karlsson, Jan Lindeberg and Hans Sandberg.
1
Introduction
1.1
Background
High density polyethylene (HDPE) pipes have been used with considerable success since the middle of the 1950s. Over the years, modifications of the molecular structure and of production processes have contributed to a significant improvement in the quality of pipes, to the extent that the classification of HDPE pipes has increased from PE63 to PE80 and, today, to PE100 and PE100+. The higher classification means that the pipes can operate at higher pressures, which in turn improves their transport performance. In order to reduce the costs of laying plastic pipes, there is a constant search for new methods of laying, accompanied by greater use of existing unspecified backfill materials. Trenchless laying methods are attracting increasing interest, with a number of different methods in use today. The methodology is based on pulling pipes through an existing pipe (pipe-bursting, relining etc.), or through a drilled or bored tunnel in the ground (controlled drilling). This means that, in comparison with the traditional methodology of an open trench, it can be possible to traverse existing obstacles such as waterways, roads or railway embankments relatively easily. Nilsson & Thörnblom (2005) showed that pulling a pipe through the ground or through a burst existing pipe subjected the new pipe to very rough treatment, with a considerable risk of causing severe scratches or scrape damage.
When using existing backfill materials for filling trenches, there is a risk of damaging the pipes by stones or rocks pressed into the pipes as the pressure from above increases. The worst scenario is thought to be that of pressing a sharp stone with a small radius into a pipe, which is fixed into or surrounded by the supporting material in such a way as to prevent it from compensating for the pressure.
Nilsson & Thörnblom (2005) also showed that damage can occur to plastic pipes even before they are delivered to the customer. Usually, however, such damage consists mainly of smaller scores and scrape marks, having probably occurred during transport. Pulling pipes along asphalt surfaces, during an installation, has a considerable effect on the pipes. Development of materials and methods of laying pipes have concentrated attention on how scratches and indentations affect the strength and service life of plastic pipes. The problems of assessing the risks of failure caused by scratches and indentations are similar for gas pipes, water pipes and district heating pipes. A joint effort to identify and quantify the risks for various materials is therefore both technically and economically justified. The parties involved in the market have sought acceptance criteria for scratches and indentations on different types of pipes and pipe applications. More recently, the question has been raised as to how it can be ensured that pipes laid by guided drilling or pipe-bursting have not been adversely affected.
1.2
Purpose, objectives and limitations
The project has aimed to evaluate the effects of scratches and indentations on the physical life of pressurised and non pressurised plastic pipes, and to develop criteria for maximum permissible depths of scratches and indentations. The intention has also been to propose one or more methods that can be used at installation sites in order to ensure that the particular pipe concerned meets the requirements. Three concrete objectives for the project were set in the light of present day knowledge:
• To investigate how the long term strength of a pipe, as determined by brittle fracture, is affected by scratches and indentations that can occur in practice. • To develop a method of evaluating the sensitivity of a pipe or piping system to
scratches.
• To develop a method of assessing the external condition of a pipe, suitable for use in the field.
In the longer term, it is thought that the results can be summarised and used as a basis for publication of guidelines and recommendations concerning choices of materials, design of piping systems and methods of laying, suitable for use in various applications and varying ground conditions.
1.3
The structure of this report
The various sub areas of the project are described separately on the following pages. An introductory section describes the overall state of knowledge concerning the effect of scratches and indentations in plastic pipes, after which each sub area is treated separately:
• Pipes for pressurised applications • Pipes for non pressurised applications • Methods of assessing sensitivity to scratches
• Methods of measuring the depth of scratches in the field.
Each sub area consists of a short introduction, together with a description of the methods and materials that have been used. It is concluded by a presentation of the results and a summary discussion. Finally, a summary of the most important conclusions from the report is presented at the end of the report.
2
Scratches and indentation in plastic pipes
2.1
Introduction
Research into scratches and indentations, trenchless methods of installation and
protectively sheathed pipes is in progress in many places. It is particularly pressurised and district heating pipes that have been the object of research and development. A number of survey studies of damage to non pressurised pipes have been carried out in the USA, but there are no more detailed analyses of how such pipes are affected by scratches and indentations. In the case of pressurised pipes, it is the internal positive pressure that is the critical load, while for non pressurised pipes it is the deformation resulting from the external pressure exerted by the surrounding earth. The external casing pipe of a district heating pipe is subjected to forces part way between the above two critical loads. Filling the outer casing pipe with insulating foam subjects it to a significant internal pressure, but in normal conditions it is probably the potential deformation from point loads that
constitutes the most critical case.
SP performed a review of the literature in 2006 (Nilsson & Sällberg [2006]), in order to determine the level of awareness of the effects of scratches and indentations on plastic pipes: how, and under what conditions, they occur, and to what extent they reduce the life of the pipes. The study showed that there was awareness of the damage caused by
scratches and indentations during installation and subsequent operation, as a result of field investigations. It was also found that there was reasonably good knowledge of how this type of damage affects the risk of rapid ductile failure. However, actual pipe failures are generally caused by brittle failure as the result of a crack gradually propagating through the wall of the pipe. Just how cracks initiate and propagate through polyethylene has been investigated for many years, so that we now have a good picture of the
phenomenon, applying the knowledge to materials testing. Notches are regularly used in order to accelerate tests and lead more quickly to failure. However, what seems to be missing is an understanding of to what extent, and under what conditions, real scratches and indentations can act as crack initiators, i.e. accelerate brittle failure. This is a piece of the puzzle that is needed before we can start to develop methods of evaluating service life and specifying thoroughly supported performance requirements in respect of the
permissible extent and magnitude of scratches and indentations.
2.2
Failure mechanisms in polyethylene
Polyethylene is a semi crystalline thermoplastic material which, in the same way as with other thermoplastics, exhibits time dependent mechanical properties. Its visco elastic behaviour can be manifested either as creep or relaxation. Creep means that deformations increase with time if the stress is maintained constant, while relaxation means that the stresses decline with time if the deformation is maintained constant.
At high stress levels, polyethylene is extremely tough. The material starts to yield somewhere around 20 MPa and 15 % strain, but then continues to be deformed up to a rupture strain of about 500 – 800 %. At low stress levels, the creep process proceeds only slowly, with the result that the time to ductile failure can be very long. The ductile process can then be overtaken by a failure mechanism under the name of Slow Crack
Growth (SCG). Slow crack growth results in brittle failure, known as Stage II failure,
occurring at modest stress levels that are not sufficient to result in plastic creep failure. The phenomenon requires a crack to have been initiated, which can be caused by a defect in the material or through mechanical damage to the surface layer.
