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High-Speed Strain Rate Dependency Testing of Materials

Significance and Purpose

High-Speed Strain Rate Dependency Testing – also termed strain rate dependence testing or strain rate dependent testing – examines how materials respond to mechanical loads applied at different speeds or strain rates. Materials often exhibit strain rate sensitivity, meaning their mechanical properties, such as yield strength, modulus, and ultimate strength, vary with the rate of loading. This is crucial for applications involving impact, crashworthiness, and high-speed manufacturing processes.

Key purposes include:

  • Understanding Strain Rate Sensitivity: Predicting performance in dynamic applications.
  • Material Modeling: Generating stress-strain curves for computational simulations.
  • Crashworthiness and Impact Analysis: Evaluating how materials behave during rapid deformation.
  • Optimizing Manufacturing Processes: Ensuring materials perform consistently under high-speed forming conditions.

Relevant ASTM and ISO Standards

High-Speed Tensile Stress-Strain

  • ASTM D638: Standard Test Method for Tensile Properties of Plastics.
  • ASTM E8: Standard Test Methods for Tension Testing of Metallic Materials.
  • ISO 527-1: Plastics — Determination of Tensile Properties.

Very High-Speed Tensile Stress-Strain

  • ASTM D412: Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers — Tension.
  • ASTM D638: Tensile Properties of Plastics.
  • ISO 527-1: Tensile Testing of Plastics.
  • ASTM E8: Tension Testing of Metals.

High-Speed Compressive Stress-Strain

  • ASTM D695: Standard Test Method for Compressive Properties of Rigid Plastics.

High-Speed 3-Point Flexural Stress-Strain

  • ASTM D790: Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials.
  • ISO 178: Plastics — Determination of Flexural Properties.

High-Speed Compressibility of Foams and Elastomers

  • ASTM D575: Standard Test Methods for Rubber Properties in Compression.
  • ISO 604: Plastics — Determination of Compressive Properties.

DatapointLabs Tests for Rate Dependency Testing

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

General Rate Dependency Testing (inquire regarding material suitability)

Test ID Test Description Standards
M-232 High Speed Tensile Stress-Strain ASTM D638, ASTM E8, ISO 527-1
M-235 Very High Speed Tensile Stress-Strain ASTM D412, ASTM D638, ISO 527-1, ASTM E8
M-233 High Speed Compressive Stress-Strain ASTM D695
M-230 High Speed 3 Point Flexural Stress-Strain ASTM D790, ISO 178

Rate Dependency Testing Specific to Elastomers, Foams

Test ID Test Description Standards
M-231 High Speed Compressibility of Foams and Elastomers ASTM D575, ISO 604

Principle of Operation

The mechanical properties of materials depend on how quickly a load is applied. High-speed testing involves:

  1. Applying a Load at Controlled Speeds: A specialized testing machine applies loads at high strain rates (e.g., 1  s−1 to 2000  s−1).
  2. Measuring Force and Displacement: Sensors record load and deformation throughout the test.
  3. Calculating Stress and Strain: Engineering and true stress-strain relationships are calculated from force-displacement data.
  4. Analyzing Rate Dependency: Data from different strain rates are compared to evaluate strain-rate sensitivity.

Typical Procedure

  1. Sample Preparation:
    • Prepare samples according to the geometry specified in the relevant standard.
    • Condition materials as required (e.g., temperature, humidity).
  2. Test Setup:
    • Select the appropriate test configuration: tensile, compressive, or flexural.
    • Calibrate force, displacement, and strain measurement devices.
  3. Test Execution:
    • Apply loads at specified strain rates.
    • Record force and displacement continuously.
  4. Data Collection & Analysis:
    • Analyze stress-strain curves to calculate mechanical properties.
    • Compare results across multiple strain rates.

Specimen Types

Specimens used by DatapointLabs in various types of high-speed strain rate dependency testing are as follows:

Specimen Type DatapointLabs Test IDs
High Speed Tensile Bars [Details] M-232, M-235
Flex Bars [Details] M-230
Prisms [Details] M-233, M-231

Extensometry Techniques

Extensometry techniques typically employed by DatapointLabs in various types of high-speed strain rate dependency testing are as follows:

Extensometry Technique DatapointLabs Test IDs
High-Speed Optical Extensometry (Axial) M-232, M-235
Crosshead Displacement (Axial) M-233
Drop Tower Load Cell Accelerometer M-230, M-231

Characterization Measurements

High-Speed Tensile Stress-Strain (ASTM D638, ASTM E8, ISO 527-1)

  • Engineering Tensile Stress-Strain Curves: Stress-strain relationship under high-speed tension.
  • Offset Yield Strain in Tension: Strain corresponding to a specific offset (e.g., 0.2%).
  • Offset Yield Stress in Tension: Stress at the offset yield strain.
  • Tensile Modulus (E): Slope of the initial linear region.
  • Tensile Strain at Yield: Strain at peak yield point.
  • Tensile Strength at Yield: Maximum stress before yielding.
  • True Tensile Stress-Strain Curves: Derived from engineering curves by accounting for changing cross-sectional area.

