- Email: [email protected]
Defect Analysis and High Density Polyethylene Pipe Durability
Defect Analysis and High Density Polyethylene Pipe Durability
Shaofu Wu, Kalyan Sehanobish, and Noor Jivraj Texas Polymer Center, The Dow Chemical Company, Freeport, Texas 77541, USA
INTRODUCTION Thermoplastics, in particular polyolefins, are gaining considerable market share in pipe applications such as gas and water supply systems. To ensure proper performance of such pipes over the required lifetime, durability analyses are needed to adequately account for the effects of loading, time, temperature, environmental conditions, as well as the role of pipe defect and imperfections on relevant polymer properties and pipe performance. Durability analysis involves defect characterization, crack initiation and propagation mechanisms, and long term performance prediction. Internal pressure testing of pipes is highly dependent on the defect properties and population, as well as the toughness of material. Thus both defect population and toughness should be used as a measure to differentiate pipe materials for durability analysis. Characterization of defects inside the extruded pipes also provides valuable information about the origin of defects. Suggestions can be made to eliminate these defects by adjusting or changing the processing conditions and environmental conditions. Thus, characterization of defects is critical for durability analysis of pipe materials. Defect analysis of high density polyethylene pipe material is discussed in this paper. Various analytical techniques, such as fractography, hot stage microscopy, energy dispersive X-ray (EDX), micro-transmittance infrared spectroscopy, scanning electron microscopy (SEM), film stretching, and transmission electron microscopy (TEM) are used to characterize the defect properties and size distribution. Some of these analytical techniques, such as optical and FTIR microscopy, have been well documented to identify defects in polyethylene. I ,2 In this paper, we will focus on defect characterization, the correlation of the defect properties and long term performance of pipe.
DEFECT CHARACTERIZATION Several PE 100 HDPE pipes which had been subjected to long-term pressure testing at 80°C with internal pressure of 5 MPa (in accordance with ISO TR 9080) have been examined by
282
Plastics Failure Analysis and Prevention
several analytical techniques in order to determine the nature of the existing defect particles, which appear to contribute to the failure of these pipes. Scanning electron microscopy (SEM), energy X-ray Figure I. (a) SEM image of fracture surface in a pipe. The primary failure initiation site is dispersive located at bottom-center in the figure and is near the inner wall of the pipe; (EDX), microtransmit(b). Large (-75 f.l m diameter) particle located at center of main failure initiation site tance infrared spectrosshown in (a). copy, and hot-stage microscopy were used to determine the size distribution and compositions of defects present in these pipes. The fracture surface shown in Figure I a was produced by freeze fracturing an Figure 2. Example of defect particle on the fracture surface of different pipes. arc-shaped section of the pipe which contained the site at which a leak was first observed when the pipe failed the long-term test. A particle approximately 75 )lm in diameter is located at the center of the round, crater-like, penny-shaped domain located at bottom-center in the SEM montage (see Figure Ib). This particle appears to have been the main initiator of failure in this pipe. Depending on the defect properties different interfacial adhesion with the matrix are observed and is shown in Figure 2. In general, material with a better adhesion between the defect and matrix has a longer lifetime with the hydrostatic test. Four tested pipes made from HDPE made using INSITPM Technology have also been evaluated. These pipes have much longer lifetime under the test conditions compared with the pipes made from HDPE made using the traditional Ziegler catalysts. Sections from the main crack sites of each pipe were cut and brittle fractured. Fracture surfaces were analyzed by optical microscopy and SEM. It was observed that fractures were initiated at the inner surface of pipes because of higher residual stress level at the inner surface. Failures initiated
283
Defect Analysis
both from small defect particles (15 to 20 flm) and at sites where no defects were observed. SEM micrographs of the fracture surface of two pipes are shown Figure 3. Example of defect particles on the fracture surface of pipe made by lNSlTE Technology. in Figures 3. The defect particles observed on the fracture surfaces also show good interfacial adhesion with the matrix. We did not observe any other pre-initiated crack/craze on these fractured surfaces. We also randomly selected several sections from these pipes and fractured these sections. Again, no clear crack initiation sites were found on these fracture surfaces. Thus the defect particles observed in these pipes can not be related to any fracture event. It is speculated that these gel particles may have similar rigidity to the matrix and good interfacial adhesion and are less responsible for fracture.
