ASTM D7774 describes the determination of dynamic full reversed fatigue properties of plastics in flexure.
Rigid or semi-rigid plastic samples are loaded in three-point (Procedure A) or four-point (Procedure B) bending configurations to determine the effect of processing, surface condition, stress, and such, on the fatigue resistance of plastic material subjected to flexural 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 D7774, it is important to read the entire specification in the relevant ASTM publication.
Prepare the specimens as described in the standard. Measure and record the specimen dimensions.
Place the sample on the three-point bend fixture if Procedure A is being followed and on the four-point bend fixture if Procedure B is being followed. Make sure that the load is being applied directly at the midpoint of the bend fixtures before starting the tests.
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.
Continue the test until the specimen failure criterion is reached or until a predetermined number of cycles has been applied to the specimen.
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).