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Key points of plastic pipe ring stiffness test (part 1)

Nov 12, 2018


The buried plastic pipes for engineering are mainly divided into two types: “pressured pipes” and“pressureless pipes” according to the types of load they bear. For example, “water pipes” and“gas pipes” are all pressurized pipes; rural domestic sewage The drainage pipe used in the treatment project is a pressureless pipe. The pressureless pipeline is mainly subjected to external loads, and the external loads are mainly composed of soil, backfill weight and loads generated by passing vehicles.

Key points of plastic pipe ring stiffness test

1 ring stiffness overview

The numerical value of ring stiffness is widely used in the world to express the ability of plastic pipes to withstand external pressure loads. In the actual engineering use, once the ring stiffness index of the plastic pipe is unqualified, it directly affects the project quality and service life.

The hoop stiffness of the pipe in the ISO standard is called “ring stiffness” and its physical meaning

Is the stiffness of a pipe section, which can also be calculated using the following formula:


In the formula: S-ring stiffness; 

                         E-material elastic modulus; 

                         I-inertia moment; 

                         D-tube ring average diameter. 

The standard for measuring the stiffness of the ring in the article is “Measurement of the stiffness of thermoplastic pipe rings (ISO9969:2007).

2. Ring Stiffness Test Method The standard stipulates that the ring stiffness is determined by the measured load and deformation at constant velocity deformation of the pipe. The specific test process is: using two parallel plates to compress a horizontally placed pipe at a constant rate in a vertical direction. The test rate is determined by the diameter of the pipe, and the load-deformation curve is obtained, which is deformed in the diameter direction of the pipe. Calculate the ring stiffness with a load of 3%. The calculation formula is:


In the formula: S-ring stiffness, in units of kN/m2;

L-sample length, positioned in millimeters (mm);

F- is equivalent to the load when the pipe is deformed by 3.0%, and the unit is kN (kN);

The amount of deformation of Y- relative to the deformation of the pipe by 3.0%, in millimeters (mm),

3 Plastic pipe ring stiffness detection points

3.1 Sample preparation

(1) The interception position of the sample

Plastic pipes are divided into the following three categories according to the type of structural wall: solid wall, water

Flat regular structural walls and vertical regular structural walls. Solid wall pipe and horizontal regular structural wall pipe

The interception position is not affected by the pipe wall, so be careful not to over-skew when intercepting;

For the interception of vertical regular structural wall pipe, it should be noted that at least one should be included when cutting the sample.

a complete rib, corrugation or other regular structure, and the cutting site should be in ribs, corrugations or

The midpoint between other rule structures (as shown in Figure 1)


   Sample interception diagram 1

L=Sample length

P=Span length

(2) Intercept length of the sample

For pipes with a nominal diameter less than or equal to 1500 mm, the average length of the specimen shall be (300 ± 10) mm; for pipes with a nominal diameter greater than 1500 mm, the average length of the specimen shall not be less than 0.2 DN. For ribs, corrugations or other regular structural wall pipes with spirals, the length of the specimen shall be equal to (d i ±20 mm), but not less than 290 mm and not more than 1000 mm. (Note: DN is the nominal outer diameter of the pipe; di is the average inner diameter of the pipe.) And because the section of the pipe is circular, it is easy to be skewed during cutting, which results in a large difference in the length of the sample taken in different circumferential directions. Provision: For each sample, the minimum length of the sample measured in different circumferential directions should not be less than 0.9 of the maximum value.

Times. This should be taken seriously when cutting the sample.

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