Creep Behaviour – Creep Internal Compression Test
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Creep behaviour – Creep internal compression test
Allgemeines
The creep internal pressure test is used to examine plastic components under internal pressure – such as pipes, fittings, hoses and the like – for long-term loading conditions and can therefore be classified as a form of component testing. The main areas of application for PVC, PE, PP and PB, as well as cross-linked PE (PE-X) or GRP materials (see: abbreviations for plastics) subjected to pressure loading are, in particular, the chemical and sanitary industries, as well as the drinking water supply sector. The key measurement requirement is the generation of a controllable internal pressure within the filled pipe section whilst simultaneously subjecting it to climatic stresses caused by temperature and the media [1].
Creep test equipment for creep internal compression tests
An example of a creep test using internal pressure loading on pipe segments is shown in Fig. 1.
| Fig. 1: | Schematic illustration of the measurement of time-dependent strain in plastics during a creep internal pressure test using a time-dependent testing equipment |
Internal pressure can be applied to the pipe sections using compressed air, with the respective pressure being monitored and regulated by pressure gauges and pressure valves. The pressurised expansion vessels serve to compensate for pressure losses resulting from leaks or the rupture of the pipe segments. The intermediate vessels, filled with a medium relevant to practical applications, are located within a temperature-controlled tank containing water or silicone oil as the heat transfer medium. Any pressure losses occurring during testing are indicated by leak detectors. Such test stands can also be used to investigate the burst behaviour of pipe or vessel sections; however, due to the compressibility of compressed air, a safety device must be in place [2].
Determination of internal pressure strength
The test and evaluation criteria for creep internal pressure tests are primarily defined by the standards [3–11], although the compilation of temperature-dependent creep internal pressure diagrams generally involves a considerable amount of time and technical effort. The long-term internal pressure strength is determined at defined time intervals by the internal pressure strength as a comparative stress value according to the modified Kessel formula, which is calculated using Eq. (1).
| (1) |
with:
| da | = | average pipe external cross-section |
| smin | = | minimum pipe wall thickness |
| σv | = | comparative stress/peripheral stress |
| pi | = | pipe internal pressure |
The creep internal pressure diagram
As these pipes must be guaranteed to have a minimum service life of 50 years, and in some cases as long as 100 years, extrapolations – usually extending beyond the service life – must generally be carried out under the specific operating conditions (minimum equivalent stress or internal pressure, temperature and medium), for example in accordance with ISO 9080 [5]. An example of a creep internal pressure diagram as a function of temperature is shown for polypropylene (abbreviation: PP) in Fig. 2, where it can be seen that the service life is significantly reduced as the test temperature increases.
| Fig. 2: | Creep internal pressure diagram for PP at different test temperatures |
See also
- Creep plastics
- Linear-viscoelastic behaviour
- Viscoelastic material behaviour
- Creep behaviour – Determination
- Tensile test overlapping creep relaxation
- BOLTZMANN's superposition principle
- VOIGT-KELVIN model
- Instrumented hardness measurement – Creep
- Stepped isothermal method, Tensile stress
- Creep behaviour – Recovery test
- Creep behaviour – Tensile creep test
References
| [1] | Höninger, H.: Component testing. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022), 3rd Edition pp. 600–624 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [2] | DIN EN 921 (1995-01): Plastic Piping Systems – Thermoplastic Pipes – Determination of Resistance to Internal Pressure at Constant Temperature (withdrawn) |
| [3] | DIN 16887 (1990-07): Determination of Long-therm Hydrostatic Pressure Resistance of Thermoplastics Pipes |
| [4] | DIN EN ISO 9967 (2016-01): Thermoplastic Pipes – Determination of Creep Ratio |
| [5] | ISO 9080 (2012-10): Plastics Piping and Ducting Systems – Determination of the Long-therm Hydrostatic Strength of Thermoplastics Materials in Pipe Form by Extrapolation |
| [6] | DIN EN 12106 (2025-07): Plastics Piping Systems – Polyethylene (PE), Crosslinked Polyethylene (PE-X) and Unplasticized Polyamide (PA-U) Pipes– Test Method for the Resistance to Internal Pressure after Application of Squeeze-off |
| [7] | DIN 53759 (1975-02): Testing of Plastic Articles – Determination of the Effect of Internal Pressure on Hollow Objects by Long-time Test (withdrawn) |
| [8] | ISO 1167-1 (2006-02): Thermoplastics Pipes, Fittings and Assemblies for the Conveyance of Fluids – Determination of the Resistance of Internal Pressure – Part 1: General Method |
| [9] | ISO 1167-2 (2006-02): Thermoplastics Pipes, Fittings and Assemblies for the Conveyance of Fluids – Determination of the Resistance of Internal Pressure – Part 2: Preparation of Pipe Test Pieces |
| [10] | ISO 1167-3 (2007-11): Thermoplastics Pipes, Fittings and Assemblies for the Conveyance of Fluids – Determination of the Resistance of Internal Pressure– Part 3: Preparation of Components |
| [11] | ISO 1167-4 (2007-11): Thermoplastics Pipes, Fittings and Assemblies for the Conveyance of Fluids – Determination of the Resistance of Internal Pressure– Part 4: Preparation of Assemblies |