the importance of axial misalignment on the long term strength of polyethylene pipe butt fusion...

The Importance of Axial Misalignment on the Long Term Strength of Polyethylene Pipe Butt Fusion Joints

JEREMY BOWMAN* and RAVINDRA PARMAR""

Department of Materials Technology Brunel University

Uxbridge, Middlesex, United Kingdom

The importance of axial misalignment at polyethylene pipe butt fusion joints has been assessed by undertaking elevated temperature lifetime tests. Both medium and high density polyethylene pipes were fusion joined to give aligned and controlled misaligned butt joints. These were tested under either a constant or fluctuating internal pressure loading using conditions that induced failure by slow, stable crack growth. It was observed that the lifetime of a butt joined system depends upon both the internal pressure or pressure range applied and the level of misalignment at the butt fusion joint. Increasing either the internal pressure (range) or the misalignment reduced system performance. These two variables of misalignment and internal pressure (range) may be incorporated into a single parameter, the amplified axial stress (or stress range) at the butt joint. This amplified or butt joint axial stress (or stress range) may be derived by considering the additional bending stresses introduced at the butt joint by virtue of misalignment combined with the axial stress loading.

INTRODUCTION EXPERIMENTAL DETAILS

arge diameter plastic pipes are most often joined L by butt fusion, and the integrity of resultant pipe systems depends upon the strength of the joints. Butt joints made with medium (MDPE) and high (HDPE) density polyethylene pipes can be as strong as the pipe itself. Techniques used to assess joint strength have included tensile testing (1, 2) and elevated temperature fatigue (3) and constant stress loading (4, 5). Certain on-site malpractices can, however, reduce the strength of butt fusion joints made with either MDPE or HDPE pipes. Heater plate contamination (6) and joint misalignment (7, 8) both lower butt joint strength.

This present contribution expands previously pub- lished work (8) on how axial misalignment influences joint strength. Elevated temperature lifetime tests were conducted on 378 butt joined MDPE or HDPE pipe systems with axial misalignments ranging from 0 to nearly 50 percent of the pipe wall thickness. Three pipe diameters were used and samples subjected to either constant or fluctuating internal pressure loading to induce failure by the propagation of stable cracks. These lifetime tests have allowed an assessment of the role of axial misalignment on joint strength.

* To whom correspondence should be sent: Fusion Plastics Ltd.. Carmood Road, Chesterfield, Derbyshire. UK. **Current address: Phillips €9 Du Pont Optical. Ltd.. Phillips Road, Blackburn. Lancashire, UK.

Materials, Pipes, and Butt Fusion Joining Two water pipe grade polyethylene compounds

from BP Chemicals Ltd. (Grangemouth, Scotland) were used. 63,90, and 125 mm SDR" 11 MDPE pipes were extruded from Rigidex PCOO2-50 R968, a blue pigmented resin. The 63 mm SDR 11 HDPE pipes were produced from a resin not now in production. Rigidex PCOO2-60. Table 1 records the melt flow rate (MFR) and mid-wall density of the various pipes. Fuller details on the characterization of the materials and pipes are contained elsewhere (9).

For a given pipe diameter the welding parameters were as listed in Table 2; these welding conditions were the same for aligned and all axially misaligned butt fusion joints. With the exception of the heat soak time, which was slightly reduced, the proce- dures, temperatures, pressures, and times for fusion joining were those recommended by the resin manu- facturer. Lengths of pipe were cut in half, the re- vealed ends prepared for joining and the two ends fused together with the permanent pipe marks aligned.

Introduction of Axial Misalignment A butt fusion joint may have three forms of misa-

lignment, see Fig. 1 . Axial misalignment (Fig. l a )

SDR is the Standard Dimension Ratio. the ratio of mean outside diameter to minimum pipe wall thickness.

1406 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER 1989, V d . 29, NO. 19

J . Bowman and R. Parmar

has the pipe axes parallel but displaced, while angular misalignment has no step at the joint but the pipe axes are non-parallel (Fig. 1b). Combined axial and angular misalignment (Fig. 1 c ] has nonparallel pipe axes and a step at the butt joint. In this study on axial misalignment the angular misalignment present was routinely measured and found to be always below 2".

