Fatigue crack growth due to spectrum load produced by trains in a bridge

The present paper studies fatigue crack growth (FCG) produced by a load pattern obtained numerically in a simulation of trains crossing a real bridge. It uses a model where the cyclic plastic deformation is assumed to be the main damage mechanism and that cumulative plastic strain at the crack tip is the driving parameter for FCG. The accumulation of damage was found to be very irregular along each load block, the major part occurring in the overload region. Plasticity induced crack closure is relatively high due to the periodic application of overloads, playing a major role. The overload produces crack tip blunting, increasing the effective load range in subsequent load cycles. The maximum elastic load range was quantified and used to eliminate load cycles not producing fatigue damage, which is important to reduce the numerical effort. The comparison of Finite Element Model (FEM) predictions with NASGRO results, showed that this gives a non-conservative difference of 23% in the number of load cycles after 1 mm of crack growth.