A model for simulating brittle crack propagation and arrest in steel plates is presented. The model considers the influence of shear lips formed on a fracture surface acting as a crack closure force and the influence of the loss of the plane-strain condition at a propagating crack tip with an increasing dynamic stress intensity factor. The shear lips are assumed not to fracture during the test, so they will yield and provide a crack closure force. The crack closure effect depends on not only the stress intensity factor but also the crack velocity. The model was applied to temperature-gradient crack-arrest tests and ultrawide duplex crack-arrest tests. The model successfully explained anomalous crack-arrest toughness under excessively high applied stress in the temperature-gradient tests and crack arrest at a stress intensity factor far above the cleavage crack-arrest toughness in the ultrawide duplex tests.