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Pipe Ring Stiffness Test

Pipe ring stiffness tester is used to determine the ring stiffness of circular cross-section thermoplastic pipes, according customer’s request, it can also execute compression, ring stiffness, ring softness and creep ratio test. This tester meet the requirement of all kinds of plastic pipes, structured-wall pipes, PVC, HDPE, LDPE, PE corrugated pipes, helically wound pipes etc.,

Conform with Standard ISO 9969, ISO 9967. ASTM D2412, EN1446, AS 1462.22 pipe ring stiffness test machine


UnitedTest Popular Pipe Ring Stiffness Testing Machine

Feature:

  • Computer control
  • $ columns + 2 leading screws, Robust frame

Key parameter:

  • Max. load: 50/100/200KN
  • Width space: 1150mm
  • Vertical space: 800~3700mm
Details

Feature:

  • Computer control
  • Floor type, Robust frame

Key parameter:

  • Max. load: 20/50/100KN
  • Width space: 500mm
  • Vertical space: 800~1400mm
Details

Feature:

  • Computer control
  • Table type

Key parameter:

  • Max. load: 10/20/50KN
  • Width space: 450mm
  • Vertical space: 800mm
Details

What's pipe pipe ring stiffness and ring flexibility?

Ring stiffness and ring flexibility are important indicators of plastic buried drainage pipes, just from the literal point of view, the difference between the two lies in "rigid" and "soft", but what are the more specific differences, many people are ambiguous, here is an all-round comparison of the two, and strive to let you understand it after reading it. 


Ring stiffness testing refers to the ability of a pipe to resist circumferential deformation and mainly measures the pipe's stability under external loads, such as soil pressure and vehicle loads. 

In the International System of Units, ring stiffness is usually expressed in kilonewtons per square meter (kN/m²), with common ring stiffness grades including SN4, SN8, SN12.5, and so on.


Pipe flexibility testing refers to the ability of a pipe to bend when subjected to external forces, reflecting the pipe's flexible characteristics. The more flexible a pipe is, the stronger its capacity to withstand cracking under external pressure.


Why do plastic pipes need to execute the ring stiffness testing?

The main purpose of conducting ring stiffness tests on plastic pipes is to evaluate their resistance to deformation under external pressure, ensure that they can withstand external loads such as soil and pressure in practical applications, and avoid rupture or failure due to excessive deformation.  ‌

The role of ring stiffness test

The ring stiffness reflects the ability of the pipe to resist deformation when subjected to vertical compression, directly affecting its stability and durability. The experiment simulates the external pressure environment to test the load bearing capacity of the pipe under specific deformation (such as 3% deformation in the diameter direction), in order to determine its ring stiffness index.  ‌
Importance in practical applications

Ensure bearing capacity: Pipes with high ring stiffness deform less under soil pressure, which can reduce problems such as pipe misalignment and leakage caused by external pressure.  ‌
Extended service life: Unqualified ring stiffness may cause the pipeline to rupture due to excessive external pressure during use, shortening its service life.  ‌
Compliant with engineering standards: like China's "Test Method for Ring Stiffness of Plastic Pipes" (GB/T 9647) explicitly requires all types of pipes to undergo ring stiffness testing to ensure their mechanical properties meet national standards.

3 load cells pipe ring stiffness test
Pipe ring stiffness tester deformation

Principle of ring stiffness and creep ratio

Principle of ring stiffness: 

The ring stiffness is determined by measuring the force and the deflection while deflecting the pipe at a constant deflection speed.

A cut length of pipe supported horizontally is compressed vertically between two parallel flat platens moved at a constant speed that is dependent upon the diameter of the pipe.

A plot of force versus deflection is generated. The ring stiffness is calculated as a function of the force necessary to produce a 3% diametric deflection of pipe.


Principle of creep ratio: 

A cut length of pipe is placed between two parallel flat horizontal plates and a constant compressive force is applied for 1008 h (42 days). The deflection of the pipe is recorded at specified intervals so as to prepare a plot of pipe deflection against time. The linearity of the data is analysed and the creep ratio is calculated as the ratio between the two years’ extrapolated deflection value and the measured 6 min (0.1 h) deflection.


