Some complex microstructured fibers (MSFs) are well known to produce poor-quality cleaves or even to break at cleavage. But to find widespread use in photonics technology, MSFs will have to be easily cleavable using mechanical cleavers, since more sophisticated techniques add complexity. In this paper, the very different, yet reproducible cleavage patterns of three high air-fraction, double-clad microstructured fibers are analyzed. Fracture faces reveal the fracture propagation paths and provide measurements of the fracture lengths in the intercapillary bridges. These lengths prove to be always shorter than the critical fracture length predicted by fracture mechanics. A criterion based on critical fracture length is thus proposed to design cleavage-robust MSFs.