Unusual behavior of long cracks at low dk: Marci effect

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A perspective on environmentally-induced cracking

The premature failure of engineering alloys in service is often associated with exposure to unintended environmental chemistry. High strength carbon steels and stainless steel will fail prematurely if exposed under tensile stress to absorbed atomic hydrogen. This is known as hydrogen embrittlement. Similarly, when aluminum alloys are exposed to liquid mercury, they are likely to crack prematurely if tensile stresses are present This is an example of liquid metal embrittlement. Silicate glass cracks in the presence of water, a phenomenon known as static fatigue of glass. Plastics fail prematurely in the presence of organic solvents. Service environments that contain soluble chlorides may lead to premature transgranular cracking of austenitic stainless steels. Interestingly, the same environments will not cause such failure in ferritic stainless steels. Likewise, caustic solutions are known to promote the premature failure of carbon steels and austenitic stainless steels. All of the above phenomena are described as environmentally-induced embrittlement or environmentally-induced cracking, EIC. These phenomena have been known for a very long time. Materials engineers are fully able to make materials selection decisions for the construction of engineering systems that see service in virtually any environment. What is also not known with certainty at this stage is the mechanism by which these examples of environmentally-induced embrittlement occur. There are multiple schools of thought regarding the mechanisms of each phenomenon mentioned above. As such, it is not uncommon to find such phenomena described broadly as stress corrosion cracking even though it is clear that corrosion is not a general prerequisite to such phenomena. It should be noted that in many cases of EIC, the alloy is virtually corrosion-free over most of its surface, including the fracture surface. As such, chemical or electrochemical dissolution has minimal if any effect on the fracture process. There has been an enormous amount of effort directed toward identifying the mechanism or mechanisms of environmentally-induced cracking, but there is wide disagreement and debate on this subject. Most involve in some fashion either the adsorption of specific embrittling species and subsequent lowering of the surface energy for fracture or localized anodic electrochemical processes such as dissolution or film formation. Our goal in this presentation is to assess the current state of knowledge of EIC in terms of the above phenomena, to identify what is known in a mechanistic sense and what remains to be understood in terms of the path forward toward a more complete mechanistic understanding

Internal stress affecting environmental fatigue of 7075-T651 alloy

Load history has been known to affect fracture and stress corrosion behavior. The degree to which it affects depends on the severity of the load history. It is known that shop peening can retard the SCC lives markedly in steels. Similarly, prestraining can reduce the KIscc and plateau velocity in high strength steels. These types of experiments are difficult to quantify their effects on the SCC behavior. One can analyze the prestarining effects in a better way by analyzing the effects of single overloads followed by constant applied load to study the behavior. Such experiments can be done by observing the ‘incubation time’ for a crack to initiate in a fatigue pre-cracked sample, at various constant applied loads in a chemical environment. Such experiments have been conducted on a 7075 aluminum alloy for both static and cyclic loads. It is observed that results are similar in behavior. The data indicates the overall behavior can be analyzed by suggesting that the total stress at the crack tip is related to the contributions from chemistry of the environment and an additional factor from “internal stress” that comes from pre-strain. Hence, we can describe the crack initiation & growth criteria in terms of: KIscc = Kapplied + Kinternal stress + Kenvironment \u3e Kthreshold Such trends in the behavior, has been observed in pre-strained steel alloys prior to environmental exposure. The general behavior suggests that the internal stress affects the threshold KIscc more than the plateau velocity. The general SCC behavior is affected by both chemistry and internal stress under external static or cyclic loads

Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and Kmax effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK-Kmax approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications

Stress corrosion cracking in Al-Zn-Mg-Cu aluminum alloys in saline environments

Copyright 2013 ASM International. This paper was published in Metallurgical and Materials Transactions A, 44A(3), 1230 - 1253, and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E a) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys (~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor