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Testing for Creep Material Models for CAE

Creep is the irrecoverable deformation of a material over time under constant mechanical stress, occurring below the yield point. While often overlooked in design due to its slow nature, creep is especially relevant for plastics, which can creep even at normal service temperatures — unlike metals, which typically only creep at elevated temperatures.

Creep occurs in three stages:

  1. Primary creep: Initial rapid deformation upon load application, involving work hardening.
  2. Secondary creep: Stable, slow deformation at a nearly constant rate.
  3. Tertiary creep: Localized strain leads to material failure or rupture.

Only secondary and tertiary stages are typically modeled in CAE; primary creep is usually handled using elastic or elastic-plastic models due to the rapidly changing strain rate in that phase.

To learn more regarding material testing and characterization in support of material models for CAE, see our TestPaks or contact us to talk with our materials testing experts.

Creep Modeling Equation

Creep is commonly modeled using a dislocation-based power law equation:

ε ˙ c r = A σ n t m
where,
ε ˙ c r = creep strain rate σ = creep stress n = stress exponent t = creep time m = time exponent

This model describes secondary creep well. Tertiary creep is harder to model and often used to establish a critical stress limit or failure envelope rather than being explicitly simulated.

Implementation Strategies

Standards such as ASTM D2990 and ISO 899 define procedures for creep testing. These tests involve:

  • Constant force applied to a specimen.
  • Continuous measurement of deformation over extended periods (typically 1000 hours).
  • Frequent data recording at the start (where most creep occurs), then less frequently later.

Tests should be run at a minimum of three stress levels to understand stress dependency. Environmental factors like temperature and humidity may also be included.

Stress selection is based on the material type:

  • Stiff, brittle materials (e.g., PEEK, PEI, fiber-filled plastics): test near failure stress to ensure measurable deformation.
  • Soft materials (e.g., HDPE, TPU): lower stresses are chosen to avoid premature failure unless rupture behavior is being studied.

The key metric is creep strain: the total strain recorded over time. This may differ from quasi-static strain values due to strain rate sensitivity or test system compliance. Plotting creep strain vs. log time yields a typical creep curve.

CAE Modeling for Creep

FEA software often uses creep models based on strain rate, but test data may need conversion to:

  • True strain (for accuracy).
  • Creep strain rate (derivative of strain vs. time).
  • Creep modulus (ratio of stress to strain over time).
  • Plastic-only creep strain (when elastic strain is subtracted out).

Sometimes it's more effective to integrate the model to match test data rather than reformatting test results. Models are typically calibrated using nonlinear regression to extract parameters, especially when considering both stress and temperature effects.

Creep Modeling Assumptions and Limitations

Creep models assume material behavior remains consistent beyond the testing range — an idealization that can overlook real-life conditions. Factors such as:

  • UV exposure
  • weathering
  • thermal cycling
  • chemical attack

are not captured by standard models. While time-temperature superposition (TTS) (from Section 8) can extend model predictions, such extrapolations must be made with caution.

Also, most creep models do not predict failure. To simulate rupture or tertiary creep, additional FEA failure criteria like:

  • element death
  • damage models
  • failure envelopes

must be implemented separately. Data from the tertiary phase may need to be excluded from the model fitting for mathematical accuracy, but should still be considered for design safety.

Conclusion

Creep is a critical, time-dependent behavior in polymers that influences long-term part performance. While simulation tools offer robust creep modeling capabilities, understanding test methods, material limitations, and modeling assumptions is essential for realistic, reliable predictions in design and analysis.

To explore the topics discussed on this page further, see Hubert Lobo (Founder, DatapointLabs) and Brian Croop (CEO, DatapointLabs), Determination and Use of Material Properties for Finite Element Analysis (NAFEMS, 2016), Ch.9.

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