Four-Point Bending Fatigue Test: Working Principle, Testing Procedure & Applications

1. Working Principle of Four-Point Bending Fatigue Test

The Four-Point Bending Fatigue Test evaluates a material’s resistance to cyclic bending loads. It is widely used for testing structural materials such as beams, bridges, aircraft components, and composite materials.

Key Aspects of the Principle:

• A rectangular or cylindrical specimen is supported at two outer points, while a cyclic load is applied at two inner points.

• This setup creates a constant bending moment between the inner loading points.

• The upper surface experiences compressive stress, while the lower surface experiences tensile stress.

• The cyclic loading causes crack initiation and propagation, leading to failure.

• The test records the number of cycles to failure (Nf) and helps determine the fatigue life and endurance limit of the material.

2. Testing Procedure for Four-Point Bending Fatigue Test

Step 1: Specimen Preparation

• A flat or beam-shaped specimen is prepared according to standards such as ASTM D7264 (for composites) or ASTM E855 (for metals).

• The surface is polished to remove defects that could act as stress concentrators.

• The dimensions and material properties are recorded.

Step 2: Mounting the Specimen

• The specimen is placed on a four-point bending fixture.

• Two outer supports are positioned at a fixed span.

• Two inner loading points are applied symmetrically within the span.

Step 3: Applying Cyclic Load

• A sinusoidal cyclic load is applied at the two inner points.

• Different loading conditions can be used:

  • Fully Reversed Loading (R = -1): Equal tensile and compressive bending stress.

  • Zero-to-Maximum Loading (R = 0): Load varies from zero to peak in one direction.

  • Partially Reversed Loading: Load oscillates between two nonzero values.

Step 4: Running the Test

• The test is conducted at a controlled frequency (typically 1–10 Hz).

• The test continues until the specimen fractures or reaches a predefined cycle limit.

Step 5: Data Analysis

• A Stress vs. Number of Cycles (S-N Curve) is plotted.

• The fatigue strength and endurance limit are determined.

• Fractography (examination of failure surfaces) is performed to study the crack propagation mechanism.

3. Applications of Four-Point Bending Fatigue Test

Industries & Components Tested:

1. Automotive Industry:

   • Testing chassis frames, suspension components, and composite body panels.

2. Aerospace Industry:

   • Evaluating aircraft wings, fuselage panels, and landing gear components.

3. Civil Engineering & Construction:

   • Assessing concrete beams, bridge decks, and reinforced structures.

4. Railway Industry:

   • Testing rails, sleepers, and train bogies for fatigue resistance.

5. Biomedical Industry:

   • Evaluating bone implants and prosthetic devices for long-term durability.