Specialty Optical Fibers Handbook

of the strengths are actually lower than the proof stress, although this occurrence

is rare. Thirdly, the proof stress cannot be removed instantaneously so that

fatigue occurs during the unloading phase. As a result, the minimum ensured

strength is a little lower than s

P

. Crack growth during unloading is minor and is

suppressed by unloading from the proof stress as quickly as possible. These three

effects have been studied in detail and reliability models have been developed.

24.7.1 Minimum Strength Design

The most common and conservative approach to reliability modeling is

known as the ‘‘minimum strength design.’’ Here, the maximum applied stress

is limited to some fraction of the minimum fiber strength, usually taken to be the

proof stress. With