HDPE Pipe Material Properties

The Stress Regression Line

The traditional method of portraying the primary mechanical property of HDPE, tensile strength, is by means of a graph of log stress vs. log time to failure. This is known as the stress regression line. It is a plot of the circumferential hoop stress in the wall of the pipe (from internal ressure) against time to failure.

Numerous actual test results, measured at 20°C and 60°C, over a range of times up to 10,000 hours, are plotted on a log scale and a regression line is calculated to fit this data. The resultant regression line is then extrapolated to 50 years (438,000 hours). The method of calculation is an internationally accepted procedure described in ISO/TR 9080. The required values of stress and time are specified in SABS ISO 4427.


HDPE Pipe Material Properties


Design Stress and Safety Factor (service factor)

Safety factors take into account handling conditions, service conditions and other circumstances not directly considered in the design. In terms of SABS ISO 4427 the minimum safety factor is 1.25. This factor, when applied to the Minimum Required Strength (MRS), for the particular material classification (e.g. PE80, PE100), gives the maximum allowable hydrostatic design stress for the designated material.

Melt Flow Index

The melt flow index of polyethylene materials is a measure of the mass of melted material, at 190°C, that will pass through a specific orifice in 10 minutes when subjected to a specific pressure. The melt flow index (MFI) is largely dependant on the molecular mass. Higher molecular masses result in lower MFI because long, well packed molecules do not flow as easily as short, less packed molecules.

Since both density and MFI are decisive for the strength properties, they are regulated in most standards for polyethylene pipes. In terms of the SABS specification the Melt Flow Index must conform to the raw material manufacturers pipe grade specification. This information can be obtained from the raw material manufacturers data sheets.

Tensile Strength

The tensile strength of polyethylene materials increases with an increase in molecular mass since long, well packed molecules are more difficult to separate. This property is also effectively regulated by standards.

Effect of Temperature Change

Working Pressure

The standard design temperature for HDPE pipes is 20°C and working pressures are usually quoted for this temperature. HDPE pressure pipes function perfectly well below 20°C right down to freezing point and can in fact, withstand higher pressures than those quoted at 20°C. As can be seen from the stress regression lines, the creep rupture strength diminishes with increasing temperature and working pressures must be down-rated if the same factors of safety are to be maintained.

Sub Zero Temperatures

Water has been known to freeze in HDPE pipes without causing fractures, but permanent strain can result, leading to severe reduction in the working life of the pipe. Hence HDPE pipes - like other pipes - should be protected against sub zero temperatures.

Expansion and Contraction

All plastics have high a co-efficient of expansion and contraction, several times those of the metals. This must be allowed for in any installation by the use of expansion joints, expansion loops etc.

Impact Resistance

It should be noted that it is possible to change the impact strength of certain plastic materials, however this usually comes at the expense of properties such as tensile strength, hardness or stiffness. This property is therefore effectively regulated (as with density and MFI) by most standards.

Ultra Violet Resistance

HDPE pipes, when manufactured to SABS ISO 4427, contain 2.5% (by mass) of carbon black. This provides exceptional protection against the effects of ultra violet light.

Flammability

As with impact resistance, it is possible to improve the fire resistance of HDPE by the addition of various compounds. Again this comes at the expense of other properties. Fire resistance is measured by a limiting oxygen index (LOI).

Abrasion Resistance

A number of international investigations to assess the abrasion resistance of various plastic materials have been carried out. Generally the results of such investigations are expressed as a loss of volume in relation to the original wall thickness. The results to date have varied in regard to the abrasion resistance of various pipe materials. However, what they all show is that plastics possess superior abrasion resistance relative to other pipe materials. For example, in one investigation, HDPE pipes suffered wear to the extent of 4mm after 1600 hours while the corresponding wear occurred in steel pipes after 1000 hours. [jod/marleypipesystems.co.za]

 

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