Tensile creep modulus determination is a critical materials testing procedure used to evaluate how plastics, composites, and other load-bearing materials deform under sustained tensile stress over time. Governed by internationally recognized standards such as ISO 899-1 and ASTM D2990, this test provides essential data for long-term performance prediction, structural design, and quality assurance. This article explains the technical principles, specimen requirements, testing methods, and typical applications of tensile creep modulus measurement.
Technical Overview
Tensile creep modulus determination measures the time-dependent stiffness of a material when subjected to a constant tensile load. Unlike short-term properties such as tensile strength or Young's modulus, which are captured within minutes, creep behavior reveals how a material slowly deforms under sustained stress over hours, days, or even years. This property is fundamental for any polymer component expected to carry load during its service life.
The tensile creep modulus, denoted Ec(t), is defined as the ratio of the applied stress to the total strain measured at a specified time t after loading:
Ec(t) = σ / ε(t), where σ is the constant applied tensile stress and ε(t) is the total strain (elastic plus viscoelastic plus plastic components) at time t. Because strain increases continuously under constant stress, the creep modulus decreases with time, and reporting the measurement time is essential for data validity.
Primary standards governing this determination include ISO 899-1 (Plastics — Determination of creep behaviour — Part 1: Tensile creep), ASTM D2990 (Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics), and the corresponding Chinese national standard GB/T 11546.1. Testing is typically performed by accredited third-party laboratories using dedicated creep frames equipped with dead-weight or lever-load systems and precision extensometers.
Testing Samples
Thermoplastic Materials
Rigid and semi-rigid thermoplastics such as polypropylene (PP), polyethylene (PE), polyamide (PA), polyoxymethylene (POM), polycarbonate (PC), and polyethylene terephthalate (PET) are among the most frequently tested materials. These polymers exhibit pronounced viscoelastic behavior, making creep characterization essential for structural applications.
Thermosetting Polymers
Epoxy resins, unsaturated polyesters, phenolics, and polyurethanes are tested to assess dimensional stability under long-term loading, particularly in adhesive joints, castings, and electrical insulation components.
Fiber-Reinforced Composites
Glass-fiber or carbon-fiber reinforced thermoplastics and thermosets (GMT, SMC, laminates) are tested to quantify the improvement in creep resistance imparted by reinforcement, with attention paid to fiber orientation and loading direction.
Specimen Geometry
- ISO 899-1 typically uses the ISO 527-2 Type 1A or 1B dumbbell specimen with a gauge length of 50 mm or 20 mm respectively.
- ASTM D2990 generally employs the ASTM D638 Type I dog-bone specimen.
- Specimens must be free of voids, notches, and processing defects, and should be conditioned to equilibrium, commonly at 23 °C ± 2 °C and 50 % ± 10 % relative humidity, unless otherwise specified.
- A minimum of three specimens per stress level is recommended, and preferably five, to account for statistical scatter.
Testing Items
A comprehensive tensile creep test program can deliver the following results:
- Tensile creep modulus Ec(t) at defined times (e.g., 1 h, 10 h, 100 h, 1000 h) and defined stress levels.
- Creep strain ε(t) as a function of time under constant stress.
- Creep compliance J(t), the reciprocal quantity describing strain per unit stress.
- Creep curves (strain–time plots) at multiple stress levels and temperatures.
- Isochronous stress–strain curves constructed from creep data at fixed times.
- Creep-rupture strength and time-to-failure under sustained tensile loading.
- Apparent modulus retention over time, often used for extrapolation to design lifetimes (e.g., 50 years for pressure pipes).
- Shift factors and master curves when time–temperature superposition is applied.
Testing Methods
ISO 899-1 Procedure
Specimens are mounted in a creep testing frame and loaded with a constant tensile force selected to produce a defined initial stress, typically within the linear viscoelastic range (commonly below 30–50 % of the yield stress). Elongation is measured continuously or at logarithmic time intervals using extensometers or optical systems. The creep modulus is calculated at each reading time and plotted against time, usually on a log-log scale. Standard preferred measurement durations include 1, 10, 100, and 1000 hours.
