Plastic Compression Creep Performance Testing: Standards, Methods, and Applications

Plastic compression creep performance testing measures the time-dependent deformation of plastic materials under sustained compressive loads, providing critical data for long-term structural design and material selection. This testing is essential for applications such as gaskets, bearing pads, seals, and load-bearing components where dimensional stability directly affects product reliability and service life. This article covers the technical principles, applicable standards, sample requirements, test procedures, and typical applications of compression creep testing for plastics.

Technical Overview

Creep is the tendency of a solid material to deform permanently under the influence of persistent mechanical stress. In plastics, this behavior is particularly pronounced because polymers are viscoelastic materials whose response to load combines elastic and viscous characteristics. When a compressive load is applied and held constant, a plastic part continues to deform gradually over time, even though the applied stress remains unchanged.

Compression creep testing quantifies this time-dependent deformation by applying a constant compressive stress to a specimen and recording the resulting strain at defined time intervals. The measured strain typically exhibits three characteristic stages: primary creep, where the strain rate decreases rapidly; secondary creep, where deformation proceeds at a nearly constant rate; and, in some cases, tertiary creep, where the strain rate accelerates toward eventual rupture or collapse.

Because creep behavior is highly sensitive to temperature, stress level, and material formulation, compression creep data are indispensable for engineers designing plastic components intended to carry sustained loads for months or years. Tests are typically conducted in accordance with ASTM D2990, while the ISO 899 series governs related creep determination methods for plastics. Compressive strength values obtained from short-term tests such as ASTM D695 are often used to establish appropriate stress levels for long-term creep studies.

Testing Samples

Molded and Machined Specimens

Standard laboratory specimens are usually prisms or cylinders, such as those prepared in accordance with ASTM D695 geometry, for example prisms of 12.7 mm by 12.7 mm cross section and 25.4 mm height, or cylinders of 12.7 mm diameter and 25.4 mm length. Specimens may be injection molded, compression molded, or machined from sheets, rods, or plaques. Care must be taken to ensure that the loading faces are flat, parallel, and perpendicular to the loading axis, as misalignment introduces bending stresses and invalid results.

Finished Components

Component-level creep testing evaluates actual products such as gaskets, seals, bushings, spacers, pipe supports, and cushioning elements in their as-manufactured geometry. This approach captures the combined effects of material behavior, part design, and processing history, making the results directly relevant to field performance.

Foam and Layered Materials

Cellular plastics, foams, and thin films are frequently evaluated in laminate stacks to achieve a measurable specimen height. Foam specimens require special attention to conditioning, since ambient humidity and temperature strongly influence cell structure stability and measured compressive creep strain.

Sample Conditioning Requirements

Before testing, specimens should be conditioned in a controlled atmosphere, commonly 23 °C and 50% relative humidity for a minimum of 40 hours, in accordance with standard plastics conditioning practice. Additional drying may be required for hygroscopic materials such as polyamides to establish a defined moisture state.

Testing Items

Compression creep testing programs can include the following measurement items:

• Compressive creep strain as a function of time at constant stress and temperature
• Creep modulus, defined as applied stress divided by measured creep strain at a given time
• Creep compliance, the reciprocal measure describing strain per unit stress
• Isochronous stress-strain curves constructed from creep data at fixed time points
• Creep recovery, measuring the elastic and viscoelastic strain recovered after load removal
• Time to creep rupture or a defined deformation limit under sustained compressive load
• Compressive deformation after load removal, relevant for gasket and seal performance
• Long-term extrapolated strain values using time-temperature superposition or curve-fitting models such as the Findley power law

Testing Methods

Compression creep testing is performed using dedicated creep frames, universal testing machines equipped with constant-load fixtures, or lever-loaded dead-weight stands. The typical procedure follows these stages:

