Geometric Reinforcement Over Material Upgrading: Integrated Modal–Static–Fatigue Assessment of Large-Scale Hydrogen Cylinder Mounting Systems for Heavy-Duty Commercial Vehicles

When a hydrogen cylinder mounting frame shows signs of inadequate strength, the conventional engineering response is to specify stronger steel. This paper argues that is the wrong diagnosis. Two Q345 frames for a dual-cylinder 510 L hydrogen storage system (1,149 kg total loaded mass) were analysed in Altair OptiStruct and nCode DesignLife under the full ECE R110 inertial load envelope and the ISO 16750-3 road vibration spectrum. The only difference between the two frames was bracket geometry. The baseline frame failed on both counts that matter. Under 6.6g forward loading, stress at the bracket–frame junction reached 396.4 MPa—a yield exceedance of 14.9% for Q345 steel (SF = 0.87). Under road vibration, the longitudinal fatigue damage index reached D = 4.424, more than four times the Palmgren–Miner limit. The two failures share a cause: flat vertical support plates force load transfer through bending rather than axial tension, concentrating stress at a poorly transitioned junction (Kt ≈ 2.8) whose first natural frequency sits uncomfortably close to the dominant road excitation range. Adding diagonal bracing along the forward load axis and widening fillet radii from R = 5 mm to R = 12 mm resolved both issues without touching the steel specification. Forward stress fell to 214.8 MPa (SF = 1.61); fatigue damage fell to D = 0.004, a reduction of roughly three orders of magnitude. The scale of that improvement may initially look implausible, but it follows directly from IIW FAT 90 weld mechanics: mean stress, RMS amplitude, and stress concentration all fell together, and under the cubic S–N damage law those effects compound multiplicatively rather than adding. For this class of structure, bracket geometry appears to govern fatigue durability more decisively than the steel grade ever could.