The Haxey Mark I1 butt welding machine (from Haxey Engineering, Doncaster, England) used in this study had specially manufactured inserts to ensure the axes of the two pipes being fusion joined were parallel but displaced as required. Fractional misalignment, x, measures this axial misalignment at the joint on the side of the pipe joint where failure was first observed. x is given by

where emax is the maximum step at the joint and t is the pipe wall thickness. The inserts for the welding machine gave nominal values for x from 0 to 0.44 for all pipe diameters.

Measurement of Joint Misalignment Butt joint alignment was assessed in three ways.

First, the butt joined HDPE pipes, tested under both constant and fluctuating pressures, had only nominal values of x recorded. Second, for all the MDPE pipes the maximum step at the joint, emax, was measured together with the pipe wall thickness, t , to allow a calculation of the actual fractional misalignment.

Table 1. Characterization of Materials and Pipes under Test.

Grade of Polyethylene

PCOO2-60 PCOO2-50 R968

Finally, for a limited number of samples, misalignment was calculated by a n extrapolation technique. Butt joined pipes were placed in V-blocks which rested on a surface plate. The heights of the pipe, either side of the joint, were measured as a function of distance from the joint, and by extrapolation joint misalignment calculated. Measuring misalignment immediately adjacent to the joint was found to agree, to within about 1 percent, with misalignment measured by the extrapolation technique (1 0).

Evaluation of the Lifetimes of Pipes Under Constant and Fluctuating Internal Pressure Loading

The performance of the variously joined MDPE and HDPE pipes was assessed by elevated temperature testing under either constant or fluctuating internal pressures. All pipe systems were free end with no axial constraint imposed. The combination of the test temperature (between 79 and 8OOC) and maximum internal pressure (0.95 MPa gage maximum for fatigue and a fixed 0.95 MPa gage for stress rupture loading) induced failure by slow, stable, crack growth (9, 10).

Nominal MFR (g/lO min) 0.2 0.2 Nominal Density (kg . m-3) 956 945 Co-monomer Type Hexene Hexene Outside Pipe Diameter (mm) 63 63 90 125 Measured MFR (g/10 min) 0.182 0.203 0.208 0.197 Mid-wall Density (kg. m-3) 957.8 942.9 945.5 945.1

Notes: 1. Both grades of polyethylene are Rigidex high density resins from BP Chemicals Ltd. 2. YFR values at lW°C and 2.16 kg load. 3. Density determined in a calibrated density gradient column.

Fig. 1 . Schematic representation of [a) axial misalignment, [b) angular misalignment, and (cJ combined angular and axial misalignment at pipe butt fusion joints.

Table 2. Welding Conditions for Butt Fusion Joining.

Grade of BP Chemicals Rigidex Polyethylene

PCOO2-60 PCOO2-50 R968

Nominal Pipe Diameter (mm) 63 63 90 125 Heater Plate Temperature ("C) 205 (k5) 205 (+5) 205 (+5) 205 (f5) Bead-up Pressure (MPa) 0.072 0.072 0.070 0.073 Bead-up Time (s) 33 33 35 40

Welding Pressure (MPa) 0.144 0.144 0.141 0.146 Heat Soak Time (s) 63 (k2) 63 (k2) 75 (+1) 110 (fl)

Welding Time (min) 5 5 5 10 Notes: 1. The bead-up time was selacted till a 2 mm wide bead was formed. 2. Heat soak timer below those recommended due to welding in a warm pipe testing laboratory. 3. Heater removal time was for 6 (Z1) s. 4. Bead-up and welding pressures are those calculated for true SDR11 pipe.

