The Mechanical Fatigue Tool uses a slightly different method to calculate life (comapared against Ansys nCode DesignLife). For Damage, the Mechanical Fatigue Tool uses Exposure Duration in the same manner as DesignLife. Essentially, it calculates the Damage in seconds and then multiplies that result by the Exposure Duration to determine the total damage. To calculate the Life, the Mechanical Fatigue Tool takes the reciprocal of total damage (just like DesignLife), but then it multiplies that quantity by the Exposure Duration. The net effect is that Exposure Duration used in the Damage calculation is canceled out in the Life calculation and the Fatigue Life is always reported in seconds, regardless of the specified Exposure Duration. If you check the Details Window under Life, you will see that the Life is reported in seconds, regardless of the specified Exposure Duration.