AWWA MANUAL M55
`
`Chapter 1
`
`Engineering Properties
`of Polyethylene
`
`INTRODUCTION
`A fundamental understanding of material characteristics is an inherent part of the
`design process for any piping system. With such an understanding, the piping
`designer can use the properties of the material to design for optimum performance.
`This chapter provides basic information that should assist the reader in understanding
`how polyethylene’s (PE’s) material characteristics influence its engineering behavior.
`PE is a thermoplastic, which means that it is a polymeric material that can be soft-
`ened and formed into useful shapes by the application of heat and pressure and which
`hardens when cooled. PE is a member of the polyolefins family, which also includes
`polypropylene. As a group of materials, the polyolefins generally possess low water
`absorption, moderate to low gas permeability, good toughness and flexibility at low
`temperatures, and a relatively low heat resistance. PE plastics form flexible but tough
`products and possess excellent resistance to many chemicals.
`
`POLYMER CHARACTERISTICS
`In general terms, the performance capability of PE in piping applications is deter-
`mined by three main parameters: density, molecular weight, and molecular weight
`distribution. Each of these polymer properties has an effect on the physical perfor-
`mance associated with a specific PE resin. The general effect of variation in these
`three physical properties as related to polymer performance is shown in Table 1-1.
`
`Density
`PE is a semicrystalline polymer composed of long, chain-like molecules of varying
`lengths and numbers of side branches. As the number of side branches increases, poly-
`mer crystallinity and hence, density decreases because the molecules cannot pack as
`
`1
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 1
`
`

`

`2
`
`PE PIPE—DESIGN AND INSTALLATION
`
`Table 1-1 Effects of density, molecular weight, and molecular weight distribution
`
`Property
`
`Tensile
`Stiffness
`Impact strength
`Low temperature brittleness
`Abrasion resistance
`Hardness
`Softening point
`Stress crack resistance
`Permeability
`Chemical resistance
`Melt strength
`
`As Density Increases
`Increases
`Increases
`Decreases
`Increases
`Increases
`Increases
`Increases
`Decreases
`Decreases
`Increases
`—
`
`As Molecular Weight
`Increases
`Increases
`Increases slightly
`Increases
`Decreases
`Increases
`Increases slightly
`—
`Increases
`Increases slightly
`Increases
`Increases
`
`As Molecular Weight
`Distribution Broadens
`—
`Decreases
`Decreases
`Decreases
`—
`—
`Increases
`Increases
`—
`—
`Increases
`
`closely together. Density affects many of the physical properties associated with the
`performance of the finished pipe. Properties such as stress crack resistance, tensile
`strength, and stiffness are all affected by the base resin density of the polymer as
`shown in Table 1-1.
`Base resin density refers to the density of the natural PE that has not been com-
`pounded with additives and/or colorants. Within this range, the materials are generi-
`cally referred to as either medium or high density in nature. PE pipe resins with a
`base resin density in the range of 0.935 to 0.941 grams per cubic centimeter (g/cc) are
`referred to as medium density PE. PE pipe base resins in the range of 0.941 to 0.945 g/cc
`are commonly referred to as high-density polyethylenes (HDPEs). Industry practice
`has shown that base resin (unpigmented) densities in the range of 0.936 to 0.945 g/cc
`offer a highly beneficial combination of performance properties for the majority of pip-
`ing applications.
`The addition of carbon black to the base PE resin does have an impact on the com-
`pounded density of the material. The addition of 2 to 2.5 percent carbon black raises
`the compounded material density on the order of 0.009–0.011 g/cc. The variability in
`the actual percentage of carbon black incorporated can have a moderate affect on com-
`parative density values. As a result, industry practice as established by ASTM stan-
`dard is to provide comparative values on the base resin density as this is a better
`indicator of the polymer crystallinity.
`
`Molecular Weight
`PE resins are composed of a number of molecular chains of varying lengths. As a
`result, the molecular weight of the resin is the average of the weight of each of these
`chains. The average weight may be determined using sophisticated scientific tech-
`niques, such as gel permeation chromatography or size-exclusion chromatography. For
`PE of a given density, the effect of increasing molecular weight on physical properties
`is shown in Table 1-1.
`A very rough indicator of the molecular weight of a polymer may be obtained using
`the melt index technique of analysis as described in ASTM D12381. The melt index tech-
`nique is an inexpensive means of comparing, in a relative manner, the molecular weight
`of PEs having similar structure. Resins with a relatively low average molecular
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 2
`
`

