Evaluating Polymer Fatigue Limits under Cyclic Mechanical Loading
Polymer fatigue limit evaluation requires separating mechanical crack growth from hysteresis heating by controlling strain amplitude, frequency, and thermal dissipation.
Strain Control Testing
Meaning
Experimental method where a material specimen is subjected to predefined, cyclic deformation limits while the resulting force or stress is measured. Conducting strain control testing is essential for evaluating low-cycle fatigue behavior in materials that experience thermal expansion and contraction during operation, such as gas turbine blades and engine pistons. This testing mode does not apply to high-cycle fatigue studies where stress levels remain well within the elastic region of the material.
Experimental Setup
Servohydraulic test machines use extensometers attached directly to the gauge section of the specimen to control the stroke. Performing strain control testing requires precise feedback loops to ensure the displacement follows a triangular or sinusoidal waveform. This system must be calibrated regularly to prevent errors.
Hysteresis Loop
Force responses are plotted against displacement to trace the energy dissipated during each cycle. Analysis of strain control testing reveals the rate of cyclic hardening or softening of the material. This loop stabilizes after a few hundred cycles.
Industrial Application
Aerospace manufacturers use these data to design turbine blades that can withstand rapid heating and cooling cycles. Utilizing strain control testing allows engineers to select alloys that resist plastic deformation under thermal stress. These tests are necessary for qualifying materials for critical safety parts.