Tensile creep evaluation measures how plastic materials deform progressively under sustained tensile stress, delivering essential data for long-term product design, lifetime prediction, and material qualification. Performed in accordance with international standards such as ISO 899-1 and ASTM D2990, this evaluation quantifies creep strain, creep modulus, isochronous stress-strain behavior, and creep-rupture performance. This article provides a comprehensive guide to the principles, specimen requirements, test procedures, data interpretation, and industrial applications of plastic tensile creep evaluation.
Technical Overview
Creep is the time-dependent, progressive deformation of a material when subjected to a constant load or stress below its short-term yield strength. Unlike metals, plastics are viscoelastic materials, meaning their mechanical response combines elastic and viscous characteristics. As a result, a plastic component that performs acceptably under an initial load may continue to deform over hours, days, or even years, potentially leading to dimensional instability, loss of preload in fasteners, leakage in pressure pipes, or eventual creep rupture.
A typical tensile creep curve consists of three distinct stages. The primary stage shows deformation at a decreasing strain rate as molecular chains orient and resist further flow. The secondary stage proceeds at an approximately constant, minimum strain rate. The tertiary stage, if the test is continued long enough, exhibits accelerating strain due to internal damage mechanisms such as crazing, void formation, and micro-cracking, ultimately ending in rupture. Understanding where a material sits within these stages under a given stress and temperature is fundamental to safe, long-life design.
Tensile creep evaluation is primarily governed by ISO 899-1 (Plastics — Determination of creep behaviour — Part 1: Tensile creep) and ASTM D2990 (Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics). The corresponding Chinese national standard is GB/T 11546.1, which is technically aligned with ISO 899-1. Short-term tensile properties used to establish initial stress levels are typically determined beforehand according to ISO 527 or ASTM D638.
Testing Samples
ISO Multipurpose Dumbbell Specimens
For testing per ISO 899-1, the preferred specimen is the ISO 3167 multipurpose test specimen, or directly the ISO 527-2 type 1A (molded thickness 4 mm) or type 1B (machined thickness 10 mm) dumbbell. These specimens provide a well-defined gauge length of 50 mm (type 1A) or 80 mm of parallel width, ensuring uniform stress distribution within the measured region.
ASTM Dogbone Specimens
ASTM D2990 testing typically uses ASTM D638 type I dogbone specimens with a nominal width of 13 mm and thickness of 3.2 mm in the narrow section. Alternative specimen geometries may be used when the material form (film, sheet, or small molded parts) requires it, provided the dimensions and gauge length are documented.
Strip and Product-Cut Specimens
For films, tapes, strapping, geotextiles, and fiber-reinforced laminates, rectangular strip specimens with long parallel gauge sections may be tested. Specimens machined from finished products can be evaluated when the objective is to verify the performance of an actual component, although anisotropy and processing history must be considered during data interpretation.
Specimen Conditioning and Quantity
Specimens should be conditioned in a standard atmosphere of 23 °C and 50% relative humidity (ISO 291 or ASTM D618) for a minimum of 40 hours, or longer for hygroscopic materials such as polyamides. Because creep data are inherently statistical, a minimum of three specimens per stress level is recommended, and a full characterization usually requires multiple stress levels, which increases the total specimen count accordingly.
Testing Items
Depending on the engineering objective, tensile creep evaluation may include the following measurable outputs:
- Creep strain versus time curves at each applied stress level and temperature
- Instantaneous (initial elastic) strain and time-dependent strain components
- Apparent creep modulus (the ratio of applied stress to total strain at any given time) as a function of time
- Isochronous stress-strain curves constructed at fixed times such as 1 h, 10 h, 100 h, and 1000 h
- Creep-rupture strength and time-to-rupture at each stress level
- Strain-at-rupture and failure mode characterization
- Creep recovery behavior measured after load removal, quantifying the residual (permanent) set
- Minimum creep rate (secondary stage slope) used for lifetime extrapolation models
Testing Methods
Test Apparatus
Creep testing is commonly performed on dead-weight lever-arm creep stands, which apply a fixed force through calibrated weights and maintain it essentially indefinitely without active control. Electromechanical or servo-hydraulic frames with closed-loop load control may also be used, particularly for higher forces or when combined with environmental chambers. Strain is measured with clip-on extensometers, long-travel contact extensometers, or non-contact video and laser systems, all of which must remain stable and drift-free over the full test duration. Temperature-controlled cabinets or chambers provide the required test environment, typically maintained within ±2 °C.
Test Conditions
The essential controlled variables are the applied tensile stress, the temperature, and the test duration. Stress levels are usually selected as fractions (for example 20%, 30%, 40%, and 50%) of the short-term yield or tensile strength measured at the same temperature. Standard test temperatures include 23 °C as a reference plus service-relevant elevated temperatures such as 40 °C, 60 °C, or 80 °C. Common test durations range from 100 hours for screening purposes to 1000 hours for design data, with extended tests of 5000 to 10,000 hours or more used for critical applications such as pressure piping.
