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Cohesive Zone Modelling for Fatigue Applications
Published in Raul D.S.G. Campilho, Strength Prediction of Adhesively-Bonded Joints, 2017
A. Pirondi, G. Giuliese, F. Moroni
The exponent d and the coefficient B depend on the material, temperature, stress ratio R = Pmin/Pmax of the cycle, and frequency (Russel and Street 1987, 1988). An accurate and efficient prediction of fatigue crack growth allows to adopt a “damage-tolerant” design philosophy, i.e. the component or structure may be safely operated even in the presence of some damage up to a limit value before a structure repair or replacement. In other words, a crack may grow in service, but it will not reach critical size before its detection. Essential ingredients of this approach are the knowledge of crack propagation as related with applied loading, and periodical inspections with a frequency ensuring that undetected damage in one inspection will not grow up to critical size before the next inspection. This approach leads to weight savings but also to increased maintenance costs, particularly those related with periodical inspections. Actually, a fail-safe design may not always possible and therefore the “safe-life design” approach must be used. This design philosophy is instead based on the intention of avoiding fatigue crack initiation during the entire lifetime.
Estimation of the Structural Reliability for Fatigue of Welded Bridge Details Using Advanced Resistance Models
Published in Structural Engineering International, 2021
Davide Leonetti, H.H. (Bert) Snijder, Johan Maljaars
The safety assessment of bridge structural details for fatigue failure, requires an appropriate resistance model representing the structural detail under consideration. Concerning bridge infrastructures, resistance models based on both S–N curves and fracture mechanics have been widely applied, either to show that the design life can be reached or to determine the safety status of such details. An S–N curve describes the empirical relationship between the applied stress range and the number of cycles to failure of a certain structural detail. The European standard EN 1993-1-9, Eurocode 3,1 recommends a fatigue resistance model based on S–N curves characterizing the fatigue resistance under constant amplitude (CA) and variable amplitude (VA) loading. The characteristic CA S–N curve is derived from laboratory tests and presents the finite life region and the fatigue limit, i.e. the stress range value assumed as the threshold for fatigue failure under CA loading, see Fig. 1. For VA loading, the CA S–N curve is extended below the fatigue limit using a log-log linear relationship with a slope modified according to the findings of Haibach.2 The Palmgren-Miner linear damage rule is used to consider the effect of VA loading, see Fig. 1. In Eurocode 3, partial factors are recommended to be applied that should be multiplied with to the characteristic values of strength to determine the design value based on the design type and consequences of failure. For a safe-life design two values of the partial factors, , are recommended: = 1.35 for high consequences of failure, = 1.15 for low consequences of failure.