`

`ENGINEERING PROPERTIES OF POLYETHYLENE
`
`3
`
`weight will have a comparatively high melt index. Conversely, resins with a relatively
`high molecular weight will yield a lower melt index. From this relationship, we can
`associate changes in physical properties (as shown in Table 1-1) with changes in melt
`index of the material. It is important not to use melt index alone as a definitive indicator
`of molecular weight because variations in polymer structure can affect both molecular
`weight and melt index.
`
`Molecular Weight Distribution
`Molecular weight distribution (MWD) refers to the statistical grouping of the individ-
`ual molecular chains within a PE resin. Resins made up of molecules that vary consid-
`erably in molecular weight are considered to have a broad MWD. When most of the
`molecules are nearly the same length, the MWD is considered narrow. The effect of
`broadening the MWD of a PE resin having a given density and molecular weight is
`shown in Table 1-1.
`
`Recent Advances
`It should be noted that recent advances in polymer technology have led to the develop-
`ment and introduction of even higher density resins for use in piping applications.
`These new materials that have base resin densities as high as 0.952 g/cc in combina-
`tion with higher molecular weight and bimodal molecular weight distribution are gen-
`erally recognized as offering higher levels of technical performance under ISO
`standards for PE piping that are common outside of North America. These higher lev-
`els of technical performance are not yet recognized within the North American stan-
`dards system.
`
`MECHANICAL PROPERTIES
`
`Viscoelasticity
`PE is characterized as a viscoelastic construction material. Because of its molecular
`nature, PE is a complex combination of elastic-like and fluid-like elements. As a
`result, this material displays properties that are intermediate to crystalline metals
`and very high viscosity fluids. Figure 1-1 is the traditional diagrammatic representa-
`tion of PE in which the springs represent those components of the PE matrix that
`respond to loading in a traditional elastic manner in accordance with Hooke’s law. The
`dashpots represent fluid elements of the polymer that respond to load much as a New-
`tonian fluid.
`As a result of the viscoelastic character of the polymer, the tensile stress–strain
`curve for PE is divided into three distinct regions. The first of these is an initial linear
`deformation in response to the load imposed that is generally recoverable when the
`load is removed. In the second stage of loading, deformation continues but at an ever
`decreasing rate. Thus, the slope of the stress–strain curve is constantly changing,
`attesting to its curvilinear nature. Deformation in the second stage may not be fully
`recoverable. The final stage of the stress–strain curve for PE is characterized by neck-
`ing down followed by distinct elongation or extension ultimately ending in ductile rup-
`ture of the material.
`The viscoelastic nature of PE provides for two unique engineering characteristics
`that are employed in the design of HDPE water piping systems. These are creep and
`stress relaxation.
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 3
`
`

