Applied Stress Affecting the Environmentally Assisted Cracking

Abstract

Stress corrosion cracking (SCC) is affected by the mode of applied stress, i.e., tension, compression, or torsion. The cracking is measured in terms of initiation time to nucleate a crack or time to failure. In a simple uniaxial loading under tension or compression, it is observed that the initiation time can vary in orders of magnitude depending on the alloy and the environment. Fracture can be intergranular or transgranular or mixed mode. Factors that affect SCC are solubility of the metal into surrounding chemical solution, and diffusion rate (like hydrogen into a tensile region) of an aggressive element into the metal and liquid metallic elements in the grain boundaries. Strain hardening exponent that affects the local internal stresses and their gradients can affect the diffusion kinetics. We examine two environments (Ga and 3.5 pct NaCl) for the same alloy 7075-T651, under constant uniaxial tension and compression load. These two cases provide us application to two different governing mechanisms namely liquid metal embrittlement (7075-Ga) and hydrogen-assisted cracking (7075-NaCl). We note that, in spite of the differences in their mechanisms, both systems show similar behavior in the applied K vs crack initiation time plots. One common theme among them is the transport mechanism of a solute element to a tensile-stress region to initiate fracture.

Peridynamic modeling of pitting corrosion damage

“…As evidenced by recent experiments, corrosion affects a subsurface region below the liquid/solid interface [16][17][18]. For example, while there are no obvious surface changes due to corrosion, a steel structure can become brittle and fail catastrophically, without going through the safer, necking deformation phase [16].…”

A comprehensive analysis of the growth rate of stress corrosion cracks

“…There may be an effect of stress on the AD rates [63], but this has not been considered. Further investigation is required for the crack growth rate near the transition point; however, the authors believe that the two mechanisms are equally important in that region because of faster diffusion through the grain boundary [64] and the existence of phases which require less work to fracture, as modelled by Novak et al [27].…”

Effect of Elastic-Plastic Stress in the Defect Tolerance Under Stress Corrosion Cracking

“…Figure 2.13: Schematic of adsorption-induced in cohesion mechanism. [28] On the other hand, Fontana [3] and Vasudevan [33] support the stress assisted dissolution shown in figure 2.14, where dissolution occurs on freshly created surfaces, as crack tips, when the passive films are broken by local plastic deformation. In this theory, the crack propagates by the solution of the solid in the liquid under the influence of an applied stress, with volume diffusion of the dissolved solute through the liquid controlling the propagation.…”

“…[33] complex phenomenon, and researchers do not arrive at unifying principles governing the embrittlement process, there are important differences in the characteristics of LME that exist from one solid metal liquid metal system to another and different results from one author to another can be found in the literature. However, some parameters have been appointed to have influence in the LME phenomenon as detailed below.…”

“…For the specific Al-Ga pair, Vasudevan [33] states that Ga transport is not stress dependent and Joseph et al [29] states that the embrittlement of aluminum by Gallium can occur without any stress, however for both cases, the phenomenon considered is not the LME.…”