Fatigue heat generation

ASTM D623,1999. Heat generation and flexing fatigue in compression. 

It must be emphasized that the fracture process during fatigue is very complex with its different influences like AK, R-ratio, load form, frequency, crack speed, heat generation, so that much more further investigations are needed. 

At a low frequency and low stress level, the temperature inside the polymer specimen will rise and eventually reach thermal equilibrium when the heat generated by hysteretic heating equals the heat removed from the specimen by conduction. As the frequency is increased, viscous heat is generated faster, causing the temperature to rise even further. After thermal equilibrium has been reached, a specimen eventually fails by conventional brittle fatigue, assuming the stress is above the endurance limit. However, if the frequency or stress level is increased even further, the temperature will rise to the point that the test specimen softens and ruptures before reaching thermal equilibrium. 

Thermal fatigue is observed mainly in stress-controlled loading because the strain amplitude increases in this case due to the reduction of stiffness with increasing temperature. The heat generated per cycle thus increases with time. If the loading is strain-controlled, thermal fatigue is usually not problematic because the stress decreases in this case. 

---