`

`4
`
`PE PIPE—DESIGN AND INSTALLATION
`
`ε1
`
`ε2
`
`ε3
`
`Figure 1-1 Traditional model of HDPE
`
`Strain = ε1 + ε2 + ε3
`
`δ0
`
`Creep is not an engineering concern as it relates to PE piping materials. Creep
`refers to the response of PE, over time, to a constant static load. When HDPE is sub-
`jected to a constant static load, it deforms immediately to a strain predicted by the
`stress–strain modulus determined from the tensile stress–strain curve. The material
`continues to deform indefinitely at an ever decreasing rate. If the load is high enough,
`the material may yield or rupture. This time-dependent viscous flow component of
`deformation is called creep. This asserts that the long-term properties of PE are not
`adequately predicted by the results of short-term testing, such as tensile strength. As
`such, PE piping materials are designed in accordance with longer-term tests such as
`hydrostatic testing and testing for resistance to slow crack growth, which when used
`in accordance with industry recommended practice, the resultant deformation caused
`by sustained loading, or creep, is not sufficiently large to be an engineering concern.
`Stress relaxation is another unique property arising from the viscoelastic nature of
`PE. When subjected to a constant strain (deformation of a specific degree) that is
`maintained over time, the load or stress generated by the deformation slowly
`decreases over time. This is of considerable importance to the design of PE piping
`systems.
`Because of its viscoelastic nature, the response of PE piping systems to loading is
`time-dependent. The effective modulus of elasticity is significantly reduced by the
`duration of the loading because of the creep and stress relaxation characteristics of
`PE. An instantaneous modulus for sudden events such as water hammer can be as
`high as 150,000 psi at 73°F (23°C). For slightly longer duration, but short-term events
`such as soil settlement and live loadings, the effective modulus for PE is roughly
`110,000 to 120,000 psi at 73°F (23°C), and as a long-term property, the effective long-
`term modulus calculates to be approximately 20,000 to 38,000 psi. This modulus
`becomes the criteria for the long-term design life of PE piping systems.
`This same time-dependent response to loading is also what gives PE its unique
`resiliency and resistance to sudden, comparatively short-term loading phenomena.
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 4
`
`

`

`ENGINEERING PROPERTIES OF POLYETHYLENE
`
`5
`
`Such is the case with PE’s resistance to water hammer, which will be discussed in
`more detail in subsequent sections.
`PE is a thermoplastic and, as such, its properties are temperature dependent as
`well as dependent on the duration of loading. Therefore, the absolute value of the engi-
`neering properties of PE will vary in relation to the temperature at which the specific
`tests are conducted. Industry convention is to design PE piping systems using engi-
`neering properties established at the standard temperature of 73°F (23°C) and then
`employ industry established temperature compensating multipliers to provide for the
`service condition temperatures.
`
`Tensile Strength
`Tensile strength is a short-term property that provides a basis for classification or
`comparison when established at specific conditions of temperature and rate of loading
`but is of limited significance from a design perspective. The tensile strength of PE is
`typically determined in accordance with ASTM D6382. In this test, PE specimens are
`prepared and pulled in a controlled environment at a constant rate of strain.
`Any material will deform when a force is applied. The amount of deformation per
`unit length is termed the strain, and the force per cross-sectional area is termed the
`stress. As it relates to tensile testing of PE pipe grades, strain is generally approxi-
`mated by assuming a straight-line relationship to stress at lower stress levels (up to
`30 percent of the tensile yield point), and it is reversible. That is, the material deforms
`but will over time recover its original shape when the stress is removed. The strain in
`this region is referred to as the elastic strain because it is reversible. The Modulus of
`Elasticity (or Young’s Modulus) is the ratio between the stress and strain in this
`reversible region.
`At stress levels generally greater than 50 percent, strain is no longer proportional
`to stress and is not reversible, that is, the slope of the stress–strain curve changes at
`an increasing rate. At these higher stress levels, the materials begin to deform such
`that the original dimensions are not recoverable. In actual testing of PE pipe grade
`materials, this stage is characterized by initiation of a distinct “necking” of the tensile
`specimen. This is called the plastic strain region. The point at which stress causes a
`material to deform beyond the elastic region is termed the tensile strength at yield.
`The stress required to ultimately break the test specimen is called the ultimate tensile
`strength or the tensile strength at break. (See Figure 1-2.)
`Of equal importance is the percent elongation obtained during tensile testing
`because this information can provide a relative indication of the ductility of the poly-
`mer being evaluated. Materials with relatively high levels of elongation are indicative
`of highly ductile performance as pipe. Modern pipe grade PEs will demonstrate elon-
`gations of 400 to 800 percent or more between yield and ultimate tensile rupture. It is
`also typical that tensile strength at yield and tensile strength at break are similar
`values; that is, once the material yields, the load required to continue specimen elon-
`gation and eventually break the specimen changes very little.
`
`Compressive Properties
`Compressive forces act in the opposite direction to tensile forces. The effect of com-
`pressive force on PE can be measured on a tensile test apparatus using the protocol
`described in ASTM D6953. At small strains (up to 3 percent for most PE pipe resins),
`the compressive modulus is about equal to the elastic modulus. However, unlike tensile
`loading, which can result in a failure, compression produces a slow and infinite yielding
`that seldom leads to a failure. For this reason, it is customary to report compressive
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 5
`
`

