Why static yield criteria consistently fail to predict cyclic fatigue failure in structural weldments, and how critical plane algorithms (Findley, Dang Van) deliver accurate life cycle estimates.
1. Why Static Stress Criteria Fail Under Dynamic Cycles
The von Mises equivalent stress is an invariant of the deviatoric stress tensor derived strictly for monotonic yielding in ductile isotropic metals. Because von Mises is a scalar magnitude without sign or directional phase, it ignores:
- • Hydrostatic hydrostatic mean stress effects that accelerate micro-crack opening.
- • Non-proportional out-of-phase loading where principal stress axes rotate dynamically during each vibration cycle.
- • Distinct shear and normal stress amplitudes along the critical grain boundary plane.
2. Critical Plane Formulations: The Findley Criterion
To overcome this, CurlVee implements critical plane algorithms that sweep the 3D unit sphere to identify the exact plane θ,φ experiencing the maximum damage combination of alternating shear stress and peak normal stress:
Where Δτ is the alternating shear stress amplitude, σ_n,max is the maximum normal tensile stress acting perpendicular to the crack plane, and k is the material sensitivity factor calibrated from pure torsion and bending tests.