Polyethylene differs from brittle materials, in which a crack is normally either stationary or propagating at high speed, by being able to exhibit stable and slow crack growth, with propagation controlled by the formation and deterioration of greatly extended fibrilles at the crack tip. Such regions of orientated material are referred to as crazes. Failure occurs as a result of the molecules that hold several areas of crystalline material together breaking or becoming unattached. This means that materials having a high proportion of
tie molecules have a better strength in terms of resisting slow crack propagation (Nilsson
& Sällberg 2006).
The service life of the material is also limited by its chemical stability. Polyethylene is affected by thermal oxidation as a result of degradation of its molecular structure by oxygen, with the formation of carbonyl groups. Anti oxidants are added to the polymer in order to make the material less sensitive to this effect. However, when the anti oxidants have been consumed, it is the polyethylene molecules that become susceptible instead, with the result that the material properties quickly deteriorates. This means that it is the time until the anti oxidant materials are used up that sets an upper limit for the service life of the material, with this time also being affected by the temperature in accordance with an Arrhenius relationship: see Karlsson et al. (1992).
Fractures resulting from ductile failure, slow crack propagation or thermal degradation are often referred to as Stage I, II or III fractures respectively. Figure 2-1 illustrates the schematic relationship between the applied stress and time to failure of the various stages.
log (failure time)
A p p lied stre ss Stage I: Ductile
Stage II: Brittle
Stage III: Degradation
Stadium II: Brittle Stadium I: Ductile
Stadium III: Degradation
log (time to rupture)
A
p
plied str
ess
Figure 2-1. Failure mechanisms for polyethylene and their schematic relationship with the applied stress level.
2.3
How scratches and indentations occur
Plastic pipes are exposed to the risk of damage in the form of scratches or indentations during transport, installation and operation. As the use of trenchless installation methods such as directional drilling, pipe-bursting etc. become more common, or that of
conveniently accessible backfill materials, but with unspecified characteristics, becomes more common, increasing attention has been focused on the problems of external damage. When a pipe is pulled through ground, or through a burst existing pipe, it is exposed to quite severe treatment, with a considerable risk of receiving significant scratches or scrapes. Stokes et al. (2000 & 2001) investigated the depth of scratches in plastic pipes that had been laid by directional drilling or pipe bursting in a number of different installation projects. The results clearly showed that pipe-bursting normally results in deeper scratches than in case of directional drilling, being of the order of 1 mm
as against 0.2 mm respectively. The conclusion was drawn that it is only when employing pipe bursting with small pipe diameters, and thus with low wall thicknesses in absolute terms, that there is a risk of the depth of scratches exceeding 10 % of the pipe wall thickness. See Table 2-1 for details.
Table 2-1. Statistics of the most prominent scratches in investigations carried out by Stokes et al. (2000 & 2001).
Directional drilling Pipe-bursting ∅90 SDR11 ∅90 SDR11 ∅180 SDR11 No. of pipes investigated 5 19 5 Deepest scratch 0.157 0.899 0.916 Average value (mm) 0.107 0.343 0.433 Standard deviation (mm) 0.028 0.234 0.287
A similar investigation was carried out by Nilsson & Thörnblom (2005), and confirmed Stokes’ results. It was also noted that normal handling of the pipes, between manufacture and delivery to site, resulted in scratches with a depth of the same order as that caused by directional drilling: see Figure 2-2. In addition, handling at the installation site – pulling tubes across gravel, asphalt, the edges of excavations etc. – can produce a considerable number of scratches: see Figure 2-3.
R1 R5/R6 P1 P2 P3 P4 R6/R7 1.6 1.2 0.8 0.4 0 D epth of dee pest s c ra tch, mm First reference pipe Last reference pipe before
test pipes Wavin TS
Uponor Profuse Hallingplast PipeLife Robust First reference pipe after test pipes HORIZONTAL DRILLING PIPE BURSTING AT DELIVERY
Figure 2-2. Measured maximum depth of scratches of pipes, as received on delivery, after directional drilling and after pipe-bursting. Nilsson & Thörnblom (2005).
Nilsson & Thörnblom also investigated how the condition of the surface of the pipes affected the occurrence of scratches. Four different multi layer pipes from Wavin, Uponor, Pipelife and Hallingsplast were investigated, showing a clear relationship
noted that Uponor Profuse, having the hardest but thinnest sheath, was the only pipe of which the sheath was penetrated.
Figure 2-3. Scrape damage to pipes and weld beads after dragging along an asphalt road surface. Nilsson & Thörnblom (2005).
Stones or other objects in the ground can also cause external damage to pipes if they act as point loads and cause indentations in the pipe wall. Bergström and Nilsson (1999) performed field trials with polyethylene gas pipes laid in coarse backfill material. Their investigations included indentations in the pipe wall caused by three different backfill materials: 0 – 8 mm crushed material, 0 – 100 mm crushed material and 0 – 100 mm natural material. The pipes were laid in a road embankment, where they were crossed about 8000 times by heavy vehicles. Using a video camera, it was found that the most severe indentation had occurred in a 160 mm diameter SDR11 pipe, made of PE80 grade material, and was about 8 mm deep. When the pipes were dug up, the particular stone that had caused this indentation was recovered, and was found to be a natural gravel fraction (having rounded edges), underneath the pipe and having a largest dimension of about 125 mm. The same project also carried out trials of district heating pipes (Molin et al. 1997) with the deepest residual indentations found after recovering the pipes having a depth of about 4 mm: see Figure 2-4. These indentations were probably somewhat deeper when the pipes were still in the ground. It is interesting to note that the coarse natural material caused considerably more and deeper indentations than did the crushed material.
Crushed material 0-100 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Nu m b er of Natural material 0-100 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Crushed material 0-8 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Crushed material 0-100 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Nu m b er of Crushed material 0-100 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Nu m b er of Natural material 0-100 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Natural material 0-100 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Crushed material 0-8 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm) Crushed material 0-8 mm 0 1 2 3 4 5 6 0-1 1-2 2-3 3-4 Indentation depth (mm)
Figure 2-4. Number and depth of residual indentations in one of the district heating pipes that was tested. Molin et al. (1997).
