DOE–RSM based multi-objective geometry optimisation of a heavy-duty ladder chassis under fatigue safety constraints

A DOE–RSM-based multi-objective optimisation framework was developed using ANSYS Mechanical simulation results. The optimisation simultaneously minimised deformation and structural mass while maximising the fatigue safety factor (FoS) through optimisation-function approach (desirability function), using side rail thickness (X ₁ ), cross-member thickness (X ₂ ), and side rail height (X ₃ ) as design variables. A Central Composite Design (CCD) has been used to establish second-order regression models representing the design space. Response surface models were developed and evaluated for statistical significance using ANOVA, while predictive accuracy was independently verified through ANSYS validation simulations at the optimised design conditions. The subsequent models developed exhibited high predictive capability, with coefficients of determination exceeding 98% and validation errors below 5% when compared with ANSYS validation FEA simulations. The outcomes highlighted that side rail height played a major role in governing total deformation and fatigue behaviour through its influence on bending stiffness, whereas variations in thickness of the frame primarily affected the structural weight. HSLA steel exhibited approximately 68% lower deformation (7.13 mm) and a fatigue safety factor of approximately 2.45. Meanwhile, aluminium succeeded in about 67% weight reduction but could not meet the required fatigue safety criterion (FoS > 1) for the loading cases considered.