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Fatigue Testing of Materials

Note: For cyclic loading testing of materials, see the following section: Cyclic Loading Testing of Materials.

Significance and Purpose

Fatigue Testing evaluates a material’s resistance to cyclic loading, measuring its ability to withstand repeated stress or strain before failure. This type of testing is critical for engineering applications where materials are subjected to fluctuating loads over time, such as in aerospace, automotive, and structural components. Various ASTM standards define test procedures for different loading conditions and material types to ensure consistency and accuracy in fatigue life assessment.

Fatigue testing is used for:

  • Material Characterization: Determines fatigue life and endurance limit under cyclic loading.
  • Structural Integrity Analysis: Helps predict component lifespan and failure modes.
  • Quality Control: Ensures materials meet performance standards for fatigue resistance.
  • Engineering Design: Supports safe and reliable product design based on fatigue performance.
  • Failure Prevention: Identifies weaknesses in materials that could lead to catastrophic failure in service.
  • Research & Development: Aids in developing materials with improved fatigue resistance.

Relevant ASTM Standards

General Fatigue Testing

  • ASTM D7791: Tensile fatigue cycles to failure and tensile fatigue S-N curves for plastics.
  • ASTM D671: Flexural fatigue cycles to failure and flexural fatigue S-N curves for plastics.
  • ASTM D7774 Procedure A: Three-point flexural fatigue S-N curves for plastics and other non-metallic materials.

Metal Fatigue Testing

  • ASTM E466: Tensile fatigue cycles to failure and tensile fatigue S-N curves for metals.
  • ASTM E606: Strain-controlled tensile fatigue cycles to failure and strain-controlled tensile fatigue S-N curves for metals.

DatapointLabs Tests for Fatigue Testing

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

General Fatigue Testing (inquire regarding material suitability)

Test ID Test Description Standards
M-512 Tensile Fatigue ASTM D7791
M-508 Tensile Fatigue S-N Curve ASTM D7791
M-502 Flexural Fatigue ASTM D671
M-504 Flexural Fatigue S-N Curves ASTM D671
M-509 3 pt. Flexural Fatigue S-N Curve ASTM D7774

Fatigue Testing Specific to Metals

Test ID Test Description Standards
M-512M Tensile Fatigue of Metals (up to 60 Hz) ASTM E466
M-508M Tensile Fatigue S-N Curve of Metals (up to 60 Hz) ASTM E466
M-513M Strain Controlled Fatigue of Metallic Materials (Up to 2 Hz) ASTM E606
M-515M Strain Controlled Tensile Fatigue E-N curves for Metals (Up to 2 Hz) ASTM E606

Principle of Operation

Fatigue testing involves applying cyclic loading to a specimen until failure occurs. The test setup varies based on loading type (tensile, flexural, or strain-controlled fatigue).

  1. Specimen Preparation: Specimens are machined to standard geometries with polished surfaces to minimize stress concentrations.
  2. Mounting in the Testing Machine: The specimen is secured in grips or fixtures.
  3. Application of Cyclic Loading:
    • Tensile Fatigue (ASTM D7791, ASTM E466): Specimen is subjected to cyclic axial tension.
    • Flexural Fatigue (ASTM D671, ASTM D7774): Specimen is subjected to cyclic bending loads.
    • Strain-Controlled Fatigue (ASTM E606): Loading is controlled based on strain rather than stress.
  4. Monitoring and Data Collection: The number of cycles to failure is recorded along with stress and strain data.
  5. Post-Test Analysis: Fatigue life curves (S-N and/or E-N) are generated to describe material behavior.

Typical Procedure

  1. Specimen Preparation
    • Shape, size, and surface finish per relevant ASTM standard.
    • For metals, polished surfaces minimize stress concentrations.
    • For plastics and composites, environmental conditioning may be required.
  2. Test Setup
    • Specimen mounted in a fatigue testing machine.
    • Extensometers applied if strain measurement is required.
  3. Loading and Testing
    • Cyclic load applied in tension, flexure, or strain-controlled modes.
    • Frequency and load ratio set according to material and application.
    • Testing continues until specimen fails or reaches a predefined cycle limit.
  4. Post-Test Analysis
    • Fatigue life curves (S-N and/or E-N) generated.
    • Fracture surfaces analyzed for failure modes.

Specimen Types

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

Specimen Type DatapointLabs Test IDs
Tensile Bars [Details] M-512, M-508
Flex Fatigue Specimens [Details] M-502, M-504
Flex Bars [Details] M-509
Contact Us [Details] M-512M, M-515M

Extensometry Techniques

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

Extensometry Technique DatapointLabs Test IDs
Not Relevant M-512, M-508, M-502, M-504, M-509, M-512M, M-508M
Contact Extensometry (Axial) M-513M, M-515M

Characterization Measurements

Cycles to Failure (Nf)

  • Definition: Number of cycles a material withstands before failure.
  • Significance: Key measure of material durability under cyclic loads.

S-N Curves (Stress vs. Number of Cycles to Failure)

  • Definition: Plot of applied stress (σ) versus number of cycles to failure (N).
  • Significance: Describes fatigue behavior of materials under stress-controlled conditions.
  • Characteristics:
    • Typically plotted on a semi-logarithmic scale.
    • Slope of the curve indicates sensitivity to fatigue loading.

E-N Curves (Strain vs. Number of Cycles to Failure, ASTM E606)

  • Definition: Plot of applied strain (ε) versus number of cycles to failure (N).
  • Significance: Used for strain-controlled fatigue testing, applicable for low-cycle fatigue (LCF).
  • Characteristics:
    • Typically plotted on a semi-logarithmic scale.
    • Separates elastic and plastic strain components.
    • More relevant for metals and ductile materials experiencing plastic deformation under cyclic loading.

Typical Data Reported (see test descriptions for exact details)

  • Cycles to Failure (Nf).
  • S-N Curves: Stress vs. cycles to failure.
  • E-N Curves: Strain vs. cycles to failure, for strain-controlled tests.

Suitable Material Types

  • Metals: ASTM E466, ASTM E606.
  • Plastics and Polymers: ASTM D7791, ASTM D671, ASTM D7774.
  • Composites: ASTM D7791 (for tensile fatigue), ASTM D671 (for flexural fatigue).
  • Elastomers and Rubber-like Materials: ASTM D7791.

Suitable Applications

  • Material Selection: Evaluating a material’s resistance to cyclic loading, stress fluctuations, and crack propagation.
  • Component Design: Assessing durability of parts like springs, fasteners, and joints subjected to repeated stress.
  • Quality Control: Ensuring consistent fatigue performance in materials such as polymers, composites, and alloys.
  • Research & Development: Testing new materials and designs to improve fatigue life and endurance limits.
  • Failure Analysis: Identifying causes of fatigue-related fractures, surface cracks, or structural weakening.
  • Product Certification: Verifying compliance with industry standards for fatigue strength and lifespan.
  • Process Optimization: Evaluating how manufacturing techniques influence fatigue resistance in finished components.
  • Environmental Testing: Assessing material performance under varying conditions such as temperature cycles, corrosion, or vibration.

Conclusion

Fatigue testing is essential for predicting material performance under real-world cyclic loading conditions, ensuring durability, reliability, and safety across various industries. By following ASTM D7791, D671, D7774, E466, and E606, engineers can generate S-N and E-N curves, determine fatigue life, and evaluate failure mechanisms. With applications spanning aerospace, automotive, biomedical, and structural engineering, fatigue testing remains a critical tool for preventing catastrophic failures in load-bearing materials and components.

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