DEFECT SIZE DISTRIBUTION Figure 4 shows the particle size distributions obtained from fracture surfaces in two tested pipes. These two pipes have a quite different lifetime under the hydrostatic test (70 hours vs. 500 hours with 5.5 MPa hoop stress at 80°C). Particles were observed on fracture surfaces which were produced by freeze fracturing sections of pipe containing visible cracks at the
1,,10
1H5 Slze(jun}
1G-2fJ
>20
$AO
11·15
Figure 4. Particle size distribution of particles observed at fracture surfaces in one pipe.
1$-2fJ
$t«I fJiml
21$
>5fi
284
Plastics Failure Analysis and Prevention
outer surface of the pipe. Most of the particles below 10 f.l m were located on a fracture surface produced by notching and fracturing pipe in liquid nitrogen, where no externally visible cracks were located. While the relative populations of defects in these two pipes cannot be determined accurately from this information, it is clear that a pipe which has a longer lifetime also contains a significant number of defects, often as large or larger than pipes which exhibited a shorter lifetime. 3
----"'"1W!------.........,
r ..
2.5
·S
•
CRACK SIZES VS. DEFECT SIZE IN TESTED PIPES
For the tested pipes, there are many natural cracks which initiated during testing, in addition to the .~ 1.,5 major crack that caused failure of the pipe. In '"r:u most cases, these natural cracks were initiated <..l from defects distributed inside the pipe. Natural Q.5 cracks in two tested pipes have been analyzed by 0--1-----.........- - -.........- - - - - 1 cryogenic fracture of tested pipes. The results and 0,1 0,2 0,3 their correlation with defect sizes are shown in Defectsix& (mml Figure 5. These results suggest that the crack size Figure 5. The defect sizes vs. the crack sizes in two tested pipes, is not proportional to the defect size in these pipes. The crack size depends not only on the size of defect, but also the geometry, rigidity, connectivity with surrounding materials, and location in the pipe. A defect having a poor attachment with the surrounding material will form a natural void, therefore becoming a site for crack initiation; while a defect with a good interfacial adhesion with the surrounding material will generate a lower stress concentration, therefore delays the crack initiation. Examples of these defects are shown in Figure 7a and b. Failure analyses of tested pipes show that cracks usually initiate from a singular defect particle located near the inner surface of pipes. Crack initiations in these sites are primarily caused by a relative higher stress state in the inner wall of the pipe. However, pipes with a similar defect population have shown a quite different lifetime under internal pressure testing. These results suggest that the long-term performance depends not only on defect size, but also on defect properties and residual stress induced by the process. 2
·fili
'll
•
•••
°
DEFECT STIFFNESS Since the defect particle stiffness and interfacial adhesion is critical to the long term performance of pipe materials, film stretching is used in this study to characterize these properties. Development of this method is based on the analysis of stress state around a sphere inclusion. 3 For the case of plane stress, the hoop stress near the interface can be expressed as:
285
Defect Analysis
oo
A - 3B 4cos2e ) r r
[1 ]
= 2E I ( 2
where A _
2V2 ) ( 2V I ) ( 1 - - - E)- 1 - - - E 2 I-v 2
P
a2 - 4E I
li_
I-v)
(12 V2 ) E +E
I-v 2
I
[2]
2
E) -E 2
p
a 4 - 4E)
(
[3]
4V I )
E I + 3 - - - E2 I-v)
and E 1, vI and E 2, v2 are the modulus and Poisson's ratio of matrix and inclusion, separately. For a special case of cavity (E 2=0), the hoop stress will be -p for e = 0 and 2p for e = 90°. For the perfectly rigid inclusion (E 2=) and assume the Poisson's ratio of the matrix is 0.4, the hoop stress will be 1.67p for e = 0 and -1.33p for e = 90°. Thus by examining the deformation behaviors near the defect particle during stretching, the stiffness and interface adhesion of defect can be characterized. A sketch of test setup is shown in Figure 6. Micrographs of the deformation behavior of gel defect particles at three stages of stretching are shown in Figures 7. All of pictures were taken under optical microscopy and with a magnification of 20x. Each figure illustrates the behavior of gel particle before stretching, after necking passing the gel particle, and after film fracture. The non-uniform deformation around the gel particle shown in Figure 7a, for example, indicates that there was stress concentration at upper and lower points of interface after necking passing the gel particle. Based on the stress analysis that soft gel particles will cause high stress concentration at upper and low points, it is concluded that the
,
Optical microscope
Inclusion p II(
t09
\
ii!:'l .. film with
Figure 6 (a). Sketch of an inclusion inside polymer and (b). Sketch of setup of gel particle property evaluation by film stretching method.
