Fatigue crack growth rate in ultrafine-grained Al 5483 and 7475 alloys processed by hydro-extrusion

Fatigue crack growth tests are performed to determine materials service life using standardized geometry of samples and test procedures, which are difficult to fulfill in the case of ultrafine-grained metals, such as obtained by hydro-extrusion. This calls for the use of mini-samples. However, mini-samples require a special approach to displacement measurements.The aim of the present study was to demonstrate the possibility of fatigue crack growth rate tests with mini-samples made of Al 5483 and 7475 alloys in as delivered states and subjected to the grain size refinement by hydro-extrusion. The optical Digital Image Correlation, DIC, was used to this end. The results of the measurements were transformed into maps of displacements near to the crack tip registered at the maximum load for pre-selected loading cycles. The displacement fields were subsequently used to determine stress intensity factor values, positions of the crack tip and to develop Paris plots. The results allowed to compare fatigue crack growth rates in standard and ultrafine-grained aluminum alloys.