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# HDPE Physical Properties

## Table of physical properties

The properties given below ae for HDPE grades used to manufacture pipe. It should be noted that many of these properties are relative to temperature and the duration of stress application.

## 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 pressure) 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 ten 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 SANS ISO 4427.

The internationally accepted method for calculating circumferential hoop stress is derived from Barlow’s formula and is as follows:
σ = p (d – t)/2t

• Where
• p = internal pressure (MPa)
• t = minimum wall thickness (mm)
• d = mean external diameter (mm)
• σ = circumferential hoop stress in wall of pipe (MPa)

The Stress Regression Line for HDPE is given below.

σ Burst Stress MPa ## Design stress and Safety Factor

Safety factors take into account handling conditions, service conditions and other circumstances not directly considered in the design. In terms of SANS 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. PE 100), gives the maximum allowable hydrostatic design stress for the designated material. The table below illustrates the relationship between MRS and σ for various design coefficients at 20°C. The design engineer may wish to apply a greater safety factor depending on operating conditions and environmental considerations. Applying Barlow’s formula (below) it is possible to calculate the minimum wall thickness for any given size and pressure class of pipe.
T = p x d / (2σ + p)

• Where
• T = minimum wall thickness (mm)
• p = internal pressure (MPa)
• d = mean external diameter (mm)
• σ = design stress (MPa)

For example, the minimum wall thickness for a 250mm Class 10 HDPE pipe made from PE 80 material is:
T = 1.0 x 250/ ((2×6.3) + 1.0)
= 18.38 mm

Round up to 18.4 mm for manufacture and / or the appropriate SDR for the Class and Material designation.

## 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 relating to the original wall thickness. The result 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 in steel pipes after 1000 hours.

The graph below provides a further indication of relative wear rates. Back To Top