286
Plastics Failure Analysis and Prevention
Atlerncok
passing 'h<: gtl
(h)
Figure 7. Micrographs (x20) of gel particle deformation behaviors in films; a). relatively soft particle; and b). relatively rigid particle.
gel particle shown in Figure 7a is relatively softer comparing to the matrix (HDPE). Figure 7a also showed that the gel particle shape starts to change when we continue stretching the film that is also the evidence of a soft gel particle. Figure 7b shows an example of a relatively hard gel particle in HDPE matrix. The gel particle shape still remained the same or became sharper after necking passing the gel particle and non-uniform deformation around the gel particle was observed. This suggests the gel particle had a higher elastic modulus comparing with the matrix (hard gel particle).
CONCLUSIONS Fractographic analysis of tested pipes shows that cracks are mostly initiated from large (>50 f.l m) defect particles. The results show that pipes with smaller defect particle size (less than 20 f.l m) have a better long-term property. However, pipes with a similar defect particle size do not necessarily have a similar long-term performance. Besides the size, defects with high rigidity, sharp edges, and low connectivity with the matrix are more susceptible to crack ini-
Defect Analysis
287
tiate during testing or service. Residual stress generated during processing can also affect the long term performance.
REFERENCES 2 3
A. Chudnovsky, K. Sehanobish, and S. Wu, Methodology for durability analysis of HDPE pipe, AS ME 1999 PVP Conference, August, Boston, 1999. K. Sehanobish et aI., Fractographic analysis of field failure in polyethylene pipe, J. Mater. Sci. Left., 4, R90-R94, 19R5. J.N. Goodier, Concentration of stress around spherical and cylindrical inclusions and flaws, J. Appl. Mechanics, Trans. of ASME, Vol. 55, No. 39, pp 39- 44,1933.
Shaofu Wu, Kalyan Sehanobish, and Noor Jivraj Texas Polymer Center, The Dow Chemical Company, Freeport, Texas 77541, USA
INTRODUCTION Thermoplastics, in particular polyolefins, are gaining considerable market share in pipe applications such as gas and water supply systems. To ensure proper performance of such pipes over the required lifetime, durability analyses are needed to adequately account for the effects of loading, time, temperature, environmental conditions, as well as the role of pipe defect and imperfections on relevant polymer properties and pipe performance. Durability analysis involves defect characterization, crack initiation and propagation mechanisms, and long term performance prediction. Internal pressure testing of pipes is highly dependent on the defect properties and population, as well as the toughness of material. Thus both defect population and toughness should be used as a measure to differentiate pipe materials for durability analysis. Characterization of defects inside the extruded pipes also provides valuable information about the origin of defects. Suggestions can be made to eliminate these defects by adjusting or changing the processing conditions and environmental conditions. Thus, characterization of defects is critical for durability analysis of pipe materials. Defect analysis of high density polyethylene pipe material is discussed in this paper. Various analytical techniques, such as fractography, hot stage microscopy, energy dispersive X-ray (EDX), micro-transmittance infrared spectroscopy, scanning electron microscopy (SEM), film stretching, and transmission electron microscopy (TEM) are used to characterize the defect properties and size distribution. Some of these analytical techniques, such as optical and FTIR microscopy, have been well documented to identify defects in polyethylene. I ,2 In this paper, we will focus on defect characterization, the correlation of the defect properties and long term performance of pipe.
