Structural and Dynamic Finite Element Analysis of Ladder-Type Chassis Frames: A Comparative Study of Structural Steel and A710 Steel for On-Highway and Off-Highway Vehicle Applications
Abstract - Ladder-type chassis frames remain the dominant load-bearing architecture for commercial, off-highway, and military vehicle platforms because of their high load-carrying capacity, modularity, and structural robustness. Field observations, however, indicate that a significant proportion of in-service chassis failures occur despite computed stresses remaining within allowable static limits, implicating dynamic phenomena — particularly resonance and vibration-induced fatigue — as the dominant failure mechanism in on-highway commercial vehicles. This work presents an integrated, system-level finite element investigation of two independent ladder-type chassis configurations: (i) an on-highway commercial chassis rated for a 12.5-tonne payload, evaluated through static structural, modal, harmonic response, and fatigue analyses, and (ii) an off-highway military-type chassis integrated with axle and leaf-spring suspension components, evaluated through modal and static structural analysis under a 2500 kg distributed payload and gravitational loading. Both models were discretised using hexahedral-dominant 3D solid elements (~630,000 nodes, ~830,000 elements) with bonded contact idealisation, and were benchmarked against two candidate materials — conventional Structural Steel (E = 200–210 GPa, σy = 250 MPa) and A710 high-strength low-alloy steel (E = 205 GPa, σy = 415 MPa). Analytical validation of the fundamental natural frequency using Dunkerley's and Rayleigh's methods showed agreement with the FE-predicted values to within 3%. Results show that the off-highway chassis exhibits stable global bending and torsional modes below 18 Hz and remains within the elastic regime under worst-case static loading, confirming adequate structural robustness. For the on-highway chassis, A710 Steel reduced peak deformation and von-Mises stress by approximately 20% relative to Structural Steel, shifted the first three natural frequencies away from the 2000–2200 rpm engine operating band, reduced harmonic resonance amplitude by up to 80%, and improved minimum fatigue life at the critical leaf-spring mounting zone from 7,160 to 19,871 cycles (a 177% improvement), raising the minimum fatigue safety factor from 0.453 to 0.505. These findings demonstrate that static analysis alone is insufficient to predict real-world ladder-chassis failures, and that a combined static–modal–harmonic–fatigue FE framework, supported by analytical cross-validation, provides a reliable and economical basis for material selection and durability-driven chassis design. A710 Steel is identified as a viable drop-in material substitution capable of eliminating resonance-driven fatigue failures without any geometric redesign. Key Words: Ladder Chassis, Finite Element Analysis, Modal Analysis, Harmonic Response, Fatigue Life, Resonance, A710 Steel, Structural Steel, Dunkerley's Method, Rayleigh's Method.