POLYMER ENGINEERING AND SCIENCE, MIPOCTOBER 1989, Vd. 29, No. 19 1407

J . Bowman and R. Parrnar

Each sample was filled with tap water, immersed in a large, temperature controlled, water bath, and 24 h allowed to elapse before testing commenced. For both constant and fluctuating internal pressure loading, stresses were generated in the wall of the pipes by top loading, with compressed air, the water within the pipes. The equipment used to pressurize the pipes and identify failure has been previously described (10- 12). For fatigue a trapezoidal-like loading profile was applied with a stress ratio of zero. Figure 2 shows a typical internal pressure profile; typical cycle times were 12 s.

EXPERIMENTAL RESULTS

Butt Joint Strength Under Constant Pressure Loading

The influence of axial misalignment on the elevated temperature, stress rupture lifetimes of 63 and 125 mm SDR 11 MDPE and 63 mm SDR 1 1 HDPE pipe systems is seen in Figs. 3a and b. For butt joined MDPE pipes measured misalignments were recorded, while only nominal values were annotated for HDPE systems.

All failures of stress rupture tested MDPE and HDPE pipes were brittle in nature (9, lo), with only modest misalignment (x = 0.09) being required to focus failure at the joint by circumferential cracking (9). For all pipe systems joint misalignment, as a “welding variable”. had a significant influence on the stress rupture lifetimes. Increasing the fractional misalignment progressively reduced the lifetime.

Joint Strength Under Fluctuating Internal Pressures

For 63 and 125 mm SDR 1 1 MDPE butt joined pipes Figs. 4a and b respectively highlight the influ-

0 0.4 0.8 1.2 gauge pressure (MPa)

Fig. 2. Pressure-time plots for 125 mrn S D R l 1 MDPE pipes tested a t 80°C.

lo2 2 5 lo3 2 5 104 stress rupture l i fe t ime lhours)

a

s t ress rupture lifetime Ihours)

Fig. 3. The influence ofjoint misalignment on the static stress Zifetimeof (a)MDPE and@) HDPE buttjointedpipes. Joint misalignment is expressed as a percent of the wall thickness of the pipe.

b

ence of joint axial misalignment on the elevated temperature fatigue lifetime. For butt joined MDPE systems tests were undertaken using at least two pressure ranges (0 to 0.65 and 0 to 0.95 MPa gage) with the fatigue lifetime correlated with the measured fractional misalignment. For the 63 mm HDPE pipe systems only the nominal misalignment was annotated and the systems were tested at a pressure range of 0 to 0.95 MPa gage only.

Elevated temperature fatigue tests induced brittle failures in both the HDPE and MDPE systems, the systems failing by stable crack growth ( 9 , l O ) . Modest joint misalignment (x = 0.09) focused the failure at the joint by circumferential cracking (9). The combined evidence from fatigue testing four different MDPE or HDPE pipe systems at elevated temperatures is the same; butt joint alignment has a major influence on the strength and quality of the joint, with increasing misalignment progressively reducing fatigue strength.

DISCUSSION Bending Stresses in Butt Joints Between Flat Plates

The evidence presented here, and particularly in a sister paper on the fractography of joint failures (9), shows the introduction of axial misalignment fo-

1408 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER 7989, YO/. 29, No. 19

Long Term Strength of Polyethylene Pipe Butt Fusion Joints

lo4 2 5 lo5 2 5 106 2 cycles to failure

a -. 0 * 0.65MPa

I I 1 I 1 I I I I I 1 I 1 1 1 1 1 1 I 104 2 5 105 2 5 106 2

cycles to failure b

Fig. 4. The influence of joint misalignment on the 80°C fatigue lifetime of (a) 63 mm and (b) 125 mm SDRl1 MDPE pipes. Joint misalignment is expressed as a percent of the wall thickness of the pipe.

cused failure at the joint by circumferential cracking for unconstrained fatigue and stress rupture testing at elevated temperatures. The circumferential cracks that precipitated failure in the misaligned butt joined pipes may be considered to grow principally because of the axial stresses in the pipe (13). A first route to consider how misalignment induced failure at butt joints is to examine the simpler case of misaligned flat plates.