Very High-Speed Tensile Stress-Strain (ASTM D412, ASTM D638, ISO 527-1, ASTM E8)

  • Engineering Tensile Stress-Strain Curves: Captures behavior under very high strain rates (up to 2000 s−1).
  • Offset Yield Strain in Tension: Measured during rapid deformation.
  • Offset Yield Stress in Tension: Indicates strain rate sensitivity.
  • Tensile Modulus (E): Evaluates elastic behavior at high speeds.
  • Tensile Strain at Yield: Strain at yield point for comparison with lower-speed tests.
  • Tensile Strength at Yield: Critical for crashworthiness applications.
  • True Tensile Stress-Strain Curves: Provides stress-strain data accounting for material thinning.

High-Speed Compressive Stress-Strain (ASTM D695)

  • Compressive Modulus: Slope of stress-strain curve during elastic compression.
  • Compressive Strain at Yield: Strain at which plastic deformation begins.
  • Compressive Strength at Yield: Peak stress before permanent deformation.
  • Compressive Ultimate Strength: Maximum stress before catastrophic failure.
  • Engineering Compressive Stress-Strain Curves: Graphs stress-strain response.
  • Offset Yield Strain in Compression: Strain at specified offset (e.g., 0.2%).
  • Offset Yield Stress in Compression: Stress corresponding to offset yield strain.
  • Strain at Ultimate: Maximum strain reached before failure.
  • True Compressive Stress-Strain Curves: Calculated to account for changing sample dimensions.

High-Speed 3-Point Flexural Stress-Strain (ASTM D790, ISO 178)

  • Flexural Modulus (Ef): Slope of stress-strain curve during elastic flexure.
  • Flexural Strength at Peak: Maximum stress measured during bending.
  • Flexural Stress-Strain Curves: Graphs flexural behavior over time.
  • Offset Yield Strain in Flex: Strain at a defined offset stress (e.g., 0.2%).
  • Offset Yield Stress in Flex: Stress corresponding to the offset yield strain.

High-Speed Compressibility of Foams and Elastomers (ASTM D575, ISO 604)

  • Compressive Modulus: Elastic stiffness of foams under high-speed compression.
  • Compressive Ultimate Strength: Maximum stress during deformation.
  • Engineering Compressive Stress-Strain Curves: Stress-strain data under compression.
  • Offset Yield Strain in Compression: Strain at specified offset stress.
  • Offset Yield Stress in Compression: Stress corresponding to the yield strain.
  • Strain at Ultimate: Maximum strain before failure.
  • True Compressive Stress-Strain Curves: Derived from engineering curves to account for cross-sectional changes.

Typical Data Reported (see test descriptions for exact details)

  • Stress-Strain Curves (Engineering & True): Captures mechanical response across strain rates.
  • Yield Properties (Stress & Strain): Offset yield stress and strain at multiple rates.
  • Elastic Modulus (Tensile, Compressive, Flexural): Material stiffness for various deformation modes.
  • Ultimate Strength (Tensile, Compressive, Flexural): Maximum load before catastrophic failure.
  • Rate Sensitivity Analysis: Compares mechanical properties across different loading rates.

Suitable Material Types

  • Plastics: Thermoplastics, thermosets, fiber-reinforced composites.
  • Metals: Aluminum, steel, and alloys used in structural applications.
  • Foams: Rigid and flexible foams in cushioning and packaging.
  • Elastomers: Rubber materials used in seals, tires, and vibration damping.

Suitable Applications

  • Automotive: Crashworthiness analysis of body panels, bumpers, and seat components.
  • Aerospace: High-speed deformation in fuselage and wing structures.
  • Defense: Impact-resistant materials for protective equipment.
  • Consumer Products: Evaluating impact performance of helmets, packaging, and electronics.
  • Medical Devices: Dynamic behavior of prosthetics and orthopedic components.
  • Manufacturing: Optimizing high-speed forming processes in metal stamping and plastic molding.

Conclusion

Rate dependency testing is essential for characterizing high- and very high-speed mechanical properties of materials across different deformation modes. By following ASTM D638, ASTM E8, ISO 527-1, ASTM D695, ASTM D790, ASTM D575, and ISO 604, engineers can accurately assess material performance under dynamic loading, aiding in material selection, process optimization, and failure prevention in high-performance applications like automotive, aerospace, and impact-resistant product design.

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