Fatigue fracture surface morphology for alloy 718

Abstract

Fatigue crack growth rate testing of Alloy 718 was conducted in room temperature air and the fracture surface morphology was characterized to establish operative cracking mechanisms. Crack growth rates exhibit a bilinear power-law relationship with a transition to lower cracking rates occurring at a stress intensity factor range (ΔK) of 18 MPa√m. This transition is associated with a deformation mechanism transition from homogeneous to heterogeneous slip. Three types of fracture surface morphologies are observed and the operative cracking mechanism is dependent on ΔK. A faceted growth mechanism is operative at ΔK levels below 18 MPa√m, striation formation is dominant between 18 and 70 MPa√m, and a combination of striation formation and dimple rupture is dominant above 70 MPa√m. Detailed descriptions of these mechanisms and the conditions responsible for the transition from homogeneous to heterogeneous deformation are provided.