ISO 9967 Creep Ratio Test for Thermoplastic Pipes

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ISO 9967 Thermoplastics pipes — Determination of creep ratio

ISO 9967 specifies a uniform test method to measure the creep ratio of circular cross-section thermoplastic pipes for long-term structural design calculations.

What is the Creep Ratio?

Imagine putting a heavy book on a plastic ruler overnight. Initially, it bends a little, but by morning, it has bent even further under the same weight. This slow, continuous deformation under a constant load is called creep.

Test Principle:

The ISO 9967 test procedure begins by preparing a ring-shaped specimen from a thermoplastic pipe. The ring is typically cut to a length equal to the pipe’s outer diameter and must be free of visible defects. Before testing, the sample is conditioned, usually at 23°C for at least 24 hours. During the test, the specimen is placed vertically between two flat, parallel plates in a compression testing machine. A constant external force is applied to the ring to compress it until a deformation equal to 3% of its mean diameter is reached. This loading should occur gradually, typically within one minute.

The test apparatus must maintain this compressive load over an extended period, most commonly 10,000 hours (42 days), under controlled environmental conditions. The deformation of the ring is measured at defined intervals during the test using a precise displacement measurement device. Initial and long-term measurements are used to calculate the creep ratio. This ratio quantifies how much the ring deforms over time under constant load, which indicates the material's long-term behavior and suitability for buried, non-pressure pipe applications. Accurate time tracking and temperature control are critical throughout the process to ensure valid results. The test concludes by comparing the deformation at 30 minutes and at the final time point to calculate the creep ratio according to the formula provided in the ISO 9967 standard.

Test Specimen Information

To ensure accurate and repeatable results, the standard dictates strict guidelines for preparing the pipe samples:

Quantity & Marking: Three test pieces (labeled A, B, and C) must be cut from the pipe. The pipe must be marked along its entire length to ensure consistent orientation.

Dimensions:

For pipes with a nominal diameter (d_n) ≤ 1500 mm, the standard length is 300 ± 10 mm.

For larger pipes (d_n> 1500 mm), the length must be at least 0.2 × d_n.

Special Structures: If the pipe has ribs or corrugations, the cuts must be made so that the test piece contains a whole number of these structures.

Age Requirement: Samples must be aged for 21 ± 2 days before testing begins.

Conditioning: Samples must be kept at the test temperature (usually 23°C) for at least 24 hours prior to the test to stabilize the material.

ISO 9967 Creep Ratio Test Equipment:

Equipment	Requirements
Compressive loading machine (UnitedTest Ring stiffness tester)	Maintains pre-load F₀ and loading force F with ±1% precision; direct/lever-arm loading allowed
Parallel flat plates	Non-deforming; length ≥ specimen length; width ≥ loaded contact width +25 mm Two perfectly flat, smooth, and rigid plates through which the force is applied to the pipe. They must be wide enough to fully support the pipe plus an extra 25 mm.
Dimensional measuring devices	Must measure length within ±1 mm. Must measure the inside diameter within ±0.1 mm (or 0.2% of the diameter). Must track the ongoing deflection (squishing) of the pipe within 0.1 mm or 0.1% of the total deflection.
Timer	Accuracy ±1 s for first 6 min; ±0.1% for later readings

Test Parameters & Stipulations

Strict rules are in place to ensure the data isn't corrupted by human or mechanical error:

Temperature: Standardized at 23 ± 2°C (or 27 ± 2°C in specific regions).

Pre-load Force (F₀): A tiny initial weight is placed to settle the pipe.

If the pipe diameter is ≤ 100 mm, F₀= 7.5 N.

If > 100 mm, F₀is calculated based on the pipe's dimensions.

Target Deflection: The main load must be adjusted so that after exactly 6 minutes (360 seconds), the pipe has deflected by 1.5% (with a strict tolerance of ±0.2%). If the pipe is too stiff or too soft and misses this target, the test must be restarted.

Force Application: The full load must be reached smoothly within 20 to 30 seconds.

Data Collection Points: Deflection must be recorded at: 6 min, 1 hr, 4 hrs, 24 hrs, 168 hrs (1 week), 336 hrs (2 weeks), 504 hrs (3 weeks), 600 hrs, 696 hrs, 840 hrs, and finally at 1008 hrs (42 days).

