Bolt Load Relaxation and Fatigue Prediction in Threads with Consideration of Creep Behavior for Die Cast Aluminum

Bolt load retention is an important consideration in developing highly loaded structures with die cast aluminum due to its creep behavior. During development of a new cylinder block design, an objective was established to optimize the main bearing bolted joint design for bolt load retention. A creep model was developed from literature data, applied using detailed thread sub-models, and calibrated to produce results in good agreement with observed bolt load loss. This creep model was applied in sensitivity studies to investigate the effect of variation in thread engagement and installation load on bolt load loss. Selected thread sub-models were used with and without creep considerations to estimate high cycle fatigue safety in the bulkhead thread roots of a highly loaded cylinder block. Results from the investigation demonstrate that bolt clamp load loss due to creep can be simulated, and sensitivity studies can provide practical design guidance. Further, including the effects of creep can provide estimates for stress relaxation in threads, and its influence in fatigue predictions is assessed.