An engineering methodology for the mechanical reliability of optical fiber is developed within a fracture-mechanics framework. The model expresses allowable in-service and installation stresses as a fraction of fiber strength in a fatigue environment for a range of n values and fiber types. Failure probability is incorporated into the model by the measurement of the fiber-strength distribution appropriate to the application. For long-length applications, strength distributions of hundreds to thousands of kilometers of fiber are needed. A 400-km strength distribution captures the beginnings of the truncated portion of the distribution.