This work examines the fatigue crack-arresting capability of thin PEI and PVDF thermoplastic interlayers in short-glass fiber-modified epoxy and core-shell rubber-modified epoxy materials. Singleedge notch bending epoxy specimens with and without interlayers are fabricated and subjected to mode-I fatigue loading under increasing ∆𝐾 and constant ∆𝐾 conditions. In the PVDF interlayered epoxy materials, the fatigue crack is initially accelerated and then shielded for at least 2×10 6 cycles, thanks to crack-bifurcation at the epoxy-thermoplastic interface.