PD IEC/TR 62048:2014 pdf free.Optical fibres 一Reliability 一 Power law theory.
First, the equivalence of the growth of an individual crack and its associated weakening is shown. This is related to applied stress or strain as an arbitrary function of time. Applied stress can be taken to fracture, from which the lifetime of the crack is calculated. Next, the destructive tests of static and dynamic fatigue are reviewed, along with their relationship to each other. These tests measure parameters useful in the theory. This also shows the difference between 9nert strength and dynamic strength.
The above single-crack theory is then extended to a statistical distribution of many cracks. This is done in terms of a survival (or failure) Weibull probability distribution in strength. It can allow for several deployment geometries in testing and service. The inert distribution and the distributions obtained by static or dynamic fatigue testing are derived for before and after proof-testing. The latter is sometimes done with approximations that may not require knowing the B-value explicitly. Finally, the various parameters measured by the above testing are related to formulae for fibre reliability, that is, lifetime and failure rate.
Some of the main assumptions in the development are as indicated below.
— The relationship between the stress intensity factor, applied stress, and flaw size is given by Equation (29): while at fracture, the relationship between the critical stress intensity factor, strength, and flaw depth is given by Equation (30).
— The crack growth velocity is related to the stress intensity factor by Equation (32).
— The Weibull distribution of stress (before any proof-testing) is unimodal according to Equations (85) and (86), or bimodal according to Equation (91). The (m, S0) pair appropriate to the desired survival probability level and length shall be used. Deployment lengths will differ upon the application such as fibre on reels, in cable, splice trays, or within a connector or other component. Because of the low failure probabilities desired, however, the low-strength extrinsic mode must usually be used.
— The values of the fatigue parameters, both static and dynamic, depend upon the fibre environment, fibre ageing and fibre preconditioning prior to testing. In theory, they are taken to be independent of time, so that some engineering judgement is needed to decide the practical values to be used in the calculations. This also implies that the corresponding static and dynamic fatigue parameters equal each other (for the same environment and time duration).
— Zero-stress ageing is not accounted for. Since the above parameters are independent of time, the strength decreases due only to stress fatigue following the power law according to 8.1.
5 Formula types
The formulae utilize parameters obtained from fatigue testing-to-failure, and from proof-testing with potential random failures. In the service condition of interest, a fibre of effective length L (dependent upon deployment geometry) is subjected to a constant applied service stress that does not change with time. (This stress is tensile, including bending stress. Torsional or compressive stresses are not covered.) The lifetime as a function of failure probability or failure rate as a function of time are given.PD IEC/TR 62048 pdf download.