Test Procedure
After conditioning and dimensional measurement, the specimen is mounted in the grips with proper alignment to eliminate bending stresses. The extensometer is attached to the gauge section, and the load is applied smoothly and rapidly, without impact or overshoot, to reach the target stress. The initial strain is recorded immediately upon full load application, and strain readings are then logged on a schedule that captures the rapidly changing early region, for example at 1, 2, 5, 10, 20, 50 min, then 1, 2, 5, 10 h, and thereafter at logarithmic intervals. Temperature and humidity are recorded continuously, and testing continues until the target duration is reached or the specimen ruptures.
Data Presentation and Analysis
Raw data are plotted as strain versus logarithmic time for each stress level. Creep modulus is calculated and plotted as Ec(t) = σ / ε(t), which decreases monotonically with time. Isochronous stress-strain curves are generated by taking the stress-strain pairs at each fixed time across all stress levels; these curves are a convenient design tool because they behave nearly linearly at small strains. For lifetime prediction, empirical models such as the Findley power law, ε(t) = ε0 + A·t^n, or viscoelastic models such as the four-element Burgers model are fitted to the data. Time-temperature superposition may be applied to shifted data from several temperatures to construct master curves extending far beyond the actual test duration, supporting long-term design extrapolation.
Applications
Tensile creep data directly support engineering decisions across many industries where plastics carry sustained loads:
- Pipes and fittings: long-term hydrostatic strength design of PE, PP, and PVC pressure systems relies on creep-based regression analysis for 50-year service life predictions.
- Automotive: under-hood brackets, clips, fuel system components, and interior trim must retain dimensional stability under sustained stress at elevated temperatures.
- Construction and infrastructure: anchors, fasteners, cable ties, geogrids, and geomembranes are qualified using creep data to ensure decades of load-bearing performance.
- Packaging: strapping and pallet-wrap films must resist elongation under sustained pallet loads during storage and transport.
- Consumer goods: chairs, shelving, handles, and housings are designed so that creep deformation remains imperceptible over the product lifetime.
- Electrical and electronic: connector housings and cable management components maintain contact force and mechanical integrity under permanent spring loading.
- Medical devices: structural and load-bearing polymer components in implants and instruments are evaluated for creep where sustained physiological loads exist.
Accredited third-party laboratories routinely perform tensile creep evaluations to support material qualification, supplier comparisons, failure analysis, regulatory submissions, and design verification for these applications.
FAQ
What is the difference between creep and stress relaxation?
Creep is the increase in strain over time under a constant applied stress, whereas stress relaxation is the decrease in stress over time under a constant imposed strain. Both phenomena originate from the same viscoelastic behavior of polymers, but they represent different loading scenarios and require different test setups.
How long should a tensile creep test run?
The required duration depends on the application. Screening tests of 100 hours can rank materials, while 1000-hour tests are the common baseline for design data. For critical, long-service-life products such as pressure pipes, tests of several thousand to over 10,000 hours are typical, often combined with elevated-temperature data and time-temperature superposition for extrapolation.
What are the key differences between ISO 899-1 and ASTM D2990?
The two standards are conceptually equivalent but differ in specimen geometry, preferred grip and extensometer practices, conditioning references, and some reporting details. ISO 899-1 uses ISO 3167 or ISO 527-2 type 1A/1B specimens, while ASTM D2990 generally uses ASTM D638 type I specimens. Results from the two standards are usually comparable in trend but should not be mixed without verification, and the standard used must always be stated with the data.
Why must multiple stress levels be tested?
A single stress level provides only one point on the creep response surface. Testing at several stress levels allows construction of isochronous curves, identification of the stress threshold below which creep remains acceptably low, determination of creep-rupture envelopes, and reliable fitting of predictive models.
Can short-term creep data be extrapolated to predict long-term performance?
Yes, within recognized limits. By fitting validated models such as the Findley power law and applying time-temperature superposition to accelerated, elevated-temperature data, engineers can extrapolate to service lifetimes well beyond the test duration. However, extrapolation beyond one to two decades of time in logarithmic terms requires caution, because mechanisms such as physical aging, oxidation, and environmental stress cracking may not be captured by short-term data alone.
What factors most strongly influence creep behavior in plastics?
Temperature is the dominant factor, since molecular mobility increases significantly with heat. Other key factors include the applied stress level, the polymer type and molecular weight, the degree of crystallinity, the presence and orientation of fillers or fibers, absorbed moisture in hygroscopic materials, and the loading history of the part.