Analytical Approach to Assess the Strength and Utilization Rate of a Case-Hardened Components based on FKM Guideline

The major challenge for developing a gearbox for an electric hypercar is to confine it within a tight design space whilst carrying enormous power density and torque. This unique situation poses a design challenge for fatigue failures. The intent of the paper is to develop analytical methodology to estimate the fatigue life using Forschungskuratorium Maschinenbau (FKM) approach with 97.5 percentage survival probability for the given duty cycle. Case hardened materials are common in powertrain engineering for shafts and gears and FKM provides adequate material information to estimate the utilization. A method is derived from FKM guideline to estimate component fatigue limit for variable amplitude with mean stress correction based on the material fatigue limit. Stress gradient and stress concentration factor are accounted to estimate variable amplitude fatigue strength of the component. The basic Miner approach is used to evaluate the cumulative damage of the duty cycle. Based on the stresses from the FEM simulation, along with the fatigue limit, utilization rate is obtained with the recommended safety factors. Design optimization is performed to achieve the utilization rate target. The results show the importance of the Wöhler curve exponent. Fatigue assessment on the surface and interior point is performed to ensure the robustness of the design. Although the approach is conservative, it proved useful to develop the product for the niche segment where the cost of error is enormous.