2.4
Effects of scratches and indentations
2.4.1
Pressurised pipes
It is not at present fully clear what the consequences of a real scratch may be in terms of affecting the service life of pipes intended for pressurised applications such as water and gas distribution. A scratch in a pressurised pipe automatically results in a stress
concentration, in that the hoop stresses in the pipe must be carried by a thinner section of the pipe wall. In addition, depending on its geometry, the scratch can act as a notch from which a brittle failure crack can propagate. In their own technical materials, the
manufacturers (Pipelife Sverige [2000], Nordisk Wavin [2000], Wavin Plastics [2001] and Rix [2005]) recommend that the depth of the scratch should not exceed 10 % of the wall thickness, and that its geometry must not be too sharp. This has become a rule of thumb, but is still not confirmed.
A report published by Zhou & Chang (2006) describes the effect of scratches on the service life of pressurised PE100 and PE80 grade pipes. The project investigated SDR 11 pipes, extruded from bimodal PE100/PE4710 and unimodal PE80/PE3408 material, in order to investigate the effect of scratches on the pipes’ hydrostatic properties. The pipes were scratched longitudinally with a 60° V-shaped notch in order to simulate the type of scratches that are likely to occur during installation. The depth of the notch in relation to the thinnest part of the wall thickness was checked as 5 %, 10 % and 15 %. Pressure tests were carried out on both scratched and unscratched pipes at temperatures of 80 °C and 90 °C. The results were extrapolated to room temperature by a three parameter Rate Process Method (RPM), related to that specified in ISO 9080. The concept of scratch sensitivity was introduced in order to investigate the effect of scratches on time to failure, being defined as the time to failure of scratched pipes divided by the time to failure of unscratched pipes when exposed to the same stress level and at the same temperature. The report shows that scratch sensitivity increases with increasing stress levels.
Zhou & Chang used the three-parameter Rate Process Method (RPM) in order to extrapolate the pressure test results to room temperature. According to this method of analysis, both pipes were expected to survive for thousands of years at the maximum operative pressure (MOP). Pipes with scratches to a depth of 10 % of their wall thickness were expected to have a considerably longer life than pipes having 5 % deep scratches at 20 °C and 23 °C. The fact that pipes with a deep scratch were expected to have a
considerably longer life than pipes with a less deep scratch, as presented by Zhou & Chang, conflicts with previous experience and reasoning. An expected life of several thousand years also conflicts with previous experience. This indicates that there is a need for a further investigation in which pipes of various grades are scratched by defined depth scratches, with the results being extrapolated in accordance with SS-EN ISO 9080.
2.4.2
Non pressurised pipes
There is at present no internationally accepted method for assessing the service life of the material of plastic pipes used for drainage, waste water or other non pressurised
applications.
In 2007, SP carried out a research project, Thörnblom et al. (2007), with the aim of developing a method for assessing the life of non pressurised plastic piping systems. The main method of evaluation was what is known as an ovalisation test, subjecting aged and non aged pipe samples to constant deformations while measuring the stress relaxation in the pipe wall.
In the case of non pressurised pipes, indentations caused by stones generally result in a deformation controlled load case. This means that the stone causes a deformation of the pipe wall, which remains constant in terms of size, while the associated stresses in the material gradually relax with time. It is difficult to accelerate the relaxation mechanisms, as any increase in the temperature - as is normally used in order to accelerate crack propagation - also results in an acceleration of the viscoelastic processes, and thus also of the stress relaxation. This has been investigated experimentally by Nilsson (2004), who demonstrated that, at high strain rates, notched samples fail more quickly in PE100 material than they do in PE63 material, which is probably a result of more rapid stress relaxation.
2.4.3
Casings on district heating pipes
Scratches have also been investigated in terms of their effects on district heating pipes. Under certain conditions, district heating pipes are subjected to substantial axial movements as a result of the pipes expanding and contracting as the temperature of the district heating water rises or falls. This means that scratches are a clear risk, even if the pipe has been laid by ‘traditional’ methods instead of by trenchless methods.
This has become of particular interest since it was realised that trenches might be back filled around the pipes using unspecified material, which could contain relatively large sharp edged stones. Molin et al. (1997) investigated to what extent scratches were caused to district heating pipes laid in three different fill materials: 0 – 8 mm crushed material, 0 – 100 mm crushed material and 0 – 100 mm natural material. The pipes were then subjected to 50 cycles of a reciprocating motion with a stroke length of about 40 mm. The deepest scratch was about 0.5 mm deep, and occurred in the coarse crushed material, as shown in Figure 2-5.
The results of similar investigations have been published by Göhler (2004). He found scratches of the order of 0.1 mm deep after carrying out joint tests in accordance with SS-EN 489 (2003) in a backfill material with grain sizes up to about 56 mm.
Figure 2-5. A scratch, about 0.5 mm deep, in the casing of a district heating pipe, caused by 0 – 100 mm crushed material. Molin et al. (1997).
Further investigations of stone indentations in district heating pipe casings have been carried out by Nilsson (2000) and Bergström & Nilsson (2001). These investigations were concerned primarily with lateral displacement of district heating pipes by stones
embedded in the backfill material. An interesting conclusion is that the depth of
indentations causing lateral displacement is largely dependent on how firmly the backfill material has been compacted. With poor compaction, no great earth pressure can be applied to the stone, and so no particularly high contact forces can be established between the stone and the wall of the pipe. If, on the other hand, the backfill material is very densely compacted, there is no concentration of earth pressure and so no point loads. The critical case seems to be that of ‘normal hard’ compaction.
Nilsson (2000) has also investigated the stresses in district heating casing pipes exposed to indenting stones, and states that the tensile stresses on the inside of the pipe wall increase with increasing wall thickness. The load case for a district heating casing pipe is different to that for an ‘ordinary’ plastic pipe, insofar as the former is internally filled with foamed polyurethane plastic. This supports the pipe wall from the inside, with the result that the wall is more inclined to bend around the stone and take up the shape of the stone. In the case of a gas or water pipe, containing a fluid, a deeper indentation is necessary if this is to occur.
