THE EFFECT OF PRESSURE-OVERLOAD ON FATIGUE-CRACK GROWTH IN PRESSURE-VESSELS

The pressure-overload method for proof-testing pressure-vessels was in vestigated for hydrofilters made of an Al-alloy. After a vessel had be en overloaded using a hydrostatic fluid, the fatigue crack growth rate of the material was studied at three levels of hydrodynamic pressure. The period of fatigue crack growth N-p and the uniaxial equivalent st ress sigma(e) were calculated on the basis of fractographic analyses f or various combinations of the hydrostatic overload and the hydrodynam ic pressure. Increasing the overload level increased N-p, and decrease d sigma(e). For a pressure overload of 3 times the normal pressure the initiation of a fatigue crack did not occur after 2 x 10(6) cycles of loading at an hydrodynamic pressure of 20.6 MPa. Striation spacing co uld not be correlated against the value of the hydrodynamic pressure a fter an overload of 2.5 times the normal pressure. A unified relations hip between the durability N-d and the ratio N-p/N-d (=N-p(d)) at diff erent hydrodynamic fluid pressures was demonstrated.