Compressive creep testing of plastics measures the time-dependent deformation of polymeric materials under sustained compressive loads, providing critical long-term performance data for design engineers and material specifiers. Governed by internationally recognized standards such as ASTM D2990, ISO 7616, and EN 1606, this test method quantifies creep strain, creep modulus, and long-term load capacity. This article explains the principles, specimen requirements, test procedures, data analysis methods, and typical industrial applications of plastic compressive creep testing, as performed by accredited third-party laboratories worldwide.
Technical Overview
Creep is the tendency of a solid material to deform progressively under a sustained mechanical stress that is well below its short-term yield or rupture strength. In plastics, this behavior arises from their viscoelastic nature: polymer chains slip, disentangle, and rearrange over time when a constant load is applied. Compressive creep specifically refers to this time-dependent deformation when the sustained stress is a compressive one, such as the load experienced by a foam insulation board beneath a concrete slab or a plastic pallet at the bottom of a stacked load.
Compressive creep testing applies a constant compressive force to a conditioned plastic specimen and records deformation as a function of time, typically at a controlled temperature. The resulting creep curve generally exhibits three stages: primary creep, in which the strain rate decreases as the material strain-hardens; secondary creep, in which deformation proceeds at a nearly constant minimum rate; and, at sufficiently high stress levels, tertiary creep, in which damage accumulation accelerates deformation until rupture or densification occurs.
Because creep deformation can continue for years or decades, short-term compression properties such as those measured by ASTM D695 or ISO 604 cannot be used directly for long-term design. Compressive creep data allow engineers to predict deformation at the intended service life, establish allowable design stresses, and compare candidate materials under realistic loading conditions.
- Temperature: creep rates increase dramatically with temperature, often by an order of magnitude over a few tens of degrees.
- Stress level: higher sustained stresses shorten time-to-failure and increase total strain.
- Polymer structure: semi-crystalline and highly crosslinked or filled materials generally exhibit lower creep than unfilled amorphous thermoplastics.
- Load duration: deformation accumulates continuously, making long-duration or extrapolated data essential.
Testing Samples
Solid Thermoplastic Specimens
Solid specimens are typically cylinders or rectangular prisms with a 2:1 length-to-width (or diameter) aspect ratio, for example 12.7 mm × 12.7 mm × 25.4 mm in the style of ASTM D695, machined or molded from granulate or plaques. The 2:1 geometry, combined with anti-buckling guidance where necessary, prevents column buckling and ensures that failure occurs by compressive yielding or rupture rather than instability.
Cellular Plastic (Fo) Blocks
Rigid cellular plastics such as expanded polystyrene (EPS), extruded polystyrene (XPS), and polyurethane (PUR) foams are tested as rectangular blocks, commonly 50 mm × 50 mm × 25 mm or larger square-edged slabs in accordance with ISO 7616, EN 1606, or ASTM D3574 test specimen configurations. Flexible foams used in seating and cushioning may be tested as larger blocks appropriate to their cellular structure.
Finished Product Sections
Sections cut from actual products, such as pipe rings, fitting walls, structural foam panels, enclosure walls, or component ribs, may be tested to evaluate real-world performance. Specimen geometry must be recorded and the loading direction must correspond to the dominant service stress direction.
Specimen Conditioning
Prior to testing, specimens are conditioned in a standard atmosphere, typically 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 40 hours, in accordance with ISO 291 or ASTM D618, unless the governing product standard specifies otherwise. Specimens are inspected for voids, sink marks, machining damage, and non-parallel end faces, all of which can distort creep measurements.
Testing Items
A typical compressive creep test program quantifies the following parameters, each of which characterizes a different aspect of long-term load-bearing performance.
- Compressive creep strain: the time-dependent reduction in specimen height expressed as a percentage of the original dimension.
- Creep (apparent) modulus: the ratio of applied constant stress to measured strain at a given elapsed time, documenting the progressive loss of stiffness.
- Creep compliance: the reciprocal quantity, expressing strain per unit stress as a function of time.
- Creep rupture strength and time-to-failure: the stress at which the specimen fails within a defined period, or the elapsed time to failure at a given stress.
- Compression set and recovery: the permanent residual deformation remaining after load removal, particularly relevant for flexible and rigid foams.
- Isochronous stress–strain curves: stress–strain relationships constructed at fixed elapsed times, widely used in design calculations.
- Creep model coefficients: parameters fitted to models such as the Findley power law or the four-element Burgers model for prediction and extrapolation.
Testing Methods
Test Apparatus and Procedure
The core principle of the test is simple: a constant, dead-weight or lever-amplified compressive load is applied to the specimen through rigid platens, and the deformation is measured continuously or at prescribed intervals. Practical implementations include dedicated creep stands, constant-load compression frames located inside environmental chambers, and electro-mechanical testing machines fitted with static load-holding control. Deformation is captured by LVDT transducers, dial compressometers, or non-contact optical methods, and measurements are typically logged on a logarithmic time schedule, for example at 1, 6, 30, 180, and 1,000 minutes, followed by daily or weekly readings for extended tests. Because a 1 % error in load or temperature propagates directly into the creep prediction, calibrated load verification and chamber uniformity checks are essential.
