Tensile creep testing quantifies the time-dependent elongation of rubber materials under a constant tensile load, delivering essential data for the design, material selection, and lifetime prediction of elastomeric components such as seals, mounts, hoses, and belts. This article explains the testing principles, specimen requirements, measurable parameters, and evaluation procedures used by accredited third-party laboratories to determine the tensile creep characteristics of rubber in accordance with ISO 899-1, ASTM D2990, and GB/T 11546.1. Read on to understand how creep strain, creep compliance, and creep rupture results support quality control and long-term performance assessment across the rubber industry.
Technical Overview
Tensile creep is defined as the progressive, time-dependent increase in strain that occurs when a rubber specimen is subjected to a constant tensile stress below its ultimate strength. Because rubber is a viscoelastic material, its deformation under load combines an almost instantaneous elastic response with a delayed molecular rearrangement involving chain slippage, disentanglement, and gradual orientation of polymer segments. Measuring this delayed deformation is therefore essential for predicting the long-term dimensional stability of elastomeric parts that carry sustained loads in service.
A typical creep curve recorded during a tensile creep test can be divided into three regions. In primary creep, the strain rate decreases continuously as the material undergoes molecular rearrangement. In secondary creep, deformation proceeds at an approximately constant rate over an extended period. In tertiary creep, void formation, micro-cracking, or localized necking accelerates the deformation and may ultimately lead to creep rupture. Unless rupture data are specifically required, the test is usually terminated before or at the onset of the tertiary stage.
The principal deliverables of the determination are the tensile creep strain as a function of time, the creep compliance, the apparent creep modulus, and, where applicable, the time to creep rupture and the elongation at rupture. Testing is typically performed in accordance with ISO 899-1, ASTM D2990, or the Chinese national standard GB/T 11546.1, with rubber test pieces prepared and conditioned following the general requirements of ISO 23529.
Testing Samples
Dumbbell (Dog-Bone) Specimens
The preferred test piece for rubber tensile creep determination is the narrow-waisted dumbbell specimen, die-cut from vulcanized sheets of 2 mm ± 0.2 mm thickness in line with the geometry requirements of ISO 37 or ASTM D412. The reduced central section ensures a uniform uniaxial stress distribution within the gauge length, so that the measured elongation reliably reflects the creep response of the material rather than grip effects. Dumbbell Type 1 and Type 2 geometries are the most commonly selected configurations.
Strip Specimens
Straight-sided strip specimens may be used when the available material is limited, for example when evaluating small molded parts, thin-walled tubing sections, or product samples cut from finished components. When strips are used, the gauge length is marked on the parallel section, and grip reinforcement or tabbed ends may be applied to prevent premature failure at the clamps. The specimen width-to-thickness ratio should be reported together with the results because it can influence the measured strain.
Specimen Conditioning and Quantity
Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 16 hours before testing. For materials known to be sensitive to moisture, a longer conditioning period or a controlled dry atmosphere may be required. A minimum of three specimens is tested at each stress level, and additional specimens are recommended when creep rupture statistics or scatter analysis are required.
Testing Items
The determination of tensile creep characteristics covers the following measurable parameters, each of which characterizes a different aspect of the time-dependent mechanical behavior of rubber:
- Tensile creep strain: the total strain recorded as a function of elapsed time under a constant applied tensile stress, expressed as the ratio of gauge-length extension to the initial gauge length.
- Creep compliance: the ratio of creep strain to the applied constant stress, describing the softening tendency of the rubber over time.
- Apparent creep modulus: the reciprocal quantity defined as the applied stress divided by the total strain at a given time, used to compare the stiffening or softening behavior of different compounds.
- Isochronous stress-strain curves: stress-strain relationships constructed at fixed elapsed times, typically 1 h, 10 h, 100 h, and 1000 h, obtained by testing several stress levels.
- Creep rupture time and elongation at rupture: the elapsed time to specimen failure under sustained load and the corresponding strain at the moment of rupture.
- Creep recovery: the residual strain recorded after removal of the load, quantifying the irreversible flow component of the deformation.
Testing Methods
Apparatus and Test Setup
The test is performed on constant-load creep frames of the dead-weight or lever-amplified type, in which the load remains stable within ±1 % throughout the test duration. Elongation is measured with a clip-on extensometer or a non-contact video extensometer attached to the gauge marks, with a resolution sufficient to detect strain changes of 0.1 % or better. An environmental chamber provides temperature control within ±2 °C for tests conducted at elevated temperatures, and the entire loading train is designed to apply the force concentrically so that bending stresses are minimized.
