The calculated life for random vibration fatigue is not affected by the exposure duration, because the Mechanical Fatigue Tool always reports the life in seconds, regardless of the specified exposure duration. The damage is reported as damage per exposure duration, but the life is reported in seconds. If you check the Details Window, you will see that the life is reported in seconds (unit is greyed out and cannot be changed). To calculate the fatigue life, the Mechanical Fatigue Tool takes the reciprocal of total damage, but it then multiplies that quantity by the specified Exposure Duration. The net effect is that Exposure Duration used in the damage calculation is canceled out in the life calculation. Note: This method is not consistent with the method used by ANSYS nCode DesignLife. DesignLife reports both the life and damage in terms of the exposure duration.