This paper presents a fundamental study for the characterization of formability in bending/hemming, or “bendability/hemmability,” of automotive aluminum alloys. Based on the strain/stress nature of the hemline (bent corner) surfaces, maximum surface strain was proposed as the hemming fracture criterion, and it is believed that bendability/hemmability of aluminum alloys can be directly approximated by the fracture strains from plane-strain tensile tests. The criterion was verified by comparing hemming and plane-strain tensile experiments on AA6111-T4 and AAx611-T4 with fracture strains measured, respectively, via a modified angled line method and thickness reductions. After a verification of material hardening laws, the identified failure criterion was implemented into 2D and 3D finite element hemming simulations for fracture predictions. The findings provide the basis for the design of aluminum hems under formality constraints, and it is expected that the fundamental theory and established methodology are applicable to bendability/hemmability evaluations for general sheet materials.