DEFECT CHARACTERIZATION Several PE 100 HDPE pipes which had been subjected to long-term pressure testing at 80°C with internal pressure of 5 MPa (in accordance with ISO TR 9080) have been examined by
282
Plastics Failure Analysis and Prevention
several analytical techniques in order to determine the nature of the existing defect particles, which appear to contribute to the failure of these pipes. Scanning electron microscopy (SEM), energy X-ray Figure I. (a) SEM image of fracture surface in a pipe. The primary failure initiation site is dispersive located at bottom-center in the figure and is near the inner wall of the pipe; (EDX), microtransmit(b). Large (-75 f.l m diameter) particle located at center of main failure initiation site tance infrared spectrosshown in (a). copy, and hot-stage microscopy were used to determine the size distribution and compositions of defects present in these pipes. The fracture surface shown in Figure I a was produced by freeze fracturing an Figure 2. Example of defect particle on the fracture surface of different pipes. arc-shaped section of the pipe which contained the site at which a leak was first observed when the pipe failed the long-term test. A particle approximately 75 )lm in diameter is located at the center of the round, crater-like, penny-shaped domain located at bottom-center in the SEM montage (see Figure Ib). This particle appears to have been the main initiator of failure in this pipe. Depending on the defect properties different interfacial adhesion with the matrix are observed and is shown in Figure 2. In general, material with a better adhesion between the defect and matrix has a longer lifetime with the hydrostatic test. Four tested pipes made from HDPE made using INSITPM Technology have also been evaluated. These pipes have much longer lifetime under the test conditions compared with the pipes made from HDPE made using the traditional Ziegler catalysts. Sections from the main crack sites of each pipe were cut and brittle fractured. Fracture surfaces were analyzed by optical microscopy and SEM. It was observed that fractures were initiated at the inner surface of pipes because of higher residual stress level at the inner surface. Failures initiated
283
Defect Analysis
both from small defect particles (15 to 20 flm) and at sites where no defects were observed. SEM micrographs of the fracture surface of two pipes are shown Figure 3. Example of defect particles on the fracture surface of pipe made by lNSlTE Technology. in Figures 3. The defect particles observed on the fracture surfaces also show good interfacial adhesion with the matrix. We did not observe any other pre-initiated crack/craze on these fractured surfaces. We also randomly selected several sections from these pipes and fractured these sections. Again, no clear crack initiation sites were found on these fracture surfaces. Thus the defect particles observed in these pipes can not be related to any fracture event. It is speculated that these gel particles may have similar rigidity to the matrix and good interfacial adhesion and are less responsible for fracture.
DEFECT SIZE DISTRIBUTION Figure 4 shows the particle size distributions obtained from fracture surfaces in two tested pipes. These two pipes have a quite different lifetime under the hydrostatic test (70 hours vs. 500 hours with 5.5 MPa hoop stress at 80°C). Particles were observed on fracture surfaces which were produced by freeze fracturing sections of pipe containing visible cracks at the
1,,10
1H5 Slze(jun}
1G-2fJ
>20
$AO
11·15
Figure 4. Particle size distribution of particles observed at fracture surfaces in one pipe.
1$-2fJ
$t«I fJiml
21$
>5fi
284
Plastics Failure Analysis and Prevention
outer surface of the pipe. Most of the particles below 10 f.l m were located on a fracture surface produced by notching and fracturing pipe in liquid nitrogen, where no externally visible cracks were located. While the relative populations of defects in these two pipes cannot be determined accurately from this information, it is clear that a pipe which has a longer lifetime also contains a significant number of defects, often as large or larger than pipes which exhibited a shorter lifetime. 3
----"'"1W!------.........,
r ..
