Fatigue limit assessment on seamless tubes in presence of inhomogeneities: Small crack model vs. full scale testing experiments

The existence of crack-like flaws cannot be excluded in pressure vessels and piping due to the presence of non-metallic inclusions and surface imperfections. These inhomogeneities reduce the fatigue strength of the pressure vessel. In this work, the detrimental effect of the surface quality on the fatigue strength of cylinder housings is analyzed by means of a fatigue model available in the literature using values obtained by crack propagation tests. The comparison in terms of fatigue limit between model and experimental full scale (FS) tests has shown an underestimation of the fatigue strength if the prediction based on ΔKth,LC values for long cracks is used. This is due to a flattening effect of the ΔKth,LC values at high stress ratios R under full scale tests that the standard crack propagation tests do not anticipate. Further investigations need to be carried out for more realistic prediction by the mentioned model. In order to obtain an accurate and more realistic fatigue limit, an alternative small scale (SS) fatigue test has been implemented. The goal was to reproduce as much as possible the real condition of a full scale (FS) state during the small scale (SS) fatigue limit test; and this has led to a good agreement between the FS and SS fatigue limit tests.