Sub-critical flaw growth.

: 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

Some aspects of environment-enhanced fatigue-crack growth

The kinetics of sustained-load subcritical crack growth in hydrogen were determined for 18Ni(200) and 18Ni(250) maraging steels over a range of hydrogen pressures and temperatures. Crack growth in each steel was characterized by an apparent threshold stress intensity, a domain where the growth rate increased sharply with stress intensity (K) (Stage I), and a range where the growth rate was independent ofK (Stage II). The rate-limited Stage II crack growth in these steels exhibited three distinct regions of temperature dependency, with a different isothermal pressure dependence in each region. In the low temperature region, Stage II crack growth was thermally activated with δH = 18.2 ±1.7 kj/mol; (δH being independent of hydrogen pressure and yield strength). The growth rates at a givenK were proportional to the square root of hydrogen pressure. In the intermediate temperature region, Stage II growth rates increased at slower rates, passed through a maximum and then decreased with increasing temperature. Within this region, the pressure dependence for crack growth increased from 1/2-power to 2.0-power with increasing temperature. Above a transition temperature, each grade of maraging steel became essentially immune to gaseous hydrogen embrittlement for the hydrogen pressure range considered. The transition temperature was strongly affected by yield strength and hydrogen pressure. Plausible explanations for these phenomenological results are considered.

Gaseous hydrogen embrittlement of high strength steels

Metal fatigue has been well recognized as an important cause for failure of engineering structures. In most applications, fatigue damage results from the conjoint actions of the cyclically applied stress and the external (chemical) environment, and is therefore time dependent. Understanding of this load-environment interaction is essential to the formulation of rational life prediction procedures and to the development of realistic materials evaluation and qualification tests. Research over the past 15 years has led to the suggestion that the rate of fatigue crack growth in an aggressive environment, (da/dN)e, is the sum of three components—the rate of fatigue crack growth in an inert environment, (da/dN)r, which represents the contribution of “pure” fatigue, a cycle-dependent component, (da/dN)cf, that requires the synergistic interaction of fatigue and environmental attack, and the contribution by sustained-load crack growth (i.e., stress corrosion cracking) at K levels above K
Iscc, (da/dN)scc. Recent fracture mechanics and surface chemistry studies have provided a clearer understanding of the cycle-dependent term, and, hence, a more complete understanding of environment assisted fatigue crack growth. (da/dN)cf results from the reaction of the environment with fresh crack surfaces produced by fatigue, and is a function of the extent of reaction during one loading cycle. For highly reactive alloy-environment systems, this contribution depends also on the rate of transport of the aggressive environment to the crack tip. The experimental basis and the development of models for transport and surface reaction controlled fatigue crack growth are reviewed. Interpretation of the effects of partial pressure of the aggressive environment and cyclic load frequency in terms of surface reaction and transport processes is discussed. Implications in terms of service performance and life prediction procedures are considered.

C.D. Beachem

Papers

A study of the stress factor in corrosion cracking by use of the pre-cracked cantilever beam specimen