: Two important advances over the past 40 years enable the optimization and management of the structural integrity of components in high performance applications. First, the solid mechanics conununity established linear elastic fracture mechanics as the premier framework for modeling the damage tolerance of fracture critical components (Irwin and Wells, 1997; Paris, 1998). Second, materials scientists developed metals with outstanding balances of high tensile strength and high fracture toughness (Garrison, 1990; Wells, 1993; Boyer, 1993; Starke and Staley, 1995; Olson, 1997; Kolts, 1996). An example of achievable strength-toughness properties is provided in Fig. 1, a plot of plane strain fracture toughness vs. tensile yield strength (sigma-YS) for ultra-high strength steels (UHSS) and beta-Ti alloys precipitation hardened with a phase (Gangloff 2001). New nano-scale characterization and high performance computational methods provide for additional advances in the mechanical performance properties of structural metals. These modem alloys and analysis tools satisfy technological needs for optimization and management of component performance in demanding fatigue and fracture critical applications in the aerospace, marine, energy, transportation, and defense sectors.

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Hydrogen Assisted Cracking of High Strength Alloys

This chapter focuses on two kinds of environmental fracture, stress corrosion cracking (SCC) and hydrogen embrittlement, and discusses the role in such failures of a number of metallurgical variables. These include chemical composition; microstructural components such as precipitate type and structure, and grain size and shape; crystallographic texture; heat treatment and its effect on the foregoing variables; and processing, particularly the thermomechanical treatments (TMT), which are attracting increased attention for property optimization. These variables are expected to be of great importance in the development of new engineering materials to meet demanding service conditions.

The Role of Metallurgical Variables in Hydrogen-Assisted Environmental Fracture

6.02 – Hydrogen-assisted Cracking

Recent studies have revealed that the mechanism of stress-corrosion cracking (SCC) of high-strength Al-Zn-Mg alloys involves both dissolution and hydrogen embrittlement (HE); moreover, under tensile-loading conditions, evidence exists that the hydrogen mechanism is dominant. In the present study, the role of HE in the SCC of Al-Mg alloys was investigated using commercial Al-4.4 wt pct Mg alloy, 5083. The susceptibility of this alloy to SCC in a saline environment was evaluated in Mode I (tension) and Mode III (torsion), using precracked fracture toughness specimens. The greater susceptibility found in Mode I indicates that HE is involved in SCC. As further evidence that HE is operating, susceptibility increased when As, a hydrogen recombination inhibitor, was added to the test solution under Mode I conditions. Issues related to the overall validity of the loading mode experiment are also addressed.

The effect of loading mode on the stress-corrosion cracking of aluminum alloy 5083

A model to estimate the reduction of effective crack tip Mode III stress intensity factors by frictional and asperity interaction of an idealized fracture surface is described An extension of the model is used to calculate the Mode I stress intensity factors due to the crack tip opening displacement induced by the mismatch of the fracture surface asperities The results of calculations based on a “reasonable” fracture surface profile are used to analyze experimental studies designed to determine the relative significance of hydrogen embrittlement and crack tip dissolution in stress corrosion crack growth in Al alloys by comparison of Mode I and Mode III stress corrosion cracking (SCC) resistance It is concluded that a pure Mode III stress state is not possible for cracks with microscopically rough surfaces and that the magnitude of the induced Mode I stress intensity factor is sufficient to cause stress corrosion crack growth

Mode I stress intensity factors induced by fracture surface roughness under pure mode III loading: Application to the effect of loading modes on stress corrosion crack growth

An 18Ni (280) maraging steel has been tested in 3.5% NaCl solution under various modes of loading. The most severe condition was plane strain loading of fatigue precracked samples. The severity of stress corrosion cracking (SCC) of unnotched samples decreased in the sequence: plane strain bending, tension, plane stress bending, and torsion. The most consistent explanation for the various observations is that SCC results from hydrogen embrittlement. Under certain circumstances anodic dissolution processes can result in beneficial crack blunting effects rather than SCC failure.

H. W. Hayden

Papers

Effect of Various Modes of Loading on the Stress Corrosion Cracking of a Maraging Steel