Figure 5 shows two flat plates parallel but displaced by e. The plates of equal thickness, d , are subjected to an in-plane force, F. This creates at the joint (which is considered free to rotate) a couple eF. The joint is thus a beam with a centrally applied

couple, eF, creating bending moments (-) either side

of the joint. These two bending moments induce in a plate of width band nominal thickness d a bending stress, uB, which is given by

eF 2

and since the in-plane force, F, is acting remote from

the joint to give an axial stress, uA, given by -, the

above equation simplifies to

F bd

Us = 3 UA(e/d)

The total stress at the joint uAJ, is the sum of the bending and axial stresses and is given by

This represents an upper-bound value since constraint and limiting the freedom of rotation at the joint reduce the amplification of stress due to misalignment (1 4).

Stresses at Axially Misal igned Pipe Butt Fusion Joints

Consider two large diameter pipes of equal wall thickness but unequal diameter, butt joined with aligned and parallel pipe axes. For this symmetrical case a uniform step and a uniform stress is created around the circumference at the joint. Equation 2 is a reasonable description of the stress at the joint due to the axial stress.

The pipe systems tested for this study differed from the above case in the following ways:

1) the pipes had equal diameters, 2) the pipes were displaced relative one to another, 3) the step was not uniform around the circumfer-

4) the diameters of the pipes were relatively small, 5) the pipes were pressurized internally to gener-

ate hoop stresses in addition to the longitudinal or axial stresses.

These differences lead to a non-uniform amplified stress around the circumference at the joint and to some constraint.

Despite the limitations noted above the worth of using Eq 2 will be examined. Cracks that precipitated

ence at the joint,

Fig. 5. Schematic representationof two plates buttjoined together with some axial misalignment.

POLYMER ENGlNEERlNG AND SCIENCE, MID-OCTOBER 1989, Vol. 29, No. 19 1409

J. Bowman and R. Parmar

6 I

failure of axially misaligned joints lay in a circumferential plane (9) and apparently opened in response to the axial stresses. Equation 2 has been found to be of some value in exploring the influence of misalignment (8). And the influence of misalignment on hoop stresses at the joint, U ~ J , is less pronounced (1 5)

I I

where OH is the hoop stress. The above factors propel us to consider first the value of Eq 2.

Analysis of the Stress Rupture Data Figures 3a and b showed how joint misalignment

influenced the elevated temperature stress rupture lifetime of HDPE and MDPE pipe systems. Fractional joint misalignment, x, can now be replaced by the amplified axial joint stress, uAJ, as calculated by Eq 2. Figures 6a and b, for 63 and 125 mm MDPE pipe systems, plot lifetime as a function of OAJ. For these single pressure tests reasonable plots are produced, particularly for the 125 mm systems.

Since it has been recorded that constraint a t the joint may lower the amplification of the axial stress due to misalignment, the more general equation below is proposed

UAJ = a A ( 1 + a ’x ] (3)

where LY is a constant. To investigate the most appro- priate value for a, the linear correlation coefficient

was calculated for double logarithmic plots of uAJ

against stress rupture lifetime for a range of values of a. The results are contained in Fig. 7a for the 90 and 125 mm pipe systems. Note that the axial pipe stress, uA. was calculated using the equation

where P is the internal pressure and do,, is the pipe outside diameter.

Figure 7a shows the correlation coefficient varies with both diameter and the value of the constant, a. At all values of a the larger diameter pipe system exhibited a better correlation coefficient, a pattern of behavior repeated with the fatigue data. For both the 90 and the 125 mm MDPE pipe systems tested under constant stress loading, the preferred value for a is between 1 and 2. To see if the same conclusions apply to the larger set of data generated under fatigue, we apply a similar analysis.