3
Pipes for pressurised applications
3.1
Introduction
As the expected life of most plastic piping systems is of the order of 50 – 100 years, it is necessary to employ accelerated methods in order to evaluate the long term strength of pipes and piping materials. As far as thermoplastic materials for pipes intended for pressurised applications are concerned, an evaluation method as described in ISO 9080 is used more or less exclusively in most parts of the world outside the USA. The method is based on a time-temperature-acceleration model that assumes an Arrhenius relationship between temperature and service life. This has been found to work well for both ductile and brittle failure mechanisms in polyethylene. The principle of the method involves pressure testing to failure at various temperatures over the proposed operating temperature range and at various stress levels (pressure levels): see Figure 3-1.
101 102 103 104 105 106 107 Log (Time, h ) Log (Stress ) 50 year 4380 h 20 °C 40 °C 60 °C 80 °C k e= 1 00 Ductile failure B rittle fa ilu re 101 102 103 104 105 106 107 Log (Time, h ) Log (Stress ) 50 year 4380 h 20 °C 40 °C 60 °C 80 °C k e= 1 00 Ductile failure B rittle fa ilu re
Figure 3-1. Schematic stress/time diagram for polyethylene, with application of extrapolation factor ke=100, in accordance with ISO 9080.
All the measured data is processed by a regression calculation to produce a function relationship that determines the parameters of a material model for the particular material concerned. If the material model is to be able to serve as a basis from which to
extrapolate the material properties to very long periods of time, results must be available from temperatures sufficiently above the normal operating temperatures, and also from sufficiently long test periods. The standard specifies a relationship between, on the one hand, the difference between the test temperature and the intended operating temperature, and on the other hand the maximum permissible time extrapolation that may be applied to the results obtained for the particular test temperature.
For tests carried out at a temperature of 60 ºC above the intended operating temperature, the standard permits an acceleration factor (ke) of 100, in terms of time. This means that, for a conventional arbitrary service temperature of 20 °C, tests of polyethylene materials carried out at 80 °C must be run for at least 4380 hours if the results are to be
This methodology has been applied in this project in order to investigate to what extent scratches in pipes reduce the strength of pipes intended for a service life of 50 years. All the tests were carried out at 80 °C, with the aim of being able to investigate changes in pipe strengths up to 4380 hours in order to represent expected strength changes over a period of 50 years at 20 °C. The work has therefore been concentrated on investigating how the strength of the pipe, in terms of resisting internal pressure, is reduced, and particularly on whether there is any reason to suspect that brittle failure mechanisms may be initiated by scratches and so drastically reduce the service life.
3.2
Pipe materials
This investigation has covered pressure testing of pipes made from four different polyethylene materials. They are identified in this report as V, X, Y and Z, having some characteristic properties as shown in Table 3-1. The abbreviation SCG refers to Slow Crack Growth, i.e. the failure mechanism that results in brittle fracture.
Table 3-1. Properties of four different materials.
Dencity MFR5 E-modulus
(kg/m3) (g/10
min)
(MPa)
V PE80 Black Bimodal 951 0,8 800 MDPE with extra good SCG
properties
X PE100 Orange Bimodal 951 0,3 1100 Standard
Y PE100 Black Bimodal 959 0,3 1100 Extra good SCG properties Z PE80* Uncolourd Bimodal 947 0,3 1000 Older type * Classification unsure
Comment Material
Classifi-cation
Colour Type
Apart from the non coloured PE80 material, all the materials are available on the Nordic market, and are marked with the voluntary Nordic Poly Mark quality symbol for
materials and plastic pipe products. See Appendix 5 for further information on certification of plastic pipes.
The non coloured PE80 material (Z) is an older type of PE material, which can be assumed to be in widespread use in existing pipes. Material X is intended for gas pipes, while the other materials are intended mainly for water pipes.
3.3
Pipe dimensions, type of scratches and depth
of scratches
Tests were carried out on 151 pipes having an external diameter of 32 mm, and on 24 pipes with an external diameter of 110 mm. The accepted practice when performing pressure testing in accordance with ISO 9080 is to carry out the tests on 32 mm pipes, with an associated presumption that the results can be used for determining the design ratings of pipes of all sizes. Performing the pressure tests on small pipes simplifies the procedure and reduces costs. This approach has been used in this work for testing notched pipes as well. The investigation carried out by Zhou & Chang (2006) was based on 32 mm pipes.
The effect of scratches on the strength of pipes can also depend not only on the depth of the scratch, expressed as a percentage of the wall thickness, but also on the absolute size of the scratch. It was in order to attempt to obtain information on this aspect of the problem that the larger 110 mm pipes were included in the investigation.
The investigation has studied the effect of three different depths and two different shapes of scratches (notches), longitudinally milled in the outside surface of the pipe. Reference samples have consisted mainly of 32 mm pipes without scratches.
For the 32 mm pipe samples, all materials had sharp scratches milled into them in the form of longitudinal V-notches to depths of 5 %, 10 % and 20 % of the thickness of the pipe wall. In addition, materials X and Z were tested by milling U-shaped notches to a depth of 20 %, and further by planing off a 20 % V-notch. 110 mm pipes of all four materials were tested with a V-notch of a depth of 10 %. Figure 3-2 shows cross sections of the V-notches and U-notches. Table 3-2 shows the combination of materials and sizes that were pressure tested.
Table 3-2. Materials, pipe sizes and notch types and dimensions.
Testing Reference sample X X X X 5 % V-notch X X X X 10 %V-notch X X X X X X X X 20 % V-notch X X X X 20 % U-notch X X
Planed -off scratch X X
32 mm 110 mm 110 mm 110 mm 110 mm 32 mm 32 mm 32 mm
Material V Material X Material Y Material Z
´
Figure 3-2. Cross-sections of typical V-notches and U-notches.
3.4
Test method and procedure
The external diameters of the pipes were measured using a circometer. The maximum and minimum wall thicknesses of the 32 mm pipes were measured ultrasonically.
Corresponding measurements for the 110 mm pipes were made using a thickness gauge carrying a dial gauge.
Scratches were produced in the pipes by means of a milling machine in order to ensure well defined and repeatable notches. The 60° V-notches were made using a 12-tooth V-cutter with an external diameter of 63 mm, running at 230 r/min and with a feed of 44 mm/min. The U-notches, with a bottom radius of 1 mm, were milled in a
corresponding manner. The length of all notches was three times the diameter of the pipe, and the notches were positioned where the minimum wall thickness had been measured.
For each individual pipe, the notch depth was calculated in relation to the measured minimum wall thickness. The depth of the scratches on some of the samples was checked using the scratch depth gauge as described in Section 6: see Table 3-3.