`

`6
`
`PE PIPE—DESIGN AND INSTALLATION
`
`Region of Strain Hardening
`
`Tensile Yield Point
`
`Plastic Strain Region
`
`Elastic Strain Region
`
`Elongation
`
`Force
`
`Figure 1-2 Generalized tensile stress–strain curve for PE pipe grades
`
`strength as the stress required to deform the test specimen to a specific strain. Under
`conditions of mild compression, the general engineering assumption is that the effec-
`tive compressive modulus is essentially equivalent to the effective tensile modulus.
`
`Flexural Properties
`The flexural strength of a material is the maximum stress in the outer fiber of a test
`specimen at rupture. Because most PE pipe resins do not break under this test, the
`true flexural strength of these materials cannot be determined. As such, the flexural
`modulus is typically calculated on the basis of the amount of stress required to obtain
`a 2 percent strain in the outer fiber. The prevailing test method is ASTM D7904.
`Depending on the density of the base resin, the effective flexural modulus of PE can
`range from 80,000 to 160,000 psi. The flexural modulus of PE is a short-term property
`that provides a basis for classification but is of limited significance from a design
`perspective.
`
`Impact Properties
`The amount of energy that a material can absorb without breaking or fracturing is
`referred to as the impact strength of that material. ASTM D2565 describes the two
`most commonly used tests for PE pipe compounds, the Izod Impact Test and the
`Charpy Impact Test. Both test methods measure the ability of a PE specimen to
`absorb energy on failure. Obviously, test information such as this is used to make a
`relative comparison of the material’s resistance to failure on impact under defined cir-
`cumstances. In this regard, PE is a very tough material demonstrating Izod impact
`resistance values in the range of 10–12 ft-lbf/in. at standard room temperature. This
`is the range in which PE pipe grades will bend or deflect in response to Izod impact
`testing. These values will change to some degree as the temperature at which the test
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 6
`
`

`

`ENGINEERING PROPERTIES OF POLYETHYLENE
`
`7
`
`is conducted changes. When Izod impact testing is conducted at very low temperature
`(< 0°F), fracture may occur.
`
`Abrasion Resistance
`PE demonstrates outstanding abrasion resistance under potable water flow condi-
`tions. Moreover, the abrasion resistant nature of this material has resulted in the
`widespread use of PE pipe for liquid slurry handling applications. However, the fac-
`tors that affect the wear resistance of liquid slurry pipelines are diverse. In addition to
`flow velocity, one must consider the type of flow regime: laminar (single phase or dou-
`ble phase) or turbulent flow; presence, size, angularity, and concentration of sus-
`pended solids; and angle of impingement. While these factors are germane to slurry
`handling applications, they will have little or no effect on the abrasion resistance of
`PE pipe used in the transport of clean potable water. At higher flow velocities typical
`of potable water distribution, there is no erosional effect on PE pipe.
`
`OTHER PHYSICAL PROPERTIES
`
`Permeability
`The rate of transmission of gases and vapors through polymeric materials varies with
`the structure of both the permeating molecules and the polymer. Permeability is
`directly related to the crystallinity of the PE and the size and polarity of the molecule
`attempting to permeate through the matrix. The higher the crystallinity (the higher
`the density), the more resistant is the polymer to permeation. PE resins used for the
`manufacture of water pipe in accordance with ANSI/AWWA C9066 possess density
`ranges that make them highly resistant to most types of permeation.
`The designer should be aware, however, that all piping systems are susceptible to
`permeation of light hydrocarbon contaminants that may be present in the soil. With
`continued exposure over time, these contaminants can permeate from the soil into the
`pipe itself either through the wall of a plastic pipe or through the elastomeric gas-
`keted joint of a mechanically joined piping system. For this reason, special care should
`be taken when installing potable water lines through contaminated soils regardless of
`the type of pipe material (concrete, plastic, ductile iron, etc.).
`From ANSI/AWWA C906, Sec. 4.1:
`
`“The selection of materials is critical for water service and distribution pip-
`ing in locations where the pipe will be exposed to significant concentrations
`of pollutants comprised of low molecular weight petroleum products or
`organic solvents or their vapors. Research has documented that pipe mate-
`rials, such as PE, polybutylene, polyvinyl chloride, and asbestos cement and
`elastomers, such as used in jointing gaskets and packing glands, are subject
`to permeation by lower molecular weight organic solvents or petroleum
`products. If a water pipe must pass through a contaminated area or an area
`subject to contamination, consult with pipe manufacturers regarding per-
`meation of pipe walls, jointing materials, etc., before selecting materials for
`use in that area.”
`
`Temperature Effects
`PE is a thermoplastic polymer. As such, its physical properties change in response to
`temperature. These property changes are reversible as the temperature fluctuates.
`The physical properties of PE are normally determined and published at standard
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 7
`
`

