The effect of water and isooctane absorption on the fatigue behavior of random fiber sheet molding compounds is investigated by two approaches.

In the engineering approach, S-N curves for SMC-R30 and SMC-R65 are established under three environmental conditions:​ ambient air, water and isooctane saturation. Water reduces the fatigue strength of SMC-R independently of maximum stress and mean stress. Consequently, a modifying factor is suggested to predict the fatigue strength of SMC-R30 and SMC-R65 under any moisture condition, from their performance in ambient air. On the other hand, isooctane reduces by an irregular manner the fatigue strength of SMC-R.

In the analytical approach, three dimensional microstresses in a short fiber model as a function of loading direction are calculated. Results are used to analyse the fatigue failure mechanisms of SMC-R under two moisture conditions:​ dry and water saturation. For a uniform composite strain, the loading direction making an angle of 30$\sp\circ$ with the longitudinal axis of the group of fibers produces the highest shear stress at the interface between the group of fibers and the matrix. Fiber density, direction of the group of fibers and composite strain are three factors which control the failure mechanisms. Debonding of the group of fibers from the matrix that are caused by water absorption can influence the propagation of the microcracks.