PSI - Issue 4

S. Beretta et al. / Procedia Structural Integrity 4 (2017) 64–70

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S. Beretta / Structural Integrity Procedia 00 (2017) 000–000

Fig. 7. Resultant processed images by the system.

process. In detail, a lognormal process has been adopted for the small crack growth under corrosion fatigue according to the model by Moretti (2013). For describing the phase when cracks become su ffi ciently long (corresponding to ∆ K > ∆ K th ), fatigue crack growth has been described by the NASGRO propagation equation, by taking ∆ K th as a lognormal random variable as suggested by Beretta and Carboni (2006). An example of the prospective growth rate corresponding to a few simulations is shown in Fig. 6.a The crack growth models have been incorporated into a Monte Carlo software suite able to simulate corrosion fatigue crack propagation under a randomized service stress spectrum starting from initial crack sizes equal to d p − t − c . The activity has been concentrated onto a fine tuning of the parameter B in Eq (1) in order to correctly reproduce a series of measurements of corrosion-fatigue damage obtained on full-scale axles subjected to constant amplitude loading. The result of the simulation tools is shown in Fig. 6.b, where the simulated distribution of surface cracks perfectly matches the crack detected on a full-scale axle made of A4T.

Table 1. Probabilistic model . phase

equation

parameter

description

pit-to-crack transition diamater

d p − t − c

gaussian

µ ptc , σ ptc

dl dN = B · ∆ σ

β · l n

small crack growth (corrosion-fatigue)

lognormal process

B

fatigue crack growth

Nasgro propagation equation

∆ K th

lognormal random variable

4. Concluding remarks

The novel method for estimating the residual service life of a corroded axle, developed with the RAAI project, is the combination of new automated scanner plus a new software tool able to accurately reproduce results so far obtained on full-scale corrosion-fatigue experiments. The combination of surface damage quantification (observations + surface measurements of the distribution of pits / cracks) and life predictions has been successfully applied in trials for assessing series of axles exposed to corrosion during their service. The results (in terms of residual service lifetime), that could not be obtained in previous projects, look to be interesting for for new NDT services and maintenance planning.