Rising Step Load Testing of AA7085-T7452

Abstract

Abstract : The phenomena of environmentally assisted cracking (EAC) can be described as stable, subcritical crack growth that occurs at stress intensities below the fracture toughness of a material due to the combined actions of stress and corrosion. Much as the same way fatigue performance is considered in the presence of alternating loads, a material's susceptibility to EAC must be considered in material selection when the possibility of corrosion exists. EAC performance can be evaluated in terms of crack initiation resistance using smooth specimen testing, resulting in limiting loads as a function of failure time. This information can be used much in the same way S-N curves are used in to design against fatigue. However, the subject of this study involves test methods that support the damage tolerance approach to lifetime prediction, and utilize pre-cracked specimens. As in fatigue crack growth testing, the threshold stress intensity at which EAC initiates from a sharp crack can be measured (Kth). In addition, the rate of environmentally assisted crack growth (V) as a function of applied stress intensity (K), can be obtained and are often referred to as V-K curves. These are analogous to the crack growth rate (da/dN) versus applied stress intensity range (deltaK) curves observed in the fatigue literature. As new and corrosion resistant alloys and alloy conditions become available, there is a need for testing procedures that clearly characterize and compare EAC performance. Due to the simplicity and low cost of crack initiation type testing, alloy manufacturers primarily provide data of this type. However it is not clear that EAC crack initiation performance for smooth specimens is a reliable indicator of material performance where the possibility of sharp defects exists. In addition, a high resistance to crack initiation does not necessarily imply slow propagation rates once EAC has initiated.