In eddy current testing (ECT), a thin-opening crack is frequently encountered. In this paper, such a crack is modeled by using an ideal crack, in which the eddy current field perturbation due to the crack is described as the effect produced by an equivalent current dipole layer located on its surface. The dipole density can be found by solving a boundary integral equation. By following a moment method scheme, numerical predictions are made for steam generator ECT benchmark models, where a conductor plate and a tube wall, respectively, are detected. The probe impedance changes are evaluated and are compared with experiments, and good agreements are found. By using rapid algorithms to compute the dyadic Green's function, the total required CPU time is only several seconds for each problem.