Table 3-3. Calculated scratch depths and measured scratch depths as indicated by the scratch depth gauge.
Test sample Calculated
scratch depth (mm) Measured scrath depth (mm) V1 0.16 0.15 V2 0.16 0.17 X4 0.15 0.15 X8 0.31 0.30 Y1 0.16 0.27 Y17 0.63 0.66 Z22 0.63 0.61 Z28 0.16 0.14
Pressure testing was carried out in accordance with SS-EN 921:1995 at a temperature of 80 °C, with water both inside and outside the pipe. 32 mm pipes were conditioned for one hour, and 110 mm pipes for six hours. For the tests, the pipes were connected to a
pressure test equipment that automatically recorded the time to failure.
In order to reflect the principles of ISO 9080, a series of stress levels was chosen such as to produce failure times for the pipes varying from a few hours up to several thousand hours. The stresses concerned are the hoop tensile stresses in the wall of the pipe, as shown in Figure 3-3.
Figure 3-3. Stress conditions in a pressurised pipe.
The internal pressure, p, was calculated from the hoop tensile stress in accordance with the equation given in EN ISO 1167-1:2006, Section 7.2:
min min 2 e d e p em − =
σ
Equation 3-1.where σ is the hoop tensile stress in MPa, dem is the average external diameter of the pipe in mm, and emin is the minimum wall thickness of the pipe in mm.
A scratch in a pressurised pipe results in a stress concentration as shown in Figure 3-4. However, in this investigation, the test pressure for notched pipes has been calculated as if the pipe was undamaged, i.e. without making any allowance for the notch effect of the scratch.
Figure 3-4. An example of stress concentration around a scratch.
In pipes made from materials with a low toughness, brittle failure can be initiated at the position of defects in the wall of the pipe beside a visible scratch. Exactly where the fracture starts and develops most rapidly depends on the position and size of the defect, together with the stress intensity at the tip of the internal or external scratch that initiates the brittle failure.
In materials with a high toughness, failure occurs as ductile failure. Such failures are not located to a particular defect position or to a particular crack, but occur in the area where the creep velocity in the whole area is most rapidly causing creep towards the material’s yield strength. If there is some larger area of thinner pipe wall ahead of, beside or after the scratch, creep there can be greater than in the area immediately adjacent to the scratch.
3.5
Results
All the results of the work are shown in tabular form in Appendix 1, which also shows the type and position of failures. In some cases, it was not possible clearly to decide whether failure was of ductile or brittle type: the Appendix also shows examples of typical failures of each type.
The following four stress/time diagrams, Figures 3-5 to 3-8, show the results of the pressure tests of 32 mm pipes of materials V, X, Y and Z. Each diagram shows the results for reference pipes without scratches, pipes with 5% notches, pipes with 10 % V-notches, and pipes with 20 % V-notches. Filled dots in the diagrams show pipes which have failed, while unfilled dots (rings) show pipes which, at the time of writing this report, had not yet failed.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoo p st ress , MPa Reference, original 5% V-notch 10% V-notch 20% V-notch + 80 °C 50 years/100=4380 hours Material V 32 mm pipe
Figure 3-5. 32 mm pipe, material V, with 0, 5, 10 and 20 % V-notches.
Failure mode of pipes made from material V was ductile for all samples, with failure being initiated at the scratch for 10 % and 20 % notches. Pipes with the 5 % notch failed in line with the scratch. Pipes with 5 % or 10 % V-notches showed no reduction in their times to failure in comparison with unnotched pipes. It was not until a V-notch depth of 20 % that the scratch had a clear effect on the strength of the pipe.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a Reference, original 5% V-notch 10% V-notch 20% V-notch + 80 °C 50 years/100=4380 hours Material X 32 mm pipe
At higher stress levels, failures in pipes made from material X occurred in ductile mode, although at lower stress levels, and for times to failure of more than about 500 hours, the failures also exhibited elements indicative of brittle fracture. All failures occurred at the notch. No strength reduction could be observed for pipes with 5 % or 10 % notches, as failures in these pipes were all of ductile mode, i.e. at the highest stress levels. At lower stress levels, with the fracture having brittle indications, the results increasingly differ from the line matched to the results for the unnotched pipes. All the pipes with 20 % V-notches clearly indicated a reduction in strength.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a Reference, original 5% V-notch 10% V-notch 20% V-notch + 80 °C 50 years/100=4380 hours Material Y 32 mm pipe
Figure 3-7. 32 mm pipe, material Y, with 0, 5, 10 and 20 % V-notches.
The results for pipes made from material Y show considerable similarities with those for pipes made from material V. All failures were of brittle type. In pipes with 5 % notches, failure occurred in line with a scratch, while for deeper scratches it occurred at the notch. Pipes with 5 % and 10 % notches did not exhibit any reduction in strength for failure times up to 5000 hours: it is only for pipes with 20 % notches that there is a clear reduction in strength.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoo p st ress , MPa Reference, original 5% V-notch 10% V-notch 20% V-notch + 80 °C 50 years/100=4380 hours Material Z 32 mm pipe
Figure 3-8. 32 mm pipe, material Z, with 0, 5, 10 and 20 % V-notches.
Pipes made of material Z exhibit a clearly lower toughness than pipes made of the other materials. Ductile failure occurred only in the unnotched pipes at the highest stress levels. All the notched pipes, and also the unnotched pipes at lower stress levels, exhibited brittle failure. In pipes with 5 % and 10 % V-notches, failure was initiated both within and outside the scratch, while all failures in pipes with 20 % V-notches started in the notch. Brittle failure mode becomes very marked for failure times over 300 hours, with failure occurring at considerably lower stress levels than would have been the case if the material had behaved as a tough material.
Figures 3-9 and 3-10 show the results for materials X and Z respectively of pressure testing reference pipes without notches, pipes with 20 % U-notches, pipes with a planed off 20 % V-notch, and pipes with a 20 % V-notch.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a Reference, original 20% U-notch Planed-off 20% V-notch 20% V-notch + 80 °C 50 years/100=4380 hours Material X 32 mm pipe
Figure 3-9. Results for 20 % V-notch, 20 % U-notch and planed off 20 % V-notch for material X.
