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A.1 GeneralA.1.1 Influence of fatigue on design(1) P Structures subjected to frequently fluctuating service loads may be susceptible to failure by fatigue andshall be checked for that limit state.(2) The degree of compliance with the ultimate or serviceability limit state criteria given in EN 1999-1-1 shouldnot be used as a measure of the risk of fatigue failure (see A.1.3).(3) The extent to which fatigue is likely to govern the design should be established at the conceptual stage ofdesign. To obtain sufficient accuracy in prediction of the safety against fatigue failure it is necessary to:a) Make an accurate prediction of the complete service load sequence throughout the design life;b) assess the elastic response of the structure under the predicted loads sufficiently accurately;c) perform constructional detail design, prescribe methods of manufacturing and degree of quality controlappropriately. These issues can have a major influence on fatigue strength, and may need to becontrolled more precisely than for structures designed for other limit states. For information onrequirements to execution, see EN 1090-3.

A.1.2 Mechanism of failure(1) It should be assumed that fatigue failure usually initiates at a highly stressed point (due to abrupt geometrychange, tensile residual stress or sharp crack-like discontinuities). Fatigue cracks will extend incrementallyunder the load of cyclic stress change. They normally remain stable under constant load. Failure occurs if theremaining cross section is insufficient to carry the peak applied load.(2) It should be assumed that fatigue cracks propagate approximately at right angles to the direction ofmaximum principal stress range. The rate of propagation increases exponentially. For this reason crackgrowth is often slow in the early stages, and fatigue cracks tend to be inconspicuous for the major part of theirlife. This may give rise to problems of detection in service.A.1.3 Potential sites for fatigue cracking(1) The following initiation sites for fatigue cracks associated with specified constructional details should beconsidered:a) Toes and roots of fusion welds;b) machined corners;c) punched or drilled holes;d) sheared or sawn edges;e) surfaces under high contact pressure (fretting);f) roots of fastener threads.