Meaning
Fatigue life predictions under complex multiaxial strain states evaluate combination shear and normal strains on critical material planes during mechanical vibration testing. The analytical model known as the Brown-Miller criterion formulates fatigue damage accumulation in ductile metallic materials subjected to out-of-phase cyclic loading. In battery pack structural engineering, the Brown-Miller criterion calculates cumulative damage in vehicle subframes, module retaining straps, and busbar connections exposed to simultaneous multi-axis road vibration.
The formulation governs maximum shear strain amplitude combined with normal strain acting on the plane of maximum shear. The criterion stops applying to brittle materials dominated by pure tensile fracture or applications operating exclusively within the linear elastic high-cycle fatigue regime without cyclic plastic strain.
Plane Selection
Critical plane algorithms scan all spatial orientations at a stress-strain location to locate the plane experiencing maximum shear strain range. The formulation asserts that cyclic shear strain drives fatigue crack initiation while normal strain opens crack faces and accelerates growth. Physical orientation of maximum damage changes as out-of-phase loading ratios alter principal stress angles over time.
Numerical implementation tracks strain histories along candidate planes to identify the maximum damage value.
Strain Integration
Combining shear strain range with normal strain via a material sensitivity coefficient yields an equivalent shear strain amplitude. This equivalent amplitude correlates directly with uniaxial fatigue strain-life curves to forecast cycles to crack initiation. Material constants calibrated from simple tension and torsion fatigue tests establish the slope of the fatigue life curve.
Battery pack enclosure designers utilize this calculation to evaluate welded joints and structural ribs under random road input profiles.
Verification Protocol
Validating multiaxial fatigue calculations requires physical shake table testing of battery structures under simulated durability road profiles. Strain gage rosettes mounted on high-stress enclosure nodes provide empirical strain tensor histories for comparison against finite element models. Quality assurance protocols require damage accumulation predictions to remain within conservative bounds established by physical testing.
Discrepancies between predicted and actual crack initiation locations indicate incorrect boundary constraint modeling or uncaptured mean stress effects.