`

`8
`
`PE PIPE—DESIGN AND INSTALLATION
`
`laboratory conditions of 73°F (23°C) with the understanding that the absolute values
`may change in response to temperature.
`For example, the pressure rating of a PE pipe relates directly to the hydrostatic
`design basis (HDB) of the material from which it is produced. Traditionally, this
`design property is established at 73°F (23°C). However, as temperature increases, the
`viscoelastic nature of the polymer yields a lower modulus of elasticity, lower tensile
`strength, and lower stiffness. As a result, the hydrostatic strength of the material
`decreases, which yields a lower pressure rating for a specific pipe DR. The effect is
`reversible in that once the temperature decreases again to standard condition, the
`pressure capability of the product returns to its normal design basis. However, the ele-
`vated temperature pressure rating is always applied for elevated temperature service
`conditions.
`Buried potable water systems typically operate in a range below 73°F (23°C). In
`these situations, the pressure capability of the pipe may actually exceed the design
`pressure class ratings listed in ANSI/AWWA C901 and C906. The current industry
`practice is to set the pressure rating of the pipe at 73°F (23°C) as the standard and
`consider any added strength at lower service temperatures as an additional factor of
`safety for design purposes.
`The coefficient of linear expansion for unrestrained PE is generally accepted to be
`1.2 × 10–4 in./in./°F. This suggests that unrestrained PE will expand or contract con-
`siderably in response to thermal fluctuation. It should be pointed out, however, that
`while the coefficient of expansion for PE is fairly high compared to metal piping prod-
`ucts, the modulus of elasticity is comparatively low, approximately 1/300 of steel for
`example. This suggests that the tensile or compressive stresses associated with a tem-
`perature change are comparatively low and can be addressed in the design and instal-
`lation of the piping system. Thermal expansion and contraction effects must be taken
`into account for surface, above grade, and marine applications where pipe restraint
`may be limited. But with buried installations, soil friction frequently provides consid-
`erable restraint against thermal expansion and contraction movement. In smaller
`diameter installation, such as those less than 12-in. nominal outside diameter, soil
`friction restraint can be enhanced by snaking the pipe side to side in the trench prior
`to backfilling. Additional restraint against movement can be provided with in-line
`anchors. (See Chapter 8.)
`In consideration of its thermal properties, PE pipe must be joined using methods
`that provide longitudinal thrust restraint such as heat fusion, electrofusion, flange
`connections, and restrained mechanical connections. Additionally, fittings used within
`the system should possess sufficient pull-out resistance in light of anticipated move-
`ment caused by thermal expansion or contraction. Finally, PE pipe should be stabi-
`lized or anchored at its termination points to other, more rigid piping or
`appurtenances to avoid potential stress concentration at the point of transition or to
`avoid excessive bending moments on system fittings. The reader is referred to Chap-
`ter 8 of this manual for more information regarding control of pull-out forces.
`
`Electrical Properties
`PE is an excellent insulator and does not conduct electricity. The typical electrical
`properties of PE are shown in Table 1-2.
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 8
`
`