It can be seen that there is a considerable spread in the results for material X pipes with a 20 % V-notch. This makes evaluation more difficult, but it is nevertheless clear that the results are largely unaffected by whether the pipe wall thickness is reduced by means of a V-shaped scratch, a U-shaped scratch or a planed off scratch. For all notched pipes, failure occurred at the notch.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a Reference, original 20% U-notch Planed-off 20% V-notch 20% V-notch + 80 °C 50 years/100=4380 hours ( ) Material Z 32 mm pipe
Figure 3-10. Results for 20 % V-notch, 20 % U-notch and planed off 20 % V-notch for material Z.
For material Z, it can be seen that the life of pipes having U-notches or a planed off notch is considerably greater than that for pipes having a V-notch. Failure of pipes with
U-notches occurred in both brittle and ductile mode, with the ductile failures occurring at the high stress levels. Failure started from the U-notch.
The result position enclosed in brackets has not been included when plotting the trend line. The intention was to plane off a 20 % V-notch, but the end result was to thin the pipe wall more than intended.
The following four stress/time diagrams, Figures 3-11 to 3-14, show the results of pressure testing of 32 mm reference pipes without notches, 32 mm pipes with 10 % V-notches, and 110 mm pipes with 10 % V-notches, for all four materials.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a
Reference, original 32 mm pipe 10% V-notch, 32 mm pipe 10% V-notch, 110 mm pipe
+ 80 °C
50 years/100=4380 hours
Material V
Figure 3-11. Unnotched 32 mm pipes, and 32 mm and 110 mm pipes with 10 % V-notches. Material V.
Both 32 mm and 110 mm pipes made from material V exhibit ductile failure starting from the notch. The stress/time curve for 110 mm pipes is about 2 % lower than the level for notched and unnotched 32 mm pipes.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a
Reference, original 32 mm pipe 10% V-notch, 32 mm pipe 10% V-notch, 110 mm pipe Reference, original 110 mm pipe
+ 80 °C 50 years/100=4380 hours Material X ( ) ) (
Figure 3-12. Unnotched 32 mm and 110 mm pipes, and 32 mm and 110 mm pipes with 10 % V-notches. Material X.
Where failure occurred after a short time, both 32 mm and 110 mm pipes of material X with 10 % notches failed by ductile failure mode. However, with longer failure times, failure exhibited elements of brittle failure mode. All failures occurred at the notch. It can be seen from Figure 3-12 that the stress/time curve for 110 mm pipes is about 4 % lower than the corresponding curve for the notched 32 mm pipes. The two points enclosed in brackets are regarded as belonging entirely to the ductile failure section of the curve at a higher stress level than of other results.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a
Reference, original 32 mm pipe 10% V-notch, 32 mm pipe 10% V-notch, 110 mm pipe
+ 80 °C
50 years/100=4380 hours
Material Y
Figure 3-13. Unnotched 32 mm pipes, and 32 mm and 110 mm pipes with 10 % V-notches. Material Y.
All the failure points for pipes made in material Y are closely gathered around a ductile failure mode line. Failure for all alternatives occurred in ductile mode, starting from the notch for both pipe sizes. The plotted trend line for 110 mm pipes is somewhat less than 2 % below the lines for the 32 mm pipes.
1 10 100 1000 10000 100000 1000000 Test time, h 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Hoop s tr es s, MP a
Reference, original 32 mm pipe 10% V-notch, 32 mm pipe 10% V-notch, 110 mm pipe
+ 80 °C
50 years/100=4380 hours
Material Z
Figure 3-14. Unnotched 32 mm pipes, and 32 mm and 110 mm pipes with 10 % V-notches. Material Z.
Failures of 110 mm pipes made of material Z occurred in both ductile and brittle mode, initiating at the V-notches.
The points in the next three diagrams, Figures 3-15 to 3-17, correspond to the
extrapolated levels (to 4380 hours) in the stress/time diagrams shown in Figures 3-5 to 3-8. The vertical axis is a pressure reduction factor, which provides a measure of the effect of the notches in the strength of the pipes, indicating by how much the pressure in a scratched pipe must be reduced if the pipe is to have the same service life as a similar pipe without scratches. In the same way, the relevant European Standards (EN) for polyethylene pressure pipes give reduction factors for the pressure when the pipes are to be used at elevated temperatures. The horizontal line at reduction factor 1.0 represents the resistance performance of an unscratched pipe exposed to internal pressure. The dotted lines show the relationship between the residual and original wall thickness of an
unscratched pipe, which corresponds to the reduction in wall thickness resulting from the notches. If the pressure reduction curve falls above this line, the effect is less than that resulting from the reduction in wall thickness, while if the curve falls below the line the effect is thus greater than that caused by the reduction in wall thickness.
Figure 3-15 shows the necessary reduction factors for various scratch depths for pipes made of the different materials.
0,7 0,8 0,9 1,0 1,1 0 5 10 15 20 25 Scratch depth, % P res su re r e d u c ti o n f a ct o r Material V Material X Material Y Material Z
Figure 3-15. Pressure reduction factors for pipes of materials V, X, Y and Z at different scratch depths.
The pressure reduction factor for material V is very close to 1.0 for V-notches of depths up to 20 % of the wall thickness. For material Y, the measurements indicate a reduction factor of 0.95 for a 20 % V-notch, while material X has a reduction factor of 0.90 for the same scratch depth. However, for material Z, it is necessary to reduce the pressure to a greater extent in order to maintain the target service life, down to 0.74 for a 20 % scratch depth. It is therefore only this material for which the necessary pressure reduction is greater than the reduction in the wall thickness caused by the notch.
In the same way as in Figure 3-15, Figure 3-16 shows the reduction factors for material X with a 20 % V-notch and a 20 % U-notch, in comparison with the performance of a pipe having a planed off 20 % V notch.
0,7 0,8 0,9 1,0 1,1 0 5 10 15 20 25 Scratch depth, % P re s s ur e r e du c tion fa c tor 20 % V-notch 20 % U-notch Planed-off 20% V-notch
Figure 3-16. Pressure reduction factors for material X with a 20 % V-notch, 20 % U-notch and a planed off 20 % V-notch on 32 mm pipes.
It can be seen from Figure 3-16 that the reduction factor for pipes of material X is 0.94 for a 20 %U-notch, but 0.90 for a 20 % V-notch. In other words, as expected, the U-notch is less dangerous than a V-notch. For this particular material, planing off the notch gave a marginal improvement in the reduction factor, to 0.95.
