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Single-fiber fragmentation tests were done on AS4 carbon fiber/epoxy and E-glass/epoxy specimens. Using a new interpretation of the photoelasticity fringes around fiber breaks we measured debonds that occurred instantaneously after each fiber break. The new techniques led to measured debond lengths that were longer than in prior studies. An energy balance analysis of the debond size when the breaks are far apart was used to investigate the interfacial fracture toughness. The best analysis was one that accounted for both residual stress effects and interfacial friction. It was not possible to determine all effects by debonding experiments alone and thus the most accurate results for interfacial toughness require supplemental experiments such as Raman spectroscopy or additional fragmentation observations. The best estimate for interfacial toughness was 220 J/m2 for carbon-fiber/epoxy and 120 J/m2 for glass-fiber/epoxy.