The compressive stress–strain relation in optical fiber, which is important for a fiber during bending, is shown experimentally to be an asymmetric continuation of the already known nonlinear relation in the tensile region. Measured strengths of optical fibers in fatigue experiments can be 50% higher because of nonlinear corrections. Consequently decreases in measured corrosion susceptibilities, up to 10%, result in estimated fiber lifetimes, which decrease by an order of magnitude. The elastic line of a fiber inserted in a two‐point‐bending testing apparatus, however, is hardly influenced by nonlinear elastic ef