AbstractHydraulic components in mobile machines are subjected to varying conditions and loads, which limit their lifetime. In particular, high pressure drop rates are suspected to increase fatigue crack growth rates. Existing studies on the fluid‐structure interaction inside fatigue cracks during fast pressure drops do not explain increased crack growth in hydraulic systems. A laminar flow model was developed to simulate the fluid flow inside a crack under periodic pressure loads. The viscosity restrained the oil inside the crack, and the mechanical resistance of the closing crack led to a high pressure increase. Knowledge of the pressure profiles inside fatigue cracks helps to estimate the effective fatigue damage during transient loads.