Meaning
Structural fracture starting at the bottom or root of a welded joint propagates upward through the weld metal when subjected to mechanical stress. Preventing a weld root tear is essential during the design and assembly of battery casings, as it leads to a complete loss of hermetic sealing. This failure occurs when the concentrated strain at the weld root exceeds the ultimate tensile strength of the cast alloy.
It is a common result of insufficient penetration or high tensile stress.
Physical Mechanism
Loading forces acting on the joint concentrate at the sharp internal notch formed by incomplete weld penetration or a poor root profile. When these forces exceed the local strength, a microcrack initiates at the root of the weld and grows quickly toward the face of the bead. This crack propagation is accelerated by the presence of brittle intermetallic phases or micro-porosity within the fusion zone.
The result is a clean separation of the joined parts under tension or bending.
Structural Hazard
Sudden loss of the hermetic seal allows the internal electrolyte to leak out and expose the battery compartment to corrosive liquids. This leakage can cause electrical short circuits across adjacent cells and increase the risk of thermal runaway. It also allows moisture to enter the cell and degrade the chemical performance of the active materials.
Mitigation Method
Optimizing the laser welding parameters to ensure full root penetration and a smooth profile reduces the strain concentration.