• Specimen measurement: The initial height and cross-sectional dimensions are recorded to calculate applied stress and strain.
• Preflex and preload: A small preload is applied to seat the specimen and eliminate surface irregularities, after which the reference zero position is established.
• Load application: A constant compressive load is applied smoothly and rapidly, typically within seconds, to reach the target stress level, which is commonly set at 10% to 30% of the material short-term compressive strength.
• Environmental control: Testing is conducted at a controlled temperature, often 23 °C, 40 °C, 70 °C, 80 °C, or 100 °C depending on the intended service environment, with humidity control where required.
• Deformation recording: Displacement is measured using LVDT transducers, extensometers, or optical systems, with readings taken on a logarithmic time schedule, for example at 1, 2, 5, 10, 20, 50, 100 hours, and beyond.
• Test duration: Common durations range from 24 hours to 1000 hours or longer; accelerated evaluation may use elevated temperatures with time-temperature superposition to estimate multi-year behavior.
• Recovery phase: For creep recovery studies, the load is removed and the residual and recovered deformation is tracked over time.

Data reduction involves calculating compressive strain as the change in specimen height divided by the initial height. Strain-time curves are plotted, and parameters such as creep modulus at specified times are reported. Reports normally include material identification, specimen geometry, conditioning history, stress level, temperature, strain-time data, and a description of any specimen failure or buckling observed.

Applications

Compression creep test data support design and quality decisions across many industries:

• Automotive: engine mounts, suspension bushings, fuel system seals, headlight housings, battery module cushions, and under-hood components exposed to sustained load and elevated temperature
• Construction and infrastructure: bridge bearing pads, pipe gaskets, expansion joint fillers, structural foam panels, and geomembranes
• Electronics and electrical: connector insulators, switch housings, cable supports, and thermal interface cushioning pads under sustained clamping forces
• Medical devices: sterilization-resistant components, implant bearing surfaces, tubing under compression, and device closures requiring long-term dimensional stability
• Packaging: stackable container bases, closure liners, and protective foam cushioning evaluated for long-term load retention
• Furniture and consumer goods: chair feet, caster stems, appliance feet, and threaded fastening points subject to persistent compressive stress
• Material development: comparison of polymer grades, filler loadings, and reinforcements for load-bearing applications

Accredited third-party laboratories routinely perform these tests to support product certification, supplier qualification, failure analysis, and regulatory submissions.

FAQ

What is the difference between creep and stress relaxation?

In creep testing, a constant stress is applied and the resulting strain increases over time. In stress relaxation testing, a constant strain is imposed and the internal stress decreases over time. Both phenomena reflect the same underlying viscoelastic behavior of plastics but require different test setups and data interpretation.

Which standards govern plastic compression creep testing?

ASTM D2990 is the principal standard covering tensile, compressive, and flexural creep and creep-rupture of plastics. The ISO 899 series addresses creep determination for plastics, and short-term compressive properties referenced during test setup are commonly measured according to ASTM D695 or ISO 604. National standards such as GB/T and JIS K series documents provide equivalent or related creep test methods.

How long does a compression creep test take?

Test duration depends on the design objective. Screening tests may last 24 to 168 hours, while standard long-term tests commonly run 1000 hours. For service life predictions spanning several years, elevated-temperature testing combined with time-temperature superposition is used to construct extrapolated master curves.

What stress level should be used for testing?

The applied compressive stress is usually selected as a fraction, often 10% to 30%, of the material short-term compressive yield or failure strength. Testing at multiple stress levels allows construction of isochronous curves and identification of the linear viscoelastic region.

Why is temperature control so important?

Polymer chain mobility increases with temperature, which accelerates creep dramatically. A material that performs well at room temperature may exhibit severe deformation at 80 °C. Accurate, stable temperature control, often within ±2 °C, is essential for generating valid and reproducible creep data.

Can finished parts be tested instead of laboratory specimens?

Yes. Component-level compression creep testing on actual gaskets, pads, or structural parts is widely performed and often more meaningful, because it accounts for geometry, processing conditions, and assembly constraints in addition to the raw material behavior.