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Creep Testing of Materials

Significance and Purpose

Creep Testing evaluates the time-dependent deformation of materials under constant stress over extended periods. It is critical for assessing long-term performance in structural, aerospace, automotive, and energy applications where materials operate under sustained loads, particularly at elevated temperatures or in varying humidity conditions. The data obtained from creep tests inform material selection, design safety, and service life predictions.

Relevant ASTM and ISO Standards

General Materials (Polymers, Composites, and Other Non-Metallics)

  • Tensile Creep:
    • ASTM D2990: Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics
    • ISO 899-1: Plastics—Determination of Creep Behavior—Part 1: Tensile Creep
  • Compressive Creep:
    • ASTM D2990
  • Flexural Creep:
    • ASTM D2990
    • ISO 899-2: Plastics—Determination of Creep Behavior—Part 2: Flexural Creep

Metals (High-Temperature Applications)

  • Tensile Creep at Elevated Temperatures:
    • ISO 204: Metallic Materials—Uniaxial Creep Testing in Tension
    • ASTM E139: Standard Test Method for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials

DatapointLabs Tests for Creep Testing

Tests in the DatapointLabs test catalog that reference creep testing are as follows:

General Creep Testing (inquire regarding material suitability)

Test ID Test Description Standards
M-250 Tensile Creep ASTM D2990, ISO 899-1
M-250H Tensile Creep With Humidity ASTM D2990, ISO 899-1
M-251 Compressive Creep ASTM D2990
M-252 Flexural Creep ASTM D2990, ISO 899-2

Creep Testing Specific to Metals

Test ID Test Description Standards
M-250HT High Temperature Tensile Creep ISO 204, ASTM E139

Principle of Operation

Creep testing involves applying a constant load (or stress) to a specimen at a fixed temperature and measuring strain over time. The test typically follows these three primary creep stages:

  1. Primary Creep (Transient Creep): Strain rate decreases as material work-hardens.
  2. Secondary Creep (Steady-State Creep): Strain rate stabilizes due to a balance between work-hardening and recovery mechanisms.
  3. Tertiary Creep: Strain rate accelerates due to internal damage (e.g., void formation, microcracking), leading to failure.

Typical Procedure

  1. Sample Preparation: Specimens are machined to standard dimensions.
  2. Test Setup: The specimen is mounted in a creep testing machine, where a constant load or stress is applied.
  3. Environmental Conditioning: Temperature and humidity (for non-metallics) are controlled as per test requirements.
  4. Data Acquisition: Strain is measured over time using extensometers.
  5. Test Termination: The test may run until rupture (creep-rupture test) or for a predefined time to assess strain accumulation.
  6. Data Analysis: Strain vs. time curves and other characterization parameters are extracted for material evaluation.

Specimen Types

Specimens used by DatapointLabs in various types of creep testing are as follows:

Specimen Type DatapointLabs Test IDs
Tensile Bars [Details] M-250, M-250H, M-250HT
Compressive Creep Specimens [Details] M-251
Flex Bars [Details] M-252

Extensometry Techniques

Extensometry techniques typically employed by DatapointLabs in various types of creep testing are as follows:

Extensometry Technique DatapointLabs Test IDs
Contact Extensometry (Axial) M-250, M-250H, M-250HT
Dial Indicator (Contact) M-251, M-252

Characterization Measurements

Creep testing provides multiple characterization metrics to assess material behavior:

Tensile Strain vs. Time

  • Measures elongation over time under constant tensile stress.
  • Used to determine creep strain rate, primary/secondary/tertiary stages, and rupture time.
  • Applicable to both general materials (ASTM D2990, ISO 899-1) and metals (ASTM E139, ISO 204).

Isochronous Tensile Stress-Strain Curves

  • Constructed by plotting strain at fixed time intervals for different stress levels.
  • Useful for comparing materials and predicting long-term behavior under different load conditions.
  • Commonly used in polymeric and composite material studies.

Compressive Strain vs. Time

  • Evaluates deformation under sustained compressive load.
  • Important for structural materials, including plastics and elastomers, where long-term stability under compression is required.
  • Governed by ASTM D2990 for general materials.

Flexural Strain vs. Time

  • Assesses creep deformation in bending, particularly for polymers and composites.
  • Follows ASTM D2990 and ISO 899-2 standards.
  • Important for beams, panels, and other flexural load-bearing components.

Typical Data Reported (see test descriptions for exact details)

  • Creep Strain vs. Time Curves: For tensile, compressive or flexural modes of deformation.
  • Time to Rupture: For creep-rupture tests.
  • Isochronous Stress-Strain Curves: Material studies for design reference.
  • Steady-State Creep Rate: Secondary creep rate, ε̇.
  • Creep Modulus: Ratio of applied stress to creep strain at a given time.

Suitable Material Types

General Materials (Polymers, Composites, and Non-Metallics)

  • Thermoplastics and thermosets: polyethylene, polypropylene, epoxy.
  • Fiber-reinforced composites.
  • Elastomers: rubber, silicone.

Metals (High-Temperature Applications)

  • Superalloys: Inconel, Hastelloy.
  • Stainless steels.
  • Titanium alloys.
  • Refractory metals: tungsten, molybdenum.

Suitable Applications

  • Aerospace & Power Generation: Turbine blades, heat exchangers, jet engine components.
  • Automotive: High-performance engine parts, structural polymers in vehicles.
  • Civil Engineering & Infrastructure: Polymers and composites in bridges, piping, and structural panels.
  • Medical Devices: Long-term implant materials (e.g., polymeric prosthetics, titanium implants).
  • Electronics & Semiconductors: Polymers and metals used in high-temperature electronic enclosures.

Conclusion

Creep testing provides essential long-term performance data for materials under sustained loads. By adhering to standardized test methods such as ASTM D2990, ISO 899, ASTM E139, and ISO 204, engineers can predict material behavior, optimize design choices, and ensure reliability in demanding applications.

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