ASTM D2990 Procedure
ASTM D2990 follows a similar philosophy: constant load application, strain monitoring over time, and modulus calculation at specified intervals. The standard also covers creep-rupture testing, in which specimens are loaded at increasing stress levels until failure, generating stress-rupture envelopes for lifetime prediction.
GB/T 11546.1 Method
The Chinese national standard is technically aligned with ISO 899-1 and specifies specimen preparation, conditioning at 23 °C/50 % RH, loading sequences, and data reporting requirements for tensile creep of plastics, ensuring compatibility of results across domestic and international test reports.
Key Testing Considerations
- Temperature and humidity must be tightly controlled, since creep of polymers is highly sensitive to environmental conditions.
- Loading should be applied smoothly and rapidly (typically within 1–5 seconds) without shock or overshoot to avoid transient artifacts.
- Extensometer resolution should be sufficient to detect strain differences of 0.01 % or better, and frame compliance must be minimized or corrected.
- Multiple stress levels (at least three) enable construction of isochronous curves and identification of the linear viscoelastic limit.
- Elevated-temperature testing (e.g., 40 °C, 60 °C, 80 °C) combined with time–temperature superposition allows extrapolation of creep behavior to long service lifetimes.
Applications
Tensile creep modulus data directly support design and material selection decisions across many industries:
- Pipes and fittings: long-term hydrostatic strength design of PE, PP-R, and PVC pressure pipes relies on creep-based extrapolation to 50-year performance per ISO 9080 and related methodologies.
- Automotive engineering: under-hood plastic components, brackets, clips, and fuel system parts must retain stiffness after years of sustained vibration, thermal cycling, and constant preload.
- Construction and infrastructure: geomembranes, geogrids, anchor fixings, and load-bearing polymer elements require verified creep limits to guarantee structural safety over decades.
- Electrical and electronics: connector housings, terminal blocks, and insulating supports must maintain contact force and dimensional stability under continuous clamping stress.
- Medical devices: load-bearing polymer implants, tubing, and sterilization-reusable components are evaluated for long-term deformation under constant physiological loads.
- Packaging and consumer goods: pallets, crates, storage containers, and strapping tapes are validated for stacking strength and load retention during storage and transport.
- Material development: resin producers and compounders use creep modulus data to benchmark new grades, assess filler or nucleating agent effects, and generate datasheet values for design engineers.
FAQ
Why does the tensile creep modulus decrease over time?
Under constant stress, polymers undergo progressive molecular rearrangement—chain slippage, segment rotation, and viscoelastic flow—which increases strain over time. Since modulus equals stress divided by strain, the calculated modulus decreases as strain accumulates.
How long does a tensile creep test take?
Standard test points range from 1 hour to 1000 hours. Design-critical applications may require tests of 5,000 to 10,000 hours or more, though time–temperature superposition can reduce the practical testing time by running measurements at elevated temperatures.
Can short-term modulus be used instead of creep modulus for design?
No. Short-term tensile modulus can overestimate stiffness by a factor of two or more at long lifetimes. For any component under sustained load, design calculations should use creep modulus data at the relevant temperature, stress level, and design lifetime.
Which stress level should be selected for testing?
The applied stress should reflect the actual service stress of the component. To characterize the linear viscoelastic region, a minimum of three stress levels is recommended; a stress-independent creep modulus indicates linear viscoelastic behavior.
Are creep test results transferable between ISO and ASTM methods?
Results are broadly comparable when specimen geometry, conditioning, stress level, temperature, and measurement times are matched, but differences in specimen dimensions and tolerances mean the specific standard used should always be stated in the test report.
What information appears in a typical test report?
A compliant report includes material identification, specimen type and preparation, conditioning and test atmosphere, applied stress, temperature, strain–time and modulus–time data, isochronous curves where applicable, and reference to the test standard applied by the accredited third-party laboratory.