Procedure for Design Optimization and durability Life Approval of Truck Axles and Axle Assemblies

The method of fatigue life evaluation and validation testing under operational conditions is a prerequisite to achieve optimal designs with respect to weight and long-term durability of the structure considered.

Criteria like driving behavior and braking performance, influenced by global stiffness as well as fatigue life evaluation were taken into account when optimizing a forged 6.5 front axle beam of a heavy truck. The procedure for weight optimization includes the following steps:

• Stress analysis e.g. using strain gage techniques and/or Finite Element Method,
• Road load data acquisition and derivation of design spectrum describing customer usage,
• Fatigue testing under constant amplitude and/or variable amplitude loading to establish component related SN-curves and/or fatigue life curves,
• Fatigue life evaluation using damage accumulation hypothesis (Miner's Rule),
• Optimization and weight reduction (in this case 14% weight savings were achieved) based on fatigue life evaluation.
• Validation testing using multi-axial computer controlled, servohydraulic test rigs.

Since vehicle manufacturers tend to leave more of the development responsibility to the component manufacturer, the methodology described is directed to the component manufacturer and is condensed, introducing the “service strength evaluation diagram” used for design optimization. Using this technique a designer can get a good feel for the relative impact of design changes with respect to durability life under service conditions.