`

`Table 1-2 Electrical properties of PE
`
`ENGINEERING PROPERTIES OF POLYETHYLENE
`
`9
`
`Electrical Property
`Volume Resistivity
`Surface Resistivity
`Arc Resistance
`Dielectric Strength
`Dielectric Constant
`Dissipation Factor
`
`Units
`ohms-cm
`ohms
`seconds
`volts/mil
`—
`—
`
`Test Method
`ASTM D257
`ASTM D257
`ASTM D495
`ASTM D149
`ASTM D150
`ASTM D150
`
`Value
`> 1016
`> 1013
`200 to 250
`450 to 1,000
`2.25 to 2.35 @ 60 Hz
`> 0.0005 @ 60 Hz
`
`CHEMICAL PROPERTIES
`
`Chemical Resistance
`An integral part of any piping system design is the assessment of the chemical envi-
`ronment to which the piping will be exposed and the impact it may have on the design
`life of the pipe. Generally, PE is widely recognized for its unique chemical resistance.
`As such, this piping material has found extensive utilization in the transport of a vari-
`ety of aggressive chemicals.
`To assist the designer in the selection of PE for piping applications, chemical resis-
`tance charts have been published that provide some basic guidelines regarding the
`suitability of PE as a piping material in the presence of various chemicals. A very com-
`prehensive chemical resistance chart has been published by the Plastics Pipe Insti-
`tute (PPI) in the Handbook of Polyethylene Pipe7.
`It is important to note that chemical resistance tables are only a guideline. Data
`such as this is generally developed on the basis of laboratory tests involving the evalu-
`ation of tensile coupons immersed in various concentrations of the reference chemi-
`cals. As such, these charts provide a relative indication of the suitability of PE when
`exposed. They do not assess the impact that continual exposure to these chemicals
`may have on various aspects of long-term performance nor do they address the effect
`produced by exposure to various combinations of the chemicals listed. Additionally,
`these chemical resistance tables do not take into consideration the affect of stress
`(loading), magnitude of the stress, or duration of application of such stress. In light of
`this, it is recommended that the designer use responsible judgment in the interpreta-
`tion of this type of data and its utilization for design purposes. Additional information
`is available from PPI Technical Report TR-198. Alternatively, the reader is referred to
`the pipe manufacturer who may have actual field experience under similar specific
`service conditions.
`
`Corrosion
`PE used in water piping applications is an electrically nonconductive polymer and not
`adversely affected by naturally occurring soil conditions. As such, it is not subject to
`galvanic action and does not rust or corrode. This aspect of PE pipe means that
`cathodic protection is not required to protect the long-term integrity of the pipe even
`in the most corrosive environments. Proper consideration should be given to any
`metal fittings that may be used to join the pipe or system components.
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 9
`
`