0,7 0,8 0,9 1,0 1,1 0 5 10 15 20 25 Scratch depth (%) P ress u re r e d u ct io n f a c to r 20 % V-notch 20 % U-notch Planed-off 20% V-notch
Figure 3-17. Reduction factors for material Z with a 20 % V-notch, a 20 % U-notch and a planed off 20 % V-notch, on 32 mm pipes.
Figure 3-17 shows corresponding results for 32 mm pipes made from material Z. Its performance differs from that of the other materials, in that a reduction factor of 0.75 is required in order to compensate for the effects of either a 20 % V-notch or a 20 % U-notch. However, a substantial improvement in the strength of the Z material can be achieved by planing off the notch, improving the reduction factor from 0.75 to 0.91. Removing scratches in this way can thus restore some of the strength of older pipes against internal pressure.
3.6
Discussion
Figure 3-15 showed the reduction factors arrived at by directly processing the
measurement results. However, in some cases, the spread of the underlying test results is so considerable that the reduction factors shown in the diagram in fact incorporate a considerable measure of uncertainty, which comes to expression in the form of some clearly unrealistic results. Reduction factors in excess of 1.0, for example, were obtained for materials V and X with 5 % scratch depths, while material Z returned a lower
reduction factor for 5 % scratch depths than it did for 10 % scratch depths. Figure 3-18 more schematically illustrates the main points of the results of the investigation. It shows the reduction factors that must be applied to the pipes’ pressure classes in order to ensure a 50 year life, together with the differences in performance of the materials and how the different scratch depths affect the pipes’ ability to withstand internal pressure.
The results show how the pipes made from modern materials withstand scratches in their walls much better than do pipes made of older materials.
0,7 0,8 0,9 1,0 1,1 0 5 10 15 20 25 Scratch depth, % P res su re r e d u ct io n f a ct o r Material V Material Y Material X Material Z
Figure 3-18. Schematic diagram of reduction factors for different materials and scratch depths, 32 mm pipes
It is clear that the effect of scratch damage on a pipe cannot be evaluated only on the basis of the depth and sharpness of the scratch. In order to be able to assess the effect of the scratch on the pipe’s utility, it is necessary also to know what material has been used to produce the pipe and what safety factors have been applied when determining its rating. For pipes manufactured from modern bimodal PE80 or PE100 materials, scratches up to 10 % depth can be accepted without having to reduce the working pressure. A reduction of the permissible working pressure, or alternatively planing off the scratch, should be considered for pipes made of older materials but having a design safety factor of 1.25. However, for pipes having a design safety factor of 1.6, it should be possible to accept scratches up to 10 % depth without reducing the working pressure.
As mentioned in the introduction, this investigation has been concentrated on
understanding the conditions to allow scratched pipes to achieve a service life of 50 years. In addition, the conclusions in this report are based mainly on the results of tests on 32 mm pipes. This means that, when aiming for service lives of 100 years or more, or for larger pipe sizes, it will probably be necessary to apply lower reduction factors in order to ensure the desired lengths of life.
As only a few reference results are available for unnotched 110 mm pipes, the results from the notched 110 mm pipes must be related to those from the 32 mm pipes. All the results show that the performance of 110 mm pipes with 10 % notches is close to, but somewhat below, the corresponding results for 32 mm pipes with the same relative notch depth, equivalent to a reduction of the reduction factor of 2-5 %. Whether this is a trend that indicates that successively larger pipes require increasing pressure reductions cannot be decided on the basis of this material, but can be ascertained only by tests carried out on larger pipes.
4
Pipes for nonpressurised applications
4.1
Introduction
In the case of nonpressurised pipes, it is the deformation from the external earth pressure load that is the critical load. The downward load from the earth above the pipe tries to ovalise it, while the transverse support from the backfill material opposes ovalisation. The transverse support increases as the backfill material is more compacted, so that good compaction results in less ovalisation. Ovalisation causes bending stresses in the wall of the pipe, which are highest at the top, bottom and sides of the pipe. The risk of a crack developing increases if there is an external scratch in the side of the pipe, where the bending stresses are highest. Poor preparation of the pipe bed, and/or large stones in the backfill material, can result in spot loads which, with time, can induce cracking in the wall of the pipe.
In 2007, SP carried out a research project, Thörnblom et al. (2007), with the aim of developing a method of assessing the expected life of non pressurised plastic piping systems. The main method of evaluation that was used was an ovalisation test (Janson test), which involved subjecting aged and nonaged pipe samples to constant deformations and measuring the associated stress relaxation in the wall of the pipes. The results showed that the polypropylene pipes that were investigated could withstand very substantial deformation up to 45 % without showing any signs of cracking during the time for which the test continued.
The project described in this report used essentially the same method. It also included remaining polypropylene pipes from the earlier project for evaluation, although now with attention concentrated on the effect of scratches and indentations. Tests were conducted both on nonaged and aged notched pipe samples, together with ovalisation trials involving application of a sphere pressed into both nonaged and aged samples.
4.2
Pipe materials
Pipes of two different materials have been investigated. The pipes were 110 mm diameter single wall types, with a wall thickness of 5 mm.
The pipes were supplied by Borealis, being remaining parts of the test pipes used in the earlier Thörnblom et al. (2007) project. In the time between the two projects, of about two and a half years, the pipes have been stored at room temperature. The polypropylene materials in the pipes has the following characteristics:
• A commercial polypropylene material of high rigidity, identified as PP-B(2) in the earlier project, and designated as Material B in this project. For the
ovalisation tests, the test samples were designated JB, with a test number, while the pipes in the indentation tests were designated as KB with a test number. • A mineral filled polypropylene material, referred to as PP filled in the earlier
project. In this current project, the material is referred to as Material M, with the samples marked JM and a test number for the ovalisation trials, and KM with a test number for the indentation trials. The material is stated to contain 30 % of calcium carbonate, CaCO3.
4.3
Test method and test procedure
4.3.1
General
Two methods have been used in order to investigate how scratches and indentations affect the physical life of the pipes:
Method 1: Tests of scratched pipe samples maintained under constant ovalisation (vertical deformation, known as Janson tests), with scratches simulated by means of longitudinal notches of a specific shape and depth. After setting the notched pipe samples under constant deformation, the reaction force was measured so that the stress relaxation in the pipe walls can be monitored throughout the test period.