Analysis of the Fatigue Lifetime Data

The influence of axial misalignment on the elevated temperature lifetimes of butt fusion joints has been most extensively explored using fatigue loading, see Fig. 4. An equation similar to Eq 3 is used to

P e b o r 0% 9% 18% 44%

1 I I I I I I l l I I I l l 1

lo2 2 5 103 2 5 104 2 stress rupture lifetime (hours)

a

tesf temperature BOOC nominal misalignment

lt%

l- 1( lo2 2 5 103 2 5 lo4 2

stress rupture lifetime (hours)

Fig. 6. For (a) 63 rnm and (b) 125 rnm S D R l 1 MDPE pipe systems the static stress lgetime is correlated with the amplified axial stress at the buttjoint, see E q s 2 and 3.

b

g -0.90

0 1 2 3 M (see equation 3)

1410 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER 1989, Vol. 29, No. 19

Long T e r m Strength of Polye thy lene Pipe But t Fusion Joints

analyze the fatigue data. The amplified axial stress range at a misaligned butt joint, AuAJ, is linked to the applied axial stress range, AUA and the fractional misalignment, x by

(4) AUAJ = A u A [ ~ + P * x ] where p is a constant which when set to a value of 3 leads E q 4 to parallel E q 2.

Figures 8a and b replot the data contained in part in Fig. 4, replacing x by AUAJ with 0 = 3. The tendency for the data to fall on a single line is encouraging. However, it is important to examine if Eq 2 is a correct expression of the influence of misalignment, that is p = 3 in Eq 4. By calculating the linear correlation coefficient for a range of values for 0 (from 0.5 to 4) the optimum value can be identified. This was undertaken for the 63.90, and 125 mm MDPE pipes, and the results are presented in Fig. 7b. Some trends exist:

For all three pipe diameters the correlation coefficient increases with increasing values of /? then declines, for most values of p, the correlation coefficient increases with increasing pipe diameter, the preferred value for ,L? from the calculated linear correlation coefficients varies with pipe diameter, being about 1.5 for the 63 mm pipes and rising to about 2.5 for the 90 and 125 mm systems, the improved correlation coefficient and the higher preferred values for 0 with increasing pipe diameter, infer less constraint may exist with larger diameter butt joined pipe systems.

These observations encourage the use of Eq 4 to describe how misalignment influences fatigue lifetime. At the larger diameters /? tends to a value of 3 to infer Eq 2 may hold for large diameter, misaligned, butt joined, polyethylene pipes.

Performance of Aligned Butt Joints

In the sister paper (9) to this publication crack initiation and propagation paths were explored. It was seen that small degrees of axial misalignment induced failure by circumferential cracking. How- ever, with the aligned butt fusion joints failure was sometimes remote from the joint or by radial cracking (9). The mechanism was not necessarily the same.

The observations above suggest that the sharp notch at the butt joint, introduced by the weld bead rolling towards the pipe bore (see Figs. 3 and 4 of the paper by Parmar and Bowman (9)). becomes particularly active when the butt joint is misaligned. It is worth considering whether E q s 3 and 4 may apply only over the range of misalignments that induce circumferential cracking at the joint. In other words, the type of plots shown in Figs. 6 and 8 may underestimate the strength of aligned butt fusion joints if extrapolated from high to low axial misalignments. There is some evidence for this statement for the fatigue tested 125 mm pipe systems, see Fig. 4b. This

POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER 1989, VOl.

cycles to failure a -. 2 10

z nominal misalignment 2 5

w O h 0 I ? 0% 9% 18% U%

7 5 2

!3

< VI

2 $ 1 < lo4 2 5 lo5 2 5 106 2

m

6 cycles to failure b

Fig. 8. For (a) 90 mm and (b) 125 mm butt joined MDPE pipe systems plots of amplified axial stress range against fat igue cycles to failure.

aspect is worthy of further study on large diameter pipe systems.

CONCLUSIONS

The influence of axial misalignment at butt fusion joints in MDPE and HDPE pipes has been assessed by undertaking elevated temperature lifetime testing. Pipe systems were tested under both fatigue and stress rupture loading, and the resultant failures were by slow, stable crack growth. The measured lifetimes were correlated with the fractional misalignment at the joint.

For both loading modes and all four pipe systems tested, the fractional misalignment at the butt joint exerted a powerful influence on lifetime as indicated in previous publications (6, 7, 8, 16). For fractional misalignments up to about 0.5, increasing the misalignment progressively reduced performance. Misa- lignment is therefore an important "welding variable" worthy of some study.

