Method for the Determination of Residual Stress

“Specification for Polyethylene Pipes for Sewer Lining (Non Pressure Applications)” and W.I.S 4.32.09 “Specification for Black Polyethylene Pressure Pipes for Portable Water above Ground or Sewerage”. By J.S Hornsey VSR(Africa)cc

Joint Tolerance:

The target set is to specify that pipe does not distort significantly at its cut ends (barrel) and that remains stable when machined. The cause of both these distortions is the residual stresses induced during normal manufacture by the vacuum process. These stresses are balanced within the wall and the pipe ring structure.

If the pipe is produced with the residual stresses balanced just in the wall of the pipe, or the pipe is annealed, the effects of barreling and distortion during cutting and machining are reduced.

To achieve this only pipe complying with the following simplified test is used for joint manufacture:

A length of pipe equal to the outside diameter is cut and a longitudinal section of pipe width equal to 0.15D is removed from the sample.The cut section is then allowed to stand for one hour at the end of this period the ends of the cut must not have touched.

(A more thorough test is given in Appendix K of W.I.S 4.32.09)

Joints are machined to a tolerance of 0.2mmAllowing for the variation in wall thickness of PE pipes the target joint lip is 0.7% x OD

Appendix K – Method for the Determination of Residual Stress

K 1 Test Pieces

Cut adjacent sections of pipe of length equal to the outside diameter immediately after extrusion and maintain them at 23 ± 2ºc laying on their sides.

Mark each one in line with one of the marking strips.

K 2 Procedure

K 2.1 Recommended preliminary experiment to establish optimum size of pipe section to be removed

The day following production (but less than 23 hours elapsed time), by sawing longitudinally in the vicinity of the marked line, remove a section of pipe (from one or more test pieces) covering an arc of between 30º and 60º. Lay the test piece(s) on its side and observe the closure of the gap in the pipe ring for 60 minutes. If the ring is near to closing completely within this period, remove a further segment. From this experiment, estimate the amount of material which would be required to be removed to allow the closure of the gap to 8mm after 60 minutes.

Method for the Determination of Residual Stress

W.I.S 4.32.05 “Specification for Polyethylene Pipes for Sewer Lining (Non Pressure Applications)” and W.I.S 4.32.09 “Specification for Black Polyethylene Pressure Pipes for Portable Water above Ground

or Sewerage”.

K.2.2 Determination

Take an uncut test piece and determine the mean wall thickness around the circumference near one end. Measure the outside diameter at the centre of the test piece using a steel tape OD tape (Dì).

24 hours after original sampling, remove the necessary section of the pipe in the vicinity of the marked line and immediately commence timing; lay the test piece on its side. After 60minutes, measure the outside diameter at the centre of the test piece altered by closure (D2).

NOTE

K3 Calculation

Assuming a linear stress distribution across the pipe wall, the residual circumferential stress (ÓR) may be determined from curved beam theory using the following equation.

Since conventionally extruded and cooled PE pipe normally has a parabolic stress profile across the wall rather than a linear profile, the maximum tensile stress on the bore is approximately two-thirds of the apparent value OR. For the purpose of this specification the residual stress (ÓR) shall be calculated from the following:

The modulus of the material (E Mpa) shall be taken as the one-hour creep value from a constant stress (4Mpa) three-point bending test on compression moulded sheet of the pipe material. The preferred method is given in Appendix D of W.I.S No. 4-32-05: Issue 1.Complying certificated results from the pipe manufacturer’s polymer supplier will be acceptable.

The experiment in K2.1 should allow the gap almost to close after 60 minutes during the determination. However it must not close completely and if, for any reason, this appears imminent a further small section will need to be removed