Method 2: Indentation tests, in which a steel sphere is pressed into the pipe wall and maintained at a specific indentation depth. Over the test period, the pipe samples are regularly inspected visually.
The tests were carried out both on nonaged pipe samples and on pipe samples aged for twelve months. The nonaged pipes had been stored at room temperature since they were manufactured, while the aged pipes were aged in a water bath at a temperature of 95 ºC throughout the aging period. The pipes had been placed vertically in the water bath, with the bottom ends sealed so that the hot water came into contact only with the outside of the pipes. This meant that the insides of the pipe were exposed only to air at a temperature of 95 ºC. After aging, the pipes were stored at room temperature.
4.3.2
Ovalisation
The tests of the scratched pipe samples maintained under constant deformation were carried out for over 6000 hours. The pipe samples were 170 mm long, with two sharp V-shaped notches milled into the wall of the pipe. The notches were positioned opposite to each other on the sides of the sample pipe. The scratch depths were 10 % and 20 % of the pipe wall thicknesses.
Stresses in the pipe walls resulting from ovalisation were assumed to be highest in the thickest parts of the wall. Based on this, the first notch in each sample was milled in that part of the wall where the greatest wall thickness had been measured. The second notch was then milled to the same percentage depth, at 180º from the first notch. As the pipe wall in the position of the second notch could be thinner than in the position of the first notch, the absolute depth of the second notch could be somewhat less.
The pipe samples were then compressed to an ovalisation equivalent to 25 % of each pipe’s external diameter by clamping it in a test rig, as shown in Figure 4-1, that would maintain constant deformation. The picture shows a notched pipe sample in the rig.
Figure 4-1. A pipe sample in a test rig.
The test rig consisted of three square tubes (30 x 30 x 2 mm), with two M20 threaded rods and a number of nuts and washers. Two of the square tubes were 260 mm long, with a third being somewhat shorter at 200 mm. The length of the threaded rods was such as to enable the load cell, the shorter tube and the plastic pipe sample to be positioned between two longer square tubes. Holes were drilled through the ends of the longer tubes, so that the pipe samples could be held and secured by the nuts and washers on the threaded rods. The pipe samples were then ovalised by mounting the test rigs in a compression testing machine, between the flat bed of the machine and a load cell with a flat face above the test rig. An HBM S9 load cell was fitted between the two upper square tubes, with the pipe sample between the two lower tubes (see Figure 4-1).
Samples were compressed by the compression testing machine at a speed of 5 mm/min until the required degree of ovalisation was reached, continuously measuring and recording the applied force as measured by the test machine's load cell. When the intended degree of ovalisation had been reached, the position of the upper beam was locked relative to the lower beam by means of the nuts. The relaxation force in each sample was then continuously recorded by means of the individual load cell connected to a Datascan 7000 data logging system, connected in turn to a PC in which the measured data was stored. The relaxation forces were logged at least once an hour over the entire test period. Application of the ovalisation load and relaxation measurements were carried out at room temperature, +23 °C.
4.3.3
Spherical indentation
The pipe samples for the sphere indentation tests were 190 mm long. Testing was performed both on nonaged pipes and on pipes aged for twelve months, with deformations of 10 %, 15 % and 25 % of the pipes’ external diameter. As the pipe samples were much shorter than pipes in reality, the ends of the pipe samples were closed and supported by cylindrical plugs, as shown in Figures 4-3 and 4-2. The indentation was applied by a 12 mm diameter steel ball, pressed into the sample pipe where the wall thickness was greatest.
Figure 4-2. Schematic diagram of ovalisation testing by means of spherical indentation.
The pipe samples for indentation testing were mounted in test rigs that enabled a constant ovalisation to be maintained. Figure 4-3 shows a test rig with the steel ball pressed into the sample. The rig consisted of two square steel tubes, 30 x 30 x 2 mm, two M20
threaded rods, nuts, washers and a short steel cylinder. This cylinder was flat at both ends, with a slight depression in the centre of one end in order to centre the steel ball. The steel tubes were 260 mm long, and the length of the threaded rods was sufficient to allow the steel cylinder and the pipe test sample to be fitted in between the two steel tubes. Holes were drilled through the ends of the tubes to enable the test rig to be held together and secured by the threaded rods, the nuts and washers.
The steel ball was pressed into the pipe samples at a velocity of 5 mm/min until the selected indentation depth, calculated in relation to the external pipe diameter, was achieved. While doing so, the necessary force was measured and logged continuously, using the compression test machine’s load cell. When the required indentation had been reached, the position of the upper tube was locked in relation to the lower tube by means of the nuts. Application of the load, and the rest of the test procedure, was carried out at room temperature, +23 °C.
4.4
Results
4.4.1
Ovalisation
Table 4-1 shows the aging status, sample length, pipe diameter, scratch depth and maximum deformation force for the ovalisation tests for all samples.
Table 4-1. Summary of the ovalisation tests performed.
Identif-ication Aged Length Diameter Scratch depth
Maximum deformation
V-notch emax e180º force
(months) (mm) (mm) (%) (mm) (mm) (kN) JB5 - 170 110,0 10 0,523 0,502 2,1 JB2 - 170 110,0 20 1,054 1,050 2,0 JB3 12 170 109,6 10 0,535 0,510 2,2 JB4 12 170 109,8 20 1,054 0,976 2,0 JM1 - 170 111,7 10 0,540 0,500 1,4 JM2 - 170 111,6 20 1,096 0,970 1,3 JM3 12 170 111,5 10 0,545 0,480 0,9 JM4 12 170 111,8 20 1,084 1,040 0,9
Figure 4-4 is a plot of the deformation force for the notched pipe samples. It can be seen from the curves that the notches do not seem to affect the characteristic for the JB
material, the samples of which do not show any fall off in the amount of force required as indentation increases: instead, the force increases steadily throughout the indentation increase. For the JM2 and JM3 materials, a maximum value of deformation force occurs at about 23 mm displacement, after which the necessary force for further deformation declines somewhat. For JM1 and JM4 materials, the maximum value occurs just before the target deformation, after which the curve flattens out. The aged JM3 and JM4 samples required less applied force compared to the other samples.
0,0 0,5 1,0 1,5 2,0 2,5 0 5 10 15 20 25 30 Ovalisation (mm) Fo rce (kN) JB5 JB2 JB3 JB4 JM1 JM2 JM3 JM4