Test Procedures of ISO 9967 Plastic Creep Ratio: Step-by-Step

Preparation	Measure the exact length and inside diameter of the three pipe samples. Condition them at the test temperature for 24 hours.
Setup	Place the first pipe ring between the compression plates. Apply the pre-load (F₀) and zero the deflection gauge. According the pipe diameter: inner diameter size choose the pre-load force（di small 0.1m f₀=7.5N, If di large than 0.1m, calculate as below: f₀=56.3N
Loading	Apply the main compressive force gradually over 20-30 seconds. Start the timer the moment full force is achieved.
Initial Measurement	Exactly 6 minutes after the force is applied, record the initial deflection (y₀). If this is not within the 1.5% ± 0.2% range, stop, recondition the pipe for 1 hour, and restart.
Test holding	Leave the machine to run for 42 days (1008 hours). The machine must automatically record the deflection at all the stipulated time intervals.
Data Analysis	Plot the 11 deflection points on a semi-logarithmic graph (Deflection vs. Log of Time). Use the least squares method(linear regression) to find the best-fit straight line.
Extrapolation	Use the derived mathematical formula to calculate the projected deflection after 2 years (Y₂).
Final Calculation	Calculate the Creep Ratio (γ) for all three pipe samples and average them to get the final result for the batch.

Test Applications (Industry Fields)

The ISO 9967 test is a cornerstone of quality assurance and engineering in several major industries:

Civil Engineering & Infrastructure: Used by municipalities and engineers when designing sewer systems, water mains, and drainage pipes to ensure they won't collapse under soil weight over decades.

Plastics & Polymer Manufacturing: Essential for R&D departments developing new thermoplastic blends (like PVC-O or HDPE) to improve their structural integrity and resistance to long-term deformation.

Construction & Building Services: Ensures that conduits for electrical cables or HVAC systems buried underground will maintain their shape and functionality throughout the building's lifespan.

Related test standard:

ISO 9969	Thermoplastics pipes — Determination of ring stiffness
ISO 13968	Plastics piping and ducting systems — Thermoplastics pipes — Determination of ring flexibility
GB/T 18042	Thermoplastics pipes--Determination of creep ratio
ISO 899-2	Plastics - Determination of creep behavior - Part 2: Flexural creep by three-point loading
ISO 7684	Glass-reinforced thermosetting plastics (GRP) pipes – Determination of the creep factor under dry conditions
EN 761	Glass-reinforced thermosetting plastics (GRP) pipes — Determination of the creep factor under dry conditions
EN 1862	Glass-reinforced thermosetting plastics (GRP) pipes – Determination of the relative flexural creep factor following exposure to a chemical environment

Plastic pipe ring stiffness testing machine is a testing device specially designed for plastic and composite pipes, used to measure the stiffness of circular cross-sections. It is also suitable for testing the static mechanical properties of metals and non-metals (including composites).

Q1: What exactly does the ISO 9967 test measure?

A:It determines the creep ratio of thermoplastic pipes. Creep is the tendency of a material to deform permanently under a constant load over time. The test quantifies how much a pipe ring will deform under sustained pressure, allowing engineers to predict its long-term behavior when buried underground.

Q2: Why is this test so important for infrastructure projects?

A:Underground pipes are constantly squeezed by the weight of the soil above them. If a pipe creeps too much, it can lose its circular shape, reduce flow capacity, or even collapse. ISO 9967 provides a scientifically validated way to predict a pipe’s performance two years into the future based on just 42 days of laboratory testing. This prevents costly failures and ensures public safety.

Q3: How long does the test take to complete?

A:The standard test duration is 1,008 hours, which equals exactly 42 days (6 weeks). However, if the data doesn’t meet the required statistical correlation, the standard allows extending the test up to 4,000 hours (about 5.5 months) to get reliable results.

Q4: What is the “1.5% deflection rule” mentioned in the test?

A:The test force is carefully calibrated so that after exactly 6 minutes of loading, the pipe deflects by 1.5% of its original inside diameter (with a strict tolerance of ±0.2%). This standardized starting point ensures that comparisons between different pipe materials are fair and meaningful.

Q5: Why extrapolate 2-year deflection instead of testing for 2 full years?

A: Real-world buried thermoplastic pipes stabilize within ≤2 years. Extrapolation from 42-day test data via linear regression saves test time while meeting engineering design accuracy requirements.

Q6: What to do if R < 0.990 even with the last 5 data points?

A: Extend the test and measure deflection at 1200 h, 1400 h, 1680 h, etc. (±48 h allowed) until R ≥ 0.990 using the last 5 measurements.

Q7: How is the final creep ratio calculated?

A: Calculate creep ratio for each of 3 specimens, then take the arithmetic average and round to two significant figures as the final result.

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