Dear Pablo,

For epoxy polymers, tensile and compressive strengths are different. Yield strength of epoxy polymers exhibits dependency on hydrostatic stress (pressure), which can be captured using Drucker-Prager or Mohr-Coulomb model. Since, their yield strength is a function of hydrostatic stress, fatigue life is expected to be influenced by hydrostatic stress.

If we know tensile and compressive strength, friction angle can be computed using Drucker-Prager model (https://en.wikipedia.org/wiki/Drucker–Prager_yield_criterion). This can give a fair idea about the actual variation in yield strength with respect to hydrostatic stress. However, the exact effect of hydrostatic stress on fatigue life need to be obtained using experiment for predetermined stress state.

Regards,

Chandrashekhar

Dear Pablo,

Yo will alos find interesting experimental observations and considerations about epoxy fatigue behavior in the following paper. The use of Arcan test permits a variation of the hydrostatic stress under fatigue.

https://ac.els-cdn.com/S1877705815045415/1-s2.0-S1877705815045415-main.pdf?_tid=e403c2fe-d718-11e7-9401-00000aab0f6b&acdnat=1512188945_d956e5994121fa5ca2b2f45e252d49be

Regards

Yves