Test Conditions
Stress levels are normally selected as a fraction, commonly 10 % to 50 %, of the short-term compressive strength determined by ASTM D695 or ISO 604, with several stress levels tested in parallel to map the stress–time–strain relationship. Reference tests are performed at 23 °C, while elevated-temperature points such as 40 °C, 60 °C, 80 °C, or 100 °C reflect service environments such as under-hood automotive or rooftop conditions. Standard test durations of 1,000 hours are common for comparative purposes, while qualification programs for building products frequently extend to 5,000 hours or more, after which results are extrapolated to design horizons of 10 to 50 years using accepted statistical methods.
Data Analysis and Reporting
Primary results are plotted as strain versus time, usually on linear–logarithmic or log–log axes, from which creep modulus decay curves and isochronous stress–strain curves are derived. Analytical models such as the Findley power law, in which strain is expressed as the sum of an instantaneous elastic term and a time-dependent term proportional to time raised to a fractional power, are fitted to the data to enable interpolation and controlled extrapolation. Time–temperature superposition may be applied to shift short-term elevated-temperature data into a master curve covering long service lives. A complete report documents material identification, specimen geometry and conditioning, stress level and temperature, the full deformation–time record, calculated creep strain and creep modulus values, and any failure mode observed.
Reference Standards
- ASTM D2990: Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics, the principal US standard for compressive creep of solid plastics.
- ISO 899 (Parts 1 and 2): Determination of creep behaviour of plastics under tensile and flexural loading, whose methodology is widely adapted for compressive creep programs.
- ISO 7616: Cellular plastics, rigid — Determination of compressive creep under specified load and temperature conditions.
- EN 1606 and EN 826: European standards for thermal insulating products, covering long-term compressive creep and short-term compression behavior respectively.
- ASTM D3574: Standard test methods for flexible cellular materials, including compression set and creep-related determinations for polyurethane foams.
- GB/T 1041 and GB/T 11546: Chinese national standards covering compression properties and creep behavior of plastics.
- JIS standards harmonized with the ISO 899 series, applied within the Japanese national framework for plastic creep evaluation.
Applications
Compressive creep data directly support safe, economical design wherever plastics must sustain static loads over long periods. Accredited third-party laboratories provide these tests for product qualification, regulatory compliance, and research and development across many industries.
- Building and construction: determination of long-term compressive creep values for EPS, XPS, and PUR insulation boards used beneath floor slabs, foundations, and inverted roofs, where deformation limits of 2 % over 50 years are commonly specified through EN 1606 based qualification.
- Pipes and buried infrastructure: evaluation of buried thermoplastic pipes subjected to constant soil and live loads, ensuring ring stiffness is maintained over the design life.
- Materials handling and logistics: validation of plastic pallets, bottle crates, stacking containers, and storage racks that must carry sustained stacking loads in warehouses.
- Automotive and transport: assessment of seat foams, NVH components, underbody supports, and battery module housings exposed to clamping loads and elevated temperatures.
- Electrical and electronic equipment: qualification of insulating supports, busbar spacers, and enclosure features under permanent fastening or assembly loads.
- Medical devices: characterization of load-bearing implant-grade polymers such as PEEK used in spinal cages and bearing components.
- Protective packaging: confirmation that foam cushioning maintains its thickness and energy-absorbing capability under stacked storage over extended transport times.
FAQ
What is the difference between creep and compression set?
Creep describes the increasing deformation of a material while a load is still applied, whereas compression set describes the permanent deformation that remains after the load has been removed. Both are measured in many foam-related programs because a cushion may need both low creep under load and good recovery after unloading.
Why can short-term compression tests not replace creep tests?
Short-term tests measure the instantaneous response of the material, while plastics continue to deform under sustained load due to chain relaxation and viscous flow. A material showing adequate strength in a ten-minute test may deform excessively or fail after months or years at the same stress level, so only long-duration or properly extrapolated creep data are suitable for structural design.
How long does a compressive creep test take?
Standard comparative tests commonly run for 1,000 hours, while qualification tests for building products may run 5,000 hours or longer. Accelerated tests at elevated temperatures, combined with time–temperature superposition or validated model fitting, are used to estimate deformation at service lives of 10 to 50 years without decade-long experiments.
What loads and temperatures are used?
Applied stresses are typically set at 10 % to 50 % of the material's short-term compressive strength, with several levels tested to construct a complete stress–time–strain map. Reference tests are performed at 23 °C, and additional tests at 40 °C to 100 °C simulate hot service environments.
How is creep modulus calculated?
Creep modulus is the constant applied compressive stress divided by the total measured strain at a specific elapsed time. Because strain increases with time, creep modulus decreases continuously, and design values must be quoted together with the corresponding time, temperature, and stress level.
Can creep data be extrapolated reliably?
Yes, provided that extrapolation is limited to moderate time extensions, typically one to two decades beyond the measured data, and that validated models such as the Findley power law or time–temperature superposition are used. Test reports from accredited third-party laboratories should always state the basis and limitations of any extrapolated value.