Abstract: A computer-implemented method for designing a mechanical product formed in one or more materials including obtaining inputs including a finite element model representing the mechanical product and having an initial value of design variables, one or more load cases, a corresponding load history for each load case, boundary conditions, fatigue properties of the one or more materials, and a fatigue calculation scheme. The method further includes computing a distribution of fatigue damage over the finite element model based on the inputs, computing a distribution of sets of fatigue damage sensitivities over the finite element model based on the computed distribution of fatigue damage, each fatigue damage sensitivity of each set approximating a derivative of the fatigue damage relative to a respective design variable, and updating the value of the design variables based on the fatigue damage sensitivities.