PD IEC TR 62048 pdf download

PD IEC TR 62048 pdf download.Optical fibres – Reliability – Power law theory
1 Scope
This technical report provides guidelines and formulae to estimate the reliability of fibre under a constant service stress. It is based on a power law for crack growth which is derived empirically, but there are other laws which have a more physical basis (for example, the exponential law). All these laws generally fit short-term experimental data well but lead to different long-term predictions. The power law has been selected as the most reasonable representation of fatigue behaviour by the experts of several standard-formulating bodies. Reliability is expressed as an expected lifetime or as an expected failure rate. The results cannot be used for specifications or for the comparison of the quality of different fibres. This document develops the theory behind the experimental principles used in measuring the fibre parameters needed in the reliability formulae. Much of the theory is taken from the referenced literature and is presented here in a unified manner. The primary results are formulae for lifetime or for failure rate, given in terms of the measurable parameters. Conversely, an allowed maximum service stress or extreme value of another parameter may be calculated for an acceptable lifetime or failure rate. For readers interested only in the final results of this technical report – a summary of the formulae used and numerical examples in the calculation of fibre reliability – Clauses 5 and 6 are sufficient and self-contained. Readers wanting a detailed background with algebraic derivations will find this in Clauses 7 to 1 2. An attempt is made to unify the approach and the notation to make it easier for the reader to follow the theory. Also, it should ensure that the notation is consistent in all test procedures. Clause 1 3 has a limited set of mostly theoretical references, but it is not necessary to read them to follow the analytical development in this technical report. NOTE Clauses 7 to 1 1 reference the B-value, and this is done for theoretical completeness only.
3 General approach
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 “inert” 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 prooftesting. 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 prooftesting) is unimodal according to Equations (85) and (86), or bimodal according to Equation (91 ). The (m, S 0 ) pair appropriate to the desired survival probability level and length must 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.