Calculation method

Ring Stiffness

The calculation formula of ring stiffness includes experimental and theoretical.

The first is according to the GB 9647-2015 standard:

S=F/ΔY⋅L*k

In this formula:

S is the ring stiffness (kN/m²), F is the load at 5% deformation (kN), ΔY is the vertical deformation (m), L is the length of the specimen (m), and k is the coefficient (about 0.149, adjusted according to the standard).


Ring Flexibility

"Ring flexibility has no direct calculation formula and is usually evaluated experimentally (according to ISO 13968:2008). In the test, radial pressure is applied to a specific deformation (e.g., 30% diameter compression) to observe cracks or yield, focusing on measuring the material's flexibility limit.

Important factors for ring stiffness

Pipe material (modulus of elasticity)

The higher the elastic modulus of the material, the greater the ring stiffness of the pipe. For example, rigid materials like steel and concrete improve ring stiffness, while more elastic materials like plastic pipes have relatively low stiffness.

In contrast to ring stiffness, materials with lower elastic modulus (e.g., PE, rubber) generally have higher ring flexibility because they are more prone to deformation and return to their original shape, while materials like steel are more rigid and have lower ring flexibility.


Pipeline structure

Structural wall pipes (such as HDPE double-wall bellows, steel strip reinforced spiral bellows, etc.) not only provide high ring stiffness through corrugated and reinforced layer design, can withstand large external loads without excessive deformation, but also have strong ring flexibility, which can adapt to external deformation such as foundation settlement and external force to avoid pipe rupture.

 

In contrast, solid-walled pipes (such as PE water supply and drainage pipes) have low ring stiffness, which is prone to large deformation in high-load environments, and lacks elasticity, and has poor ring flexibility, which is easily affected by external deformation and leads to brittle fracture.

 

Pipe wall thickness

When the thickness of the pipe wall increases, the ring stiffness of the pipeline increases, and the thicker pipe wall can improve the compressive ability of the pipeline, but at the same time, it will lead to a decrease in the flexibility of the ring of the pipeline, making the pipeline inelastic and weakening its adaptability to external deformation.


Test procedure of pipe ring stiffness test

1. Sample preparation

2. Specimen specifications

Length: 300 mm ± 10 mm, cut flat at both ends and free of defects.

Pretreatment: 23°C±2°C environment conditioning for at least 24 hours to eliminate temperature effects.

3. Special structural pipe requirements

The corrugated pipe needs to verify the bonding strength between the corrugated layer and the inner wall, and the steel strip reinforced pipe needs to detect the peeling of the steel strip.


Test process

1. Sample installation

Place the specimen horizontally, and the contact surface of the platen at both ends should be parallel to avoid axial displacement.

2. Loading phase

Preloading: Apply an initial load of 50 N to eliminate the gap.

Formal loading:  Compressed at a rate of (5±1) mm/min, the force value (F) when the       deformation reaches 3% (ΔY=0.03D, D is the initial inner diameter) is recorded.

3. Repeatability verification

The test was repeated after rotation of the same specimen by 90°, and the average of the results was taken twice.

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Data processing and calculation

1. Ring stiffness (S) calculation

o Formula: S=FΔY×LS=ΔY×LF (unit: kN/m²), where L is the length of the specimen.

2. Result Judgment If the pipe ruptures before 3% deformation, it is judged to be unqualified, and the material or process problem needs to be investigated. 


Related standadrd: 

ISO 9969  Thermoplastics pipes -- Determination of ring stiffness

ISO 9967  Thermoplastics pipes -- Determination of creep ratio

ASTM D2412 Standard Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading

EN 1446 Plastics piping and ducting systems - Thermoplastics pipes - Determination of ring flexibility

DIN 16961 Thermoplastics pipes and fittings with profiled wall and smooth pipe inside

AS/NZS 1462.22 Methods of test for plastics pipes and fittings Method 22: Thermoplastics pipes – Determination of ring stiffness

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