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This work presents a novel membrane deflection fracture experiment (MDFE) to investigate the fracture toughness of microelectromechanical systems (MEMS) and other advanced materials in thin film form. It involves the stretching of freestanding thin-film membranes, in a fixed-fixed configuration, containing preexisting cracks. The fracture behavior of ultrananocrystalline diamond (UNCD), a material developed at Argonne National Laboratory, is investigated to illustrate the methodology. When the fracture initiates from sharp cracks, produced by indentation, the fracture toughness was found to be 4.5±0.25 MP m12/. When the fracture initiates from blunt notches with radii about 100 nm, machined by focused ion beam (FIB), the mean value of the apparent fracture toughness was found to be 6.9 MPa m12/. Comparison of these two values, using the model proposed by Drory et al., provides a correction factor of two-thirds, which corresponds to a mean value of ρ/2x=1/2.