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ASTM D7791 | Plastics | Fatigue Testing

ASTM D7791 describes the determination of dynamic fatigue properties of plastics in uniaxial loading.

Rigid or semi-rigid plastic samples are loaded in tension (Procedure A) and  rigid plastic samples are loaded in compression (Procedure B) to determine the effect of processing, surface condition, stress, and such, on the fatigue resistance of plastic material subjected to uniaxial stress for a large number of cycles. The results are suitable for application to design only when the specimen test conditions realistically simulate service conditions.

Before conducting ASTM D7791, it is important to read the entire specification in the relevant ASTM publication.

ASTM D7791 | Plastics | Fatigue Testing

  1. Prepare the specimens as described in the standard. Measure and record the specimen dimensions.
  2. Place the sample in the test machine, making sure the centerline of its axis is aligned with the loading axis. Place the extensometer or the compressometer on the specimen.
  3. Program the testing machine to follow a dynamic load such as sinusoidal (default), square, or trapezoidal or deflection waveform with the amplitude set to the selected load or deflection level. Set the test frequency and start the test.
  4. Continue the test until the specimen failure criterion is reached or until a predetermined number of cycles has been applied to the specimen.
  5. Plot the maximum stress level vs. the logarithm of the number of cycles to failure (S-N curve) and the maximum strain level vs. the logarithm of the number of cycles to failure (r-N curve).
Calculations:
  • Strain or Stress Level
  • Number of Cycles

ASTM D7791 | Plastics | Fatigue Testing

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