2.5
·S
•
CRACK SIZES VS. DEFECT SIZE IN TESTED PIPES
For the tested pipes, there are many natural cracks which initiated during testing, in addition to the .~ 1.,5 major crack that caused failure of the pipe. In '"r:u most cases, these natural cracks were initiated <..l from defects distributed inside the pipe. Natural Q.5 cracks in two tested pipes have been analyzed by 0--1-----.........- - -.........- - - - - 1 cryogenic fracture of tested pipes. The results and 0,1 0,2 0,3 their correlation with defect sizes are shown in Defectsix& (mml Figure 5. These results suggest that the crack size Figure 5. The defect sizes vs. the crack sizes in two tested pipes, is not proportional to the defect size in these pipes. The crack size depends not only on the size of defect, but also the geometry, rigidity, connectivity with surrounding materials, and location in the pipe. A defect having a poor attachment with the surrounding material will form a natural void, therefore becoming a site for crack initiation; while a defect with a good interfacial adhesion with the surrounding material will generate a lower stress concentration, therefore delays the crack initiation. Examples of these defects are shown in Figure 7a and b. Failure analyses of tested pipes show that cracks usually initiate from a singular defect particle located near the inner surface of pipes. Crack initiations in these sites are primarily caused by a relative higher stress state in the inner wall of the pipe. However, pipes with a similar defect population have shown a quite different lifetime under internal pressure testing. These results suggest that the long-term performance depends not only on defect size, but also on defect properties and residual stress induced by the process. 2
·fili
'll
•
•••
°
DEFECT STIFFNESS Since the defect particle stiffness and interfacial adhesion is critical to the long term performance of pipe materials, film stretching is used in this study to characterize these properties. Development of this method is based on the analysis of stress state around a sphere inclusion. 3 For the case of plane stress, the hoop stress near the interface can be expressed as:
285
Defect Analysis
oo
A - 3B 4cos2e ) r r
[1 ]
= 2E I ( 2
where A _
2V2 ) ( 2V I ) ( 1 - - - E)- 1 - - - E 2 I-v 2
P
a2 - 4E I
li_
I-v)
(12 V2 ) E +E
I-v 2
I
[2]
2
E) -E 2
p
a 4 - 4E)
(
[3]
4V I )
E I + 3 - - - E2 I-v)
and E 1, vI and E 2, v2 are the modulus and Poisson's ratio of matrix and inclusion, separately. For a special case of cavity (E 2=0), the hoop stress will be -p for e = 0 and 2p for e = 90°. For the perfectly rigid inclusion (E 2=) and assume the Poisson's ratio of the matrix is 0.4, the hoop stress will be 1.67p for e = 0 and -1.33p for e = 90°. Thus by examining the deformation behaviors near the defect particle during stretching, the stiffness and interface adhesion of defect can be characterized. A sketch of test setup is shown in Figure 6. Micrographs of the deformation behavior of gel defect particles at three stages of stretching are shown in Figures 7. All of pictures were taken under optical microscopy and with a magnification of 20x. Each figure illustrates the behavior of gel particle before stretching, after necking passing the gel particle, and after film fracture. The non-uniform deformation around the gel particle shown in Figure 7a, for example, indicates that there was stress concentration at upper and lower points of interface after necking passing the gel particle. Based on the stress analysis that soft gel particles will cause high stress concentration at upper and low points, it is concluded that the
,
Optical microscope
Inclusion p II(
t09
\
ii!:'l .. film with
Figure 6 (a). Sketch of an inclusion inside polymer and (b). Sketch of setup of gel particle property evaluation by film stretching method.
286
Plastics Failure Analysis and Prevention
Atlerncok
passing 'h<: gtl
(h)
Figure 7. Micrographs (x20) of gel particle deformation behaviors in films; a). relatively soft particle; and b). relatively rigid particle.
gel particle shown in Figure 7a is relatively softer comparing to the matrix (HDPE). Figure 7a also showed that the gel particle shape starts to change when we continue stretching the film that is also the evidence of a soft gel particle. Figure 7b shows an example of a relatively hard gel particle in HDPE matrix. The gel particle shape still remained the same or became sharper after necking passing the gel particle and non-uniform deformation around the gel particle was observed. This suggests the gel particle had a higher elastic modulus comparing with the matrix (hard gel particle).
CONCLUSIONS Fractographic analysis of tested pipes shows that cracks are mostly initiated from large (>50 f.l m) defect particles. The results show that pipes with smaller defect particle size (less than 20 f.l m) have a better long-term property. However, pipes with a similar defect particle size do not necessarily have a similar long-term performance. Besides the size, defects with high rigidity, sharp edges, and low connectivity with the matrix are more susceptible to crack ini-
Defect Analysis
287
tiate during testing or service. Residual stress generated during processing can also affect the long term performance.
REFERENCES 2 3
A. Chudnovsky, K. Sehanobish, and S. Wu, Methodology for durability analysis of HDPE pipe, AS ME 1999 PVP Conference, August, Boston, 1999. K. Sehanobish et aI., Fractographic analysis of field failure in polyethylene pipe, J. Mater. Sci. Left., 4, R90-R94, 19R5. J.N. Goodier, Concentration of stress around spherical and cylindrical inclusions and flaws, J. Appl. Mechanics, Trans. of ASME, Vol. 55, No. 39, pp 39- 44,1933.