Test Procedure
The standard test sequence is carried out as follows:
- Measure the width and thickness of the narrow section of each specimen at several points and calculate the cross-sectional area; mark the initial gauge length.
- Mount the specimen in the grips, apply a small preload to remove slack, and zero the extension measurement system.
- Apply the selected constant tensile load smoothly and without shock, typically within 1 s to 5 s as specified in ISO 899-1, and start the timing at the moment of full load application.
- Record the extension at logarithmically spaced time intervals, for example at 1, 2, 5, 10, 20, 50, 100, 200, 500, and 1000 minutes, and continue according to the specified test duration.
- Conduct the reference test in the standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity; perform accelerated tests at elevated temperatures such as 70 °C, 100 °C, or 125 °C as agreed with the client.
- For creep recovery measurement, remove the load at the end of the loading period and record the residual strain over a comparable time interval.
Data Evaluation
The tensile creep strain is calculated as the extension at time t divided by the initial gauge length. The creep compliance is then obtained by dividing the creep strain by the applied constant stress, and the apparent creep modulus is calculated as the applied stress divided by the total strain at the same time point. Results are conventionally presented as strain versus logarithmic time, or as double-logarithmic plots, which linearize the response of many rubber compounds and facilitate extrapolation. Long-term behavior is commonly modeled with the Findley power law or estimated by time-temperature superposition, in which short-term tests at several elevated temperatures are shifted to construct a master curve representing the service temperature. Any extrapolation beyond the measured time window should be clearly identified as such in the test report.
Applications
Tensile creep data are used throughout the rubber industry whenever a component must sustain a static or quasi-static tensile load over months or years of service. Typical application areas include:
- Seals, gaskets, O-rings, diaphragms, and bellows that must maintain dimensional stability to preserve sealing performance.
- Vibration isolators, engine mounts, and suspension bushings where sustained loading leads to progressive settling and preload loss.
- Tires, where creep in the sidewall and belt-edge regions contributes to growth and long-term durability concerns.
- Hoses, conveyor belts, and power transmission belts operating under constant tension.
- Elastomeric bridge bearings and structural bearing pads subject to long-term compressive and tensile stress components.
- Medical rubber products such as tubing, tourniquets, and elastic components that must retain force over their shelf life and use period.
- Wire and cable accessories, including elastomeric jackets and strain-relief elements under permanent tension.
Beyond product design, the determination is widely used for compound qualification, incoming material inspection, batch-to-batch quality control, comparative evaluation of formulation changes, and the generation of material constants for finite element analysis and lifetime prediction models. Accredited third-party laboratories routinely perform these tests to provide clients with independent, standards-based evidence of long-term performance.
FAQ
What is the difference between creep and stress relaxation in rubber?
In a creep test, a constant stress is applied and the strain increases progressively with time, whereas in a stress relaxation test, a constant strain is imposed and the stress decays over time. Both phenomena originate from the same viscoelastic nature of rubber, but they represent different loading scenarios, and the appropriate test is selected according to how the component is loaded in service.
Which standards govern the determination of tensile creep characteristics?
The most commonly applied standards are ISO 899-1 and GB/T 11546.1 for the determination of tensile creep behavior, together with ASTM D2990 for tensile, compressive, and flexural creep and creep rupture. Specimen geometry, conditioning, and dimensional measurement generally follow ISO 37, ASTM D412, and ISO 23529.
What test durations and temperatures are typically used?
A reference test is performed at 23 °C ± 2 °C in the standard laboratory atmosphere, and durations of 100 h to 1000 h are common. Because rubber creep is thermally activated, accelerated tests at 70 °C, 100 °C, or 125 °C are frequently added so that long-term behavior can be estimated through time-temperature superposition within a practical laboratory timeframe.
How many specimens are required for a reliable determination?
At least three specimens should be tested at each stress level. When creep rupture is to be characterized, additional specimens are necessary because rupture times scatter significantly, and a statistical treatment of the failure data is normally required.
Can short-term creep data be extrapolated to predict service life?
Yes, within limits. The Findley power law and time-temperature superposition are widely accepted techniques for extending predictions beyond the measured window, provided the extrapolation does not cross a transition region such as crystallization or thermal degradation of the rubber. The basis and limits of any extrapolation should always be stated in the test report.
Why is tensile creep important for product design?
Excessive creep leads to loss of preload, loosening of clamped joints, drift of critical dimensions, reduction of sealing force, and, in extreme cases, delayed rupture of the component. Quantifying the creep characteristics of the rubber compound at the design stage allows engineers to select materials, define maintenance intervals, and set safety factors that ensure reliable long-term performance.