Abstract
Abstract Fatigue failure of tungsten carbide inserts (TCI) in roller cone and hybrid drill bits represents a major limitation on durability when drilling in heterogeneous formations. This study develops a finite element-based numerical framework to model cumulative fatigue damage in WC-Co inserts under cyclic, variable-amplitude loading representative of interbedded soft–hard sequences. The approach integrates nonlinear Chaboche kinematic hardening for binder plasticity with Lemaitre continuum damage mechanics and a Fatemi–Socie critical-plane parameter to capture multiaxial stress states from Hertzian contact, torsional oscillations, and axial vibrations. Simulations reveal that crack initiation occurs predominantly subsurface near the chamfer or edge radius, driven by shear-dominated damage accumulation. In heterogeneous rock, fatigue life to initiation is reduced by factors of 2–10 relative to homogeneous conditions, primarily due to transient load spikes at layer transitions that amplify stress range and maximum stress, accelerating nonlinear damage progression. Parametric analyses demonstrate strong sensitivity to strength contrast, layer thickness ratio, and interface sharpness, with tougher grades (higher Co content, coarser WC grains) providing 1.5–3× longer life in severe heterogeneity. The modeling framework enables quantitative prediction of fatigue life reduction in realistic heterogeneous environments, bridging laboratory WC-Co fatigue data with field-relevant downhole dynamics. Results support selection of high-toughness inserts for interbedded sections and optimized drilling practices (stable WOB, reduced RPM in transitions, enhanced stabilization, and real-time MSE-vibration monitoring) to mitigate premature failure. Future extensions should incorporate thermal effects, full-bit simulations, field dull-bit validation, and probabilistic uncertainty quantification.