To explain how misalignment influences performance, consideration was given to the additional bending stresses introduced at the joint. Equations 3 and 4 within this text describe how fractional misalignment can raise the local or joint axial stress above that in the pipe remote from the joint. If this proce- dure is adopted some of the data can be explained. This applies particularly to the fatigue tested MDPE pipe systems, where the large sets of data, obtained over a number of applied pressure ranges, gave good linear correlation coefficients. The agreement was particularly good with the largest diameter pipes sys-

29, No. 19 1411

R. Parmar and J. Bowman

tems where constraint would be less when compared to the 63 mm pipes.

For stress rupture tested MDPE and HDPE systems the correlation coefficients were generally not as good as the fatigue tested samples. The authors feel better agreement may have resulted if a wider range of test pressures had been employed. Future work in this area should use multiple pressure testing and this should be, in preference, on large diameter butt joined pipe systems.

Finally, there is some evidence that back extrapo- lating from highly misaligned joints to predict the performance of aligned joints may underestimate the lifetimes of aligned joints. The introduction of axial misalignment appears to activate a notch or flaw created when the weld bead rolls toward the bore of the pipe (9). In aligned welds this notch is much less “active.” It is this that may explain the high quality of well made and aligned butt fusion joints (3-5). This gives further encouragement to the use of butt fusion as the method to joint large diameter, plastic pipes.

ACKNOWLEDGMENTS

The authors wish to thank the following. The Poly- mer Engineering Directorate of the Science and En- gineering Research Council for provision of monies, and WRC Swindon (of Swindon, England) and The Associated Octel Company Ltd. (of Ellesmere Port, Merseyside) for the provision of additional funds and an intellectual input to the program. Mr. W. Price is thanked for help with the maintenance of the lifetime testing laboratory. Dr. A. Yettrum on stresses at

joints and Mrs. Jolliffe for statistical analysis both gave advice and help.

REFERENCES

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2.

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4. 5. 6.

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D. R. de Courcy and J. R. Atkinson, J . Mater. Sci. , 12, 1535 119771. H. Poiente And P. Tappe. Materials & Design, 5 , 273 [ 1985). J. Bowman, paper 11 of Proc. Int. Conf. Plastics Pipes VII, Bath England, Sept. 1988 (The Plastics and Rubber Institute), London, 1988. J. N. Mallinson and D. P. Gwynn, paper 9 idem. G. Diedrich and E. Gaube, Kunststoffe, 60, 74 (1970). L. Maine and T. G. Stafford, “In-field Quality Control of Fusion Jointing Methods.” E614 report from Engineer- ing Research Station of British Gas plc., (British Gas plc. Newcastle on Tyne, England) Sept. 1988. T. G. Stafford and L. Ewing. p40 of Proc. 9th Plastic Fuel Gas Pipe Symposium, New Orleans USA (American Gas Association, Arlington VA), Nov. 1985. R . Parmar and J. Bowman. paper 25 of Proc. 6th h t . Conf. on Plastics Pipes, York, England, March 1985. (The Plastics and Rubber Institute), London, 1985. R. Parmar and J . Bowman, Polyrn. Eng. Sci., 29, 1396 (1 989). R. Parmar. PhD Thesis, Brunel University, 1986. M. B. Barker, J. Bowman, and M. Bevis, J. Mater. Sci., 18, 1095 (1983). G. C. Ford, M. B. Barker, S. Bentley, K. Batchelor, and J. Bowman, Polyrn. Test. , 3, 161 (1983). J. Bowman and R. Parmar, Proc. 1 l t h Plastic Fuel Gas Pipe Symposium, San Francisco, USA (American Gas Association, Arlington, VA) Oct. 1989. S. Berge and H. Myhre, Norwegian Maritime Research, 5. 29 (April 1977). M. Reinke and H. Potente, SPE ANTEC Tech. Papers, 28, 59 (1982). P. John and J. Hessel, Kunststoffe, 7 5 . 770 (1985).

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the importance of axial misalignment on the long term strength of polyethylene pipe butt fusion...

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