Meaning
Progressive structural degradation of threaded tension rods under cyclic mechanical loads leads to sudden mechanical failure of bolted battery modules. Engineers analyze tie-rod fatigue to ensure that the structural retention system can withstand the expansion forces generated by the cells during charge and discharge cycles. The cyclic swelling of the cells acts as a repeated tensile load on the rods.
Stress Concentration
Threaded regions and transitions in the diameter of the rod act as stress raisers where micro-cracks can initiate. In service, tie-rod fatigue is accelerated by thermal expansion and contraction of the aluminum battery module housing. Applying a precise initial torque during assembly helps to keep the cyclic stress amplitude below the fatigue limit of the material.
Material Degradation
High-strength steel or titanium alloy rods are selected for their resistance to cyclic loads and environmental stress corrosion. Surface finishes and coatings are applied to prevent corrosion, which would otherwise drastically reduce the resistance of the rod to tie-rod fatigue. Routine maintenance protocols for heavy-duty battery packs include the non-destructive inspection of these tension members.
System Failure
Complete fracture of a tension rod can result in the rapid decompression of the cell stack, leading to electrical disconnection or cell swelling. The loss of clamping force associated with tie-rod fatigue allows the individual cells to shift, increasing the risk of short circuits from vibration. Module designs often incorporate redundant tension members to prevent catastrophic collapse of the assembly.
