Fitting of an FCG test of the WE43C and AA7050 alloys using two phase-fields and a Constant- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si421.svg" display="inline" id="d1e1051"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>K</mml:mi> <

Fatigue crack growth critically influences the lifespan of structural components in high-demanding engineering applications. Despite advances in phase-field fracture models, cycle-by-cycle simulations remain computationally prohibitive and often rely on extrapolation techniques. This work introduces a novel energy-based fatigue degradation evolution equation within a phase-field framework, enabling direct recovery of the Paris-law behavior without the need for explicit cycle-jumping algorithms. We implement a staggered solution scheme and employ a constant- Δ K loading procedure to compute crack growth rates in selected stress intensity ranges. The proposed strategy is effective in calibrating simulation parameters, resulting in a reduction of up to 97% in simulated cycles. Additionally, we utilize an automatic crack length measurement algorithm based on the A* pathfinder heuristic, which minimizes user intervention and mesh dependence. Validation with experimental data for the WE43 and AA7050 alloys shows excellent agreement in the Paris plots, while reducing computational costs. The proposed methodology offers a robust and efficient tool for material characterization and fatigue analysis in brittle-to-ductile materials. • Thermodynamically consistent phase-field model using two scalar variables for damage and fatigue. • Efficient constant- Δ K methodology to calibrate phase-field parameters and recover Paris law behavior with minimal computation. • Automated crack length measurement algorithm using path finding heuristics in finite element meshes. • Validated model for WE43 magnesium and AA7050 aluminum alloys with very good agreement with experiments.