`

`10
`
`PE PIPE—DESIGN AND INSTALLATION
`
`Tuberculation
`The potential for tuberculation of PE pipe is minimal. Tuberculation typically occurs
`in response to the deposition of soluble encrustants onto the surface of the pipe and
`subsequent corrosive action with the base material of the pipe. Properly extruded, PE
`pipe has an extremely smooth surface, which provides minimal opportunity for the
`precipitation of minerals such as calcium carbonate and the like onto the interior sur-
`face. PE itself is inert and therefore not prone to galvanic action, which these solubles
`may initiate in other piping materials.
`
`Resistance to Slow Crack Growth
`PE piping manufactured in accordance with the requirements of ANSI/AWWA C901
`or C906 is resistant to slow crack growth when used in typical potable water systems.
`Research in the area of slow crack growth combined with continual advancements in
`material science have resulted in HDPE piping products that when manufactured and
`installed in accordance with these standards are designed to provide sustained resis-
`tance to slow crack growth phenomena such as environmental stress cracking. To
`understand the significance of this statement, one must first understand the nature of
`slow crack growth and pipe failure in general.
`Excluding third party damage phenomena, such as dig-ins, etc., pipe failure may
`occur in one of three ways. First is the sudden yielding of the pipe profile in response
`to a stress level beyond the design capability of the material itself. Generally, this is
`referred to as Stage I type failure and is typically ductile-mechanical in nature and
`appearance. The pressure class designations and working pressure-rating methodol-
`ogy presented in ANSI/AWWA C906 are developed within the constraints of these
`material capabilities. The material requirements stipulated in ANSI/AWWA C906
`combined with additional pipe requirements, such as workmanship, dimensional spec-
`ifications for each pressure class, and the five-second pressure test, provide a basis for
`resistance to this type of failure over the design life of the PE piping system.
`The second mode of pipe failure is the result of slow crack growth. Generally, this
`is referred to as Stage II brittle-mechanical type failure. In this mode, pipe failure is
`characterized by very small slit-type failures in the pipe wall that initiate at points of
`mechanical stress concentration associated with inhomogeneities in the pipe wall or at
`imperfections on the inner pipe surface. Typically, these types of failures are slower in
`nature and occur as a three-stage process: crack initiation, crack propagation, and
`final ligament yield that results in pipe failure. This type of failure phenomena may
`be the result of exposure to more aggressive conditions such as elevated temperature
`(> 140°F [60°C]) or the oxidation reduction potential (ORP) of the water system, which
`is a function of chemical concentration (chlorine, chloramines, chlorine dioxide, ozone,
`dissolved oxygen, etc.) or other factors that are not typical of the majority of potable
`water applications. ANSI/AWWA C906 places specific requirements on the pipe manu-
`factured in accordance with this standard to guard against Stage II type failures
`while in potable water service.
`ANSI/AWWA C906 requires that all pipe must be produced from a material for
`which a PPI hydrostatic design basis (HDB) has been recommended. This requirement
`ensures that stress-rupture data for pipe specimens produced from the listed material
`is reviewed in accordance with the protocol in PPI’s TR-3 to ensure that it meets the
`stress-rating requirements of ASTM D2837. The stress-rupture data is further analyzed
`to ensure that it “validates.” That is, additional higher temperature stress-rupture
`tests are conducted to validate that the slope of the regression curve obtained at a spe-
`cific temperature does not change until some time after the 100,000-hour requirement
`
`AWWA Manual M55
`
`Copyright © 2005 American Water Works Association. All Rights Reserved.
`
`Eve Energy Co., Ltd v. Varta Microbattery Gmbh
`
`Eve Ex. 10033, p. 10
`
`

`

`ENGINEERING PROPERTIES OF POLYETHYLENE
`
`11
`
`established within ASTM D2837. Second, ANSI/AWWA C906 also has specific perfor-
`mance requirements for the manufactured pipe or fittings such as thermal stability,
`the elevated-temperature sustained-pressure test, and the bend back test, which min-
`imize the potential for Type II failures in typical potable water service applications.
`As a safeguard against Type II failure phenomenon, piping products manufactured
`in accordance with ANSI/AWWA C906 are produced from PE resins that are highly
`resistant to environmental stress cracking as determined by the tests described below.
`Laboratory tests to assess resistance to environmental stress cracking include
`ASTM D16939 and ASTM F147310. These standard test methods are utilized within
`the plastic pipe industry to assess the piping material’s resistance to cracking under
`accelerated conditions of concentrated stress, aggressive chemical attack, and elevated
`temperature. According to ASTM D1693, 10 compression molded specimens of the PE
`material are prepared, deformed into a 180º U-bend, and submerged in an aggressive
`stress-cracking chemical such as Igepal CO630 (a strong detergent) at 100ºC. The
`specimens are maintained at elevated temperature and the time to failure is recorded.
`Failure is defined as cracks that are visible on the surface